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

PART ONE – Basic Principles in Pediatric Anesthesia


Chapter 1 – Special Characteristics of Pediatric Anesthesia

Etsuro K. Motoyama,
Peter J. Davis

  

 

Changing Concepts, 3

  

 

Recent Developments in Pediatric Anesthesia, 3

  

 

Perioperative Monitoring Techniques and Standards, 3

  

 

New Volatile Anesthetics, 4

  

 

Intravenous Agents, 4

  

 

Laryngeal Mask Airway, 5

  

 

Intraoperative and Postoperative Analgesia in Neonates,5

  

 

Regional Analgesia in Infants and Children, 5

  

 

Same-Day Surgery—Efficiency and Quality of Care, 5

  

 

Fundamental Differences in Infants and Children, 6

  

 

Psychological Differences, 6

  

 

Differences in Response to Pharmacologic Agents,6

  

 

Anatomic and Physiologic Differences, 6

  

 

Summary, 9

▪ CHANGING CONCEPTS

In the 1940s and 1950s, the techniques of pediatric anesthesia, as well as the skills of those using and teaching them, evolved more as an art than as a science, as Dr. Robert Smith vividly and eloquently recollects through his firsthand experiences in his chapter on the history of pediatric anesthesia (see Chapter 35 , History of Pediatric Anesthesia). The anesthetic agents and methods available were limited, as was the scientific knowledge of developmental differences in organ system function and anesthetic effect in infants and children. Monitoring in pediatric patients was limited to inspection of chest movement and occasional palpation of the pulse until the late 1940s, when Smith introduced the use of the precordial stethoscope for continuous auscultation of heartbeat and breath sounds (Smith, 1953, 1968 [87] [89]). Until the mid-1960s, many anesthesiologists monitored only the heart rate in infants and small children during anesthesia and surgery. Electrocardiographic and blood pressure measurements were either too difficult or too extravagant and were thought to provide little or no useful information. Measurements of central venous pressure were thought to be inaccurate and too invasive even in major surgical procedures. The insertion of an indwelling urinary (Foley) catheter in infants was considered invasive and was resisted by surgeons.

The introduction of soft latex blood pressure cuffs suitable for newborn and older infants ( Smith, 1968 ) encouraged the use of blood pressure monitoring in children (see Fig. 35-4 ). The “Smith cuff” remained the standard monitoring device in infants and children until the late 1970s, when it began to be replaced by automated blood pressure devices.

In the past two decades an explosion of new scientific knowledge in physiology and pharmacology in developing humans, as well as technologic advancements in perioperative monitoring, has markedly changed the concepts and techniques of pediatric anesthesia. At the same time the anesthesiologist's responsibilities have expanded well beyond the operating room and now cover the perioperative care of critically ill surgical and nonsurgical patients in intensive care settings. Resuscitative techniques, prolonged mechanical ventilatory support, and elaborate mechanical and physiologic instrumentation have become essential elements in anesthesiology. More recently, the roles of anesthesiologists have expanded to specialists in the management of acute and chronic pain beyond the perioperative period.

Significant developments in these areas over the past two decades include advances in perioperative monitoring techniques and standards; development and availability of new inhaled anesthetics, intravenous anesthetic and sedative-hypnotic agents, synthetic opioids, muscle relaxants, and other adjuvant drugs for both routine and complicated procedures; a better understanding of pain perception in neonates and advances in techniques of conduction analgesia as part of general anesthesia and perioperative pain management; parental presence during induction of anesthesia and in the postanesthesia care unit (PACU); and reevaluation of time-honored preoperative laboratory tests and fasting routines in ever-expanding, same-day (outpatient) surgery settings to improve efficiency and health care cost containment. (See Chapter 27 , Anesthesia for Same-Day Procedures.)

