Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

PART TWO – General Approach to Pediatric Anesthesia

Chapter 9 – Pediatric Anesthesia Equipment and Monitoring

Ronald S. Litman,David E. Cohen,
Robert J. Sclabassi

  

 

Latex Allergy, 273

  

 

Anesthesia Machines, 273

  

 

Anesthesia Machine Ventilators, 273

  

 

Humidifiers, 273

  

 

Anesthesia Breathing Systems, 275

  

 

Nonrebreathing and Partial Rebreathing Systems, 275

  

 

Mapleson A System,275

  

 

Mapleson D System,276

  

 

Circle Systems, 278

  

 

Anesthesia Facemasks, 278

  

 

Oral and Nasal Airways, 279

  

 

Cuffed Oropharyngeal Airway, 281

  

 

Laryngeal Mask Airway, 282

  

 

Perilaryngeal Airway, 285

  

 

Laryngeal Tube, 285

  

 

Endotracheal Tubes, 286

  

 

Endotracheal Tubes for One-Lung Ventilation,287

  

 

Univent Endotracheal Tube, 288

  

 

Selective Endobronchial Intubation, 288

  

 

Tracheostomy Tubes, 289

  

 

Laryngoscopes, 291

  

 

Devices and Techniques for a Difficult Intubation,291

  

 

Intravenous Equipment, 294

  

 

Catheters, 294

  

 

Infusion Sets, 295

  

 

Warming Devices, 295

  

 

Forced Warm Air Device, 296

  

 

Wrapping and Draping With Plastic Sheets,296

  

 

Humidification of Inspired Gases, 296

  

 

Monitoring, 296

  

 

Physical Examination, 297

  

 

Electrocardiography, 298

  

 

Systemic Arterial Pressure, 298

  

 

Central Venous Pressure, 300

  

 

Pulmonary Artery Catheters, 301

  

 

Transesophageal Echocardiography, 302

  

 

Temperature, 302

  

 

Urine Output, 302

  

 

Noninvasive Respiratory Gas Monitoring,303

  

 

Monitoring Oxygen and Carbon Dioxide, 305

  

 

Cutaneous Oxygen Tension, 305

  

 

Cutaneous Carbon Dioxide Tension,306

  

 

Pulse Oximetry, 306

  

 

Bispectral Index, 307

  

 

Neurophysiologic Monitoring, 307

  

 

Perioperative Assessment, 308

  

 

Technical Methodology, 308

  

 

Electrodes, 308

  

 

Signal Amplification,308

  

 

Stimulators, 308

  

 

Anesthetic Techniques, 309

  

 

Neurophysiologic Measures, 309

  

 

Maturational Effects, 309

  

 

General Procedures, 309

  

 

Electroencephalography, 309

  

 

Somatosensory and Motor Evoked Potentials, 310

  

 

Somatosensory Evoked Potentials (Ascending Activity),310

  

 

Median, Ulnar, and Radial Nerve Potentials, 311

  

 

Common Peroneal and Tibial Nerve Potentials, 311

  

 

Dermatomal Responses, 312

  

 

Motor Evoked Potentials, 312

  

 

Combined Ascending and Descending Activity, 313

  

 

Auditory Brainstem Responses, 313

  

 

Visual System, 313

  

 

Electromyography, 313

  

 

Evaluation of Cranial Nerve Function, 313

  

 

Electromyograms in Tethered Cord Releases and Selective Rhyzotomies,314

  

 

Summary, 314

Confronted with the task of caring for the tiny neonate or the large adolescent while providing optimal conditions for simple or complex surgical procedures, the pediatric anesthesiologist requires a wide variety of equipment and monitors often specifically adapted or developed for the pediatric patient. Specific modifications and adaptations of complex monitoring technology enhance the moment-to-moment surveillance of the anesthetized child, supplementing the anesthesiologist's direct observation of the patient. National and local standards reinforce this need for specially adapted electronic surveillance even in the simplest of anesthetic procedures and in the healthiest of children. This chapter focuses on the types of equipment and monitoring devices that are used in pediatric anesthesia.

▪ LATEX ALLERGY

With an increasing frequency, anaphylactic reactions to latex (natural rubber) have produced catastrophic problems for the pediatric anesthesiologist ( Slater et al., 1990 ; Holzman, 1993 ; Hepner and Castells, 2003 ). Children with myelodysplasia, congenital urologic abnormalities, or cerebral palsy with ventriculoperitoneal shunts are at the greatest risk for intraoperative reactions ( Kelly et al., 1991 ; Dormans et al., 1994, 1997 [79] [80]). This may be related to repeated exposure to rubber products during surgery or other procedures. The U.S. Food and Drug Administration has recommended that all patients should be questioned for latex hypersensitivity (1991). Even with a negative history for sensitivity to latex products, anaphylactic reactions have occurred, especially in the high-risk groups noted (Gold et al., 1991 ). Prophylactic management of children with known hypersensitivity or those at high risk may minimize both the incidence and severity of possible reactions ( Dormans et al., 1994 ). The benefit of this type of management is unknown and there are numerous reports of failure of premedication to prevent allergic reactions to latex ( Kwittken et al., 1992 ). Minimizing exposure to latex in the operating room is vitally important to both patients and workers. The use of a latex-safe protocol without premedication has been used successfully ( Holzman, 1997 ; Berry et al., 2001 ). Unfortunately, latex is ubiquitous in anesthetic and surgical equipment.

Substitution of latex-containing products and devices with latex-free devices minimizes exposure to the patient. The Food and Drug Administration has mandated that medical devices with natural rubber latex be identified on product and package labeling ( Hubbard, 1997 ). Careful examination of the components of each device or disposable product in the operating room complex is needed to identify problematic products. When substitute products are not available, efforts need to be made to shield or minimize contact between these devices and the child at risk.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ ANESTHESIA MACHINES

Although much of the equipment discussed in this chapter is designed specifically for pediatric use, no anesthesia machines are dedicated exclusively to pediatric applications. There are certain characteristics that should be sought in an anesthesia machine to be used in pediatric patients.

Measures should be taken to eliminate the risk of unintentional administration of an hypoxic gas mixture. The American Society for Testing and Materials (ASTM) (F-1850-00, 2005) specifies that all machines be designed to deliver a preset oxygen concentration at any flow and oxygen gas pressure. Because these safety mechanisms can fail, an oxygen analyzer should be maintained in-line as specified in the Standards for Basic Anesthetic Monitoring of the American Society of Anesthesiologists (ASA) (2003 ; Box 9-1 ).

BOX 9-1 

Standards for Basic Anesthetic Monitoring

Rights were not granted to include this content in electronic media. Please refer to the printed book.

When fragile hemodynamics or the patient's condition (e.g., necrotizing enterocolitis) precludes the use of nitrous oxide, compressed air should be available to reduce the inspired oxygen concentration (FIO2). Another example is the premature infant at risk for retinopathy of prematurity (ROP). Clinical studies implicate arterial oxygen tension as one of several variables linked to ROP in the most vulnerable population of preterm infants (<1300 grams) whose retinas are immature ( Flynn et al., 1992 ). The contribution of brief periods of intraoperative hyperoxemia to the development of ROP remains unknown; oxygen concentrations greater than necessary should be avoided in this vulnerable population. Compressed air is usually required during airway laser surgery to reduce the concentration of inhaled gas mixtures that support combustion (i.e., nitrous oxide, oxygen). Specially adapted anesthesia machines capable of delivering inspired carbon dioxide to achieve hypercarbia or increased inspired nitrogen to produce hypoxia may be indicated in the care of the neonate with specific types of congenital heart disease ( Tabbutt et al., 2001 ).

Another feature of an anesthesia machine that is useful in pediatric anesthesia is the ability to accommodate special pediatric circuits (e.g., Mapleson). This represents more than a convenience, as it avoids the inherent dangers of “modifying” vital equipment to meet the needs of these circuits. A well-designed pediatric breathing circuit reduces the risk of incorrect assembly by those who use these circuits infrequently.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ ANESTHESIA MACHINE VENTILATORS

An anesthesia machine ventilator appropriate for infants and small children should be capable of accurately delivering a range of small tidal volumes and high ventilatory rates. Newer anesthesia machines have ventilators that can precisely deliver small volumes at high rates (e.g., NAD 6000; North American Dräger, Telford, PA) ( Stayer et al., 2000 ). The standard ventilators used in pediatric intensive care units can be adapted for use in the operating room (e.g., Siemens Servo 900C; Siemens-Elema AB, Sweden). The added cost and complexity of this approach may be warranted only in children with preexisting pulmonary disease.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ HUMIDIFIERS

In the human respiratory tract, inspired gases are warmed and brought to 100% relative humidity (44 mg H2O/L at 37°C). The caloric expenditure of humidification consumes approximately five times the energy required to heat the inspired gases ( Rashad and Benson, 1967 ); this may amount to 20% of the basal metabolic rate of an infant.

Benefits of heating and humidifying anesthetic gases include prevention of intraoperative hypothermia, decreased atelectasis, improved mucociliary clearance, and reduced impairment of ciliary function caused by inhalation of dry anesthetic gases. Partial humidification of gas in the anesthesia breathing circuit takes place within the carbon dioxide absorber, which uses an exothermic reaction that may raise the water vapor content to as much as 29 mg/L. Further humidification is accomplished by reducing the amount of fresh gas flow, thereby increasing rebreathing of humidified gases, and using a heat and moisture exchanger (HME) (“artificial nose”), which uses a fine mesh to cause condensation of exhaled water vapor. The latter may increase the resistance to breathing for some infants and children, although these changes are usually tolerable. HMEs increase airway humidification and preserve temperature in anesthetized children at a lower cost than active humidification systems ( Bissonnette and Sessler, 1989 ). They require 80 minutes to achieve optimal saturation of the membrane, during which time they are less efficient. Specially designed HMEs filter out infectious pathogens and minimize the risk of cross-infection between patients ( Wilkes et al., 2000 ).

Active humidification is the most efficient means by which to heat and humidify inspired gases ( Bissonnette and Sessler, 1989 ). A servo-controlled, shielded heated wire in the fresh gas line helps to prevent cooling and condensation of the water as it passes through the inspiratory limb. The temperature should be regulated by a probe near the patient connection, because overheating of the inspired gases can produce injury to the airway ( Klein and Graves, 1974 ). Active humidifiers may also increase the compression volume of the breathing circuit ( Coté et al., 1983 ); thus, compensatory increases in the tidal volume during controlled ventilation may be necessary.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ ANESTHESIA BREATHING SYSTEMS

Since Philip Ayre's landmark article (1937) began the modern era of breathing systems for pediatric anesthesia, this has been a topic of controversy. Using Magill's technique of endotracheal anesthesia for the repair of cleft lip and palate in infants, Ayre noted adverse results. Breathing through a “closed” system, these infants often developed “rapid, ‘sighing’ respirations” and “ashy pallor and sweating.” They exhibited a “dark, congested oozing at the site of operation.” Postoperatively, the infants were “in varying degrees of shock: some … for days.” The contribution of hypotension or hypovolemia to this picture remains unknown, because blood pressure was not measured and blood loss was difficult to quantify by Ayre's account.

Ayre noted dramatic clinical improvement when he adopted an open T-piece breathing system. The T-piece, an extremely simple device, consists of an inspiratory limb, a connection to the patient, and an expiratory limb. It has no unidirectional or overflow valves, nor any breathing bag. The expiratory limb serves as a reservoir for fresh gas, a means of monitoring the infant's respirations, and, if the distal end is intermittently occluded, a means of providing positive pressure ventilation. If the volume of the expiratory limb is one third of the tidal volume, rebreathing can be virtually eliminated during spontaneous ventilation with a fresh gas flow that is twice the minute ventilation ( Ayre, 1956 ). Ayre attributed the salutary effect of the T-piece to marked reductions in resistance to gas flow and rebreathing.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ NONREBREATHING AND PARTIAL REBREATHING SYSTEMS

Despite its apparent benefits, the T-piece is far from ideal. The major flaws are its release of anesthetic gases into the operating room and its inability to provide assisted or controlled ventilation. A series of modifications followed. Jackson-Rees first proposed the addition of a breathing bag to the expiratory limb ( Jackson-Rees, 1950 ). Another system, the Magill attachment, which predated Ayre's publication, introduced fresh gas distal to a breathing bag and an overflow valve near the patient connection. These and other variations were brought together under a single classification scheme proposed by Mapleson (1954) in which each system was distinguished on the basis of the location of its fresh gas inflow and overflow valves relative to the patient connection ( Fig. 9-1 ).

 
 

FIGURE 9-1  The Mapleson circuits. Each circuit is classified on the basis of the relative position of the fresh gas inlet, overflow valve, corrugated tubing, and reservoir bag in relation to the patient connection.  (Adapted from Mapleson WW: The elimination of rebreathing in various semi-closed anaesthetic systems. Br J Anaesth 26:323, 1954.)

 

 

 

The Mapleson systems share the benefit of reduced resistance to breathing by virtue of the absence of unidirectional valves and canisters; the elimination of these components results in various degrees of rebreathing. Rebreathing is not necessarily bad, as it serves to conserve heat, humidity, and anesthetic gases. Yet in the absence of a mechanism by which to monitor the accumulation of carbon dioxide, the consequences of hypercarbia and respiratory acidosis probably outweigh these benefits.

Each system has very different rebreathing characteristics depending on the location of the fresh gas inflow and overflow valves, the fresh gas flow rate, the respiratory rate (i.e., expiratory time) and tidal volume, carbon dioxide production, and the mode of ventilation (i.e., spontaneous or controlled). The following sections describe the Mapleson A (Magill attachment) and D systems. The B and C systems are virtually never used today. The Mapleson E system is the T-piece described earlier.

▪ MAPLESON A SYSTEM

The Mapleson A system results in virtually no rebreathing during spontaneous ventilation when the fresh gas flow is more than 75% of the minute ventilation; it requires a large fresh gas flow to eliminate rebreathing during controlled ventilation ( Waters and Mapleson, 1961 ; Kain and Nunn, 1967 ) ( Fig. 9-2 ). This design is also impractical in the operating room because the proximal location of the overflow valve makes it cumbersome for scavenging waste gases, difficult to adjust during head and neck surgery, and potentially dangerous, as the heavy valve could dislodge a small endotracheal tube.

 
 

FIGURE 9-2  Rebreathing characteristics of the Mapleson A circuit. During spontaneous ventilation, the overflow valve is open and exhaled gases are discharged. There is virtually no rebreathing as long as fresh gas flow is more than 75% of the minute ventilation. With controlled ventilation, the valve is closed and rebreathing is significant.

 

 

▪ MAPLESON D SYSTEM

The Mapleson D system is characterized by a proximal fresh gas inflow and a distal overflow valve. It is a modification of the T-piece in which a breathing bag and an overflow valve have been added to the distal expiratory limb. Although it requires slightly more fresh gas flow to eliminate rebreathing during spontaneous ventilation than the Mapleson A system, it is the most economical during controlled ventilation ( Waters and Mapleson, 1961 ). On balance, considering both spontaneous and controlled ventilation, the Mapleson D requires the lowest fresh gas flow rates among all Mapleson circuits. This system has become the most widely used of the Mapleson circuits for pediatric anesthesia.

The precise flow dynamics in the Mapleson D system is a subject of controversy that has resulted in a variety of complex recommendations ( Mapleson, 1954 ; Waters and Mapleson, 1961 ; Nightingale et al., 1965 ; Bain and Spoerel, 1973 ; Rose et al., 1978 ; Spoerel et al., 1978 ; Rose and Froese, 1979 ). To eliminate rebreathing, higher fresh gas flows are needed during spontaneous ventilation than during controlled ventilation. With spontaneous ventilation, rebreathing is virtually eliminated by provision of fresh gas flow equal to the mean inspiratory flow rate ( Mapleson, 1954 ; Rose et al., 1978 ). If one assumes an inspiratory/expiratory ratio of 1:1 to 1:2, the mean inspiratory flow rate is two to three times the minute ventilation. Although Spoerel and others (1979) have demonstrated that a normal PaCO2can be maintained during spontaneous ventilation at fresh gas flows as low as 100 mL/kg per minute, an increased minute ventilation (and hence, more respiratory work) is required to compensate for rebreathed carbon dioxide.

The recommendations for fresh gas flow during controlled ventilation are complex and varied ( Waters and Mapleson, 1961 ; Nightingale et al., 1965 ; Bain and Spoerel, 1973 ). This reflects the importance of several factors that were summarized by Rose and Froese (1979) ( Fig. 9-3 ). When a high fresh gas flow (>100 mL/kg per minute) is used, the PACO2 is governed by minute ventilation (ventilation limited). At low fresh gas flow (<90 mL/kg per minute), PACO2 is independent of minute ventilation, varying instead as a function of the amount of rebreathing, which is governed by the fresh gas flow rate (flow limited).

 
 

FIGURE 9-3  Mapleson D, controlled ventilation. Illustrates factors in determining alveolar PCO2 during controlled ventilation with Mapleson D. At low fresh gas flow (FGF) (<90 mL/kg per minute), PaCO2 changes only with FGF (“flow-limited”) regardless of ventilation. At higher FGF (>100 mL/kg per minute), PaCO2 changes as a result of changes in delivered minute ventilation (“ventilation-limited”).  (From Froese AB: ASA annual refresher course, 1978.)

 

 

 

Additional important factors that govern the magnitude of rebreathing include carbon dioxide production, respiratory rate, and respiratory waveform characteristics (inspiratory flow, inspiratory and expiratory times, and expiratory pause) ( Rose and Froese, 1979 ). Adjustments to the ventilatory pattern that allow the fresh gas flow to constitute a larger proportion of the inspired gas (e.g., slow inspiratory time, low inspiratory flow) or that enable exhaled gases to be more completely washed out (e.g., long expiratory pause, slow rate) reduce the amount of rebreathing. If one were using a Mapleson D circuit with controlled ventilation and low fresh gas flow (flow limited), an attempt to reduce the PaCO2 by increasing the respiratory rate would reduce the expiratory pause and thus promote rebreathing ( Fig. 9-4 ). In this situation, the increased ventilation is offset by increased FICO2, resulting in no net change in PaCO2. To wash out the exhaled gas at this higher respiratory rate and take advantage of the increased minute ventilation, the fresh gas flow must be increased. These fresh gas flow and ventilatory recommendations are predicated on a normal metabolic rate and hence normal carbon dioxide production ( Bain and Spoerel, 1977 ; Nightingale and Lambert, 1978 ). Conditions that increase carbon dioxide production (e.g., fever, catabolic state, malignant hyperthermia) must be met with a proportional increase in fresh gas flow or ventilation.

 
 

FIGURE 9-4  Fresh gas flow-limited controlled ventilation using a Mapleson D circuit. Note that higher respiratory rate reduces the washout of exhaled gases. Rebreathing occurs so that there is no net reduction in end-tidal carbon dioxide (ETCO2). Fresh gas flow must be increased to wash out exhaled gases and lower the ETCO2.

 

 

Bain Modification of Mapleson D

The Bain modification of the Mapleson D circuit incorporates the fresh gas supply within the expiratory limb in a coaxial arrangement ( Bain and Spoerel, 1972 ) ( Fig. 9-5 ). This circuit is light and streamlined with only a single hose to the patient. It also provides some countercurrent warming of the inspired gases and effective scavenging of expired gases. Its major disadvantage lies in the inability to directly inspect the integrity of the inspiratory limb. Pethick (1975) described an indirect test of the Bain inspiratory limb integrity in which the oxygen flush is passed through the circuit for several seconds. If the inspiratory limb is intact, the rapid flow of gas through it exerts a Venturi effect on the expiratory limb, resulting in a slight negative pressure and collapse of the breathing bag. With a leak from the inspiratory limb into the expiratory limb, the pressure in the latter rises, tending to inflate the reservoir bag. The rebreathing characteristics of the Bain circuit are essentially identical to those of any other Mapleson D. The major reasons that proponents have advocated the use of these Bain circuits for pediatric anesthesia are their relatively lower resistance to breathing and the simplicity with which a heating circuit may be added.

 
 

FIGURE 9-5  The Bain circuit. The Bain modification of the Mapleson D circuit provides a low profile circuit and allows simple connection to the patient's airway.

 

 

The primary sources of resistance in an anesthesia delivery system are (1) the endotracheal tube, (2) the valves, and (3) the carbon dioxide absorber. With modern equipment, the endotracheal tube represents a major source of resistance in the neonate ( Cave and Fletcher, 1968 ; Brown and Hustead, 1969 ). Lightweight, large-diameter, modern disc valves exert resistance in two ways. There is a minimum, flow-independent resistance necessary to displace the valve, usually much less than 1 cm H2O ( Hunt, 1955 ). A much higher resistance may be required when the expiratory valve is wet. At high gas flows (>30 L/min), the valves also become a source of turbulent resistance proportional to the flow through them. Carbon dioxide canisters are also a source of turbulence. Their resistance is inversely proportional to the length of the path the gas must take through the resistor. Modern absorbers are short and wide to minimize this path of resistance.

