Handbook of Clinical Anesthesia
Although the role of the supraglottic airway (SGA) is firmly established in routine anesthetic care as well as airway rescue, the advent of video laryngoscopy promises to remove many of the failings of a technique (translaryngeal tracheal intubation) that has been in use for more than 100 years (Rosenblatt WH, Sukhupragarn WA: Airway management. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 751–792). The decrease in claims related to airway events at the induction of anesthesia is not matched with a decrease during emergence and the postoperative period. Management of the airway is paramount to safe perioperative care, and specific steps are necessary to favorably affect outcome (Table 29-1).
- Review of Airway Anatomy
- The term “airway” refers to the upper airway, consisting of the nasal and oral cavities, pharynx, larynx, trachea, and principal bronchi.
- The laryngeal skeleton consists of nine cartilages (three paired and three unpaired) that together house the vocal folds that extend in the anterior–posterior plane from the thyroid cartilage to the laryngeal cartilages.
- The larynx is innervated by two branches of the vagus nerve, superior laryngeal nerve, and recurrent laryngeal nerve.
- The recurrent laryngeal nerve supplies all the intrinsic muscles of the larynx with the exception of the cricothyroid muscle.
- Vocal cord dysfunction accompanies trauma to these nerves. Unilateral nerve injury is unlikely to impair airway function, but the protective role of
the larynx in preventing aspiration may be compromised.
Table 29-1 Steps to Favorably Affect Outcome as Related to Airway Management
- The cricothyroid membrane covers the cricothyroid space (~9 mm).
- The cricoid cartilage is at the base of the larynx and is suspended by the underside of the cricothyroid membrane.
- The trachea in adults measures about 15 cm and is supported circumferentially by 17 to 18 C-shaped cartilages with a posterior membranous aspect overlying the esophagus.
- The first tracheal ring is anterior to the sixth cervical vertebra.
- The trachea ends at the fifth thoracic vertebra (carina), where it bifurcates into the right and left bronchi. The right mainstem bronchus is larger than the left and deviates from the plane of the trachea at a less acute angle. (Aspirated materials as well as deeply placed tracheal tubes are more likely to enter the right than the left bronchus.)
- Patient History and Physical Examination
- Preoperative evaluation of the patient should elicit a thorough history of airway-related untoward events as well as related airway symptoms (obstructive sleep apnea, chipped teeth, dysphagia, stridor, cervical spine pain or limited range of motion, temporomandibular joint pain or dysfunction).
- Physical findings that may indicate subsequent difficult airway management include a short, muscular neck with full dentition; a receding mandible; protruding maxillary central incisor teeth; decreased
mobility at the temporomandibular joints; a long, high, arched palate; and increased alveolar–mental distance.
Table 29-2 Sensitivity and Specificity of Preoperative Findings for Difficult Airway Management
- No common index for difficult airway prediction has proven to be both sensitive and specific (Table 29-2).
- The Mallampati classification (based on tongue size in the intraoral cavity) is determined with the patient seated, the head in neutral position, the mouth opened as widely as possible, and the tongue protruded. The extent to which the base of the tongue is able to mask the visibility of the pharyngeal structures is the basis of classifying the airway as Mallampati I (uvula is fully visible) to IV (only the hard palate is visible). The practical value of this classification is its ease of application, but this index (like most others) has not proven to be sensitive or specific (there are many false-positive results) in identifying patients who may be difficult to intubate or the ease of using a SGA (laryngeal mask airway [LMA], fiberoptic bronchoscope).
- The multivariate index (MI) assigns relative weights to each examination finding (thyromental distance, mouth opening, Mallampati score, head
and neck movement, ability to prognath). Compared with the Mallampati classification, the MI has an improved positive predictive value and specificity.
- Clinical Management of the Airway
- This procedure entails the replacement of the lung's nitrogen volume (“denitrogenation”) with oxygen to provide a reservoir for diffusion of oxygen into the alveolar capillary bed after the onset of apnea (as associated with direct laryngoscopy for tracheal intubation).
- Breathing room air results in desaturation to below 90% after about 2 minutes of apnea.