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

▪ RECENT DEVELOPMENTS IN PEDIATRIC ANESTHESIA

▪ PERIOPERATIVE MONITORING TECHNIQUES AND STANDARDS

The introduction of pulse oximetry for routine clinical use since the early 1990s has been the single most important development in monitoring and patient safety, especially related to pediatric anesthesia, since the advent of the precordial stethoscope in the 1950s ( Smith, 1956 ) (see Chapters 9 , 10 , 11 , and 34 , Anesthetic Equipment and Monitoring, Induction of Anesthesia, Intraoperative and Postoperative Management, and Safety and Outcome). Pulse oximetry is superior to clinical observation and other means of monitoring, such as capnography, for the detection of intraoperative hypoxemia (Coté et al., 1988, 1991 [20] [21]). In addition, Spears and colleagues (1991) have indicated that experienced pediatric anesthesiologists may not have an “educated hand” or a “feel” adequate to detect changes in pulmonary compliance in infants. This report is particularly sobering for “old timers” who had always assumed that their clinical skills were sufficient to protect the safety of their young patients, without depending on monitors. Pulse oximetry has revealed that postoperative hypoxemia occurs commonly among otherwise healthy infants and children undergoing simple surgical procedures (Motoyama and Glazener, 1986 ). Consequently, the use of supplemental oxygen in the PACU has become a part of routine postanesthetic care (see Chapter 11 , Intraoperative and Postoperative Management).

Although pulse oximetry has greatly improved patient monitoring, there had been some limitations, namely motion artifact and inaccuracy in low-flow states and in children at levels of low oxygen saturation (e.g., cyanotic congenital heart disease). Advances have been made in the new generation of pulse oximetry (Masimo Signal Extraction Technology [SET]). This device minimizes the effect of motion artifact, improves accuracy, and has been shown to have advantages over the existing system in low-flow states, mild hypothermia, and moving patients ( Malviya et al., 2000 ; Hay et al., 2002 ; Irita et al., 2003 ).

The standards for intraoperative patient monitoring proposed by the Harvard group ( Eichhorn et al., 1986 ) and the American Society of Anesthesiologists (1986) (see Chapter 11 , Intraoperative and Postoperative Management) strongly recommend the routine use of pulse oximetry and capnography or their equivalents. These standard monitoring procedures have been mandated by law in many state legislatures ( New York State Hospital Code, 1988 ).

Depth of anesthesia can be difficult to assess in the pediatric patient. The bispectral index (BIS) monitor has been developed to objectively assess the depth of anesthesia continuously during general anesthesia. BIS monitor technology compresses electroencephalographic signals with use of a sophisticated algorithm into a digital readout ranging from 0 to 100 ( Sebel et al., 1997 ). By comparing the reading with previously measured results in conscious, sedated, and anesthetized patients, a normogram for level of sedation has been generated. Studies in infants and children have confirmed the validity of this instrument and have demonstrated that more accurate titration of sedation and general anesthesia in children is achieved with the use of a BIS monitor ( Denman et al., 2000 ; Bannister et al., 2001 ;Choudhry and Brenn, 2002 ; McCann et al., 2002 ). Although the BIS monitor has been used in adult patients to prevent awareness under general anesthesia ( Sebel et al., 1997 ), its use for pediatric patients is expected to increase over the coming decade (also see Chapter 11 , Intraoperative and Postoperative Management).

▪ NEW VOLATILE ANESTHETICS

More than a decade after the release of isoflurane for clinical use, two volatile anesthetics, desflurane and sevoflurane, became available in the mid to late 1990s in most industrialized countries. Although these two agents are dissimilar in many ways, they share common physiochemical and pharmacologic characteristics: very low blood-gas partition coefficients (0.4 and 0.6, respectively), which are close to that of nitrous oxide and are only fractions of those of halothane and isoflurane; rapid induction of and emergence from surgical anesthesia; and hemodynamic stability (see Chapters 6 , 10 , and 11 , Basic Pharmacokinetic Concepts, Induction of Anesthesia, and Intraoperative and Postoperative Management).

Although desflurane is not suitable for inhalation induction because of its pungent odor and airway irritability with frequent and often severe laryngospasms ( Fisher and Zwass, 1992 ), sevoflurane appears to be an excellent anesthetic for inhalation induction with hemodynamic stability ( Sarner et al., 1995 ). Indeed, in Japan, where sevoflurane has been in clinical use since 1993, and in most western European countries, where it was introduced much later than in the United States, halothane has been almost completely replaced by sevoflurane for pediatric anesthesia. Even in the United States, the clinical use of holothane has been limited almost exclusively for pediatric anesthesia over the last two decades and its future survival, commercially or otherwise, has become cloudy as of the spring of 2005. Although these newer, less-soluble inhaled agents allow for faster emergence from anesthesia, emergence excitation or delirium associated with their use has become a major concern to the pediatric anesthesiologists ( Davis et al., 1994 ; Sarner et al., 1995 ; Lerman et al., 1996 ; Welborn et al., 1996 ; Cravero et al., 2000 ). However, issues of patient temperament, separation anxiety, postoperative pain, and hunger have clouded the etiology of the stormy emergence associated with these issues. Adjuncts, such as opioids, analgesics, serotonin antagonists, and α1-adrenergic agonists, have been found to decrease the incidence of emergence agitation. In addition, risk factors such as patient age and type of surgery, in addition to the actual inhaled anesthetic agents, have also been identified as risk factors for emergence agitation (Aono et al., 1997, 1999 [6] [7]; Davis et al., 1999 ; Galinkin et al., 2000 ; Cohen et al., 2001 ; Ko et al., 2001 ; Kulka et al., 2001 ; Voepel-Lewis et al., 2003 ).