At approximately a half-century after Ayre introduced the T-piece, the extent to which his work applies remains unclear. If the infants he described were victims of rebreathed carbon dioxide in those days before the carbon dioxide absorber came into common use, it would seem that infants are more likely to be faced with rebreathing in the Mapleson circuits than in the modern circle system. Perhaps Ayre's infants were subjected to the significant resistance imposed by the valves of that era. Although the neonate has a lower proportion of fatigue-resistant fibers in the diaphragm ( Muller et al., 1979 ), infants as small as those 2 weeks old have been shown to be able to compensate for increases in resistance of 200% without changes in blood gases, at least for relatively short periods of time ( Graff, 1966 ). The benefits of the Mapleson systems must be weighed against their inherent problems on an individual basis. One must also factor in the added risks of a practitioner who uses Mapleson circuits infrequently and is unfamiliar with their characteristics or who may have to make substantial alterations to an anesthesia machine to accommodate them. Even if one uses a circle system for most children, it is important to understand the flow requirements of this circuit, as it continues to be used very commonly in intensive care units and for transport of critically ill patients.

▪ CIRCLE SYSTEMS

The circle system is standard equipment on anesthesia machines designed to meet the standards specified in ASTM 1850-00 (2005). When functioning properly, it enables lower fresh gas flows than the Mapleson D; a circle system conserves heat, humidity, and anesthetic agent. It also minimizes environmental pollution. Compared with the resistance of an endotracheal tube, the additional resistance imposed by the addition of unidirectional valves and a carbon dioxide canister seems trivial. Because the ratio of the patient's tidal volume to the total circuit volume is small in young children, changes in the anesthetic concentration can take some time to reach equilibrium unless higher fresh gas flows are used. One must be vigilant for the manifestations of stuck (resistance) or floating (rebreathing) unidirectional valves, because both have harmful consequences in small children.

Special circle breathing systems have been designed for infants and children. They incorporate the same components as the standard adult circuits but have been modified to minimize dead space and resistance to breathing. Most incorporate short, narrow-caliber hoses and Y connections and smaller carbon dioxide absorbers.

Although these circuits have been shown to minimize dead space and the resistance work of breathing, they have not gained wide acceptance. They require significant modifications of standard equipment and thus are inconvenient. Unlike the adult circle system and Mapleson circuits, these circuits have not been marketed in modern, disposable materials that readily adapt to standard anesthesia machines.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ ANESTHESIA FACEMASKS

The most appropriate anesthesia facemask for a child spans vertically from the bridge of the nose to just below the lower lip, without compressing the nasal passages ( Fig. 9-6 ). It should contain the least volume inside (i.e., dead space) possible. The pediatric facemask should be constructed as a clear (non-latex-containing) plastic that allows recognition of cyanosis, the condensation of exhaled gas, and the presence of excess secretions or vomitus. A constant challenge in pediatric anesthesia, especially for small infants, is to find a facemask that conforms to the shape of the infant's face without a significant leak. During positive pressure ventilation, the anesthesia practitioner often must twist or torque the facemask, without applying undue pressure against the child's face, to reduce the amount of air that escapes from within the mask. To achieve these purposes, a variety of facemasks have been used in the pediatric population.

 
 

FIGURE 9-6  The most effective mask ventilation technique for infants and young children is for the anesthesiologist to hold the mask over the mouth and nose with the thumb and forefinger, while the middle finger is placed on the bony portion of the mandible.  (From Litman RS: Pediatric airway management. In: Litman RS, ed: Pediatric anesthesia, The requisites. St. Louis, 2004, Mosby.)

 



The most common anesthesia facemask in use today is the plastic disposable type that contains an adjustable pneumatic cushion that, when inflated or deflated with air, can be altered to conform to the shape of the child's face. A variety of different manufacturers produce this type of facemask ( Fig. 9-7 ). An alternative variety for use in pediatric patients is the Rendell-Baker-Soucek mask, which remains in use in many centers ( Fig. 9-7 ). This mask is available in malleable rubber or non-latex-containing silicone and allows an effective seal on the child's face while minimizing internal dead space. It was originally designed on the basis of anatomic molds of a large number of children ( Rendell-Baker and Soucek, 1962 ).

 
 

FIGURE 9-7  Pediatric facemasks. Various mask sizes are available for the wide range of pediatric patients. Top row, Four different Rendell-Baker-Soucek masks (Willy Rusch, Kernen, Germany). Bottom row, Disposable bubble masks (Vital Signs, Totowa, NJ).

 

 

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ ORAL AND NASAL AIRWAYS

Oral and nasal pharyngeal airway devices are used in pediatric anesthesia to improve patency of the upper airway and to facilitate delivery of oxygen or anesthetic gases to the lungs. Minimum requirements for these devices are noted in ASTM F1573-95 (Subcommittee F29.12 on Airways, 2000). The Guedel-type oral airway is probably the airway most commonly used in pediatric patients. It contains a central lumen for the passage of airflow and for suctioning of the posterior pharynx ( Fig. 9-8 ). The oral airway device is primarily used when manual airway-opening techniques have failed to alleviate upper airway obstruction, which is usually caused by tonsillar or adenoidal hypertrophy, or normal pharyngeal tissue obstruction, as often occurs in small infants (see Chapter 2 , Respiratory Physiology).

 
 

FIGURE 9-8  Oral airways for the entire spectrum of pediatric patients from infancy through adolescence.

 

 

Oral airways are usually manufactured from plastic or polyethylene and are latex free. They are sized depending on the total length of the device (50 to 80 mm, flange to tip, for most children) or based on an arbitrary scale designated by the manufacturer. The appropriate size is determined by placing the airway adjacent to the child's face to approximate its position in the oral cavity ( Fig. 9-9 ). When appropriately placed, its distal end snugly curves around the back of the tongue, without the proximal end protruding out of the mouth. Too small an oral airway pushes the posterior portion of the tongue against the posterior pharyngeal wall, and too large an oral airway may itself cause upper airway obstruction at the laryngeal inlet by compressing or distorting the epiglottis.

 
 

FIGURE 9-9  The appropriate-size oral airway device is chosen by placing it adjacent to the face to approximate its position in the oral cavity.

 

 

The oral airway device should be inserted in its normal orientation position with the aid of a tongue depressor. In older children, insertion may be accomplished with the distal tip oriented cephalad, and then turned 180 degrees when the tip has reached the posterior aspect of the palate. In younger children, this maneuver may push the tongue posteriorly and exacerbate airway obstruction.

Complications of oral airway use in children are not infrequent and usually occur during emergence. Lip or tooth damage is possible; a loose tooth may become dislodged and lost in the oral cavity, where it may accidentally travel into the bronchopulmonary tree. Compression of oral structures by the oral airway may result in transient postoperative numbness.

The nasal airway device is made from soft latex-free rubber to allow easy insertion through the nasal passage and into the nasal or oropharynx. It can be bathed in warm or cold water to decrease or increase its stiffness, respectively. Some nasal airways contain an enlarged flange at the proximal end to prevent unintentional advancement into the nasal cavity. Others contain an adjustable ring to secure against the outside of the nasal opening ( Fig. 9-10 ). Nasal airways are available in sizes 12 F to 36 F (outer diameter). If required, a stiffer nasal airway can be fashioned out of a standard endotracheal tube by cutting it off at the appropriate length. An anesthesia breathing circuit can be connected to any type of nasal airway using an appropriately sized endotracheal tube adaptor ( Fig. 9-11 ).

 
 

FIGURE 9-10  A large selection of nasal airways is needed for pediatric patients. Those pictured (Willy Rusch, Kernen, Germany) can be adjusted for length by moving the circular disc along the tube.

 

 

 
 

FIGURE 9-11  The nasal airway can easily be connected to a standard 15-mm endotracheal tube adaptor for connection into an anesthesia breathing circuit.

 

 

Before insertion, the nasal cavities should be inspected to ensure the absence of significant septal deviation or other causes of narrowing (e.g., polyp) that obstruct passage of the nasal airway. To avoid trauma and bleeding of the delicate nasal mucosa, the nasal airway should be lubricated and gently inserted in a posterocaudad direction along the floor of the nasal cavity. A topical vasoconstrictor, such as 0.05% oxymetazoline, can be applied to the nasal mucosa before nasal airway insertion to shrink nasal mucosal tissue and reduce bleeding. The proper diameter of the nasal airway is determined by approximating the circular diameter of the nasal opening. The proper length of the nasal airway is estimated by measuring the distance from the nares to the tragus of the ear. When appropriately placed, its distal tip should lie at the level of the angle of the mandible, between the posterior aspect of the tongue and above the tip of the epiglottis.

The most common complication from nasal airway insertion is trauma to the nasal or pharyngeal mucosa that results in minor bleeding. Adenoidal tissue may be disrupted and may bleed into the oropharynx. Occasionally, a friable vessel is encountered in the nasal mucosa and bleeding is brisk. A lesser known, although not rare, complication is the insertion of the nasal airway device into a false passage beneath the posterior wall mucosa of the nasal and oral pharynx. This is not usually accompanied by bleeding and may be caused by a patent bursa of Thornwaldt ( James et al., 1968 ). Nasal airways should not be inserted in children with a coagulopathy, neutropenia, or suspicion of a traumatic basilar skull fracture.

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Copyright © 2005 Mosby, An Imprint of Elsevier

▪ CUFFED OROPHARYNGEAL AIRWAY

The cuffed oropharyngeal airway (COPA) is essentially a Guedel airway manufactured with a 15-mm anesthesia breathing circuit connector on the proximal end and an inflatable cuff on the distal end ( Fig. 9-12 ). It has an integrated bite block, which is color coded for size and to help with proper positioning. The device is inserted in a similar manner to that of an oral airway; once inserted, the cuff is inflated (with 20 to 40 mL air) to provide a low-pressure seal in the hypopharynx to facilitate spontaneous or controlled ventilation. Once inserted, the COPA can be secured in place using an accompanying head strap that attaches to the posts on the tooth/lip guard. The COPA is intended as a single-use device.

 
 

FIGURE 9-12  The cuffed oropharyngeal airway (COPA) is essentially a Guedel airway manufactured with a 15-mm anesthesia breathing circuit connector on the proximal end and an inflatable cuff on the distal end.

 

 

The principal indication of the COPA is to aid airway management by replacing the use of an anesthesia facemask, freeing up the hands of the anesthesia practitioner ( Robbins and Connelly, 2000 ;Sammartino and Ferro, 2002 ). It is primarily intended for use in anesthetized patients who are breathing spontaneously, but it also can be used with controlled ventilation in some patients.

The COPA is available in four sizes ( Table 9-1 ). The smallest (size 8) is appropriate for most school-aged children. When held adjacent to the patient's head, the appropriately sized COPA should rest with the bite block just above the teeth and the distal tip at the angle of the mandible. This is usually one size larger than the corresponding appropriately sized oral airway. Before insertion, the distal end of the device is lubricated and the cuff is tested for leaks. When sized correctly, the COPA “locks into place” behind the base of the tongue. If the proper size has been chosen, the colored bite block should “transition” at the teeth. Once inserted, the COPA is fastened to the head strap, a jaw-thrust/chin-lift maneuver is performed, and the cuff is inflated with the proper amount of air (see Table 9-1 ).


TABLE 9-1   -- Characteristics of the cuffed oropharyngeal airway

Size

Color

Amount of Air Needed to Inflate Cuff (mL)

8

Green

25

9

Yellow

30

10

Red

35

11

Light green

40

 

 

On occasion, manual airway adjustments may be required to enhance the proper functioning of the COPA; these include increased or decreased head tilt, turning the head to one side, supporting the shoulders, gentle chin lift, or application of continuous positive airway pressure (CPAP) up to 10 cm H2O ( Bussolin and Busoni, 2002 ). The COPA can be removed at any time, preferably with the cuff remaining inflated to facilitate removal of oral secretions.

When compared with the laryngeal mask airway (LMA) in children, the use of a COPA resulted in a greater number of subsequent airway maneuvers or a switch to another airway method to establish ventilation ( Mamaya, 2002 ). Its use was also associated with less airway response with cuff inflation and decreased requirement for assisted ventilation compared with the LMA.

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▪ LARYNGEAL MASK AIRWAY

The LMA has become an accepted standard device for airway management in pediatric patients ( Lopez-Gil et al., 1996 ). It consists of a rigid tube with a standard 15-mm connector at the most proximal end and a fenestrated, elliptical cuff cavity at the distal end ( Brain, 1983 ; Brain et al., 1985 ) ( Fig. 9-13 ). When placed properly, the distal cuff overlies the laryngeal inlet, and the fenestrations prevent the epiglottis from obstructing the lumen. Once inflated through a pilot tube, the cuff creates a seal in the pharynx that permits both spontaneous and controlled ventilation without a large gas leak when the peak pressure is below 15 cm H2O ( Epstein and Halmi, 1994 ). A black line runs longitudinally along the posterior curvature to permit orientation of the tube after placement. A variety of LMA sizes are available to accommodate all pediatric age groups ( Table 9-2 ). Newer, disposable models are available in all pediatric sizes (Portex, Keene, NH). Several larger reusable sizes are made with a port that allows passage of an orogastric tube to facilitate gastric emptying (ProSeal; LMA North America, Inc., San Diego, CA).

 
 

FIGURE 9-13  A laryngeal mask airway, (A) deflated and (B) inflated.

 

 


TABLE 9-2   -- Laryngeal mask airway size characteristics

Laryngeal Mask Airway Size

Approximate Weight (kg)

Cuff Volume (mL)

1

<5

2 to 5

1.5

5 to 10

3 to 8

2

10 to 20

5 to 10

2.5

20 to 30

10 to 15

3

30 to 50

15 to 20

4

50 to 70

≤30

5

70 to 100

≤40

6

>100

≤50

 

 

The LMA is used in pediatric anesthesia as a routine airflow conduit during general anesthesia, as a tool with which to facilitate endotracheal intubation in difficult-to-intubate children, and as an airway rescue device for the difficult-to-ventilate child. The LMA is recommended as a component of the ASA difficult airway algorithm to facilitate ventilation when bag-mask ventilation or endotracheal intubation is unsuccessful in adults ( ASA Task Force on Management of the Difficult Airway, 1993 ; Benumof, 1996 ).

When the LMA was first introduced in the 1980s, its use as an elective airway device was reserved for procedures that were amenable to facemask anesthesia ( Grebenik et al., 1990 ; Johnston et al., 1990 ;Mason and Bingham, 1990 ; Watcha et al., 1994 ). Except for the emergency rescue situation, the LMA was considered a substitute for an anesthesia facemask but not for an endotracheal tube. As practitioners have become more comfortable with its use, the LMA has become a useful alternative to an endotracheal tube in certain nonemergent cases, such as tonsillectomy or strabismus repair. An LMA that is specially designed with a wire-reinforced shaft is available for use during procedures that would otherwise require an oral RAE endotracheal tube, named after its inventors, Ring, Adair, and Elwyn (1975) (Mallinckrodt, Inc., St. Louis, MO) ( Webster et al., 1993 ). In most children, positive pressure ventilation can be accomplished via an LMA, but peak inspiratory pressures greater than approximately 15 cm H2O are associated with a leak around the distal cuff into the esophagus, subsequent gastric insufflation, and possible regurgitation and aspiration ( Fawcett et al., 1991 ; Gursoy et al., 1996 ).

A variety of methods of LMA placement in children are possible. The manufacturer recommends that the mask be advanced across the hard palate with the cuff fully deflated, distal aperture facing anteriorly, and head in the classic “sniffing” position. The index finger of the right hand helps guide the LMA over the surface of the tongue. A water-based lubricant smeared onto the posterior surface of the LMA may decrease the resistance to insertion. On meeting the characteristic resistance of the upper esophageal sphincter, the cuff is inflated through the pilot tube. With cuff inflation, the tube usually moves outward a short distance as the tube centers itself over the laryngeal inlet.

The LMA can also be inserted with the cuff partially inflated or with the aperture facing posterior and then turned 180 degrees once in the larynx ( Chow et al., 1991 ; McNicol, 1991 ; O'Neill et al., 1994 ;Nakayama et al., 2002 ). There are no advantages to one particular insertion method, but one method may be easier than another in any particular child. Additional maneuvers that may facilitate ease of insertion include increasing head extension, pushing the tongue forward with a jaw-thrust maneuver, inserting the LMA slightly laterally to avoid the uvula, or using a laryngoscope to lift the tongue anteriorly ( Brain, 1989 ; van Heerden and Kirrage, 1989 ; Cass, 1991 ). Nevertheless, in some children, insertion is difficult and associated with pharyngeal bleeding ( Marjot, 1991 ). The LMA is usually placed in an anesthetized child; insertion in a conscious or sedated child (after use of topical analgesia) is occasionally necessary when one desires to gain control of the child's airway in a potentially difficult-to-ventilate situation ( Denny et al., 1990 ; Markakis et al., 1992 ).

When inserted correctly, the distal end of the LMA cuff rests in the proximal end of the esophagus, the proximal portion of the LMA cuff pushes the epiglottis anteriorly, and the grille lies over the laryngeal inlet ( Goudsouzian et al., 1992 ) ( Fig. 9-14 ). The relatively cephalad location of the pediatric larynx does not lend itself to ideal LMA placement. Fiberoptic bronchoscopy and magnetic resonance imaging (MRI) studies in pediatric patients demonstrate a high incidence of malpositioning of the LMA after placement in children ( Keidan et al., 2000 ). In general, the smaller the size of the LMA, the higher the incidence of malposition, usually with the epiglottis contained within the grille of the LMA ( Rowbottom et al., 1991 ; Mizushima et al., 1992 ; Dubreuil et al., 1993 ). Studies have revealed that even in the presence of this malpositioning, ventilation is not usually impaired ( Mason and Bingham, 1990 ; Rowbottom et al., 1991 ). Ventilation may become impaired more easily in children subsequent to seemingly minor movements of the child or LMA.

 
 

FIGURE 9-14  When properly inserted, the distal outlet of the laryngeal mask airway is situated over the laryngeal inlet.

 

 

LMA use with spontaneously ventilating patients results in fewer episodes of arterial oxygen desaturation than does standard mask ventilation ( Johnston et al., 1990 ; Watcha et al., 1994 ). In addition, the work of breathing through an LMA is less than the work of breathing through a standard facemask ( Keidan et al., 2000 ). Unlike a facemask, the LMA can be secured to the face with tape in the manner of a tracheal tube, thereby freeing the anesthesiologist to attend to other care responsibilities such as drug and fluid administration or record-keeping.

Compared with tracheal intubation, the LMA produces less sympathetic stimulation, despite a lighter plane of anesthesia, and is associated with fewer postoperative sore throats than tracheal intubation (Alexander and Leach, 1989 ; Hickey et al., 1990 ; Wilkins et al., 1992 ). Compared with an endotracheal tube, the LMA is associated with less laryngeal stimulation and a decreased incidence of airway complications in children with upper respiratory tract infections ( Tait et al., 1998 ).

The cardiovascular response produced with LMA insertion is not as pronounced or prolonged as that observed with tracheal intubation ( Hickey et al., 1990 ; Wilson et al., 1992 ). Changes in intraocular pressure are similarly blunted ( Lamb et al., 1992 ; Watcha et al., 1992 ). Accurate intraocular pressure measurements may be obtained during LMA use in children.

For the pediatric anesthesiologist, the LMA has significantly improved the care of children with congenital facial anomalies, such as Robin's syndrome or Treacher Collins syndrome, who in the past would have been difficult to ventilate and intubate ( Beveridge, 1989 ; Ebata et al., 1991 ; Hansen et al., 1995 ; Stocks et al., 2002 ; Bucx et al., 2003 ; Gandini and Brimacombe, 2003 ). Initial reports of successful LMA placement in these children while conscious led to the widespread use of the device after the induction of general anesthesia. It is unusual that the LMA cannot provide adequate ventilation in an anesthetized child with a congenital facial anomaly. Once inserted, the LMA has also been used to facilitate endotracheal intubation using flexible fiberoptic bronchoscopy ( Heard et al., 1996 ). This technique is limited by the small size of the LMA relative to the size of the flexible bronchoscope and endotracheal tube ( Table 9-3 ). Blind intubation of the trachea with an endotracheal tube or an intubating guide that subsequently serves as a stylet can often be accomplished without fiberoptic visualization because the apertures of the LMA and larynx are usually closely approximated ( Chadd et al., 1992 ; Kihara et al., 2000 ). This blind technique is problematic in small children, in whom the approximation of aperture and larynx is perfect in only 27% of patients ( Dubreuil et al., 1993 ).


TABLE 9-3   -- Pediatric-sized laryngeal mask airways and compatible endotracheal tubes[*]

Laryngeal Mask Airway Size

Maximum Lubricated Uncuffed Standard Endotracheal Tube Inner Diameter (mm)

Maximum Lubricated Cuffed Standard Endotracheal Tube Inner Diameter (mm)

Maximum Flexible Bronchoscope Size[†]

1

3.5

3.0

2.7

1.5

4.0

4.0

3.0

2

5.0

4.5

3.5

2.5

6.0[‡]

5.0

4.0

3

6.0

5.0

4

6.0

5.0

5

7.0[†]

5.0

6

7.0[†]

5.0

With permission from Litman RS: The difficult pediatric airway. In Litman RS, editor: Pediatric anesthesia: The requisites, St. Louis, 2004, Mosby.