- Breathing oxygen for 5 minutes maintains oxyhemoglobin saturation at 90% for about 6 minutes. As an alternative to breathing oxygen for 5 minutes, the patient may take four vital capacity breaths of oxygen over 30 seconds (or eight vital capacity breaths over 60 seconds).
- Alveolar carbon dioxide increases during any period of apnea independent of preoxygenation.
- Support of the Airway with the Induction of Anesthesia
- With the induction of anesthesia and the onset of apnea, ventilation and oxygenation are supported by the anesthesiologist using traditional methods (face mask, tracheal tube) or a newer SGA device (LMA).
- The anesthesia face maskis the most ubiquitous device used to deliver anesthetic gases and oxygen as well as to ventilate a patient who has been rendered apneic. Appropriate positioning of the patient's head and neck (“sniffing position”) is paramount to successful mask ventilation.
- After induction of anesthesia, the face mask is held firmly on the patient's face with downward pressure on the mask applied by the anesthesiologist's thumb and first or second fingers with concurrent upward displacement of the mandible with the other fingers (known as a jaw thrust), which raises the soft tissues of the anterior airway off the pharyngeal wall and allows for improved ventilation.
- In patients who are obese, edentulous, or have beards, two hands may be required to ensure a
tight-fitting face mask (when two hands are required, a second operator is needed).
- In the presence of normal lung compliance, the lung inflation pressure should not exceed 20 to 25 cm H2O.
- If more pressure is required, it may be prudent to consider other devices to aid in the creation of a patent upper airway (oral airway, nasal airway, LMA) to create an artificial passage between the roof of the mouth, tongue, and posterior pharyngeal wall.
- Oral airways may provoke coughing, vomiting, or laryngospasm when placed into the pharynx of a semiconscious patient.
- Supraglottic Airways.The advent of the LMA and other SGA devices has led some to question the relative safety of tracheal intubation (vocal cord edema, increased airflow resistance). Pharyngeal mucosal changes as a result of SGA use appear to be delayed compared with the effects of tracheal intubation.
- The Laryngeal Mask Airway Classicis composed of a small “mask” designed to sit in the hypopharynx with an aperture overlying the laryngeal inlet. The rim of the mask is composed of an inflatable silicone cuff that fills the hypopharyngeal space, creating a seal that allows positive-pressure ventilation with up to 20 cm H2O pressure and tidal volumes of 8 mL/kg.
- The Laryngeal Mask Airway Flexibleis designed to permit sharing of the airway with the surgical team (designed to be used with a tonsillar mouth gag).
- The Laryngeal Mask Airway and Bronchospasm.As a SGA, the LMA appears to be well suited for patients with a history of bronchospasm (asthma) who are not at risk for reflux and aspiration.
- Laryngeal Mask Airway Removal.The timing of removal of the LMA at the end of surgery is critical. The LMA should be removed when the patient is deeply anesthetized or after protective reflexes have returned and the patient is able to open the mouth on command.
- Contraindications to Laryngeal Mask Airway Use(Table 29-3)
- Complications of Laryngeal Mask Airway Use(Table 29-4)
Table 29-3 Contraindications to use of a laryngeal mask airway
- The Laryngeal Mask Airway Proseal. This device incorporates a gastric drain and increases the maximum airway seal during positive-pressure ventilation (possible use in obese patients during laparoscopic cholecystectomy) (Table 29-5).
- The laryngeal tubeconsists of a single-lumen tube and two (distal and proximal) low-pressure cuffs. When inserted correctly, the proximal cuff seals the oral and nasal pharynx, and the distal cuff sits within the upper esophageal sphincter.
- The Cobra pharyngeal laryngeal airwayis a disposable supralaryngeal device with a single lumen that terminates in a widened distal end. A fiberscope or tracheal tube may be passed through this device.
Table 29-4 Complications with Use of a Laryngeal Mask Airway
Table 29-5 Features of the Laryngeal Mask Airway Proseal
- Tracheal Intubation
- Routine Laryngoscopy.Repeated attempts at tracheal intubation often result in edema and bleeding of the anterior upper airway structures, hindering subsequent attempts at visualization and causing increased airway obstruction (first attempt at laryngoscopy in a best attempt).