▪ INTRAVENOUS AGENTS

Propofol has increasingly been used in pediatric anesthesia as an induction agent, for intravenous sedation, or as the primary agent of a total intravenous technique ( Martin et al., 1992 ). Propofol has the advantage of rapid emergence and causes less nausea and vomiting during the postoperative period, particularly in children with a high risk of vomiting, such as those who have undergone strabismus surgery ( Wacha et al., 1991 ).

The eutectic mixture of local anesthetics (EMLA cream) for skin analgesia ( Soliman et al., 1988 ) has become available since the 1990s and has made intravenous cannulation and intravenous induction of anesthesia less threatening for children.

The development of shorter-acting synthetic opioids and intermediate- and short-acting nondepolarizing muscle relaxants, as well as a better understanding of their pharmacokinetics and pharmacodynamics (see Chapter 6 , Pharmacology of Pediatric Anesthesia), has increased the opportunities for pediatric anesthesiologists to provide safe and stable anesthesia with various approaches and less dependence on volatile anesthetics. For example, Anand and Hickey (1992) found that neonates undergoing cardiac surgery with a high-dose of sufentanil had a significantly better outcome than those who received morphine and halothane. Availability of shorter-acting opioids and muscle relaxants has also changed the approach to more routine pediatric procedures such as inguinal herniorrhaphy, tonsillectomy, and bronchoscopy for foreign body aspiration.

The most recent development has been the use of ultrashort-acting opioids in pediatric patients. Specifically, remifentanil, a μ agonist, is metabolized by nonspecific plasma and tissue esterases. The organ-independent elimination of remifentanil, coupled with its clearance rate, which is highest in neonates and infants compared with older children, makes its kinetic profile different from that of any other opioids ( Davis et al., 1999 ; Ross et al., 2001 ). In addition, its ability to provide hemodynamic stability, coupled with its kinetic profile of rapid elimination and nonaccumulation, makes it an attractive anesthetic option for infants and children. Numerous clinical studies have described its use for pediatric anesthesia ( Wee et al., 1999 ; Chiaretti et al., 2000 ; Davis et al., 2000, 2001 [24] [25]; German et al., 2000 ; Dönmez et al., 2001 ; Galinkin et al., 2001 ; Keidan et al., 2001 ; Chambers et al., 2002 ; Friesen et al., 2003 ).

▪ LARYNGEAL MASK AIRWAY

The laryngeal mask airway (LMA) ( Brain, 1983 ), from the United Kingdom, has been used widely in pediatric anesthesia since the 1990s. Although it is not a substitute for the endotracheal tube, the LMA maintains upper airway patency in anesthetized patients who are breathing spontaneously (Keidan et al., 2000). It also serves as an emergency airway when the patient cannot be adequately ventilated with a conventional bag-and-mask system or when intubation is not successful. The LMA is also used as a conduit for endotracheal intubation with a fiberoptic bronchoscope (see Chapter 10 , Induction of Anesthesia). The LMA may or may not be suitable for positive pressure ventilation because of air leaks and possible regurgitation of gastric contents, although a number of reports indicate its relative safety as long as the peak inspiratory pressure is limited to less than 10 to 15 cm H2O ( Barker et al., 1992 ; Devitt et al., 1994 ; Keidan et al., 2001 ). The LMA has been used in infants and children ( Grevenik et al., 1990 ; Johnston et al., 1990 ; Mizushima et al., 1992 ; Wilson, 1993 ) (see Chapter 10 ).

In addition to LMA, other airway devices have been used in children that help decrease the need for tracheal intubation; the cuffed oropharyngeal airway (COPA) and the perilaryngeal airway (PLA) are newer airway devices that have been successfully used in children (see Chapter 9 , Anesthetic Equipment and Monitoring and Chapter 27 , Anesthesia for Same-Day Procedures).

