LARYNGEAL MASK AIRWAY

Design and Function

The laryngeal mask airway (LMA) is an ingenious supraglottic airway device that is designed to provide and maintain a seal around the laryngeal inlet for spontaneous ventilation and allow controlled ventilation at modest levels (≤15 cm H₂ O) of positive pressure. This device is available in seven sizes for neonates, infants, young children, older children, and small, normal, and large adults ( Fig. 42-9 ). After establishing an adequate depth of anesthesia and lubrication of the cuff, the appropriately sized LMA is inserted into the mouth, with the aperture facing the base of the tongue and the cuff tip pressed against the posterior pharyngeal wall. Although the standard method involves total cuff deflation, other clinicians prefer to insert the LMA with the cuff partially inflated. An excellent instruction manual is available from the manufacturer (LMA of North America, 9360 Towne Centre Drive, San Diego, CA 92121). The requirement for adequate anesthesia makes the LMA generally unsuitable for use in the conscious emergency room patient. The index finger of the dominant hand is used to guide the LMA into the hypopharynx until resistance is felt, and the cuff then is inflated with the appropriate volume of air ( Fig. 42-10 ). The resistance indicates that the cuff tip has reached the upper esophageal sphincter ( Fig. 42-11 ). A bite block, usually a folded 4 × 4-inch gauze, is inserted in the mouth to protect the LMA before the gauze and LMA are secured with tape. It is important to check by capnography, auscultation, and visualization of air movement that the cuff is correctly positioned and has not produced obstruction from downward displacement of the epiglottis. Because of the limited ability of the LMA to seal off the

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Figure 42-9 The laryngeal mask airway is available in seven sizes (upper left to lower right): size 1, neonates or infants up to 5 kg (4 mL); size 1.5, infants 5 to 10 kg (7 mL); size 2, infants or children 10 to 20 kg (10 mL); size 2.5, children 20 to 30 kg (14 mL); size 3, children or small adults weighing more than 30 kg (20 mL); size 4, normal and large adults (30 mL); size 5, large adults (40 mL). Maximum cuff inflation volumes in milliliters are given in parentheses for each size of LMA. (Courtesy of LMA North America, Inc., San Diego, CA.)

Figure 42-10 Insertion of the laryngeal mask airway (LMA). A, The tip of the cuff is pressed upward against the hard palate by the index finger while the middle finger opens the mouth. B, The LMA is pressed backward in a smooth movement. Notice that the nondominant hand is used to extend the head. C, The LMA is advanced until definite resistance is felt. D, Before the index finger is removed, the nondominant hand presses down on the LMA to prevent dislodgment during removal of the index finger. The cuff is subsequently inflated, and outward movement of the tube is often observed during this inflation. (Courtesy of LMA North America, Inc., San Diego, CA.)

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Figure 42-11 Intubating laryngeal mask airway (ILMA), illustrating the rigid curve and handle. Notice the different window compared with a standard LMA. (Courtesy of LMA North America, Inc., San Diego, CA.)

The LMA may be used as a substitute for the classic mask airway to eliminate the presence of a relatively large mask and practitioner's hand that may interfere with surgical access. A flexible LMA (LMA-Flexible, LMA of North America) may allow for easier connection at any angle from the mouth while resisting kinking and displacement. The LMA may be inserted to establish an emergency airway in awkward settings for intubation such as the lateral or prone positions. The device may also be employed to establish an airway in the patient in whom mask ventilation or tracheal intubation is difficult.^{[38] [39] [40] [41]} This role is discussed in a later section of this chapter. The LMA may be used to provide a conduit to facilitate fiberoptic, gum bougie-guided, or blind oral tracheal intubation. The size of the LMA dictates the size of endotracheal tubes that can be employed: cuffed 6.0-mm internal diameter (ID) for the size 3 and 4 and 7.0-mm ID for the size 5.^[13] Problems include inadequate endotracheal tube length as dictated by the presence of the LMA, limitation on endotracheal tube size, and inability to remove the LMA without risking extubation. The use of the fiberoptic scope to assist in placing a gum elastic bougie rather than direct placement of the endotracheal tube is one possible approach to this situation. A laryngeal mask specifically designed to facilitate tracheal intubation is available (LMA of North America, Inc.) (see Fig. 42-11 ).

With increasing use, problems have been reported with the LMA. They include pulmonary aspiration, laryngospasm, need for neck extension in the patient with cervical spine disorder, and failure to function properly in the presence of local pharyngeal or laryngeal disease.^[14] In patients with diminished pulmonary compliance or increased airway resistance, adequate ventilation may not be possible because of the high inflation pressures required and the resultant leaks. A newer modification, the ProSeal LMA, has been developed to overcome this limitation by means of its larger cuff ( Fig. 42-12 ) and

Figure 42-12 The ProSeal laryngeal mask airway (LMA), illustrating its larger cuff and drain tube. The drain tube inside the cuff is positioned (top) to prevent the epiglottis from occluding the airway tube in the absence of aperture bars. The LMA ProSeal is depicted (bottom) with the introducer in place.

The overall role of the LMA in clinical anesthesia appears to lie between that of the facemask and that of the endotracheal tube, because it provides more airway security (when properly positioned) than the former but not the reliable airway protection and maintenance of the latter. The LMA has become a nearly essential backup airway device to provide emergency ventilation when conventional mask ventilation and intubation attempts fail.