*

Based on experiments performed by the author.

As per LMA North America.

Largest available uncuffed endotracheal tube available at The Children's Hospital of Philadelphia.

 

A specially designed LMA (Fastrach; The Laryngeal Mask Company, Henley-on-Thames, UK) is available in larger sizes to facilitate intubation in individuals with difficult airways ( Brimacombe, 1997 ;Ferson et al., 2001 ) ( Fig. 9-15 ). The Fastrach is shorter than the standard LMA and has a rigid handle to allow easy manipulation and a different distal aperture to allow easier elevation of the epiglottis when passing an endotracheal tube.

 
 

FIGURE 9-15  Laryngeal mask airway Fasttrach is designed to facilitate intubation in individuals with difficult airways (Fastrach, Henley-on-Thomes, UK).

 

 

Because of its inability to adequately seal off the trachea, the LMA is not indicated for use in children at risk for pulmonary aspiration of gastric contents. The extent of protection provided by the LMA against regurgitation of stomach contents into the trachea or aspiration of oral contents is unclear. In adults without significant risk for reflux or aspiration, there was a 25% incidence of regurgitation of methylene blue into the laryngeal mask ( Barker et al., 1992 ). No tracheal soiling was noted. Nevertheless, multiple cases of aspiration of gastric contents have been reported in conjunction with LMA use (Cyna, 1990 ; Wilkinson, 1990 ; Maroof et al., 1993 ). Aspiration of stomach contents may result from high intragastric pressure, a decrease in lower esophageal sphincter tone, or inclusion of the esophageal opening within the rim of the LMA cuff. In contrast, the study of Williams and Bailey (1993) suggests that the LMA may offer protection from aspiration of oral contents. In this study of patients undergoing adenotonsillectomy, fiberoptic inspection of the laryngeal trachea after surgery did not reveal any blood ( Williams and Bailey, 1993 ).

During emergence from general anesthesia, the LMA can be removed at any time; compared with adults, children demonstrate more problems (7% to 13%), including laryngospasm, coughing, biting on the tube, or breath holding, when the LMA remains until the child has recovered his or her protective reflexes ( Mason and Bingham, 1990 ; McGinn, 1993 ; Laffon, 1994 ; O'Neill et al., 1994 ). Removal with the cuff inflated facilitates removal of blood or secretions that have collected above the cuff. If one chooses to wait until the child is strong and awake before removing the LMA, a soft bite block should be inserted between the patient's teeth to prevent compression of the lumen of the LMA, which, as a result of a completely obstructed airway, may result in negative pressure pulmonary edema. During the excitement phase of emergence, removal of the LMA is associated with a lower incidence of airway complications compared with removal of an endotracheal tube. Nevertheless, airway complications during emergence are least when the LMA is removed before the child regains airway reflexes and full consciousness. Rapid removal of the LMA may cause displacement of loose teeth.

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Copyright © 2005 Mosby, An Imprint of Elsevier

▪ PERILARYNGEAL AIRWAY

The perilaryngeal airway (PLA) (CobraPLA; Engineered Medical Systems, Inc., Indianapolis, IN) is an addition to the supply of laryngeal masklike devices that aid in airway management. The PLA consists of a distal softened tip that has slotted openings for ventilation and is designed to be positioned in the hypopharynx overlying the laryngeal inlet ( Fig. 9-16 ). It is secured in place by a more proximal cuff, and it contains a proximal 15-mm connector that attaches to the anesthesia breathing circuit.

 
 

FIGURE 9-16  Perilaryngeal airway consists of a distal softened tip that contains slotted openings for ventilation, a more proximal cuff to secure the airway, and a proximal 15-mm connector that attaches to the anesthesia breathing circuit.  (Reproduced with permission of Engineered Medical Systems, Inc., Indianapolis, IN.)

 



The PLA is available in eight different sizes, of which five are suitable for pediatric age and weight ranges ( Table 9-4 ). It is suitable for use during spontaneous or controlled ventilation, and the larger sizes can accommodate an appropriately sized fiberoptic bronchoscope and endotracheal tube for difficult intubations.


TABLE 9-4   -- Classification of the Cobra perilaryngeal airway

 

PERILARYNGEAL AIRWAY CHARACTERISTICS

Perilaryngeal Airway Size

Patient Size

Weight (kg)

Internal Diameter (mm)

Cuff Volume (mL)

0.5

Neonatal

>2.5

5

<8

1

Infant

>5

6

<10

1.5

Child

>10

6

<25

2

Child

>15

10.5

<40

3

Child/small adult

>35

10.5

<65

4

Adult

>70

12.5

<70

5

Large adult

>100

12.5

<85

6

Large adult

>130

12.5

<85

Reproduced with permission from Engineered Medical Systems, Inc., Indianapolis, IN.

 

 

 

The PLA is inserted in a similar manner to the LMA. A preliminary study in adults demonstrated that the PLA frequently requires readjustment up or down to effect adequate ventilation once inserted (Agro et al., 2003 ). There are no published studies on efficacy or complications with the use of this device in children.

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Copyright © 2005 Mosby, An Imprint of Elsevier

▪ LARYNGEAL TUBE

The Laryngeal Tube (VBM Medical Inc., Noblesville, IN) is another supraglottic ventilatory device that is used for airway management assistance. It has been described as a single-lumen, shortened Combitube. It consists of an oval ventilation aperture placed between two distal low-pressure cuffs ( Fig. 9-17 ), and it is inserted in a similar manner as the LMA. The distal (esophageal) balloon is designed to seal the airway distally and protect against regurgitation. The proximal (oropharyngeal) balloon is designed to seal off the pharynx above the ventilation port. The two balloons are inflated sequentially via a unique connector at a pressure of 60 cm H2O by using a manometer. There are six sizes that encompass all age ranges.

 
 

FIGURE 9-17  The laryngeal tube consists of an oval ventilation aperture placed between two distal low-pressure cuffs. The distal (esophageal) balloon is designed to seal the airway distally and protect against regurgitation.  (Reproduced with permission of VBM Medical Inc., Noblesville, IN.)

 



Like the LMA and PLA, the laryngeal tube is designed as an alternative to the anesthesia facemask and as a potential tool for providing ventilation in patients with a difficult airway. It can be used during spontaneous or controlled ventilation. In adult studies, the rate of successful placement exceeds 90% ( Cook et al., 2003 ), but pediatric trials have not been performed.

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▪ ENDOTRACHEAL TUBES

In evaluating endotracheal tubes for use in infants and children, one must consider their influence on dead space, resistance to breathing, and tracheal or laryngeal injury. Endotracheal intubation reduces the dead space of the natural extrathoracic airway ( Glauser et al., 1961 ) but can have a dramatic negative impact on the resistance to breathing. Resistance to laminar flow through a tube is governed by Hagen-Poiseuille Law, which dictates that resistance is proportional to the length of the tube and inversely proportional to the fourth power of the radius. Thus, small changes in the lumen of a tube can dramatically increase the resistance to flow through it. Assuming that flow is laminar and that all other variables are constant, one can predict that a reduction in internal diameter (ID) from 7.5 to 7.0 mm increases resistance 24%, whereas a reduction from 3.5 to 3.0 mm increases resistance by nearly 50%. Because luminal changes between the endotracheal tube and the adapter promote turbulent flow, measured differences are even more exaggerated. The resistance to breathing through the natural airway of a neonate is greater than that through a 3.5-mm-ID endotracheal tube but significantly less than that through a 2.5-mm-ID endotracheal tube ( Polgar and Kong, 1965 ). A small amount of secretions or debris can increase resistance substantially, yet these accumulations are difficult to avoid in small-lumen tubes. The use of 2.5-mm-ID endotracheal tubes should be restricted to situations in which no other tube fits.

Resistance to breathing is governed by the ID of a tube, but the potential for laryngeal or tracheal mucosal injury is related to the outer diameter (OD). Mild trauma to the airway producing as little as 1-mm mucosal edema can result in significant narrowing of the infant's airway. As little as 25 mm Hg pressure on the lateral wall of the trachea causes local ischemia and mucosal injury in adults ( Ching et al., 1974 ) and presumably in children as well.

Endotracheal tubes are made of nonreactive polyvinylchloride (PVC). Biologically inert materials reduce the risk of airway inflammatory reactions. PVC tubes are flammable and cannot be used in airway laser surgery. One must substitute a tube that is either nonflammable (e.g., metal) or laser resistant (e.g., red rubber tube wrapped with aluminum foil or manufactured with special surface treatments) (seeChapter 23 , Anesthesia for Pediatric Otorhinolaryngologic Surgery). These special tubes should have labeling indicating they are intended for use in laser surgery ( Subcommittee F29.18 on Operating Room Fire Safety, 1995 ). Patients with difficult airways or tracheal abnormalities often require a spiral wire-embedded silicone, or “anode” tube ( Fig. 9-18 ). This tube has the flexibility to assume virtually any position, while the wire coil embedded in its wall preserves the lumen. Extreme caution must be taken with anode tubes, because their flexibility makes accidental extubation more likely.

 
 

FIGURE 9-18  The flexibility of the anode tube and its ability to maintain a patent lumen with extremes in position make this endotracheal tube useful in surgery of the face and the airway. This is only a representative sample of the sizes of anode tubes that are available.

 

 

There are a variety of endotracheal tubes for special needs. An RAE tube has a preformed contour that facilitates access to the surgical field ( Fig. 9-19 ). Oral RAE tubes were initially developed for cleft palate surgery. The proximal end of the oral RAE tube rests over the middle of the mandible, whereas the proximal end of a nasal RAE tube rests on the forehead ( Fig. 9-20 ). These preformed tracheal tubes can be used for any type of head or neck surgery in which the operating room table is turned away from the anesthesiologist and the head is maintained in a neutral position. The greatest disadvantage of preformed tubes is that the flexion point is fixed. The length may be inappropriate, especially in patients whose cricoid diameter is unusually large or small, increasing the risk of endobronchial intubation or accidental extubation, respectively. Similarly, in a patient with a laryngeal or proximal tracheal stenosis, a standard endotracheal tube of a caliber small enough to be admitted to the airway may be too short. Tubes of small caliber with extra length are commercially available for this purpose.

 
 

FIGURE 9-19  Oral RAE tubes. Preformed oral RAE tubes are made in a variety of sizes. They are especially useful in facial surgery.  (From: Litman RS: Pediatric airway management. In: Litman RS, ed: Pediatric anesthesia: The requisites. St. Louis, 2004, Mosby.)

 



 
 

FIGURE 9-20  The nasal RAE tube is preformed to sit on the forehead for procedures in the oral cavity or neck.  (From Litman RS: Pediatric airway management. In: Litman RS, ed: Pediatric anesthesia: The requisites. St. Louis, 2004, Mosby.)

 



Double-Lumen Endotracheal Tubes

A double-lumen endotracheal tube is commonly used to attain lung separation in adults. Its advantages include rapid and easy separation of the lungs, access to both lungs to facilitate suctioning, the ability to rapidly switch to two-lung ventilation if needed, and the ability to administer CPAP or oxygen insufflation to the operative lung, when necessary. The smallest commercially available double-lumen endotracheal tube is size 28 F, which precludes its placement in children weighing less than 30 to 35 kg or younger than 8 to 10 years ( Table 9-5 ). With this smallest double-lumen endotracheal tube, bronchoscopic confirmation of its appropriate location within the trachea and bronchus requires use of an ultrathin flexible bronchoscope.


TABLE 9-5   -- Comparison of tracheal tube dimensions used for one-lung ventilation

Tracheal Tube

Tracheal Lumen (mm)

Bronchial Lumen (mm)

Outside Diameter (mm)

Broncho-Cath *

28 F (L)

4.5

4.5

9.8

35 F (L)

6.0

6.0

12.1

37 F (L)

6.5

6.5

13.2

39 F (L)

7.0

7.0

14.3

41 F (L)

7.4

7.4

15.4

Univent[†]

6.0

 

11.0

Standard *

7.0

7.0

 

9.5

9.0

9.0

 

11.1

10.0

10.0

 

13.2

*

Nellcor, Pleasanton, CA.

Fuji Systems, Inc., Tokyo, Japan.

 

▪ ENDOTRACHEAL TUBES FOR ONE-LUNG VENTILATION

Because of mechanical difficulties of one-lung ventilation in small children, pediatric surgeons historically have used retractors and surgical packs to improve surgical exposure during thoracic surgery. With the popularity of thoracoscopic surgical techniques in the pediatric population, there is an increasing need to provide one-lung ventilation to facilitate surgical exposure ( Rowe et al., 1994 ; Tobias, 1999 ;Hammer, 2001 ). The pediatric anesthesiologist has a number of choices with which to provide one-lung ventilation.

▪ UNIVENT ENDOTRACHEAL TUBE

The Univent endotracheal tube (Fuji Systems Corporation, Tokyo, Japan) is a single-lumen endotracheal tube with a moveable bronchial blocker built into its side wall ( Kamaya and Krishna, 1985 ;MacGillivray, 1988 ; Hammer, 1998 ) ( Fig. 9-21 ). The bronchial blocker contains a low-pressure high-volume cuff and has a central channel used to suction the blocked lung and to insufflate oxygen. The Univent tube is inserted into the trachea like a standard tracheal tube with the bronchial blocker withdrawn into the main tube. The bronchial blocker can then be advanced into the main stem bronchus of the operative lung under bronchoscopic guidance. Rotation of the tracheal tube determines the direction that the blocker takes as it is advanced. Inflation of the endobronchial cuff on the balloon tip catheter enables isolation of the operative lung. At the end of the procedure, the blocker tube can be withdrawn into the main tube, permitting postoperative ventilation without the need to change to a separate single-lumen tube.

 
 

FIGURE 9-21  (A) Univent tube. A bronchial blocker is incorporated into this tracheal tube to allow for isolation of one of the lungs during an anesthetic. It can be withdrawn and ventilation can be continued in the postoperative period without the need to change to a single-lumen tube. (B) The blocker in its retracted state.

 

 

Advantages of the Univent tube include ease of placement and the ability to easily change from one-lung ventilation to two-lung ventilation. In addition, unlike the standard double-lumen endotracheal tube, the bronchial blocker can be pulled back into its channel with the Univent tube left in place for postoperative ventilation. The Univent tube is available in sizes 3.5-(uncuffed), 4.5-, 6.0-, 6.5-, 7.5-, 8.0-, 8.5-, and 9.0-mm ID. The OD is larger than that of a conventional endotracheal tube of the same ID ( Table 9-5 ). The IDs of the 3.5- and 4.0-mm Univent tubes limit the passage of a standard pediatric bronchoscope with an OD of 3.5 mm or greater, thereby requiring an ultrathin pediatric bronchoscope to visualize the bronchial blocker.

▪ SELECTIVE ENDOBRONCHIAL INTUBATION

In infants and young children whose small size precludes placement of a double-lumen endotracheal tube or Univent tube, there are two additional options for one-lung ventilation: selective endobronchial intubation with a standard endotracheal tube or placement of a separate “bronchial blocker.”

Endobronchial Intubation With a Standard Endotracheal Tube

An endotracheal tube can be inserted into either main bronchus with the use of bronchoscopic guidance. In children older than 2 years, a cuffed endotracheal tube maintains an effective seal of the lung while maintaining the tube in a proximal position within the main bronchus. The major disadvantage of selective endobronchial intubation is that it is not possible to quickly change from one-lung ventilation to two-lung ventilation because it requires repositioning the endotracheal tube from the bronchus into the trachea and vice versa. Furthermore, with unintentional movement of the endotracheal tube and minimal cephalad displacement, selective intubation may be lost because of bronchial extubation.

Bronchial Blocker Devices

A bronchial blocker device consists of a small balloon that is purposefully inflated within the proximal portion of the main bronchus to isolate one of the lungs under bronchoscopic guidance. Several different devices can be used as bronchial blockers, including a Fogarty embolectomy catheter and the Arndt (Cook Critical Care, Bloomington, IN) endobronchial blocker ( Fig. 9-22 ). These devices contain a central channel that allows suctioning (for lung deflation) and the application of oxygen and CPAP.

 
 

FIGURE 9-22  The Arndt bronchial blocker is inserted under fiberoptic guidance. (A) With a special multi-port adapter that is attached to the tracheal tube, the blocker is inserted through a side port into the main body of the connector. Ventilation with 100% oxygen is enabled through a second side port. (B) A flexible bronchoscope is inserted through the remaining port engaging the wire loop at the end of the bronchial blocker. (C) The bronchoscope is advanced into the bronchus to be isolated. (D) The blocker is advanced into the bronchus sliding down the bronchoscope. (E) The bronchoscope is withdrawn leaving the blocker in place. (F) The blocker's balloon is inflated and placement is verified using the bronchoscope before it is withdrawn completely.  (Drawings courtesy of Cook, Inc., Bloomington, IN.)

 



A Fogarty embolectomy catheter contains a balloon at the distalmost end to place within the proximal main bronchus to isolate the lung. A disadvantage of the embolectomy catheter is that the tip can be displaced proximally during the course of a surgical procedure, especially if the patient's position changes. Total airway obstruction can result if the inflated balloon slips back into the trachea.

The Arndt Endobronchial Blocker is a bronchial blocker with an inflatable cuff and a central lumen, through which a wire with a looped end has been passed ( Arndt et al., 1999 ; Hammer et al., 2002 ) (seeFig. 9-22 ). The bronchial blocker is passed through a specialized adapter that is placed at the proximal end of the endotracheal tube. This adapter contains four ports: (1) a connection to the endotracheal tube, (2) a standard 15-mm adaptor for the anesthesia circuit, (3) a port for the bronchial blocker with a self-sealing diaphragm that can be tightened around the bronchial blocker to hold it in place, and (4) a port for the flexible bronchoscope. The bronchial blocker is passed through the port and placed at the entrance of the endotracheal tube. The bronchoscope is passed through the port and then through the wire loop at the end of the bronchial blocker. The bronchoscope and bronchial blocker are passed under direct vision as a single unit into the main bronchus of the operative side. The bronchoscope is withdrawn into the trachea, and the balloon is inflated under direct visualization. When correct placement has been confirmed, the wire loop is removed from the central channel. Once the wire guide is removed from the channel, it cannot be replaced. The Arndt blocker is currently available in 3 sizes (5 F, 7 F, and 9 F) with the 9 F recommended for endotracheal tubes of 7.5 mm and above, 7 F for endotracheal tubes of 6.0 to 7.0 mm, and 5 F for endotracheal tubes of 4.5 to 5.5 mm.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ TRACHEOSTOMY TUBES

The three most common indications for tracheostomy in children are (1) prolonged mechanical ventilation (>50%), (2) upper airway obstruction (40%), and (3) pulmonary toilet (10%) ( Wetmore et al., 1999 ). Within each category are congenital, traumatic, metabolic, infectious, and neoplastic conditions that require tracheostomy. Although the underlying medical conditions may be numerous, the most common diagnoses in pediatric tracheostomy patients are bronchopulmonary dysplasia and neurologic disorders.

The ideal tracheostomy tube should be made of a material that causes minimal tissue reactivity, can be easily cleaned and maintained, and is available in a variety of shapes, diameters, and lengths ( Fig. 9-23 ). The tube needs to be rigid enough to prevent kinking or collapse, yet soft enough to be comfortable for the patient. Early tracheostomy tubes were made of stainless steel or silver ( Downes and Schreiner, 1985 ). These tubes had the advantages of causing minimal tissue reaction and avoiding tracheal collapse. Their rigidity caused significant discomfort to the patient because of injury to the tracheal mucosa. Most manufacturers use silicone tubes that have minimal tissue reactivity and conform to the structure of the airway. The ideal tube also contains an inner cannula that can be removed and cleaned. Modern tracheostomy tubes have a 15-mm male connector for the attachment of standard respiratory equipment. To improve the patient's comfort and ease of care, a low-profile swivel is commonly added to the tracheostomy tube ( Schreiner, 1986 ). This allows unrestricted neck movement and easy care through a suction port. Tracheostomy tubes for infants are uncuffed; in larger children and adolescents, cuffed tracheostomy tubes are preferred.

 
 

FIGURE 9-23  Representative models of pediatric tracheostomy tubes include Shiley (left) and Bivona (right).

 

 

The appropriate tracheostomy tube is selected on the basis of the ID and OD and the length. The OD determines the size of the tube that may be inserted, whereas the ID determines the actual airway size. The diameter of the tube should be large enough to allow adequate air exchange, easy suctioning, and clearance of secretions. If the indication for tracheostomy is assisted ventilation, the size of the tube should be adjusted to prevent excessive air leak. Predictors of the appropriate tube size include the child's age and the size of a preexisting endotracheal tube. A tube that is too large compromises the capillary blood flow in the tracheal wall, which may result in mucosal ischemia, ulceration, and development of fibrous stenosis. Overinflation of a cuffed tracheostomy tube for a prolonged period of time may produce similar injuries. This complication may be avoided by selecting the proper size of tracheostomy tube ( Table 9-6 ) and adjusting the cuff pressure to less than 20 cm H2O. The choice of the tube size is also influenced by visualization of the size of the tracheal lumen.