▪ INTRAOPERATIVE AND POSTOPERATIVE ANALGESIA IN NEONATES

It has long been thought that newborn infants do not appreciate pain the way older children and adults do and therefore do not require anesthetic or analgesic agents ( Lippman et al., 1976) . Later studies, however, have indicated that pain, such as that caused by circumcision without analgesia, is felt by the newborn infant and causes prolonged disruption of behavioral development ( Dixon et al., 1984 ). In a landmark study, Anand and others (1987) reported that premature infants showed marked endocrine responses to surgically induced stress, as in the ligation of a patent ductus arteriosus. Pretreatment with fentanyl completely abolished these responses ( Booker, 1988 ).

With this increased awareness has come new interest in the prevention and management of perioperative pain, particularly in neonates. Because of increased cardiovascular sensitivity to inhaled anesthetics and delayed elimination of opioids in the neonate and premature infant, conduction anesthesia is a suitable technique for those patients who are expected to be awake and breathing at the end of surgery and anesthesia. The development of remifentanil, an ultra short-acting opioid, has markedly improved the safety of general anesthesia for neonates and infants. (See Chapters 6 and 11 , Pharmacology of Pediatric Anesthesia, and Intraoperative and Postoperative Management)

▪ REGIONAL ANALGESIA IN INFANTS AND CHILDREN

Conduction analgesia has been used in infants and children since the beginning of the twentieth century, when open-drop ether and chloroform were the anesthetics of choice ( Bainbridge, 1901 ). During the first half of the twentieth century, virtually all of the regional anesthetic techniques available for adults were applied to pediatric patients, mostly by surgeons. By the 1950s, however, when well-trained anesthesiologists were available and general anesthesia was considerably safer for children, the use of regional anesthesia went out of fashion and rapidly declined.

Since the mid-1980s there has been a resurgence of interest in regional anesthesia among pediatric anesthesiologists. One important reason is the difficulty of anesthetizing the increasing number of prematurely born infants who are being cared for in the newly organized neonatal intensive care units. These infants often have severe cardiopulmonary compromise and histories of apnea. Regional analgesia with or without supplemental inhalation or intravenous anesthesia has been used almost exclusively in these situations ( Abajina et al., 1984) .

As newer local anesthetic agents with less systemic toxicity become available, their role in the anesthetic/analgesic management of children is increasing. Studies of the use of levobupivacaine and ropivacaine have demonstrated safety and efficacy in children greater than that of bupivacaine, the standard regional anesthetic in the 1990s (Ivani et al., 1998, 2002, 2003 [54] [55] [56]; Hansen et al., 2000, 2001 [49] [50]; Lönnqvist et al., 2000 ; McCann et al., 2001 ; Karmakar et al., 2002 ).

Pediatric anesthesiologists have been paying much closer attention to postoperative analgesia than they did even a few years ago as part of an overall anesthetic strategy. The pain management plan, either conduction analgesia or patient-controlled analgesia, is discussed with the surgeon and with the parents (and the child if he or she is old enough to understand) preoperatively. A single dose of local anesthetics through the caudal and epidural spaces is most often used for a variety of surgical procedures as part of general anesthesia and for postoperative analgesia. Insertion of an epidural catheter for continuous or repeated bolus injections of local anesthetics often with opioids and other adjunct drugs for postoperative analgesia has become a common practice in pediatric anesthesia. The addition of adjunct drugs, such as midazolam, neostigmine, tramadol, ketamine, and clonidine, to prolong the neuroaxial blockade from local anesthetic agents has become more popular even though the safety of these agents on the neuroaxis has not been determined ( Ansermino et al., 2003 ; de Beer and Thomas, 2003 ) (see Chapters 13 and 14 , Pain Management and Regional Anesthesia). By the beginning of the twenty-first century, pediatric pain service has been organized and practiced by pediatric anesthesiologists in most pediatric institutions, and the pain service rotation has become an integral part of the pediatric anesthesia fellowship training program.

▪ SAME-DAY SURGERY—EFFICIENCY AND QUALITY OF CARE

Efficiency and health care cost-containment concerns, particularly in the United States, have resulted in an astonishing increase in same-day (outpatient) surgery in relatively healthy adult and pediatric patients. Most pediatric centers encourage children and families to participate in presurgical preparatory programs within a few weeks before scheduled surgery. On the day of surgery, all patients are seen by anesthesiologists and screened for acute illness and fasting status. Laboratory tests in healthy children are usually kept to a minimum. The necessity of a routine hemoglobin and hematocrit has been questioned, and these are mostly eliminated from practice (see Chapter 27 , Anesthesia for Same-Day Procedures). The preoperative fasting requirement has been reevaluated and liberalized. In mostinstitutions, clear fluid is now offered to infants and children up to 2 hours before admission to the same-day surgery unit. The safety of such a practice has been confirmed in the anesthesia literature (seeChapter 27 , Anesthesia for Same-Day Procedures).