- Direct Laryngoscopy.Successful laryngoscopy involves the distortion of the normal anatomic planes of the supralaryngeal airway to produce a line of direct visualization (“sniff position of the patient's head”) from the operator's eyes to the larynx (this requires alignment of the oral, pharyngeal, and laryngeal axes) (Fig. 29-1).
- Lateral external pressure over the larynx may be applied in an attempt to improve the laryngoscopist's view.
- The mouth can be opened by hyperextension of the atlanto-occipital joint using the laryngoscopist's hand placed under the patient's occiput or by application of downward pressure on the patient's chin.
Figure 29-1. With the patient in the supine position with no head support, the oral, pharyngeal, and tracheal axes do not overlap (A). The “sniff” position maximally overlaps the three axes (B).
Figure 29-2. Macintosh (curved) and Miller (straight) laryngoscope blades and small and regular-sized handles.
- Use of the Direct Laryngoscope Blade(Fig. 29-2)
- The laryngoscope blade is inserted into the right side of the patient's mouth (with care taken not to compress the upper lip against the teeth or to rotate the blade on the upper incisors) and advanced toward the epiglottis as the tongue is displaced to the left.
- Whereas the Macintosh blade tip is advanced into the vallecula, the Miller blade is advanced until it is positioned beneath the epiglottis while the laryngoscopist's arm and shoulder lift in an anterior and caudad direction.
- The view of the larynx may be complete, partial, or not possible (Fig. 29-3).
- The tracheal tube is inserted with the right hand and passed through the vocal folds to a depth of at least 2 cm after the disappearance of the tracheal tube cuff past the vocal folds (represents the midtrachea, which corresponds to the 21- and 23-cm external marking on the tracheal tube at the teeth for the typical adult woman and man, respectively.
- The gold standard for confirmation that the tube has been placed in the trachea is sustained detection of exhaled carbon dioxide as measured with capnography.
Figure 29-3. The Cormack and Lehane laryngeal view scoring system. A. Grade I, visualization of the entire glottic aperture. B. Grade II, visualization of only the posterior aspects of the glottic aperture. C. Grade III visualization of the tip of the epiglottis. D. Grade IV, visualization of no more than the soft palate.
- NPO Status and Rapid Sequence Induction.In the rapid sequence induction of anesthesia, intravenous administration of an induction drug is followed by a rapidly acting neuromuscular blocking drug. Direct laryngoscopy and tracheal intubation are performed as soon as muscle relaxation is confirmed.
- The Intubating Laryngeal Mask Airway (LMA-Fastrach).Blind fiberoptic-aided, stylet-guided, and laryngoscopy-directed intubation via the LMA can be accomplished in adults and children.
- A limiting factor is the size of the tracheal tube that may be passed through the airway.
- LMA-Fastrach can accommodate up to an 8-mm cuffed tracheal tube and is indicated for routine elective intubation and for anticipated and unanticipated difficult intubation.
- LMA CTrachis functionally identical to the Intubating LMA-Fastrach with the addition of integrated fiberoptic channels.
- Extubation of the tracheais often performed after the return of consciousness (after the patient can follow simple commands) and spontaneous ventilation (resolution of neuromuscular blockade). A patient who presents with a “difficult airway” at the time of induction of anesthesia must also be considered having “difficult extubation” at the time of extubation.
- Laryngospasmis a possible cause of airway compromise after tracheal extubation. (It is treated with administration of oxygen with continuous positive airway pressure and, if necessary, the use of a small dose of a rapidly acting muscle relaxant.) Negative-pressure pulmonary edema may result from airway obstruction in a patient who develops laryngospasm and who continues to create negative intrathoracic pressure as a result of voluntary respiratory effort.
- Approach to Difficult Extubation.When it is suspected that a patient may have difficulty with oxygenation or ventilation after tracheal extubation, the clinician may choose from a variety of management strategies (standby reintubation equipment, placement of a guide for reintubation or oxygenation).
III. The Difficult Airway
- The Difficult Airway Algorithm(Fig. 29-4)
- A difficult airway is defined as the situation in which a conventionally trained anesthesiologist experiences difficulty with mask ventilation (unassisted anesthesiologist unable to maintain SpO2 > 90% using 100% oxygen) or the inability to place a tracheal tube with conventional laryngoscopy (more than three attempts or more than 10 minutes).