TABLE 9-6   -- Tracheostomy tube dimensions

Model

Inner Diameter (mm)

Outer Diameter (mm)

Overall Length (mm)

Shiley *

Shiley Neonatal

3.0

3.0

4.5

30

3.5

3.5

5.2

32

4.0

4.0

5.9

34

4.5

4.5

6.5

36

Shiley Pediatric

3.0

3.0

4.5

39

3.5

3.5

5.2

40

4.0

4.0

5.9

41

4.5

4.5

6.5

42

5.0

5.0

7.1

44

5.5

5.5

7.7

46

Shiley Pediatric Long

5.0

5.0

7.1

50

5.5

5.5

7.7

52

6.0

6.0

8.3

54

6.5

6.5

9.0

56

Shiley Cuffed Pediatric

4.0

4.0

5.9

41

4.5

4.5

6.5

42

5.0

5.0

7.1

44

5.5

5.5

7.7

46

Shiley Cuffed Pediatric Long

5.0

5.0

7.1

50

5.5

5.5

7.7

52

6.0

6.0

8.3

54

6.5

6.5

9.0

56

Bivona[†]

Standard Neonatal Uncuffed

60N025

2.5

4.0

30

60N030

3.0

4.7

32

60N035

3.5

5.3

34

60N040

4.0

6.0

36

60N045

4.5

6.5

36

Neonatal FlexTend Plus[‡]

60NFP25

2.5

4.0

30

60NFP30

3.0

4.7

32

60NFP35

3.5

5.3

34

60NFP40

4.0

6.0

36

Standard Pediatric Uncuffed

60P025

2.5

4.0

38

60P030

3.0

4.7

39

60P035

3.5

5.3

40

60P040

4.0

6.0

41

60P045

4.5

6.7

42

60P050

5.0

7.3

44

60P055

5.5

8.0

46

Standard Pediatric FlexTend Plus[‡]

60PFS25

2.5

4.0

38

60PFS30

3.0

4.7

39

60PFS35

3.5

5.3

40

60PFS40

4.0

6.0

41

60PFS45

4.5

6.7

42

60PFS50

5.0

7.3

44

60PFS55

5.5

8.0

46

Extra Long Pediatric FlexTend Plus[‡]

60PFS35

3.5

5.3

40

60PFS40

4.0

6.0

44

60PFS45

4.5

6.7

48

60PFS50

5.0

7.3

50

60PFS55

5.5

8.0

52

Neonatal Cuffed[§]

N025

2.5

4.0

30

N030

3.0

4.7

32

N035

3.5

5.3

34

N040

4.0

6.0

36

Pediatric Cuffed[§]

P025

2.5

4.0

38

P030

3.0

4.7

39

P035

3.5

5.3

40

P040

4.0

6.0

41

P045

4.5

6.7

42

P050

5.0

7.3

44

P055

5.5

8.0

46

Adjustable and Extra Length[¶]

60HA25

2.5

4.0

55

60HA30

3.0

4.7

60

60HA35

3.5

5.3

65

60HA40

4.0

6.0

70

60HA45

4.5

6.7

75

60HA50

5.0

7.3

80

60HA55

5.5

8.0

85

550050

5.0

7.7

50

550055

5.5

8.3

52

550060

6.0

8.3

55

*

Shiley is manufactured by Mallinckrodt Inc., St. Louis, MO.

Bivona is manufactured by Portex, Keene, NH.

FlexTend tubes feature a flexible, kink-resistant proximal extension that allows easier access and enables distal connections away from the infant's chin, neck, and chest.

§

Bivona cuffed tubes are available in three different models: Fome-Cuf tubes (85 series) include a SidePort AutoControl Adapter that synchronizes cuff and ventilator pressure. A complete seal is possible for patients with high peak end-expiratory pressure or high inspiratory pressure needs. Aire-Cuf tubes (65 series) feature a traditional air-filled cuff. TTS cuffs (67 series) feature an uncuffed tube profile when the cuff is deflated but provide the protection of a cuff when it is inflated.

Adjustable Hyperflex (HA series) is an instantly customizable, soft, flexible, kink-resistant, silicone tube with an adjustable neck flange. The flexible shaft adapts to the unique contours of each patient's anatomy. The Adjustable Hyperflex tube is primarily used as a measuring device and temporizing measure until the child can receive a permanent tube with a fixed neck flange. The 550 series is made of polyvinylchloride that softens at body temperature. It features a long tube shaft and a traditional anatomic curve.

 

The length of the tube is important, especially in neonates and infants. A tube that is too short may result in accidental decannulation or the development of a false passage. If a tube is too long, the tip may abrade the carina or become situated in the right main bronchus. Some plastic tubes may be cut to the desired length as necessary. Extra-long custom-made tubes may be helpful in unusual situations, such as tracheomalacia or tracheal stenosis, to span the diseased area.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ LARYNGOSCOPES

A variety of pediatric-sized laryngoscope blades are available ( Table 9-7 ). The laryngoscope has three basic parts: a handle, a blade, and a light. The blade consists of a spatula, a flange, and a tip. Blades differ in length, width, and curvature. Laryngoscopes can be purchased with incandescent or fiberoptic light sources. Those with fiberoptic light sources provide extremely bright, highly focused light that occasionally can be obscured by the tongue and soft tissue. Standards for each component are specified by ASTM standard F-1195-99 ( ASTM, 2005 ). Selection of a particular laryngoscope is usually based on personal preference and experience, the size of the child, and the peculiarities of a specific airway problem.

TABLE 9-7   -- Laryngoscope blade types and sizes

 

BLADE TYPE AND SIZE

Age

Miller

Wis-Hippel

Macintosh

Premature neonate

0

Term neonate

0 to 1

1 to 12 mo

1

1

1 to 2 yr

1

1.5

2

2 to 6 yr

2

2

6 to 12 yr

2

3

Modified from Coté CJ: A practice of anesthesia for infants and children. Philadelphia, 2001, WB Saunders.

 

 

Generally, the straight Miller blade is used in children ( Miller, 1941 ). This blade allows the cephalad aspect of the larynx to be exposed more easily, because the base of the tongue can be lifted out of the line of sight, and the protruding epiglottis can be retracted with the tip. Wide blades and large-flange blades, like the Robertshaw, the Flagg, and the Wis-Hipple, allow the wide tongue of the small child to be flattened during laryngoscopy ( Flagg, 1928 ; Robertshaw, 1962 ) ( Fig. 9-24 ). The 1.5 Wis-Hipple blade is especially useful in the toddler ( Fig. 9-25 ). For older children and young adults, longer straight blades (e.g., Miller 2) or curved blades (e.g., the Macintosh) allow the anesthesiologist to achieve good exposure and avoid prominent dentition ( Macintosh, 1943 ).

 
 

FIGURE 9-24  Robertshaw blade (top) and Flagg blade (bottom).

 

 

 
 

FIGURE 9-25  Two sizes of the Wis-Hipple blade are available.

 

 

During laryngoscopy, neonates may rapidly develop hypoxemia secondary to apnea, decreased functional residual capacity, and increased oxygen consumption ( Gibbons, 1986 ). The Oxyscope is a modified Miller blade that allows insufflation of oxygen into the pharynx and decreases the rapidity with which hypoxemia occurs during laryngoscopy ( Todres and Crone, 1981 ) ( Fig. 9-26 ).

 
 

FIGURE 9-26  Oxyscope. This is a Miller O blade that is specially fitted; 2 to 3 L/min of oxygen is connected to the cannula on the left side of the blade, as shown. The oxygen is then directed by the lumen of this cannula into the trachea.

 

 

▪ DEVICES AND TECHNIQUES FOR A DIFFICULT INTUBATION

Children with micrognathia, such as those with Goldenhar's syndrome, Pierre Robin sequence, or Treacher Collins syndrome, are often difficult to intubate with standard laryngoscopy equipment. A number of different devices have been designed to facilitate endotracheal intubation in these types of children; these include the Bullard laryngoscope, the lighted stylet, and the fiberoptic flexible bronchoscope.

Bullard Laryngoscope

The Bullard laryngoscope (ACMI, Stamford, CT) uses a fiberoptic telescope attached to a rigid curved blade to enhance glottic visualization ( Borland and Casselbrant, 1990 ; Brown et al., 1993 ; Shulman et al., 1997 ; Shulman and Connelly, 2000 ) ( Fig. 9-27 ). It is available in two pediatric sizes. Tube placement can be performed using a mechanical device on the scope to position the tube or by directing the tube along the side of the laryngoscope using the telescope. Use of the device by novices is often awkward, but experienced users tout its ease of use. Comparison trials of the Bullard scope with other methods to secure the difficult pediatric airway have not been performed.

 
 

FIGURE 9-27  The Bullard laryngoscope allows a view of the larynx in children with limited ability to open the mouth and whose larynx is positioned relatively anterior (cephalad).

 

 

Lighted Stylet

The lighted stylet (“light wand”) consists of a semirigid stylet with a bright light at the distalmost end (Fiberoptic Intubation Stilette; Anesthesia Medical Specialties, Santa Fe Springs, CA; and Trachlight; Laerdal Medical, Armonk, NJ) ( Fig. 9-28 ). The lighted stylet is prepared by inserting it inside a standard endotracheal tube, which is then blindly inserted into the pharynx. In a darkened room, the transmitted light is used to guide accurate glottic location, and the endotracheal tube is slid off the lighted stylet and into the trachea. It is useful when the child has an anatomically normal larynx that is difficult to visualize with direct methods. This may occur with micrognathia ( Krucylak and Schreiner, 1992 ), temporomandibular joint disorders (or any condition that limits mandibular mobility), cervical spine instability, or facial trauma. It is particularly suited to children with limited neck and mandible mobility, but it is not useful in cases of fixed upper or lower airway obstructive pathology or in the presence of a foreign body.

 
 

FIGURE 9-28  Using a lighted stylet, the endotracheal tube can be guided into the trachea with the use of the surface anatomy of the child and projection of the lighted stylet tip.  (From Litman RS: Pediatric airway management. In: Litman RS, ed: Pediatric anesthesia: The requisites. St. Louis, 2004, Mosby.)

 



Innovative solutions to the size limitations have been overcome, and the lighted stylet can be used with endotracheal tubes as small as 2.5-mm ID ( Davis et al., 2000 ). A “home-grown” version lightwand for small infants and neonates has been created, with a 20-gauge fiberoptic illuminating lightpipe (Storz Ophthalmics Inc., St. Louis, MO) attached to any standard fiberoptic light source ( Fig. 9-29A to C ).

 
 

FIGURE 9-29  (A) A thin fiberoptic bundle is used as a lightwand. (B) The fiberoptic bundle can be attached to any commercially available light source with adjustable light intensity. (C) The fiberoptic bundle is inserted alongside a thin pliable stylet into an endotracheal tube.  (From Litman RS: The difficult pediatric airway. In: Litman RS, ed: Pediatric anesthesia: The requisites. St. Louis, 2004, Mosby.)

 



Flexible Fiberoptic Bronchoscopy

Fiberoptic bronchoscopes with flexible tips have been used successfully both in infants and in children whose intubation would otherwise have been difficult or impossible. With the child sedated or anesthetized and breathing spontaneously, a fiberoptic bronchoscope that has been passed through an endotracheal tube is inserted through the mouth or nose to visualize the larynx. The endotracheal tube is slid off the bronchoscope after its entrance into the trachea. In the past, the major limitation of this technique was the inability of large adult bronchoscopes to fit through small pediatric endotracheal tubes. This problem has been solved by the availability of new, smaller ultrathin fiberoptic laryngoscopes ( Fan et al., 1986 ; de Blic et al., 1991 ; Roth et al., 1994 ) ( Table 9-8 ). Anesthesiologists have become more adept at manipulating the ultrathin bronchoscope, which may be used inside a 2.5- or 3.0-mm-ID endotracheal tube (depending on the manufacturer). In addition, the optical aspects of the equipment have improved with these smaller fiberscopes to allow better screen resolution.


TABLE 9-8   -- Pediatric fiberoptic bronchoscopes

Manufacturer

Model No.

Outer Diameter (mm)

Suction Channel Diameter (mm)

Working Length (cm)

Field of View (°)

Angulation Up (°)

Angulation Down (°)

Fujinon, Wayne, NJ

BRO-YP2

4.8

2.0

57.5

90

160

100

Olympus, Melville, NY

LF-P

2.2

None

60

75

120

120

Olympus

BF-N20

2.2

None

55

75

160

90

Pentax, Golden, CO

FI-7P

2.4

None

60

95

130

130

Storz, Culver City, CA

BN 11301

5.0

2.3

65

110

160

160

Storz

BD 11302

3.7

1.5

65

80

120

120

 

 

These ultrathin bronchoscopes may or may not be manufactured with suction ports; secretions and blood are more likely to obscure the view in smaller children. Some ultrathin bronchoscopes contain a suction port, but it is too narrow to allow effective suctioning of secretions. Furthermore, oxygen insufflation should not be performed via this port in small children because of the possibilities of generating dangerously high intrabronchial pressures and development of a tension pneumothorax ( Iannoli and Litman, 2002 ).

Apneic ventilation is usually ineffective in small children because of their limited time to oxyhemoglobin desaturation. This is caused by the markedly reduced functional residual capacity (FRC) in anesthetized small children and their relatively high oxygen consumption. Ventilation can be accomplished during bronchoscopy by the use of a special anesthesia mask that incorporates a conduit for passage of the bronchoscope (VBM Medical Inc., Noblesville, IN).

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ INTRAVENOUS EQUIPMENT

▪ CATHETERS

Intravenous catheters appropriate for the smallest premature infant and the largest adolescent patient are made by a wide variety of companies. The appropriate catheter is usually dictated by the patient's size, expected fluid requirements, and the operator's preference. In general, a 22- or 24-gauge catheter suffices in the small infant. Patients with significant fluid requirements, such as neonates undergoing gastroschisis repair, may need two or three intravenous catheters. In older children, a 22- or 20-gauge catheter usually suffices. Butterfly needles inserted into tiny veins are not adequate for infusions during surgery because they are easily dislodged. They are convenient for performing rapid intravenous induction in a young child or an anxious adolescent. Pain can be minimized when using a 25- or 27-gauge butterfly needle.

In accordance with Poiseuille's relationship, the resistance to flow through an intravenous catheter is related most significantly to the radius of the lumen, but the length of the catheter and the viscosity of the fluid also have an impact ( Table 9-9 ). Although small differences exist between comparable catheters of various manufacturers ( Hodge and Fleisher, 1985 ; Rosen and Rosen, 1986 ), major flow reductions occur when they are lengthened to enable central venous cannulation. Viscosity of the infused fluid (e.g., blood versus crystalloid) imposes significant resistance changes only when small (<20 gauge) or long (>3 inches) catheters are used ( Hodge and Fleisher, 1985 ; Rothen et al., 1992 ). For example, an 18-gauge catheter lengthened to 8 inches to enable central venous placement exhibits the flow characteristics of a short 24-gauge catheter.

TABLE 9-9   -- Catheter sizes and their flow rates

 

 

MEAN FLOW RATE RANGE (mL/min)

Catheter Size (gauge)

Length (inches)

Crystalloid (gravity)

Crystalloid (pressure)

Blood (pressure)

24

0.75

14 to 15

42 to 47

20 to 30

22

1

24 to 26

65 to 77

44 to 50

20

1.25 to 2

38 to 42

103 to 126

69 to 81

18

1.25 to 2

55 to 62

164 to 214

150 to 164

16

2

75 to 81

248 to 280

216 to 286

14

2

92 to 93

301 to 319

334 to 410

20

8

5

16

3

18

8

13

51

22

16

8

31

97

35

Data summarized from Hodge D III, Fleisher G: Pediatric catheter flow rates. Am J Emerg Med 3:403, 1985.

 

 

 

With the widespread concern of the danger of accidental needlestick injuries, many medical centers have replaced standard intravenous and butterfly catheters with those that provide retractable needles and less danger of a needlestick after intravenous catheter insertion. A number of studies have determined that needleless intravenous catheters reduce the rate of needlestick injuries by up to 60% ( Orenstein et al., 1995 ; Lawrence et al., 1997 ). Consequently, in 2001 the U.S. Occupational Safety and Health Administration (OSHA) authored the Needlestick Safety and Prevention Act (H.R. 5178), which stipulates that the 1991 Bloodborne Pathogens Standard (29 CFR 1910.1030) be revised to strengthen the requirements related to the use of safety-engineered sharp devices (Federal Register on January 18, 2001). The Needlestick Safety and Prevention Act provides a legislative mandate that health care facility employers provide employees with safety-engineered sharp devices.

▪ INFUSION SETS

Intravenous infusion sets must also be tailored to the patient and the planned surgical procedure. Microdrip infusion sets (60 drops/mL) allow the anesthesiologist to more accurately deliver small volumes of infusate to the small child compared with the standard 15 drops/mL infusion set. For an older child (older than approximately 10 years), the standard adult infusion set is adequate. The addition of extension tubes to all pediatric infusion sets permits the intravenous catheter to be placed in any available extremity and allows a small child to be moved down on the operating table for better surgical access. Insertion of multiple stopcocks in the infusion tubing allows precise volumes of blood or colloid to be administered. The blood set is attached to the stopcock closest to the patient, and a syringe is placed proximally. The stopcocks are opened to allow blood to flow into the syringe and then adjusted to allow an accurate volume of blood to be infused from the syringe into the patient. Before their use, all infusion sets should be “de-bubbled” to prevent air entrainment into the circulation, a task especially important in a small premature infant, who is likely to have a patent foramen ovale or ductus arteriosus, or in a child with a known intracardiac defect. To decrease the incidence of accidental needlestick injuries, all manufacturers now offer injection ports throughout the length of the tubing that are accessible with needleless Luer-Loc ports. These ports can easily inject air that is trapped in the injection ports.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ WARMING DEVICES

Heat conservation is critical in the care of the newborn infant. A decrease of 2°C in the environmental temperature is sufficient to double the oxygen consumption of a full-term infant ( Hill and Rahimtulla, 1965 ). To meet this increase in oxygen consumption, the neonate must double its minute ventilation. For a critically ill premature infant who is unable to mount this increase in ventilation, an oxygen debt and progressive lactic acidosis ensue (see Chapter 5 , Thermoregulation: Physiology and Perioperative Disturbances). The rate of heat loss in newborn infants is four times that in adults because of a higher surface area-to-body weight ratio, increased curvature of body surfaces, and decreased insulation from skin and subcutaneous fat ( Adamsons and Towell, 1965 ). Special efforts must be made to maintain body temperature during anesthesia, especially in small infants, in operative procedures with large insensible losses, and with rapid fluid administration.

Heated water-filled mattresses are routinely used to prevent conductive heat loss from the infant to the operating room table and to transfer heat to the infant (see Chapter 5 , Thermoregulation: Physiology and Perioperative Disturbances). The safe temperature range is narrow-below 35°C, infants may lose heat to the mattress, and above 38°C, there is the possibility of overheating and burns. In children who weigh less than 0.5 m2 (=10 kg), circulating water warming blankets have been shown to conserve heat ( Goudsouzian et al., 1973 ). Circulating water warming blankets placed under the patient have no significant effect on temperature conservation in larger children and adults, although they do make the operating room table more comfortable. This difference in warming blanket effectiveness relative to the patient's size probably reflects the proportionally larger surface area in contact with the blanket in the smaller infant than in the larger child or adult. Placing the circulating water blanket over the patient is more effective than having the child lie on the warming device ( Sessler and Moayeri, 1990 ). Inappropriate use of warming blankets can result in overheating and burns ( Crino and Nagel, 1968 ). Burns can be minimized by placing one or two sheets between the patient and the water filled blanket; each additional sheet reduces heat transfer by 20%. To decrease further the risk of burns, the warming blanket should have a maximum temperature of 42°C and an automatic shutoff if this temperature is exceeded or if the patient's temperature exceeds a preset value. In addition, the machine should display both the patient's temperature and the water temperature so that periodic visual comparison can act as a further safety monitor. Excess surgical preparation solution should not be allowed to pool over the blanket because skin irritation can occur, especially under pressure points. The cause of this irritation is not known.

▪ FORCED WARM AIR DEVICE

The flow of warm air across a child's skin produced by a forced air warming blanket prevents heat loss to the environment and may even effectively warm patients via radiant shielding and convection. Of all devices, these are one of the most effective and should be used in all cases where hypothermia is a possibility ( Sessler et al., 1991 ; Camus et al., 1993 ; Kurz et al., 1993 ). Forced air systems inject warm air through a connecting hose into a quiltlike blanket that has small holes on one surface. Warm air escaping through these holes provides conductive and convective warming to the patient. Different size blankets permit maximum coverage of specific body areas over a range of patient sizes. The most effective units direct flow toward areas with major blood vessels like the chest, axilla, abdomen, and groin, where convective heating is most effective ( Giesbrecht et al., 1994 ). Like overhead radiant warmers and circulating water blankets, forced air systems can produce burns and overheating when used inappropriately ( Truell et al., 2000 ). Maximum air temperatures on these devices are designed to minimize the risk of burns. Ensuring that the tube carrying the warm air to the blanket does not touch the patient also decreases the likelihood of thermal injury. Furthermore, directing warm air from the hose without the blanket (“free-hosing”) has been associated with patient burns.