To reduce the anxiety of children and their parents, preoperative preparations of pediatric patients have undergone considerable changes since the 1990s. Premedication via the painful intramuscular injection, until the 1980s, has been replaced by transmucosal (oral, nasal, or rectal) midazolam, fentanyl, or ketamine and their combinations (see Chapters 8 , 10 , and 27 , Preoperative Preparation, Induction of Anesthesia, and Anesthesia for Same-Day Procedures). Midazolam, given orally, has become the most popular and successful ( Kain et al., 1997 ).

Since the 1990s, parental presence during the induction of anesthesia and in the PACU has become commonplace in most institutions to minimize separation anxiety. Both parents and anesthesiologists have had mostly positive responses to this approach, despite initial reservations and anxiety on the part of the anesthesiologists ( Hannallah and Rosales, 1983 ). Some parents, however, are exceedingly anxious and may transmit this anxiety to their child ( Bevan et al., 1990 ). Kain and others (1996, 1998, 2000, 2003a, 2003b, 2004) [59] [60] [62] [63] [64] [65] have shown that parental presence is as effective as a preanesthetic medication, and public awareness and consumer expectations place a high value on these family-centered care efforts. The value of parents being present for induction is a function of educating parents about their role in the process (see Chapter 7 , Psychological Aspects).

Progress in biotechnology, medical knowledge, and postgraduate training in anesthesiology has produced remarkable advances in pediatric anesthesia in terms of patient safety and outcome and the patients—comfort perioperatively that would have been unthinkable even a few decades ago. These advances, however, were not achieved without cost—the increasing cost of health care in most industrialized nations. In the United States, health care reform is moving quickly by market force. Cost containment has become the major focus in the minds of practitioners, which, unfortunately, tends to impede further improvement in technology and the quality of anesthesia.

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

▪ FUNDAMENTAL DIFFERENCES IN INFANTS AND CHILDREN

The reason for undertaking a special study of pediatric anesthesia is that children, especially infants younger than a few months, differ markedly from adolescents and adults. Many of the important differences, however, are not the most obvious. Although the most apparent contrast is size, it is the physiologic differences related to general metabolism and to immature function of the various organ systems (including the heart, lungs, kidneys, liver, blood, muscles, and central nervous system) that are of major importance to the anesthesiologist.

▪ PSYCHOLOGICAL DIFFERENCES

For a child's normal psychological development, continual support of a nurturant family is indispensable at all stages of development; serious social and emotional deprivation (including separation from the parents during hospitalization), especially during the first 2 years of development, may cause temporary or even lasting damage to psychosocial development ( Forman et al., 1987 ). A young child who is hospitalized for surgery is forced to cope with separation from parents, to adapt to a new environment and strange people, and to experience the pain and discomfort associated with anesthesia and surgery (see Chapter 7 , Psychological Aspects).

The most intense fear of an infant or a young child is created by separation from the parents, often conceived as loss of love or abandonment. The sequence of reaction often observed is as follows: angry protest with panicky anxiety, depression and despair, and eventually apathy and detachment ( Bowlby, 1973 ). Older children may be more concerned with painful procedures and the loss of self-control implicit with general anesthesia ( Forman et al., 1987 ). Repeated hospitalizations for anesthesia and surgery may be associated with psychosocial disturbances in later childhood ( Dombro, 1970 ). In children old enough to experience fear and apprehension during anesthesia and surgery, the emotional factor may be of greater concern than the physical condition; in fact, it may represent the greatest problem of the perioperative course ( Smith, 1980 ) (see Chapter 7 , Psychological Aspects).

All of these responses can and should be reduced or abolished through preventive measures to ease the child's adaptation to the hospitalization, anesthesia, and surgery. The anesthesiologist's role in this process is extremely important (see Chapters 7 , 8 , and 10 , Psychological Aspects, Preopetrative Preparation, and Induction of Anesthesia). Description of the normal pattern of behavior that emerges during infancy and childhood is beyond the scope of this book. The reader is encouraged to consult standard pediatric textbooks.