- The incidence of failed tracheal intubation is estimated to be 0.05% to 0.35%, and the incidence of failed intubation plus an inability to achieve mask ventilation is estimated to be 0.01% to 0.03%.
- Even if the patient's oxygen saturation remains adequate during unsuccessful attempts at tracheal intubation, it is prudent to limit the number of attempts to three because significant soft tissue trauma can result from multiple laryngoscopies. At this stage, the clinician may consider alternatives to direct laryngoscopy and tracheal intubation that include fiberoptic laryngoscopy, use of an LMA, or establishment of a surgical airway. In some instances, it may be best to allow the patient to resume spontaneous ventilation and awaken.
- Awake Airway Management
- Awake airway management (LMA, fiberoptic intubation) is indicated if, after a thorough airway examination or a review of previous anesthetics, the ability to safely control ventilation and oxygenation without the risk of pulmonary aspiration is in question.
- Important to the success of awake intubation techniques is administration of drugs to reduce anxiety and secretions.
- Local anesthetics (topical lidocaine, benzocaine spray, phenylephrine, oxymetazoline) are a cornerstone of awake airway techniques. Local anesthetic therapy should be directed to the nasal cavity or nasopharynx (cotton-tipped applicators soaked with local anesthetic solution), pharynx, base of the tongue (aerosolized local anesthetic solution or voluntary “swish and swallow,” superior laryngeal nerve block), and the larynx or trachea (transtracheal injection of local anesthetic solution).
Figure 29-4. The American Society of Anesthesiologists' difficult airway algorithm. LMA = laryngeal mask airway.
- Use of a Fiberoptic Bronchoscope in Airway Management
- The fiberoptic bronchoscope has proven to be the most versatile tool for anesthesiologists dealing with an awake or unconscious patient who is or appears to be difficult to intubate.
- Use of a Fiberoptic Bronchoscope
- A variety of intubation airways are available and designed to provide a clear visual path from the oral aperture to the pharynx, keep the bronchoscope in the midline, prevent the patient from biting the insertion cord, and provide a clear airway for the spontaneously or mask-ventilated patients.
- After successful navigation through the SGA, the endoscopist visualizes the vocal folds. After the larynx is entered, a landmark (the carina) is selected to serve as an identifying landmark as the tracheal tube is advanced.
- An estimated 20% to 30% of tracheal tube advancements are accompanied by “hang up.”
- Video-Macintosh laryngoscopeconsists of a conventional-appearing laryngoscope and a fiberoptic cable that enters the handle where the camera elements are housed. The video image is displayed, and the operator or second operator directs the tube into position.
- Retrograde wire intubationis performed with the patient in a sitting position by percutaneous placement of an 18-gauge catheter placed through the cricothyroid membrane. A radiographic guidewire is placed through the catheter until it appears in the mouth, and a 7.0 tracheal tube is place over the guidewire and guided into the trachea.
- Use of Esophageal Tracheal Combitube
- The tube is inserted blindly, and the oropharyngeal balloon and distal cuff balloon are inflated.
- In the majority of patients, tube placement results in an esophageal position, and ventilation occurs via this lumen's hypopharyngeal perforations.
- Advantages include rapid airway control, airway protection from regurgitation, easy use by an inexperienced operator, no need to visualize the glottic opening, and the ability to maintain the neck in a neutral position.
- The Laryngeal Mask Airway in the Failed Airway
- It is estimated that one in 10,000 patients cannot be ventilated by mask and cannot be intubated by traditional
means. It is also estimated that one in 800,000 patients cannot be managed with an LMA.
Table 29-6 Percutaneous Transtracheal Jet Ventilation
- The major disadvantage of the LMA is the lack of mechanical protection from regurgitation and aspiration.
- Minimally Invasive Transtracheal Procedures
- Cricothyrotomyis the procedure of choice in emergency situations.
- Percutaneous transtracheal jet ventilationis a form of cricothyrotomy that is an option in the “cannot ventilate, cannot intubate” situation (Table 29-6).
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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