▪ WRAPPING AND DRAPING WITH PLASTIC SHEETS

Because of their large surface area, infants have significant evaporative and radiant heat loss. Wrapping and draping the patient with special blankets are also effective in decreasing heat loss during prolonged surgery, especially when forced air devices are impractical. Radiant heat loss can be decreased with the use of Webril cotton wrapping (Kendall Corporation, Boston, MA) covered with plastic bags over exposed extremities. For small infants with relatively large heads, cranial heat loss can be decreased up to 73% by wrapping the head in an insulated hat or plastic bag. This is significant in neonates, because their brain is responsible for 44% of their total heat production ( Rowe et al., 1983 ). The use of reflective blankets, made from the material used in outdoor survival apparel (e.g., space blankets), is also effective in reducing heat loss ( Bourke et al., 1984 ). Cutaneous heat loss is proportional to the surface area of the patient, making the percentage of skin surface covered far more important than the region of the body covered or the type of material used for passive insulation ( Sessler et al., 1991 ).

▪ HUMIDIFICATION OF INSPIRED GASES

Humidification of inspired gases, as noted previously, is an important technique for conserving heat in anesthetized patients. Because 12% to 14% of body heat is lost through the respiratory tract, the use of warm, humidified gases decreases the potential heat drain in addition to preventing damage to the ciliated cells of the tracheobronchial tree caused by dry gases ( Clarke et al., 1954 ). The humidifier should be servo-controlled, shutting off when the preset temperature equals the patient's airway temperature, so that airway burns are avoided. Maximum temperature limits, causing automatic shutoff, should also be an integral aspect of the humidifier. Heat and moisture exchangers (e.g., Humid-Vent) can provide 80% inspired humidity when 80 minutes of use is allowed to saturate the hygroscopic membrane (Bissonnette and Sessler, 1989 ; Bissonnette et al., 1989b ).

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ MONITORING

Most studies that have determined the rate of cardiac arrests due to anesthesia have found a three- to fivefold greater risk among children compared with adults ( Graff et al., 1964 ; Keenan and Boyan, 1985). In children younger than 1 year, the incidence increases to 9.2 to 17 per 10,000 anesthetics, or 10 times the adult incidence ( Olsson and Hallen, 1988 ; Cohen et al., 1990 ). Factors contributing to cardiac arrests in anesthetized children are likely to be related to the cardiovascular or respiratory system ( Salem et al., 1975 ). The incidence of other serious complications is also greater for infants than for adults in the operating room ( Tiret et al., 1988 ) and postanesthesia care unit (PACU) ( Cohen et al., 1990 ). These data indicate that children represent a high-risk population and should be monitored with particular attention to cardiovascular and respiratory variables.

Guidelines for the intraoperative monitoring of patients under anesthesia have been published by the ASA (2003) (see Box 9-1 ). These standards mandate the continuous presence of a anesthesiologist or a nurse anesthetist throughout the conduct of anesthesia and require continuous monitoring of oxygenation, electrocardiography, and adequacy of ventilation and circulation. The minimum standard for monitoring oxygenation includes an oxygen analyzer in the anesthesia breathing circuit, sufficient illumination to evaluate the patient's color, and a quantitative method such as pulse oximetry, except under extenuating circumstances. Tracheal intubation must be verified by physical examination and qualitative detection of carbon dioxide in the exhaled gas. Regardless of whether endotracheal intubation has been performed, continuous capnography is required unless it becomes invalidated by the nature of the patient, procedure, or equipment. Furthermore, quantitative monitoring of the volume of expired gas is strongly encouraged. The ASA also recommends monitoring of ventilation using observation of chest excursion and the reservoir breathing bag, as well as auscultation of breath sounds. When ventilation is controlled by a mechanical ventilator, there shall be in continuous use a device that is capable of detecting disconnection of components of the breathing system, and the device must give an audible signal when its alarm threshold is exceeded.

ASA monitoring standards for circulation mandate that every patient receiving anesthesia shall have continuous ECG and determination of arterial blood pressure and heart rate at least every 5 minutes. In addition to these, every patient shall have circulatory function continually evaluated with use of at least one of the following methods: palpation of a pulse, auscultation of heart sounds, monitoring of an intra-arterial pressure tracing, ultrasound peripheral pulse monitoring, or pulse plethysmography or oximetry. Finally, a method by which the temperature can be measured should be readily available during general anesthesia, and patients should have their temperature monitored when clinically significant changes in body temperature are intended, anticipated, or suspected.

Many of these provisions have been extended to the PACU. In standards adopted by the ASA in 1988 and updated in 1994, PACU monitoring should emphasize oxygenation, ventilation, circulation, and temperature assessment with specific capability for quantitative determination of systemic oxygenation by pulse oximetry or its equivalent. Equipment should be readily available to enable the practitioner to meet these standards in all pediatric patients.

The anesthesiologist is the ultimate monitor. Devices that provide electronic surveillance should not distract the anesthesiologist from direct monitoring using observation, auscultation, and palpation. In healthy pediatric patients, under certain circumstances, it may be appropriate to induce anesthesia while using these senses alone. This enables smooth “steal” inductions, in which the patient enters the operating room asleep and does not awaken during the induction of general anesthesia. It also allows the practitioner to enlist the cooperation of a child who might otherwise become increasingly anxious and less cooperative if the induction were delayed or altered by the application of monitors. When anesthetizing an infant (i.e., <1 year old) or a critically ill child, the benefit of delaying the application of monitors must be weighed against the risks of not having them in place during induction of general anesthesia.

▪ PHYSICAL EXAMINATION

Observation

The anesthesiologist can gain a tremendous amount of information from observation alone. Anesthetic depth can be inferred from the rate and pattern of respiration, and airway obstruction can be detected by chest wall retractions or “seesaw” paradoxical motion. The skin and mucous membranes should be continually assessed to confirm adequate oxygenation, because a pulse oximeter reading may significantly lag behind other indices of hypoxemia when placed on an extremity ( Reynolds et al., 1993 ), or it may not detect a pulse at all during intense vasoconstriction. In rare circumstances, pulse oximetry falsely indicates normal saturations during hypoxic conditions ( Costarino et al., 1987 ).

Capillary refill can provide valuable information about the intravascular volume and cardiac output of an euthermic patient. A child with cool, mottled, poorly perfused extremities should be examined closely for additional evidence of hypovolemia or reduced cardiac output even if the systemic arterial pressure remains normal. Progression of this mottled appearance onto the trunk indicates the extreme vasoconstriction that may herald imminent cardiovascular collapse.

Auscultation

Continuous auscultation of heart and lung sounds by means of a precordial stethoscope is useful during all phases of general anesthesia, as well as during transport of the child between hospital locations. A precordial stethoscope allows the anesthesiologist to immediately detect changes in the rate and character of heart and breath sounds and is often the first warning of a physiologic alteration (e.g., right main bronchial intubation, wheezing, etc.). Crisp heart tones are produced by the flow of blood through a briskly contracting heart. Myocardial depression initially results in a muffled and then a distant quality to the heart tones. Careful auscultation may reveal arrhythmias or murmurs such as the “mill wheel” murmur that results from a venous air embolus. During the administration of halothane, the character of the heart sounds is often used to judge the depth of anesthesia. During ligation of a patent ductus arteriosus (PDA), auscultation with an esophageal stethoscope can help the surgeon identify the correct structure, because clamping the ductus results in disappearance of the murmur.

In selecting and placing a precordial or an esophageal stethoscope, one should consider the nature of the planned surgery, the proposed anesthetic, and any underlying patient condition that may affect auscultation. Breath sounds and heart tones are best heard when a precordial stethoscope is positioned near the left sternal border between the second and fourth interspaces (above the nipple line). An esophageal stethoscope is reserved for patients whose anesthetic management includes endotracheal intubation and in whom a precordial stethoscope either provides inadequate information or violates the surgical field. The proper method for accurate placement of the esophageal stethoscope is to listen while simultaneously advancing the device and placing it at the level where the heart and lung sounds are maximal. In small infants, unintentional placement of the esophageal stethoscope into the stomach can occur easily.

Esophageal stethoscopes are contraindicated in patients with esophageal atresia or in those who have a disease process involving the proximal portion of the esophagus. They confer a rigid feel to the esophagus, which might be mistaken for the trachea ( Schwartz and Downes, 1977 ). As a result, the esophageal stethoscope is relatively contraindicated in neck dissections where the trachea is a critical landmark, such as tracheostomy.

▪ ELECTROCARDIOGRAPHY

In pediatric anesthesia, the electrocardiogram (ECG) is most useful for tracking the heart rate and diagnosing intraoperative rate-related arrhythmias, of which the two most common are bradycardia and supraventricular tachycardia (SVT). The ECG is much less prone to movement-related artifact than is the original pulse oximeter; new pulse oximeter devices have eliminated most motion artifacts. In small infants, hypoxemia-related bradycardia may occur before the pulse oximeter reveals oxyhemoglobin desaturation. Conversely, resolution of hypoxemia is heralded by the transition from bradycardia to normal sinus rhythm. Premature ventricular contractions (PVCs) are commonly observed when halothane is used as the general anesthetic agent, especially during periods of hypercapnia and/or catecholamine release. The precordial stethoscope as a single monitor provides a much better indication of cardiac contractility and thus the overall hemodynamic status.

Electrolyte abnormalities may also be uncovered through the use of the ECG. Hyperkalemia produces the characteristically prominent T waves. Hypocalcemia, which may occur during rapid administration of citrated blood products, prolongs the QT interval. Because ischemic changes in normal pediatric patients are rare and lead II provides a good view of atrial activity for arrhythmia diagnosis, the latter is recommended for the routine intraoperative electrocardiographic monitoring of pediatric patients.

In children, the normal heart rate varies with age ( Table 9-10 ). The normal heart rate of the newborn ranges from 120 to 160 beats per minute, although lower rates (e.g., 70) are frequently observed during sleep, and higher rates (>200) are common during anxiety or pain. Heart rates tend to decrease with age and in parallel with decreases in oxygen consumption. In addition, many children have a noticeable variation in heart rate with respiration (i.e., sinus arrhythmia).

TABLE 9-10   -- Normal resting heart rates of infants and children

 

HEART RATE (beats/min)

Age

Mean

Range (±2 SDs)

0 to 24 hr

119

94 to 145

1 to 7 days

133

100 to 175

8 to 30 days

163

115 to 190

1 to 3 mo

152

124 to 190

3 to 12 mo

140

111 to 179

1 to 3 yr

126

98 to 163

3 to 5 yr

98

65 to 132

5 to 8 yr

96

70 to 115

8 to 16 yr

77

55 to 105

Modified from Liebman J, Plonsey R, Gilette PC, editors: Pediatric electrocardiography. Baltimore, MD, 1982, Williams & Wilkins.

 

 

 

▪ SYSTEMIC ARTERIAL PRESSURE

Noninvasive Measurement

Blood pressure is easily measured noninvasively in children and small infants using oscillotonometry. In children, oscillometric measurements of systolic arterial pressure ( Bruner et al., 1981 ; Friesen and Lichtor, 1981 ) and mean arterial pressure ( Kimble et al., 1981 ) usually correlate well with the Riva Rocci mercury column method as well as with direct arterial pressure measurement but tend to underestimate the diastolic component. During routine uncomplicated cases, measurement of blood pressure should be performed every 3 to 5 minutes while the child is anesthetized-determinations that are too frequent can result in limb ischemia. The blood pressure cuff is most commonly placed on the upper arm but can be placed on the forearm, thigh, or calf. There is inconsistent correlation of measurements obtained between the upper and lower limbs.

The width of the blood pressure cuff should cover approximately two thirds the total length of the upper arm (or other extremity portion to which it is applied). A cuff that is too small or too narrow incompletely occludes the artery, resulting in the premature return of detectable flow and hence falsely increasing the pressure measurement ( Park et al., 1976 ; Kimble et al., 1981 ). The error can be as great as 30 mm Hg. A cuff that is too wide may dampen the arterial wave and result in a falsely low pressure, but the magnitude of this error is small ( Kimble et al., 1981 ). Blood pressure increases gradually throughout childhood (Figs. 9-30 and 9-31 [30] [31]) and is dependent on the height of the child such that taller children demonstrate a higher blood pressure ( Table 9-11 ). Blood pressure ranges in premature infants have been defined ( Table 9-12 ) and vary depending on the health status of the infant and mother.

 
 

FIGURE 9-30  Age-specific percentiles of blood pressure measurements in boys, from birth to 12 months of age. Values for girls are slightly lower.  (From National Heart, Lung, and Blood Institute: Report of the Second Task Force on Blood Pressure Control in Children. Bethesda, MD, 1987, The Institute. Reproduced by permission of Pediatrics, Vol. 79, p. 1. Copyright 1987.)

 

 

 

 
 

FIGURE 9-31  Age-specific percentiles for blood pressure measurements in boys, 1-13 years of age. Values for girls are slightly lower.  (From National Heart, Lung, and Blood Institute, Bethesda, MD: Report of the second task force on blood pressure control in children, 1987. Reproduced by permission of Pediatrics. Vol. 79, p. 1. Copyright 1987.)

 

 

 


TABLE 9-11   -- Systemic arterial pressure

Age

Systolic Pressure (mm Hg) (±95% confidence limits)

Diastolic Pressure (mm Hg) (±95% confidence limits)

Mean Pressure (mm Hg) (±95% confidence limits)

Newborn (kg) *

1

47 (9)

27 (10)

35 (7)

2

54 (9)

32 (10)

40 (7)

3

62 (9)

37 (10)

45 (7)

4

69 (9)

42 (10)

50 (7)

6 wk–9 yr[†]

Boys

93 (18)

59 (18)

 

Girls

96 (24)

62 (22)

 

10–19 yr[†]

Boys

108 (20)

67 (18)

 

Girls

105 (20)

64 (22)

 

*

Adapted from Versmold HT, et al.: Aortic blood pressure during the first 12 hours of life in infants with birth weight 610 to 4,220 grams. Pediatrics 67:607, 1981.

Adapted from Adams FH, Landaw EM: What are healthy blood pressures for children? Pediatrics 68:268, 1981.

 


TABLE 9-12   -- Blood pressure ranges in healthy premature infants (birth weight between 501 and 2000 g)

 

SYSTOLIC BLOOD PRESSURE (mm Hg)

DIASTOLIC BLOOD PRESSURE (mm Hg)

Age (days)

Minimum

Maximum

Minimum

Maximum

1

48 ± 9

63 ± 12

25 ± 7

35 ± 10

2

54 ± 10

63 ± 10

30 ± 0

39 ± 8

3

53 ± 9

67 ± 10

31 ± 8

43 ± 8

4

57 ± 10

71 ± 11

32 ± 8

45 ± 10

5

56 ± 9

72 ± 14

33 ± 9

47 ± 12

6

57 ± 9

71 ± 11

32 ± 7

47 ± 10

From Hegyi T, Anwar M, Carbone MT, et al.: Blood pressure ranges in premature infants: II. The first week of life. Pediatrics 97:336–342, 1996.

Values are mean ± standard deviation.

 

 

 


Direct Measurement

Direct measurement of blood pressure via an arterial catheter is indicated when there is a need for precise beat-to-beat blood pressure monitoring or for frequent determination of arterial blood gas values. This patient population may include children who are expected to develop unstable hemodynamics or those undergoing a surgical procedure that could result in profound hemodynamic alterations related to blood loss (i.e., total loss >50% estimated blood volume [EBV] or acute loss >10% EBV), fluid shifts (i.e., third space losses >50% EBV), deliberate hypotension, or nonpulsatile blood flow (e.g., cardiopulmonary bypass). The respiratory indications for direct arterial monitoring include significant abnormalities in gas exchange due to either preexisting disease or the procedure (e.g., thoracotomy). Rarely, direct arterial monitoring is necessary because of the inability to measure systemic arterial pressure by any indirect technique.

There are no absolute contraindications to placing an arterial catheter, but a risk-benefit analysis should be performed in patients with a hypercoagulable state or bleeding disorder. The radial artery is a favored site for arterial cannulation because the vessel is superficial and easily accessible. Other anatomic sites frequently used are the ulnar, dorsalis pedis, posterior tibial, and femoral arteries. The axillary artery has gained favor because of increased collateral blood flow compared with the brachial or femoral artery ( Lawless and Orr, 1989 ; Cantwell et al., 1990; Greenwald et al., 1990 ; Piotrowski and Kawczynski, 1995 ). In general, the brachial artery should be avoided because of the risk of median nerve damage and poor collateral flow around the elbow. Umbilical vessels provide an alternate site via which the aorta and inferior vena cava may be cannulated in neonates. In determining a site, one needs to consider the history of that vessel (i.e., whether it has been cannulated before), its collateral flow, the experience of the person inserting the catheter, and special physiologic issues (e.g., whether it arises on aortic root proximal to the ductus arteriosus) or surgical issues (e.g., whether it arises from a vessel likely to be clamped or sacrificed during the procedure). Cannulation of vessels with good collateral flow, such as the arch vessels of the wrist or foot, may reduce the risk of ischemic tissue damage distal to the catheter.

As the largest superficial vessel, the femoral artery can be cannulated most predictably in situations where intense peripheral vasoconstriction may accompany low cardiac output and blood pressure. In less dire circumstances, the selection of a vessel may reflect a variety of anatomic and physiologic characteristics exhibited by certain vessels. The pedal vessels exhibit pressure wave amplification that results in pressure determinations exceeding aortic values by as much as 30% ( Park et al., 1983 ).

After palpation and localization of the artery with the nondominant hand, one can cannulate the selected artery either by inserting the catheter directly into the artery using a catheter-over-needle device or by using the Seldinger technique. The Seldinger technique involves entering the vessel with a needle, placing a guidewire through the needle after the vessel is entered, removing the needle, and then placing the catheter over the wire into the vessel. A 22-gauge catheter is appropriate for peripheral artery cannulation in infants and children younger than 5 years, whereas a 20-gauge catheter may be substituted in older children. Aseptic technique should always be followed when placing an arterial line. When cannulating a peripheral artery, it is helpful to immobilize the extremity with a board.

A Doppler flow transducer is occasionally useful to locate an artery that is difficult to palpate. Surgical cutdown may be the preferred option in patients in whom percutaneous placement is likely to be difficult or has failed. Indwelling arterial catheters are associated with several possible complications. Proximal emboli, distal ischemia, arterial thrombosis, and infection are common to all sites. Thrombosis of the radial artery is generally temporary, although it is more likely to persist after a cutdown ( Miyasaka et al., 1976 ). Although small flush volumes (0.3 mL) in radial arterial catheters can be detected in the aortic arch vessels, cerebral infarcts have not been reported ( Edmonds et al., 1980 ). The tip of an umbilical artery catheter should be placed in either a high (above the diaphragm) or a low (below L-3) position to avoid direct flushing into the renal arteries. Despite these precautions, as many as 10% of neonates exhibit hypertension as a late complication attributed to umbilical artery catheterization ( Bauer et al., 1975 ; Plumer et al., 1976 ; Horgan et al., 1987 ). Minor complications of umbilical artery monitoring include vasospasm of the lower extremity vessels, which are more common with low tip placement. Major complications (e.g., necrotizing enterocolitis, renal artery thrombosis) occur independent of location ( Mokrohisky et al., 1978 ; Umbilical Artery Catheter Trial Study Group, 1992 ). The rarity of clinical complications is remarkable given an incidence of aortic thrombosis on removal of umbilical artery catheters that approaches 95% in some series ( Neal et al., 1972 ), although most series define the incidence at 12% to 31% of neonates ( Symansky and Fox, 1972 ; Horgan et al., 1987 ; Seibert et al., 1987 ).

▪ CENTRAL VENOUS PRESSURE

There are four relative indications for central venous catheterization: inadequate peripheral venous access, central venous pressure monitoring, infusion of hyperosmolar or sclerosing substances, and a planned operative procedure with a high risk of hemodynamically significant venous air embolism. There is no absolute indication for central venous pressure monitoring in pediatrics. Unlike direct systemic arterial pressures, central venous pressure itself rarely provides the sole basis for therapeutic action. It does, however, provide useful information that, taken together with other data, helps to form a management plan. The procedures in which this monitoring deserves consideration include large estimated blood loss or fluid shifts (>50% EBV), deliberate hypotension, cardiac surgery with cardiopulmonary bypass, situations in which the usual signs of hypovolemia are likely to be misleading (e.g., renal failure, congestive heart failure), and procedures with expected moderate blood loss or fluid shifts. The normal values for central venous pressure in children are similar to those in adults (mean, 2 to 6 mm Hg).