▪ DIFFERENCES IN RESPONSE TO PHARMACOLOGIC AGENTS

The extent of the differences among infants, children, and adults in response to the administration of drugs was not fully appreciated until fairly recently. Before the 1960s, the primary concern in pediatric pharmacology was to find a formula to convert adult dosages to pediatric ones. Application of a pharmacokinetic method in which mathematical and statistical analyses are used to relate drug dosage, pharmacologic effects, and time ( Jusko, 1972 ) has been instrumental in the rapid development of pediatric pharmacology since the 1980s. During the first several months after birth, rapid development and growth of organ systems take place, altering the factors involved in uptake, distribution, metabolism, and elimination of anesthetics and related drugs. These changes appear responsible for developmental differences in the response to drugs. The pharmacology of anesthetics and adjuvant drugs and their different effects in neonates, infants, and children are discussed in detail in Chapter 6 , Pharmacology of Pediatric Anesthesia.

▪ ANATOMIC AND PHYSIOLOGIC DIFFERENCES

Body Size

As already stated, the most striking contrast between children and adults is size, but the degree of difference and the variation even within the pediatric age group are hard to appreciate. The contrast between an infant weighing 1 kg and an overgrown and obese adolescent weighing more than 100 kg appearing in succession in the same operating room is overwhelming. It makes considerable difference whether body weight, height, or body surface area is used as the basis for size comparison. As pointed out by Harris (1957) , a normal newborn infant weighing 3 kg is 1/3 the size of an adult in length but 1/9 the adult size in body surface area and 1/21 adult size in weight ( Fig. 1-1 ). Of these body measurements, body surface area (BSA) is probably the most important because it closely parallels variations in basal metabolic rate measured in kilocalories per hour per square meter. For this reason, BSA is believed to be a better criterion than age or weight in judging basal fluid and nutritional requirements. For clinical use, however, BSA proves somewhat difficult to determine, although a nomogram such as that of Talbot and associates (1952) facilitates the procedure considerably ( Fig. 1-2 ). For the anesthesiologist who carries a pocket calculator, the following formulas may be useful to derive BSA:

 
 

FIGURE 1-1  Proportions of newborn to adult with respect to weight, surface area, and length.  (From Crawford JD, Terry ME, Rourke GM: Pediatrics 5:785, 1950.)

 

 

 

 
 

FIGURE 1-2  Body surface area nomogram for infants and young children.  (Reprinted with permission of the publishers and The Commonwealth Fund, from Talbot NB, Sobel FH, McArthur JW, Crawford JD: Functional Endocrinology From Birth Through Adolescence.Cambridge, MA, 1952, Harvard University Press; copyright, 1952, by the Commonwealth Fund.)

 

 

 

Formula of DuBois and Dubois (1916)

Formula of Gehan and George (1970)

At full-term birth, BSA averages 0.2 m2, whereas in the adult it averages 1.75 m2. A table of average height, weight, and BSA is presented for reference ( Table 1-1 ). A simpler, crude estimate of BSA for children of average height and weight is given in Table 1-2 . The formula BSA (m2) = (0.02 × kg) + 0.40 is also reasonably accurate in children of normal physique weighing 21 to 40 kg ( Vaughan and Litt, 1987 ).


TABLE 1-1   -- Relation of age, height, and weight to body surface area (BSA)[*]

Age (y)

Height (cm)

Weight (kg)

BSA (m2)

Premature

40

1

0.1

Newborn

50

3

0.2

1

75

10

0.47

2

87

12

0.57

3

96

14

0.63

5

109

18

0.74

10

138

32

1.10

13

157

46

1.42

16 (Female)

163

50

1.59

16 (Male)

173

62

1.74

*

Based on standard growth chart and the formula of DuBois and DuBois (1916) : BSA (m2) = 0.007184 × Height0.725 × Weight0.425.

 


TABLE 1-2   -- Approximation of body surface area (BSA) based on weight

Weight (kg)

Approximate BSA (m2)

1 to 5

0.05 × kg + 0.05

6 to 10

0.04 × kg + 0.10

11 to 20

0.03 × kg + 0.20

21 to 40

0.02 × kg + 0.40

Modified from Vaughan VC III, Litt IF: Assessment of growth and development. In Behrman RE, Vaughn VC III (eds): Nelson's Textbook of Pediatrics, ed 13. Philadelphia, 1987, WB Saunders.

