To the Editor:—
A patient was anesthetized for an orthopedic procedure and a no. 4 laryngeal mask airway (LMA) was inserted. An air leak around the LMA was observed at approximately 12 cm H2O pressure, causing ventilation to be inadequate. I attempted to alleviate the air leak by repositioning the LMA, changing the cuff inflation volume, and repositioning the patient's head, with no success. Accordingly, I prepared to exchange the LMA with an endotracheal tube when the circulating nurse placed a thyroid shield on the patient in preparation for fluoroscopy. The air leak ceased and the case was completed using the LMA.
On several subsequent occasions, I have successfully used a thyroid shield to ameliorate an air leak around an LMA. The patient shown in figure 1
is such an example. The patient had a no. 4 LMA inserted without difficulty, but despite multiple manipulations of the LMA, LMA cuff, and the patient's head, the LMA had a persistent leak at 10–12 cm H2O. The patient was breathing spontaneously but entraining room air around the LMA cuff. Placement of a thyroid shield allowed positive pressure ventilation up to 22 cm H2O, and the room-air entrainment stopped with spontaneous ventilation. To date, I have had no instances of failure to correct an LMA cuff leak and was able to proceed with the anesthetics planned using an LMA.
The mechanism for this result seems to be that the thyroid shield forces the soft tissue of the anterior pharynx against the LMA cuff. LMA cuff pressure is adjusted to maintain an air leak around the LMA of 20–25 cm H2O to lessen the risk of pharyngeal mucosal ischemia.