Every insertion site that has been used in adults can be used in children. Access to the central circulation can be achieved from the internal and external jugular, subclavian, basilar, umbilical, and femoral veins. The site selected depends on the experience of the operator and the indication for the catheter. If venous access is the only requirement, one might elect to use visible veins (e.g., basilar, external jugular) or those with a lower risk of complications (e.g., femoral). Situations that require true intrathoracic central venous placement also require placement of the catheter into the internal jugular vein or subclavian vein. The umbilical vein can be used in neonates for volume resuscitation, but the high frequency with which these catheters enter the branch portal veins introduces a significant risk of permanent liver injury if sclerosing or hyperosmolar solutions are infused. Because a catheter tip can erode through the wall of the right atrium, care must be taken to avoid intracardiac tip placement. The catheter should be advanced only until the orifice lies in the intrathoracic great vessels, and its position should be confirmed radiographically.

Catheters of various sizes (2.5 to 10 F), lengths, and composition are available for pediatric applications (Cook Critical Care, Bloomington, IN, and other companies). Selection is based on the size of the patient ( Andropoulos et al., 2001 ) and the purpose of the catheter. The composition of the catheter depends on its intended use. Teflon is fairly resistant to thrombus formation, but concerns about perforation by catheters have prompted the development of softer catheter materials, especially for long-term use (e.g., Silastic and polyurethane). The catheters are generally inserted via a Seldinger technique using landmarks that are similar to those used in adults.

There are no absolute contraindications to placing a central venous catheter, but each site has potential risks. All sites share the common complications of infection (site cellulitis, bacteremia), venous thrombosis with potential emboli, air embolism, catheter malfunction (occlusion, dislodgment, or fractures), dysrhythmias (when the catheter tip is in the heart), and bleeding. Universal precautions and sterile technique should be used when placing a central venous catheter. The risks involved in cannulating the internal jugular vein include carotid artery puncture, Horner's syndrome, pneumothorax, and injury to the thoracic duct when the left internal jugular vein is cannulated. The high approach to the internal jugular vein, at the midpoint of the sternocleidomastoid muscle, results in comparable success with fewer complications than lower approaches ( Coté et al., 1979 ). Two-dimensional ultrasound scanning improves localization of the internal jugular vein and increases the success rate of central venous cannulation in adults and children ( Verghese et al., 2002 ; Hind et al., 2003 ) ( Fig. 9-32 ). Using this device, Alderson and others (1993) reported an 18% prevalence of anatomic variations in children younger than 6 years that would preclude or significantly hinder the successful cannulation of the internal jugular vein using anatomic landmarks alone.

 
 

FIGURE 9-32  The Site-Rite (Bard Access Systems, Pittsburgh, PA) is an ultrasound device that is used to localize the anatomic position and relationship of large vessels and to facilitate central venous access.  (Reproduced with permission.)

 



▪ PULMONARY ARTERY CATHETERS

Since its introduction in 1970, indications for the use of the flow-directed balloon-tipped pulmonary artery catheter (Swan-Ganz) in pediatric patients have been slow to evolve. While the validity and value of the data that these catheters generate remain controversial in pediatrics, the technical difficulties and complications associated with their use are significant. Pulmonary artery pressure measurement can help guide therapy in children with elevated or volatile pulmonary vascular resistance, but the interpretation of the flow data they generate is hindered by several factors. First, the desired cardiac output varies according to age, disease state, and other elements of management that alter metabolic demand in complex ways, thereby introducing significant uncertainty in assigning a target value. Second, the prevalence of intracardiac communications that permit shunting of blood causes discrepancies in pulmonary and systemic blood flow that may vary continuously and are difficult to quantify. Finally, despite several studies demonstrating reasonable accuracy when thermodilution is compared with other methods of flow determination, such as the Fick equation ( Freed and Keane, 1978 ) and dye dilution (Colgan and Stewart, 1977 ), the precision of these determinations in small infants is low and has a 25% intersample variability. In patients with congenital heart malformations, for example, measurement errors are introduced by shunting and complex anatomy, and the risks of improper placement of the flow-directed pulmonary artery catheter are increased. Alternatively, directly placed pulmonary artery catheters can provide the necessary information regarding pulmonary vascular resistance and residual left-to-right shunts, whereas left atrial catheters reflect filling and diastolic function of the left ventricle after cardiac surgery.

There are situations in which pulmonary artery catheters can provide useful information. In children who have severe coexisting pulmonary and circulatory failure, pulmonary artery catheters can help to quantify the hemodynamic impact of extreme respiratory support measures and guide complex fluid and pharmacologic regimens. They may also be useful in patients with underlying pulmonary hypertension or poorly compensated left ventricular dysfunction who undergo acute surgical stress (e.g., arteriovenous malformation clipping or aortic cross-clamping). Given the uncertainty regarding optimal systemic flow in a given child, mixed venous oxygen saturation may serve as a better indication of global perfusion. In the absence of left-to-right shunts, this sample is best obtained from the pulmonary artery.

Pulmonary artery catheters can be difficult to insert, especially in infants or in children with low cardiac output. They may be placed in any vein used for access to the central venous system, but the most reliable veins are the right internal jugular and the femoral. In infants and children smaller than 15 kg, it is technically difficult to place an introducer sheath in the neck vessels; the femoral veins are preferable. Multilumen catheters capable of thermodilution are available in two sizes, 5 and 7 F, with four options for the right atrium-pulmonary artery interluminal distance. Catheter recommendations are based on age ( Table 9-13 ). The proper placement of these catheters can take a long time, and thus the assistance of fluoroscopy is recommended in infants and children less than 30 kg and in larger children who have a low cardiac output.


TABLE 9-13   -- Guidelines for multilumen pulmonary artery catheters in infants and children

Age (yr)

Size (F)

CVP - Pulmonary Artery Port Distance (cm)

Newborn to 3

5

10

3 to 8

5

15

8 to 14

7

20

>14

7

30

CVP, central venous pressure.

 

 

 

The risks of balloon-tipped pulmonary artery catheters are numerous and include the risks of central venous catheter placement discussed previously, as well as the complications seen in adult patients with pulmonary artery catheters: infection, air emboli, thrombus, pulmonary artery rupture, acute right bundle branch block, and intracardiac knots. There are also complications that are more common with children: misleading information, paradoxical systemic emboli, disruption of an intracardiac repair, and high-grade right ventricular outflow tract obstruction because of the relatively large balloon diameter. The presence of intracardiac and extracardiac malformations may result in an aberrant catheter course leading to incorrect data as well as an increased risk of systemic emboli.

Cardiac output can be estimated in children through indicator dilution (e.g., thermodilution or dye dilution) and noninvasive techniques. Doppler determinations of aortic blood velocity can be used to quantify systemic flow if the angle of the incident ultrasound beam and the cross-sectional area of the aorta are reliably determined ( Alverson et al., 1982 ). Transthoracic and transesophageal evaluations of Doppler cardiac output in children have proved to be less promising ( Notterman et al., 1989 ; Muhiudeen et al., 1991 ). Thoracic bioimpedance, a method that estimates stroke volume on the basis of changes in thoracic impedance, has been applied to children as small as 3.6 kg. Although some correlation exists between bioimpedance and indicator dilution methods, reproducibility is poor ( O'Connell et al., 1991 ). Further details and the complexities encountered in the measurement of cardiac output in children are beyond the scope of this chapter but have been reviewed previously ( Tibby and Murdoch, 2002 ).

A noninvasive cardiac output (NICO) monitor has been developed that determines cardiac output via the Fick principle for rebreathed carbon dioxide (Respironics; Novametrix Medical Systems Inc., Wallingford, CT) ( Capek and Roy, 1988 ). The NICO monitor has been clinically validated in adults and is approved by the Food and Drug Administration for use, but it requires tidal volumes of 200 mL or greater ( Guzzi et al., 2003 ; Watt et al., 2004 ). Unpublished studies in children have demonstrated a similar validation compared with thermodilution (personal communication, Richard J. Levy, M.D., 2004) and represents a possible future method for NICO determination.

▪ TRANSESOPHAGEAL ECHOCARDIOGRAPHY

Transesophageal echocardiography (TEE) provides anatomic and physiologic information in small infants and children ( Phoon et al., 1999 ; Kavanaugh-McHugh et al., 2000 ). Because of its esophageal position, it affords a unique perspective when the transthoracic approach is either unavailable (e.g., during cardiac surgery) or not helpful. For small infants, transesophageal probes with decreased size (9 mm) are available with “omniplane” imaging, in which, once the transducer is directed at a specific site of interest, the imaging plane can be rotated 180 degrees. As the technical resolution of TEE probes has improved, posterior cardiac structures such as the pulmonary veins, left atrium, mitral valve, and left ventricular outflow tract are often better visualized from the esophagus. The wide spectrum of anatomic variability with congenital heart malformations and the judgment necessary to make physiologic determinations under the varying functional and loading conditions typical in the operating room require extensive experience ( Hsu et al., 1991 ; Muhiudeen et al., 1992 ). Unlike many anesthesia practices devoted to acquired heart disease, TEE investigations in children with congenital malformations are usually performed in conjunction with pediatric cardiologists.

▪ TEMPERATURE

Temperature monitoring is vital during pediatric anesthesia as children may exhibit hypothermia or hyperthermia, both of which can have profound physiologic consequences (see Chapter 5 , Thermoregulation: Physiology and Perioperative Disturbances). As discussed previously in this chapter, temperature maintenance in the operating room is difficult for the infant and the small child; hypothermia is common in this group. Although axillary temperature measurements are easier to obtain, core temperature measurement using rectal or esophageal probes better reflects the magnitude of hypothermia and should be used routinely, particularly in small children.

The site selected for temperature monitoring depends on one's objectives. Rectal temperature provides a good index of core temperature but lags behind temperature monitors in more vascularized locations (e.g., esophagus, nasopharynx), especially during rapid core cooling, as occurs during institution of cardiopulmonary bypass. Esophageal temperature is influenced by the great vessels and thus rapidly reflects changes in core temperature if the probe is in the middle or distal portion of the esophagus. Probes in the proximal portion of the esophagus may be influenced by the inspired gas temperature in the trachea ( Bissonnette et al., 1989a ). These probes should be used only in patients who have undergone endotracheal intubation. Nasopharyngeal temperature probes more accurately reflect the temperature of the blood perfusing the brain ( Hindman et al., 1992 ). Usually, a small probe inserted a distance equal to that between the ala nasi and the tragus of the ear reaches the mucosa of the nasopharynx. Tympanic membrane probes also accurately reflect brain temperature. In early studies, thermocouples provided tympanic membrane temperature. Because they must make contact with the tympanic membrane, the insertion of these probes was noxious and traumatic to children, resulting in a small incidence of bleeding and even tympanic rupture. Axillary temperature is usually 1°C lower than core temperature. Axillary probes should be placed over the axillary artery with the arm tightly adducted. Skin surface temperature varies from core temperature by an unpredictable and often substantial amount as a function of changes in skin perfusion. Algorithms used by manufacturers of liquid crystal skin thermography sensors tend to overestimate core temperature in anesthetized children ( Leon et al., 1990 ).

▪ URINE OUTPUT

Urine output often reflects intravascular volume status and cardiac output. Proper assessment of urine output requires recognition of the physiologic mechanisms that exert an affect on urine flow in children. During the first week of life, the glomerular filtration rate and renal plasma flow are only 25% of normal adult values ( Arant, 1978 ). The neonatal kidney is limited in its ability to concentrate the urine ( Simpson and Stephenson, 1993 ). By the end of the first week of life, the kidney begins to reach absorption thresholds for sodium and glucose that approach adult levels.

Normal newborns produce between 0.5 and 4 mL urine/kg per hour in the first 3 hours of life ( Strauss et al., 1981 ). Urine flow, which initially ranges from 15 to 60 mL/kg per day, reaches as much as 120 mL/kg per day by the end of the first week of life, with 90% of neonates producing 0.5 to 5 mL/kg per hour ( Douglas, 1972 ; Guignard, 1982 ). In the neonate less than 1 week old, urine flow alone is not a sensitive index of changes in cardiac output or intravascular volume. The limited capacity of the neonatal kidney to compensate for diminished or excessive intravascular volume demands more precise management of blood and fluid replacement in these infants.

Beyond the neonatal period, a urine flow of 0.5 to 1 mL/kg per hour usually indicates adequate renal perfusion and function. When the systemic arterial pressure exceeds a critical opening pressure for the afferent arteriole of the glomerulus (approximately 75 mm Hg systolic pressure in the adult), urine flow is directly proportional to circulating blood volume. Urine flow may be subject to several modulating factors in the operating room: the hormonal response to anesthesia and surgery, previous diuretic therapy, preexistent renal disease, hypothermia, deliberate hypotension, and the nonpulsatile flow of cardiopulmonary bypass. When these modulating factors are likely to have a significant impact on urine flow, alternative measures for estimating circulating blood volume must be used.

Intraoperative monitoring of urine output is indicated in procedures in which large shifts in fluid, blood, or hemodynamics are anticipated, including blood loss greater than 20% of the EBV, third space replacement exceeding 50% of the EBV, cardiopulmonary bypass, neurosurgery, deliberate hypotension, planned use of diuretics, or planned hemodilution. Silastic Foley catheters are available in sizes small enough (6 F) for full-term neonates. Alternatively, a small feeding tube can be used in premature infants and in those with a small urethra. In infants, urinary bladder catheters should be connected to a urinometer capable of measuring small volumes or to a vented 10- to 20-mL syringe.

▪ NONINVASIVE RESPIRATORY GAS MONITORING

Carbon Dioxide

Capnometry is the instantaneous measurement of carbon dioxide in the breathing circuit; capnography depicts this information in a continuous graphic display by which both the quality and the quantity of ventilation can be evaluated (Figs. 9-33 to 9-36 [33] [34] [35] [36]).

 
 

FIGURE 9-33  Common capnographic diagnoses-rebreathing. Top tracings, normal capnographs with the graph on the right compressed over a longer time. Note plateau suggesting valid end-tidal carbon dioxide data and the return to baseline between breaths; there is no rebreathing. Lower curves illustrate rebreathing, as there is no return to baseline. This can occur with inadequate fresh gas flow or floating unidirectional valves. The small initial deflection before exhalation (“pre-exhalation hump”) can occasionally be seen. It represents the inhalation late in the inspiratory phase of more concentrated exhaled gas from the previous exhalation.

 

 

 
 

FIGURE 9-34  Common capnographic diagnoses-poor sampling. Left, poor sampling as evidenced by absence of a plateau phase. One would not be able to make this diagnosis with a capnometer that is incapable of real-time graphics. This is typical of small neonates whose small exhaled volumes are washed out by fresh gas flow. The speckled curve to the right projects the full exhaled breath if fresh gas flow were diverted. Note that the plateau would be higher than the actual curve by an unpredictable amount. The digital information derived from a curve like that on the left is useless.

 

 

 
 

FIGURE 9-35  Common capnographic diagnoses-reduction in PETCO2. There are many reasons for sudden reduction in the PETCO2, some of which result in characteristic capnographic patterns. (A) Abrupt reduction to zero or nearly zero typically indicates mechanical disruption, disconnection, accidental extubation, or plugged sampling line. (B) Sudden reduction to a lower PETCO2 while preserving plateau and characteristics of a good trace indicates sudden increase in dead space ventilation as occurs with a pulmonary embolus (either thrombus or air). (C) Exponential reduction to zero (carbon dioxide washout curve) is characteristic of no pulmonary blood flow and thus either massive embolus or cardiac arrest.

 

 

 
 

FIGURE 9-36  Common capnographic diagnoses-irregular tracings. Irregularities in the curve are common especially at the end of exhalation when exhaled gas flow is lowest. (A) Diaphragmatic activity indicating spontaneous respiratory effort, usually the result of dissipating neuromuscular blockade. (B) Cardiac oscillations: fluctuations in intrathoracic gas volume as a result of cardiac activity, usually a benign finding.

 

 

Before 1998, capnography was considered a standard monitor by the ASA for the purpose of confirming the initial placement and continuous presence of an endotracheal tube. This section of the ASA monitoring standards was updated in 1998 and states that capnography should be used to confirm adequate ventilation during general anesthesia with or without an endotracheal tube (during laryngeal mask airway, facemask, or natural airway anesthesia). Specifically, these guidelines state:

Continual monitoring for the presence of expired carbon dioxide shall be performed unless invalidated by the nature of the patient, procedure or equipment…. Continual end-tidal carbon dioxide analysis, in use from the time of endotracheal tube/laryngeal mask placement, until extubation/removal or initiating transfer to a postoperative care location, shall be performed using a quantitative method such as capnography, capnometry or mass spectroscopy (ASA, 2003).

Most capnometers use the principle of infrared light absorption by sampling circuit gas in either a mainstream or a sidestream fashion. Sidestream analyzers aspirate a sample from the circuit and transport it via a long, narrow-bore tube to a distant analyzing chamber. Advantages include a lightweight airway adapter and the remote location of the delicate components of the analyzing chamber. Disadvantages of sidestream systems include potential occlusion of the sampling tube, distortion or dilution of the exhaled gas wave during aspiration and transport to the analyzing chamber, and the delay necessary to transport and analyze the sample. Innovations in capnography technology have allowed a sampling rate as low as 30 mL/min (“microstream technology”).

Mainstream analyzers use a sample chamber placed directly into the circuit. They have the advantage of providing virtually instantaneous analysis by avoiding transport of the sample. Such a system necessitates the addition of a delicate and bulky sensor to the proximal airway connection, where it might easily serve as a fixation point to dislodge a small tracheal tube. Solid-state innovations have dramatically reduced the weight of the mainstream sensors, but they remain significantly more hazardous when added to the circuits of neonates and small infants. Although early mainstream sample chambers added as much as 17 mL of dead space to the circuit, currently available models reduced this volume to 2 mL or less.

Capnography in pediatric anesthesia is used to confirm placement of an endotracheal tube in the correct tracheal position and to continuously assess the adequacy of ventilation. Capnography also provides information about the respiratory rate, breathing pattern, endotracheal tube patency, and, indirectly, degree of neuromuscular blockade. Capnography can assist with the diagnosis of metabolic and cardiovascular events and can provide an early warning of a faulty anesthesia delivery system. In pediatric patients, an abnormal increase in end-tidal carbon dioxide (PETCO2) most commonly signifies hypoventilation but, rarely, also indicates the presence of increased carbon dioxide production as occurs with temperature elevation or as an early sign of malignant hyperthermia. Conversely, an abnormally low PETCO2 may indicate an increase in dead space or suggest a state of low pulmonary perfusion. Sudden absence of the capnographic tracing indicates a breathing circuit disconnection, and the abnormal presence of inspired carbon dioxide signifies the presence of a faulty unidirectional valve, an exhausted carbon dioxide absorber, or, when a semiopen circuit is being used, rebreathing secondary to an insufficient fresh gas flow.

The capnographic tracing of small infants is often characterized by the lack of an apparent alveolar plateau. This is usually a result of a higher respiratory rate, an excessively high sampling flow for the volume of carbon dioxide produced, excessive dead space in the breathing circuit, or an excessive leak around an uncuffed endotracheal tube.

The degree to which PETCO2 reflects PaCO2 is subject to many variables, some technical and others physiologic. The technical issues of primary importance in the accurate measurement of mean alveolar carbon dioxide tension include the volume and flow rate of exhaled gas, the aspirating flow rate (for sidestream analyzers), the fresh gas flow rate, the type of breathing circuit, and the circuit location of the sampling chamber (mainstream analyzers) or lumen of the aspirating tubing (sidestream analyzers). These variables are of particular importance in the small neonate whose small exhaled volumes at low flow rates are often diluted by high fresh gas flows or aspirating gas flows (Badgwell et al., 1987, 1993 [18] [19]; Rich et al., 1990 ; Spahr-Schopfer et al., 1993 ). Badgwell and others (1987) demonstrated exponential increases in the discrepancy between PETCO2 and PaCO2 values with progressive reduction in patients with weight of less than 12 kg ( Fig. 9-37 ). The coaxial distal sampling tube that they advocated dramatically improved the correlation.

 
 

FIGURE 9-37  Gradient between end-tidal carbon dioxide (ETCO2) determinations made in the proximal and distal ends of a tracheal tube. Exponential increases in the gradient suggest substantial potential inaccuracy in proximal ETCO2 determinations for children under 12 kg.  (From Badgwell JM, McLeod ME, Lerman J, et al.: End-tidal PCO2 measurements sampled at the distal and proximal ends of the endotracheal tube in infants and children. Anesth Analg 66:959, 1987.)

 

 

 

The physiologic variable that introduces the most significant error in PETCO2 is dead space ventilation ( Swedlow, 1986 ). Apart from children with severe pulmonary pathology or acute events such as pulmonary embolus, the most prevalent pediatric population in whom substantial dead space ventilation occurs are those with cyanotic congenital heart disease, particularly right-to-left shunts ( Burrows, 1989 ; Fletcher, 1991 ).

Other Gases

The ability to measure other respiratory and anesthetic gases can provide important information about cardiopulmonary physiology. Confirming the elimination of nitrogen is useful in determining adequate preoxygenation, whereas its presence during anesthesia may reveal a leak in the delivery system or, in combination with a sudden decrease in end-tidal carbon dioxide, a venous air embolism. The measurement of anesthetic gases and vapors serves to illustrate the uptake and elimination of these agents and to confirm the purity and the accuracy of the tanks and vaporizers used to administer them. The quantity of residual inhaled anesthetic agent has obvious importance in the evaluation of prolonged emergence from anesthesia.