 

 

The caloric need in relation to BSA of a full-term infant is about 30 kcal/m2 per hour. It increases to about 50 kcal/m2 per hour by 2 years of age and then decreases gradually to the adult level of 35 to 40 kcal/m2 per hour.

Relative Size or Proportion

Less obvious than the difference in overall size is the difference in relative size of body structure in infants and children. This is particularly true with the head, which is large at birth (35 cm in circumference)—in fact, larger than chest circumference. Head circumference increases by 10 cm during the first year and an additional 2 to 3 cm during the second year, when it reaches three fourths the adult size.

At full-term birth, the infant has a short neck and a chin that often meets the chest at the level of the second rib; these infants are prone to upper airway obstruction during sleep. In infants with tracheostomy, the orifice is often buried under the chin unless the head is extended with a roll under the neck. In addition, infants are more prone to upper airway obstruction under anesthesia or sedation because upper airway muscles, which normally support the airway patency, are disproportionately sensitive to the depressant effect of anesthesia and sedation, resulting in pharyngeal airway collapse and obstruction (Ochiai et al., 1989 ) (see Chapter 2 , Respiratory Physiology). The chest is relatively small in relation to the abdomen, which is protuberant with weak abdominal muscles ( Fig. 1-3 ). Furthermore, the rib cage is cartilaginous and the thorax is too compliant to resist inward recoil of the lungs. In the awake state, the chest wall is maintained relatively rigid with sustained inspiratory muscle tension, which maintains the end-expiratory lung volume (functional residual capacity [FRC]). Under general anesthesia, however, the muscle tension is abolished and FRC collapses, resulting in airway closure, atelectasis, and venous admixture unless positive end-expiratory pressure (CPAP) or positive end-expiratory pressure (PEEP) is maintained.

 
 

FIGURE 1-3  A normal infant has a large head, narrow shoulders and chest, and a large abdomen.

 

 

Poor development of body support by bone and muscle, together with disproportion, creates problems in positioning the child for surgery. In the prone position, the shoulders are toosmall to provide adequate support despite attempts to build them up with rolls underneath both shoulders, thereby keeping the thorax and abdomen free for adequate ventilation. Occasionally, when the child must sit up for a craniotomy, special attention is needed to secure the head carefully, because the neck is a very weak stem for the heavy head. Structure and function of the thorax and airways, as well as respiratory physiology in infants and children, are detailed in Chapter 2 .

Central and Autonomic Nervous Systems

The brain of the neonate is relatively large, weighing about 1/10 of body weight compared with about 1/50 in the adult. The brain grows rapidly; its weight doubles by 6 months of age and triples by 1 year. At birth, about one fourth of the neuronal cells are present. The development of cells in the cortex and brain stem is nearly complete by 1 year of age. Myelinization and elaboration of dendritic processes continue well into the third year. Incomplete myelinization is associated with primitive reflexes, such as the Moro and grasp reflexes, in the neonate; these are valuable in the assessment of neural development. Inadequate nutrition during this critical period of brain growth results in impaired neuronal function, as seen with inborn errors of metabolism.

At birth the spinal cord extends to the third lumbar vertebra. By the time the infant is 1 year old, the cord has assumed its permanent position, ending at the first lumbar vertebra ( Gray, 1973 ).

In contrast to the central nervous system, the autonomic nervous system is relatively well developed in the newborn. The parasympathetic components of the cardiovascular system are fully functional at birth. The sympathetic components, however, are not fully developed until 4 to 6 months of age ( Friedman, 1973 ). Baroreflexes to maintain blood pressure and heart rate, which involve medullary vasomotor centers (pressor and depressor areas), are functional at birth in awake newborn infants ( Moss et al., 1968 ; Gootman, 1983 ). In anesthetized newborn animals, however, both pressor and depressor reflexes are diminished ( Wear et al., 1982 ; Gallagher et al., 1987 ).

The laryngeal reflex is activated by the stimulation of receptors on the face, nose, and upper airways of the newborn. Reflex apnea, bradycardia, or laryngospasm may occur. Various mechanical and chemical stimuli, including water, foreign bodies, and noxious gases, can trigger this response. This protective response is so potent that it can cause death in the newborn (see Chapters 2 and 3 , Respiratory Physiology and Cardiovascular Physiology).

Respiratory System

At full-term birth, the lungs are still in the stage of active development. The formation of adult-type alveoli begins at 36 weeks post conception but represents only a fraction of the terminal air sacs with thick septa at full-term birth. It takes more than several years for functional and morphologic development to be completed. Similarly, control of breathing during the first several weeks of extrauterine life differs notably from control in older children and adults. Of particular importance is the fact that hypoxemia depresses, rather than stimulates, respiration. The development of the respiratory system and its physiology are detailed in Chapter 2 , Respiratory Physiology.

Cardiovascular System

During the first minutes after birth, the newborn infant must change his or her circulatory pattern dramatically from fetal to adult type to survive in the extrauterine environment. Even for several months after initial adaptation, the pulmonary vascular bed remains exceptionally reactive to hypoxia and acidosis. The heart remains extremely sensitive to volatile anesthetics during early infancy, whereas the central nervous system is relatively insensitive to these anesthetics. Cardiovascular physiology in infants and children is discussed in Chapter 3 .

Fluid and Electrolyte Metabolism

Like the lungs, the kidneys are not fully mature at birth, although the formation of nephrons is complete by 36 weeks—gestation. Maturation continues for about 6 months after full-term birth. The glomerular filtration rate (GFR) is lower in the neonate because of the high renal vascular resistance associated with the relatively small surface area for filtration. Despite a low GFR and limited tubular function, the full-term newborn can conserve sodium. Premature infants, however, experience prolonged glomerulotubular imbalance, resulting in sodium wastage and hyponatremia ( Spitzer, 1982 ). On the other hand, both full-term and premature infants are limited in their ability to handle excessive sodium loads. Even following water deprivation, concentrating ability is limited at birth, especially in premature infants. After several days, neonates can produce dilute urine; however, diluting capacity does not mature fully until 3 to 5 weeks of life ( Spitzer, 1978 ). The premature infant is prone to hyponatremia when sodium supplementation is inadequate or with overhydration. Furthermore, dehydration is detrimental in the neonate regardless of gestational age. The physiology of fluid and electrolyte balance is detailed in Chapter 4 , Regulation of Body Fluids and Electrolytes.

Temperature Regulation

Temperature regulation is of particular interest and importance in pediatric anesthesia. There is a better understanding of the physiology of temperature regulation and the effect of anesthesia on the control mechanisms. General anesthesia is associated with mild to moderate hypothermia resulting from environmental exposure, anesthesia-induced central thermoregulatory inhibition, redistribution of body heat, and up to 30% reduction in metabolic heat production ( Bissonette, 1991 ). Small infants have disproportionately large BSAs, and heat loss is exaggerated during anesthesia, particularly during the induction of anesthesia unless the heat loss is actively prevented. General anesthesia decreases but does not completely abolish thermoregulatory threshold temperature to hypothermia. Mild hypothermia can sometimes be beneficial intraoperatively, and profound hypothermia is effectively used during open heart surgery in infants to reduce oxygen consumption. Postoperative hypothermia, however, is detrimental because of marked increases in oxygen consumption, oxygen debt (dysoxia), and resultant metabolic acidosis. Regulation of body temperature is discussed in detail in Chapter 5 , Thermoregulation: Physiology and Perioperative Disturbances.

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

Pediatric anesthesia as a subspecialty has evolved because the needs of infants and young children are fundamentally different from those of adults. The pediatric anesthesiologist should be aware of the child's cardiovascular, respiratory, renal, neuromuscular, and central nervous system responses to various drugs, as well as to physical and chemical stimuli, such as changes in blood oxygen and carbon dioxide tensions, pH, and body temperature. Their responses are different both qualitatively and quantitatively from those of adults and among different pediatric age groups. More important, the pediatric anesthesiologist should always consider the child's emotional needs and create an environment that minimizes or abolishes fear and distress.

There have been many advances in the practice of anesthesia to improve the comfort of young patients since the sixth edition of this book was published in 1996. These advances include a relaxation of preoperative fluid restriction, more focused attention to the child's psychological needs with more extensive use of preoperative sedation via the transmucosal route, the wide use of topical analgesia with EMLA cream before intravenous catheterization, and more generalized acceptance of parental presence during anesthetic induction and in the recovery room. Furthermore, a more diverse anesthetic approach has evolved through the combined use of regional analgesia, together with the advent of newer and less soluble volatile anesthetics, intravenous anesthetics, and shorter-acting synthetic opioids and muscle relaxants. Finally, the scope of pediatric anesthesia is expanding as pediatric anesthesiologists assume the role of pain management specialists beyond the boundary of perioperative care.

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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