The techniques enabling multigas analysis that have found clinical application are based on properties such as ionized mass separation (mass spectrometry), ultraviolet and infrared light absorption, and absorption into lipophilic substances. A variety of manufacturers produce devices that quantify respiratory and anesthetic gases in the same unit as the capnograph; their complete description is beyond the scope of this chapter.

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▪ MONITORING OXYGEN AND CARBON DIOXIDE

Because of the high oxygen consumption and small functional residual capacity of infants and small children, they are more likely to become hypoxemic during general anesthesia than adults. Careful tracking of arterial oxygenation is vitally important. Noninvasive monitors of oxygenation are ubiquitous in the perioperative setting because of technological advances that have improved their reliability and because of standards that require their application not only in the operating room but also in the PACU and sedation room. These devices are of two basic types: those that measure cutaneous (transcutaneous) oxygen tension and others that evaluate arterial oxygen saturation (pulse oximeters).

▪ CUTANEOUS OXYGEN TENSION

In 1972, a miniature Clark polarographic oxygen electrode, similar to those used in in vitro blood gas analysis, became available for application to the skin. When a probe heats the skin to 42° to 44°C, the cutaneous oxygen tension (Pso2) approaches arterial oxygen tension because the skin blood flow and permeability to oxygen are increased ( Barker and Tremper, 1985 ). The correlation may be better in neonates because their epidermis is less keratinized and their cutaneous capillary bed is denser. In fact, skin heating alters oxygen dissociation and may even result in a Pso2 value that is higher than the Pao2value ( Lubbers, 1981 ). In older children and adults, as the keratinized layer thickens, the diffusion gradient for oxygen becomes more significant. In practice, transcutaneous gas monitors are subject to the effects of these and myriad other nonlinear variables that influence skin perfusion, such as hypotension, hypothermia, and pharmacologic agents. They are the only noninvasive monitors that can provide information regarding significant hyperoxemia ( Monaco et al., 1982 ; Rafferty et al., 1982 ; Barker and Tremper, 1985 ). The correlation, especially outside the physiologic range of Pao2, is variable depending on the individual conditions, and the data produced may differ from that provided by the Pso2 by a substantial yet unpredictable amount ( American Academy of Pediatrics, 1989 ). Such results led Barker and Tremper (1985) to propose a more reasonable role for this monitor in determining peripheral tissue oxygen delivery (perfusion) rather than arterial oxygenation.

From a practical standpoint, the monitor is cumbersome. It requires calibration, a warm-up time of 10 to 20 minutes, and meticulous skin preparation and probe placement. It is sensitive to electrosurgical interference and mechanical manipulation. The technical demands of this monitor have limited its current use to special applications, such as detection of hyperoxemia in premature infants. A frequent side effect of transcutaneous monitoring is the occurrence of first- and second-degree burns.

▪ CUTANEOUS CARBON DIOXIDE TENSION

Cutaneous carbon dioxide tension (PsCO2) measurement using a variant of the Severinghaus electrode is also available ( Nosovitch et al., 2002 ). Although PsCO2 is always higher than PaCO2 as a result of tissue carbon dioxide production and the increased metabolism caused by a heating sensor, these monitors accurately follow trends in arterial carbon dioxide tension. The predictable gradient from arterial to cutaneous carbon dioxide tensions enables the monitors to calculate the gradient and display a “corrected” value. These devices are less altered by changes in skin temperature and perfusion than are cutaneous oxygen analyzers. The reasonably good correlation of end-tidal and arterial carbon dioxide tensions in all but extremely small subjects has limited the interest in cutaneous carbon dioxide tension monitoring in the operating room to very rare situations.

▪ PULSE OXIMETRY

Pulse oximetry provides an estimate of the oxyhemoglobin saturation. The pulse oximeter uses plethysmography to determine the systolic portion of the cardiac cycle. During systole, there is a greater volume of blood in a pulsatile arterial vascular bed. This vascular bed is positioned between a sensor that contains a two-wavelength (660 and 940 nm) light-emitting diode and a photodiode receptor. Less light is transmitted in systole than in diastole because of the increased volume of the arterial bed. Sophisticated algorithms based on the amount of light absorbed differentiate systole from diastole. Oxygenated and deoxygenated blood absorb different quantities of light, proportional to their concentrations or the percent saturation according to the Beer-Lambert law. Once systole is identified through plethysmography, the arterial saturation during this period is determined by using the ratio of light absorption at the two different wavelengths through this vascular bed. The ratio is matched to data acquired over a range of experimentally determined saturations and ratios of light absorption stored in the instrument's memory to determine the arterial saturation. Using the ratio of light absorption at two different wavelengths makes unnecessary individual calibration and zeroing to adjust to the size of the patient or skin pigment. Within the 80% to 100% saturation range, arterial saturation values determined by this method correlated well with in vitro measurements ( New, 1985 ).

Based on the algorithm used to determine systole and the time-averaging process used over several cardiac cycles, different pulse oximeters have slightly different responses to a variety of clinical situations. Because the device must identify the pulse-added absorption, it may confuse motion of the extremity to which the sensor is attached with pulsating motion and abort the display of saturation data or, worse, display inaccurate data. Decreased peripheral perfusion that is caused by decreased temperature or low cardiac output, a pulseless state, also makes determination of the pulse interval difficult, causing loss of data during critical episodes. Because the machine identifies the pulse as systole, high venous pressure causing venous pulse waves, as in secondary severe tricuspid regurgitation, elevated intrathoracic pressure, or obstructed venous return, can also create false readings ( New, 1985 ).

Pulse oximetry was widely introduced into pediatric practice in the United States in the 1980s ( Salyer, 2003 ). It serves as an early warning signal of impending or actual hypoxemia, often before the onset of cyanosis, and frequently reminds anesthesiologists of the alarming rapidity with which infants develop hypoxemia. Continuous use pulse oximetry is included in the Basic Monitoring Standards of the ASA (2003).

There are no outcome studies that demonstrate proved benefit from the use of pulse oximetry ( Moller et al., 1993 ; Pedersen et al., 2001 ). Anesthesiologist-blinded studies have demonstrated that the use of pulse oximetry facilitates earlier recognition and fewer episodes of hypoxemia (Coté et al., 1988, 1991 [65] [66]). Pulse oximetry has evolved into a standard monitor during pediatric anesthesia and has never been subjected to rigorous outcome studies with a true control group (an anesthetic without a pulse oximeter) ( Cohen et al., 1988 ).

There are a number of well-described limitations of pulse oximetry, in which values are dependent on ambient lighting conditions, motion, peripheral circulation to the extremity, and abnormal hemoglobins, among other factors.

Although the pulse oximeter is a continuous monitor, it does not instantaneously reflect the arterial saturation or the degree of desaturation. When fully saturated, a substantial decrease in Pao2 can occur without a change in Sao2. Reynolds (1993) detected desaturation in children 30 seconds earlier in probes placed centrally (facial) than in those placed on an extremity ( Reynolds et al., 1993 ). By the time the value indicated by a peripheral sensor had decreased 5%, the value indicated by a central sensor was 30% to 40% lower. The precise mechanism for this discrepancy remains unknown, althoughSeveringhaus and Naifeh (1987) postulated that it reflects peripheral blood transit, capillary composition, and oxygen utilization.

Pulse oximeters are designed to warn practitioners when the arterial saturation decreases below normal, not to serve as quantitative devices in hypoxemic patients. Compared with measured arterial saturation in children with cyanotic congenital heart disease, most pulse oximeters exhibit a progressively positive bias typically reaching 5% to 15% at an Sao2 of 60%, in addition to a significant reduction in precision (±8% to 10%) ( Gidding, 1992 ; Schmitt et al., 1993 ) ( Fig. 9-38 ).

 
 

FIGURE 9-38  Accuracy and precision of pulse oximetry (Nellcor N-100) in chronically hypoxemic children with congenital heart malformations. Data comparing pulse oximetry (Spo2) with co-oximetry (Sao2) reflect a 5.8% mean positive bias for Spo2 with wide discrepancies between the two techniques (±2 SDs = -3.8% to +15.4%).  (From Schmitt HJ, Schuetz WH, Proeschel PA, et al.: Accuracy of pulse oximetry in children with cyanotic congenital heart disease. J Cardiothorac Vasc Anesth 7:61, 1993.)

 

 

 

Interference with the expected spectrophotometric absorption pattern also causes errors in measurement. Low hemoglobin, less than 5 g/dL, and abnormal hemoglobin species, like methemoglobin or carboxyhemoglobin, cause inaccurate saturation estimates by the pulse oximeter ( New, 1985 ; Barker and Tremper, 1987 ; Barker et al., 1989 ; Watcha et al., 1989 ). In contrast, abnormal hemoglobin molecules, like fetal hemoglobin, apparently have little effect on the saturation measurement ( Jennis and Peabody, 1987 ). Intravenous dyes, like methylene blue and indocyanine green, affect the expected light absorption and produce spurious information ( Sidi et al., 1987 ). Aberrant radiation, like the electromagnetic energy from the electrocautery, infrared heat lamps, or operating room lights, also causes incorrect saturation determination ( Brooks et al., 1984 ; Costarino et al., 1987 ; Hanowell et al., 1987 ).

Innovative pulse oximetry technologies using signal extraction technology (SET) have claimed improved performance during extremity motion and states of poor perfusion (Masimo Corporation, Mission Viejo, CA) ( Anonymous, 2000 ; Goldman et al., 2000 ). Studies comparing SET with conventional pulse oximetry during pediatric anesthesia have demonstrated superior performance with SET ( Malviya et al., 2000 ). Its application in the perioperative setting has not been universally adopted.

▪ BISPECTRAL INDEX

In 1996, the Food and Drug Administration approved the use of the bispectral index (BIS) monitor (Aspect Medical Systems, Nattick, MA), an electroencephalogram (EEG)-based device that is used to predict the relative level of hypnosis, or unconsciousness, in anesthetized patients ( Rosow and Manberg, 1998 ). With use of a patch that affixes to the patient's forehead, the BIS monitor integrates various EEG descriptors into a single dimensionless, empirically calibrated number ranging from 0 to 100, where 0 represents electrical silence and 100 represents full wakefulness. A state of unconsciousness consistent with BIS values less than 60 usually ensures a lack of intraoperative recall ( Glass et al., 1997 ). In adults, titration of anesthetics to a targeted BIS value between 40 and 60 results in the administration of smaller doses of anesthetics and earlier awakening ( Gan et al., 1997 ). Preliminary data in adults suggest that routine BIS monitoring is associated with reduced intraoperative awareness during high-risk surgical procedures (e.g., microlaryngeal surgery, cesarean section, cardiac bypass, etc.) ( Myles et al., 2003 ).

In anesthetized children, BIS values are inversely proportional to the end-tidal concentration of sevoflurane ( Denman et al., 2000 ; Davidson et al., 2001 ; Degoute et al., 2001 ). This association weakens in infants younger than 1 year. In adolescents undergoing scoliosis surgery, BIS can predict voluntary patient movement in response to commands during the intraoperative wake-up test ( McCann et al., 2002). BIS values during sevoflurane anesthesia appear to be proportionately less in children with quadriplegic cerebral palsy and mental retardation ( Choudhry and Brenn, 2002 ).

BIS monitoring in children aged 3 to 18 years who are undergoing tonsillectomy and adenoidectomy is associated with reduced recovery times; in the same study, BIS monitoring did not affect recovery times in children younger than 3 years who were undergoing hernia repair ( Bannister et al., 2001 ).

Because of age-related differences in brain maturation and synapse formation throughout childhood, BIS monitoring may not be as useful an intraoperative monitor as for adults ( Watcha, 2001 ). Future studies are expected to further delineate the use of BIS in the pediatric population.

▪ NEUROPHYSIOLOGIC MONITORING

It has long been appreciated that the patient's physiologic status is dynamic and that rapid and life-threatening changes may occur during surgery. The comparative ability to evaluate the functional status of the nervous system by either clinical means or the commonly used physiologic monitoring tools that are available to the anesthesiologists is limited. Routine monitoring may reflect stress on the central nervous system (CNS), yet changes in heart rate related to both brainstem and vagal stimulation provide only a nonspecific, insensitive view of global function. Intraoperative neurophysiologic monitoring adds another dimension as well as specificity to assessment of the status of the patient during surgery and anesthesia.

Neurophysiologic techniques provide important and reliable alternative tools for assessment of function of the pediatric CNS ( Taylor, 1993 ). These techniques provide objective measures of the functioning of the CNS and can serve to document and localize deterioration in neuronal function. Intraoperatively, continuous monitoring of the area of the CNS that is at risk from surgical and anesthetic manipulation provides immediate insight into the effects of these operative interventions. Rather than waiting to evaluate the neurologic examination in the postoperative period of a child at risk for intraoperative diminution of neurologic function, continuous neurophysiologic monitoring provides an immediate view of the integrity of the CNS, permitting intraoperative changes in operative and anesthetic technique to minimize or correct the deleterious effects of the intraoperative manipulation. Advantages of these methods are that the results are objective and quantifiable, the site of the lesion can be identified, clinically latent or evolving lesions can be frequently demonstrated, and distinctions can often be made between disease entities. These techniques have a role to play both in the diagnostic investigation of pediatric CNS function and in the developing field of intraoperative assessment of CNS function.

It is imperative that the measures used are both specific to the neural tissue being manipulated and sensitive to changes in the functioning of the neural tissue produced by the surgical manipulations. Monitoring of the electrical activity dependent on the functioning of the brainstem (brainstem auditory evoked potentials [BAEPs] and brainstem somatosensory evoked potentials [BSEPs]), the cortex (the EEG, somatosensory evoked potentials [SEPs], and visual evoked potentials [VEPs]), the spinal cord (SEPs and motor evoked potentials [MEPs] and electromyograms [EMGs]), the various cranial nerves (EMGs), and peripheral nerves (compound action potentials and EMGs) provides a multidimensional assessment of the integrity of the neural structures at risk. In addition, many of these measures provide information not only about function itself but also about variables that directly or indirectly affect function, such as blood flow, hypoxia, and hypotension. The goal of intraoperative neurophysiologic monitoring is to provide information to the surgeon and anesthesiologist to allow them to modify their operative strategy before inducing additional deficits in the functioning of the CNS.

▪ PERIOPERATIVE ASSESSMENT

An important aspect necessary for successful intraoperative monitoring is the planning and execution of surgical procedures in such a way that the neurophysiologists are in close communication with all other members of the surgical team, including surgeons, anesthesiologists, and radiologists. This preoperative and intraoperative communication among members of the surgical team ensures that the appropriate neurophysiologic measures are used during the procedure, that the anesthesiologist is prepared to switch anesthetic technique in support of the requirements imposed by the monitoring techniques, and that the significance of observed changes is appreciated by all members of the operative team. The surgeon needs to understand the level of information that the neurophysiologist can provide as the operative procedure is evolving, and the anesthesiologist needs to understand the effects of the pharmacologic manipulations on the monitoring tools available to the neurophysiologist. In addition, preoperative and postoperative neurophysiologic studies to determine and to reaffirm baseline responses increase understanding of the significance of intraoperative findings.

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▪ TECHNICAL METHODOLOGY

The recording of high-quality neurophysiologic data is dependent on the appropriate use of technology. In particular, attention must be paid to the electrode properties, the amplifiers, and the equipment used to acquire and display the data.

▪ ELECTRODES

The bioelectrical activity at the scalp and the surface of the body is sensed using metal electrodes and transferred through conducting leads to recording amplifiers. Disk electrodes are typically used in the diagnostic laboratory, whereas subdermal needle electrodes are used in the operating room. Both disk and needle electrodes may also be used as stimulating electrodes as well as recording electrodes.

▪ SIGNAL AMPLIFICATION

Techniques of data acquisition and handling before analysis are of considerable importance in the operating room environment. Functions such as signal scaling, bias levels, prefiltering, artifact removal, and analog-to-digital conversion must be optimized by trained individuals using specialized equipment. For example, evoked potentials are typically a fraction of the size of the spontaneous brain activity appearing in the background EEG and about one-thousandth the size of the other physiologic and extraneous potentials with which they are intermixed. The most effective method for extracting the signal of interest from the noise, after amplifying the signal with differential amplifiers, is to use signal averaging, which is in effect a cross-correlation between a point-process defined by the occurrence of the stimuli and the recorded evoked activity (i.e., an optimal filter) ( Lee, 1960 ). In averaging, the signal component at each point is coherent and adds directly, whereas the background and noise components tend to be statistically independent and summate in a more-or-less root-mean-square fashion. The resultant recording highlights the signal of interest and deemphasizes the background and noise components.

▪ STIMULATORS

Evoked SEPs are usually produced by electrical stimulators that produce a shock through the skin. MEPs are produced by either electrical or magnetic stimulators. Electrical activation of the motor cerebral cortex or brainstem can be performed transcranially. Electroencephalographic scalp electrodes or electrode plates placed adjacent to the scalp or hard palette can be used to stimulate the cortex. Transcranial magnetic stimulation involves the generation of a rapidly changing magnetic field that induces an electric current in nearby conductors. The magnetic field induced by transcranial magnetic stimulation passes through the scalp and skull and induces an electric current in underlying cerebral tissues. Auditory stimulation is obtained using one of several techniques, depending on the surgical procedure involved and thus on whether the auricle is retracted, as well as other considerations. Miniature open-air high-fidelity earphones (commonly used with personal tape players or radios) that rest in the concha of the ear or a tubal insert earphone (3A; EAR Tone, Indianapolis, IN). The tubal insert is attractive by virtue of distancing the transducer from the recording electrode (producing reduced stimulus artifact) and ease of support. For stimulation of the visual system, a fiberoptic system, which is positioned directly under the eye but not on the globe, is designed to be mounted on the flash stimulator driven by a Grass-Telefactor photic stimulator. With any of the stimulators, precise synchronization with the monitoring and averaging process must occur.

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▪ ANESTHETIC TECHNIQUES

It is well known that the type of anesthesia and the patient's blood pressure, cerebral blood flow, body temperature, hematocrit, and blood gas tensions all affect the functioning of the patient's CNS and thus intraoperatively observed neurophysiologic measures (Grundy, 1983; McPherson, 1994 ). The neurophysiologist must communicate with the anesthesiologist concerning the anesthetic plan before the start of the procedure to ensure that no conflicts exist over the required anesthetic and neurophysiologic monitoring. Both neurophysiologist and anesthesiologist must understand the needs of each other and develop a plan for the monitoring and anesthesia that allows both individuals to provide care that is appropriate for the specific patient. The halogenated hydrocarbon inhalation agents tend to significantly reduce the amplitude of somatosensory evoked responses ( Salzman et al., 1986 ). The best SEPs are often recorded with use of a narcotic relaxant technique, consisting of an opioid, nitrous oxide (<65%), and a muscle relaxant. Boluses of medications produce more disruption of signals than do constant infusions. Regardless of how medications are delivered, the anesthesiologist must inform the neurophysiologist of medication administration, changes in patient temperature or blood pressure, or any other change in the patient's condition.

In many situations, the use of halogenated hydrocarbon inhalation agents is desired to help control blood pressure. Once baseline responses have been obtained and compared with the preoperative responses, many children can maintain their responses to an isoflurane level of approximately 0.3 MAC (minimum alveolar concentration), whereas many adults can maintain their responses to 0.5 MAC. This is highly variable and strongly dependent on the patient and his or her individual reaction to the inhalation agent. A slow increase in isoflurane until either the blood pressure is controlled or the responses significantly deteriorate usually leads to satisfactory results; it should be noted that there are a small number of patients who cannot maintain their somatosensory evoked potentials (SEPs) with any inhalation agent on board. Propofol, etomidate, and ketamine also appear to maintain SEPs at anesthetic concentrations and may be particularly useful when signals are expected to be difficult to obtain ( Schubert et al., 1990 ; Kalkman et al., 1991 ; Taniguchi et al., 1992 ; McPherson, 1994 ).

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ NEUROPHYSIOLOGIC MEASURES

Neurophysiologic measures that are routinely used provide a functional map of much of the entire neuroaxis. These include the EEG, an unstimulated measure of cortical function suitable for providing information concerning the degree of cortical activation related to either metabolic process (e.g., hypoxia) or to pharmacologic manipulation (e.g., pentobarbital-induced burst suppression to protect the patient's cortical function) (Niedermeyer and Lopes da Silva, 1987); SEPs and VEPs, which provide additional measures of cortical function specific to certain pathways and vasculature; BAEPs and BSEPs, which provide information about the functioning of the brainstem, again specific to certain pathways ( Regan, 1989 ); and EMGs, produced either by muscles innervated by the various cranial nerves, which provide information about both the cranial nerves themselves and their underlying brainstem nuclei ( Kamura, 1983 ), or by somatic muscles providing information about spinal cord or peripheral nerve function.

▪ MATURATIONAL EFFECTS

The functional assessment of the pediatric CNS presents difficult and unique problems compared with that of the more mature adult CNS. The pediatric CNS differs from the adult CNS in that it is maturing over the first several years of life; that is, the neural tissue, the myelin coating of the axonal processes, and the vascular supply to the CNS all show significant changes. These developmental anatomic changes are reflected in maturational functional changes as measured by both ascending SEP and descending MEP activity, VEPs, and BAEPs ( Starr et al., 1977 ; Cracco et al., 1979 ; Guthkelch et al., 1982). There are a number of factors that contribute to the maturational changes of evoked potentials, and the use of age- and size-matched normal controls is essential. For intraoperative monitoring purposes, infants act as their own controls. Central and peripheral myelination is believed to be completed by 5 years of age, and from then until maturity the dominant factor affecting SEP latency is height (Yakovlev and Lecours, 1967 ; Gilmore et al., 1985 ) ( Fig. 9-39 ).

 
 

FIGURE 9-39  Maturational changes in cortical somatosensory evoked potentials to median nerve stimulation (A), visual evoked potentials to flash stimulation (B), and brainstem auditory evoked potentials to click stimulation (C), in early infancy. Note the decreasing latency (the time measured from the initiation of the stimulus to the point of maximum amplitude of signal) and enhancing morphology for the identified waves in all three modalities.

 

 

▪ GENERAL PROCEDURES

Neurophysiologic recording during pediatric operations can rapidly become quite complex. It is not unusual to monitor several different neurophysiologic variables simultaneously, such as EEGs, BAEPs, BSEPs, SEPs, and EMGs relating to several cranial nerves. This requires a well-organized and theoretically parsimonious approach to monitoring.

The positioning of recording electrodes should be chosen in relation to the expected distribution of the responses to be recorded. Many laboratories place scalp electrodes at sites determined with use of the international 10-20 system ( Jasper, 1958 ). This system, originally devised for EEG recordings, specifies the position of 21 evenly spaced locations on the scalp. Recording electrodes are placed symmetrically to provide for control recordings from the side contralateral to the surgery, even when electrodes may not be positioned in the standard recording sites.

All recordings are usually performed using subdermal needle electrodes. Electrodes that are not in the operative field but are on the scalp and are not accessible during surgery are either sutured or stapled in place. Electrodes on the face, which are placed to record electromyographic activity, are taped in place. Electrodes in the operative field are placed by the surgeons using sterile technique, usually early in the procedure.

Baseline responses are obtained before draping the patient and compared with the preoperative evaluation. Significant differences must be accounted for, because signal deterioration may be caused by the effects of inadequate patient positioning.

▪ ELECTROENCEPHALOGRAPHY

The functioning of the cerebral cortex is extremely sensitive to changes in arterial oxygenation and insufficient cerebral blood flow or an inadequate partial pressure of oxygen; this sensitivity is rapidly reflected in the EEG ( Meyer and Marx, 1972 ). Oxidative metabolism supplies the energy for maintenance of the membrane potential of nerve cells, and the EEG is directly dependent on the transmembrane potentials of neurons; it reflects disturbances of cerebral metabolism such as hypoxia. Some factors that may contribute to ischemic events in surgical patients are decreased oxygen-carrying capacity due to hypovolemia or decreased cerebral perfusion pressure due to factors associated with decreased systemic arterial pressure, increased intracranial pressure, and mechanical obstruction of cerebral vessels ( Freye, 1990 ).

Two channels of continuous EEG monitoring is thought to be adequate, because the problems are not related to precise focality but rather are of global or hemispheric importance. The EEG can be observed both as the ongoing unprocessed signal and in a Fourier transformed representation. The electroencephalographic appearances of any ischemic or hypoxic events are similar, and differentiation between the various putative causative factors is made by being particularly attentive to the clinical situation; for example, blood pressure, ECG, oxygen saturation, administered drugs, and surgical manipulations may all have an observable effect. Other concurrent factors that may alter the EEG are changes in the depth of anesthesia, temperature changes, and changes in carbon dioxide content. These factors may be recognized by their relatively slow onset, lasting for several minutes, in contrast to the changes of ischemia, which generally occur within seconds. There are situations where the EEG may be acutely depressed on injection of an anesthetic that rapidly passes the blood-brain barrier. Such situations may be found with the use of high-dose opioid anesthesia, in which fentanyl induces an immediate and marked reduction in fast-frequency activity in the EEG, with an increase in low-frequency, high-amplitude activity in the delta range ( Freye, 1990 ).

A simple but useful summary of possible changes is that decreased frequency with increased amplitude ( Van der Drift, 1972 ) implies an ischemic event to the cortex; widespread frequency slowing and decreased amplitude usually imply brainstem ischemia (Roger et al., 1954); ischemic events affecting the thalamus and the internal capsule produce unremarkable changes in the EEG ( Van der Drift, 1972 ) but possible significant changes in the SEPs.

▪ SOMATOSENSORY AND MOTOR EVOKED POTENTIALS

The neurophysiologic measures of value in assessing the spinal cord consist of SEPs, produced by stimulating various peripheral nerves, and MEPs, which may be observed as either compound muscle or nerve action potentials and which may be produced by either transcranial or spinal cord stimulation. It is currently advantageous to think of the SEPs as characterizing the ascending activity in the spinal cord and of the MEPs as characterizing the descending activity in the spinal cord. This distinction, although not particularly important with respect to the sensory activity, is potentially extremely important with respect to the descending activity, because important questions remain as to what pathways are being stimulated ( Rose et al., 1994 ).

▪ SOMATOSENSORY EVOKED POTENTIALS (ASCENDING ACTIVITY)

SEPs are dependent on the stimulation of the large afferent fibers of peripheral nerves. Following stimulation of peripheral nerves in the arms or the legs, SEPs can be reproducibly recorded over the spine and scalp. In the spinal cord, the SEPs are conducted primarily through the dorsal columns.

SEPs are a sequence of potentials generated in the peripheral nerves, dorsal horn nuclei, dorsal column pathways, and dorsal column nuclei of the spinal cord; the medial lemniscal pathways of the brainstem; and the thalamus, thalamocortical, parietal region of the brain after the application of a transient electrical stimulus to a peripheral nerve ( Sclabassi et al., 1993 ). When recorded from electrodes on the surface of the body, the potentials of interest are very small, ranging in size from 2 to 5 μV, and occur in approximately the first 100 milliseconds after the application of a stimulus (often referred to as early and middle latency potentials). Evoked potentials are typically a fraction of the size of the spontaneous brain activity appearing in the background EEG and about one-thousandth the size of the other physiologic and extraneous potentials with which they are intermixed. Unwanted activity (noise) originates with both the signal recorded from the subject and electrical devices in the immediate neighborhood of the recording equipment. The aim of evoked potential recording is to ensure a large, clear response with the least possible noise contamination (i.e., the best signal-to-noise ratio possible); the elimination of these unwanted signal components is essential. Evoked potentials are described in terms of latency and amplitude. Latency is the time measured from the application of a stimulus to the point of maximum amplitude of the evoked potential. Some types of SEP have more than one peak and the time between peaks is the interpeak latency. The amplitude is the voltage difference between two peaks of opposite polarity or a reference potential. Measurement of latencies, amplitudes, and interpeak latencies characterize SEP recordings. Changes in these measurements during an anesthetic and surgical procedure may represent injury to the neural tissue between the stimulus generator and the recording electrode.

In all cases, the stimuli are electrical impulses applied transcutaneously, at a rate of 0.7 to 5.3 Hz, depending on the robustness of the response, which is typically a function of the patient's age and pathology. Typically, responses to 128 stimuli are averaged; in many cases, as few as 12 responses may be averaged, providing near real-time updating of the responses.

All types of SEPs are used for intraoperative monitoring, primarily during spinal surgery. Potentials can be recorded after stimulation of the median nerve at the wrist, the common peroneal nerve, the posterior tibial nerve, and the dorsal nerve of the penis and the clitoris. Multiple types of responses from different stimuli and different sources often are simultaneously recorded, allowing the entire neuroaxis to be monitored. Monitoring multiple upper and lower extremity responses simultaneously during spinal surgery allows cord injury to be distinguished from global problems or localized monitoring difficulty.

▪ MEDIAN, ULNAR, AND RADIAL NERVE POTENTIALS

The median (MSPs), ulnar (USPs), and radial (RSPs) nerve evoked potentials are all useful in assessing the brachial plexus, upper spinal cord, brainstem, and telencephalon. One important distinction is that the USPs provide information regarding level C-8 and above, whereas the MSPs provide information about level C-7 and above ( Fig. 9-40 ). At Erb's point, the response consists of an apparently triphasic (positive-negative-positive) nerve action potential, reflecting the passage of the mixed nerve volley through the brachial plexus. This component is usually labeled N11 for the large negative going component. At the cervical C-7 recording site, the main component is a negative peak occurring at 14-millisecond latency, N14, with an associated complex structure. It has been postulated that these waves are generated in the dorsal roots, dorsal horn, posterior columns, and structures of the lower brainstem. During spinal fusions, monitoring of the brachial plexus may also alert the anesthesiologist to poor positioning of the arms.

 
 

FIGURE 9-40  Median nerve potentials produced by right median nerve (MD) stimulation. Data are recorded from Erb's point (bottom), cervical C-7, C-2, and cranial C-3 and all referenced to Fz. Note the increase in latency of large negative depression noted by the N11 wave at Erb's point and N14, N15, and N20 at the other respective electrode locations as the recording electrode becomes more distant from the stimulator.

 

 

▪ COMMON PERONEAL AND TIBIAL NERVE POTENTIALS

In the lower limb, nerves used to elicit cortical SEPs include the tibial, peroneal, sural, saphenous, and others. Spinal potentials are most consistently obtained through stimulation of the tibial nerve at the medial malleolus or peroneal nerve in the popliteal fossa.

SEPs recorded over the spine reflect the afferent volley traversing the dorsal columns. These responses can be recorded from electrodes attached to the skin over the spine, and they progressively increase in latency at more rostral recording locations. Spinal SEPs are relatively easy to obtain in children, with the amplitude and definition of the waves decreasing with increasing age such that by the mid-teenage years, these responses are more difficult to obtain, as is the case with adults. More rostral recording locations reflect potentials arising in multiple ascending pathways, including the dorsal and dorsolateral columns, which lie primarily ipsilateral to the side of stimulation.

▪ DERMATOMAL RESPONSES

A disadvantage of SEPs produced by stimulation of large nerve trunks is that input to the spinal cord usually occurs over more than one level. This problem can be addressed by delivering the stimulus to a small cutaneous nerve that is believed to derive from a single dorsal root or to the “signature area” of a particular dermatome. Significant disagreement exists concerning the cutaneous distributions of dermatomes, and care should be taken to stimulate the commonly accepted receptive fields of a root.

Pudendal nerve responses are a special type of dermatomal responses that are particularly useful, especially in patients with spina bifida. The pudendal nerve carries sensory fibers from the penis, urethra, anus, and pelvic floor muscles and supplies motor innervation to the bulbocavernosus and pelvic floor muscles, the external urethral sphincter, and the external anal sphincter. Cortical responses to electrical stimulation of the dorsal nerve of the penis, the urethra, and the urinary bladder have all been described ( Badr et al., 1982 ; Haldeman et al., 1982). Pudendal nerve responses are of similar morphology to the tibial nerve SEP (TSP) and are best recorded from the same area of the scalp ( Fig. 9-41 ).

 
 

FIGURE 9-41  Pudendal nerve responses obtained from a male tethered cord patient with symptoms referable to the pudendal nerve. All responses are recorded from Pz referenced to Fz. A was obtained by stimulating the right branch of the dorsal nerve of the penis, whereas B was obtained by stimulating the left. Note the significant reduction in amplitude in response to the left-sided stimulation. C represents control data recorded during every procedure.

 

 

▪ MOTOR EVOKED POTENTIALS

Because SEPs reflect function in the dorsal columns of the spinal cord, SEPs do not directly assess the integrity of descending spinal motor tracts. It is possible to have focal damage to the motor areas in the spinal cord in which the SEPs remain normal. Accordingly, there have occasionally been misleading results when using SEPs alone for intraoperative monitoring and diagnosis ( Lesser et al., 1986 ), but this is rare. MEPs, which may be either evoked EMGs or compound action potentials, can be used to test the integrity of the motor pathways through either electrical or magnetic stimulation ( Merton and Morton, 1980 ; Barker et al., 1985 ). MEPs can be obtained via stimulation of the motor areas of the brain or spinal cord through the intact skin, direct stimulation of exposed neuronal tissue, or direct root stimulation (e.g., during the release of a tethered cord) and recording of a stimulus-related response either as a compound muscle action potential (CMAP) or as an efferent compound nerve action potential (CNAP) ( Fig. 9-42 ).

 
 

FIGURE 9-42  Compound muscle action potentials (A) and compound muscle action potentials (B) produced by transcranial simulation at C4, Pz, and C2, using a magnetic stimulator. The compound muscle action potentials are recorded from the abductor pollicis brevis, whereas the compound nerve action potentials were recorded from the left median nerve at the wrist.

 

 

Transcranial and electrical surface stimulation may be used to elicit motor responses. There is no general consensus about the location of recording electrodes, outside of specific muscle groups for evoked CMAPs or over the obvious peripheral nerves for evoked CNAPs, nor is there a general consensus concerning which class of these activities is more advantageous to record. CNAPs allow the patient to have neuromuscular blockade agents.

▪ COMBINED ASCENDING AND DESCENDING ACTIVITY

Intraoperative monitoring of CNAPs and posterior TSPs is used to provide a simultaneous measure of the ascending and descending activities in the spinal cord. Through this approach, sequential stimulation is performed of the left tibial nerve, the right tibial nerve, and the spinal cord through the spinous processes. Recording electrodes positioned on the scalp record the bilateral SEPs from the tibial nerve stimuli as well as the afferent activity induced in the spinal area via direct spinal stimulation. Recording electrodes in the popliteal fossa allow the afferent stimulus compound action potentials to be observed, and then the descending compound action potentials produced by the spinal cord stimulation. This combined technique aids in localizing spinal cord dysfunction during surgery by continuously evaluating the adequacy of both the sensory and motor components of spinal cord neural activity.

▪ AUDITORY BRAINSTEM RESPONSES

Monitoring of the function of cranial nerve VIII through the use of BAEPs assists in preserving hearing, locating cranial nerve VIII, or determining whether the overall function of the brainstem is altered. BAEPs are also sensitive to the retraction on the frontal poles, most likely due to force transmission to the brainstem.

The classic BAEP consists of a minimum of five and a maximum of seven peaks. All occur with 10 milliseconds of a brief click or tone presentation. Wave I is generated in the auditory portion of nerve VIII. Wave II is generated bilaterally at or in the proximity of the cochlear nucleus. Wave III is generated bilaterally from the lower pons near the superior olive and trapezoid body. Waves IV and V are probably generated in the upper pons or lower midbrain, near the lateral lemniscus or possibly near the inferior colliculus.

Waves I through V are relatively resistant to sedative medication and general anesthetics; these responses place no constraints on the anesthesiologist. They are sensitive to temperature changes, with absolute and interpeak latencies increasing by approximately .20 milliseconds. The latency of wave V is the primary concern in intraoperative monitoring of the BAEPs, because this is the most robust and easily identifiable of the waves in this response.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ VISUAL SYSTEM

VEPs are used to aid in determining the functional integrity of the visual system, primarily in the region of the optic nerves, chiasm, and optic radiations. The recorded activity is generated either at the retina (electroretinogram) or at the cortex.

Stimulation of the visual system using a bright flash is not recommended for diagnostic purposes due to intersubject variability ( Ciganek, 1961 ), except in selected situations; in the operating room this is a very helpful and effective technique ( Fig. 9-43 ).

 
 

FIGURE 9-43  Intraoperative visual evoked potentials (VEPs), obtained between midline occipital and vertex electrodes, to flash stimuli. Results were obtained from a 10-month-old girl who was being operated on for a chiasmal glioma, which was 90% removed. Top trace, preresection response; bottom trace, postresection response. The responses were abnormal but unchanged during the procedure.

 

 

▪ ELECTROMYOGRAPHY

The EMG is electrical activity produced in muscle fibers below the skin; it has a frequency content ranging from 1 to 150 Hz. Three different types of electrodes are used to record the EMGs: fine wire electrodes, which have the highest impedance and the narrowest field of view; subdermal needles, which have an intermediate impedance and a larger field of view; and disk surface electrodes, which have the lowest impedance and the largest field of view (field of view means the integrated level of electrical activity). The recording techniques are essentially the same for all cranial nerves and all muscle groups. Subdermal platinum needle electrodes are used in bipolar recording configurations; that is, all recordings are made between a pair of electrodes inserted into the same muscle group. Bipolar recordings are used to minimize confusion regarding which cranial nerve or branch of a cranial nerve is producing the observed EMG. The electrodes are normally placed before the start of the procedure; occasionally electrodes are placed in a sterile field by the surgeons.

These signals are listened to continuously for evaluation of nerve function both by the neurophysiologists and by the surgeons. Four categories of EMG activity are observed: (1) no activity, which in an intact nerve is the best situation but which may also be the case in a nerve that has been sharply dissected; (2) irritation activity, which sounds like soft intermittent flutter and is consistent with working near the nerve; (3) injury activity, which sounds like a continuous, nonaccelerating tapping and which can be an indicator of permanent injury to the cranial nerve; and (4) a “killed-end” response, which sounds like an accelerating firing pattern and is an unequivocal indicator of nerve injury. It is important to note that a sharply cut nerve may produce only a brief burst of activity; monitoring cannot be expected to replace extreme caution when working near the cranial nerves.

▪ EVALUATION OF CRANIAL NERVE FUNCTION

Cranial nerve function is monitored continuously during surgery for two reasons: first, to establish the location and orientation of the cranial nerves in the operative field; and second, to preserve functioning in the cranial nerves and their related brainstem nuclei ( Sclabassi et al., 1993 ). The major observed variables are the EMGs from the appropriate muscle group innervated by the cranial nerves of interest. In general, the cranial nerves ipsilateral to the operative side are monitored; when appropriate, bilateral activity is monitored.

In addition to monitoring the ongoing EMG activity related to the various cranial nerves, the various cranial nerves may also be electrically stimulated. This is usually done to determine the location of the nerve in the operative field, because many times the nerve is enveloped by tumor and may not be directly observable, or to determine the functional integrity of the nerve ( Daube and Harper, 1989 ). The most common example of this procedure is the direct stimulation of nerve VII. The return path for the stimulating current is provided by a metal electrode inserted into the adjacent muscle mass. In some situations, where very precise localization of the nerve is required, bipolar stimulating electrodes are used. The great majority of the time, the question being asked is, Is the nerve there?

▪ ELECTROMYOGRAMS IN TETHERED CORD RELEASES AND SELECTIVE RHYZOTOMIES

The EMG is a useful indicator of the integrity of descending activity in the spinal cord. The EMG is either spontaneous, such as anal sphincter activity produced by irritation of the S3-5 roots during an untethering procedure involving the lower portion of the cauda equina, or evoked of the type produced in either stimulation of nerve roots when attempting to identify the cauda equina or selective rhyzotomy for the treatment of spasticity. Anesthetic management involves the avoiding of neuromuscular blockade during this type of evaluation.

The commonly accepted principal goal of intraoperative monitoring is to prevent morbidity, and at a certain level this is true; the more fundamental goal of intraoperative monitoring is to provide the operative team with information that allows them to accomplish the desired operative objective with as optimal an anesthetic and surgical strategy as possible, while having a clear idea of what morbidity is being induced along the way. This latter goal is particularly important in cases where the degree of difficulty is high and it is virtually impossible to prevent morbidity.

Stringent time constraints exist in intraoperative monitoring of neurophysiologic function, and damage to the CNS may occur rapidly, over seconds. This constraint has inspired the development of methods for extracting and analyzing evoked potential, EMG, and EEG waveforms rapidly and efficiently. A corollary of the increased sensitivity required to decrease the monitoring time is a higher rate of individually false-positive measures. These are usually rapidly identified as such and produce no disruption in the flow of the case. Intraoperative monitoring requires rapid interpretations to be made of complex data, recorded in less-than-optimal conditions. It does no good to inform the surgeon 10 minutes after the fact that a significant change has occurred. Successful intraoperative monitoring of the pediatric CNS requires the acquisition of as many appropriate neurophysiologic variables simultaneously as possible. In addition, the correct interpretation of these responses is greatly aided by the ability to display the history of all of the acquired data in such a way as to facilitate a comparison of all of the data. This includes the baseline values acquired both at the beginning of the procedure and from the preoperative studies.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ SUMMARY

The anesthesiologist caring for infants and children has a wide array of equipment and monitors available. The exact configuration of equipment and monitors necessary depends primarily on the patient's illness, the experience of the anesthesiologist, and the proposed surgery. With the increasing complexity of anesthesia equipment and monitors, the anesthesiologist needs to understand thoroughly the operation and limitations of each device. Last, the anesthesiologist should never rely too heavily on the monitoring equipment and abandon the direct, close, personal surveillance of each patient during anesthesia and surgery.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

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