Originally designed for use in spontaneously breathing patients, it consists of a ‘mask’ that sits over the laryngeal opening, attached to which is a tube that protrudes from the mouth and connects directly to the anaesthetic breathing system.
On the perimeter of the mask is an inflatable cuff that creates a seal and helps to stabilize it. The LMA is produced in a variety of sizes suitable for all patients, from neonates to adults, with sizes 3, 4 and 5 being the most commonly used in female and male adults.
Patients can be ventilated via the LMA provided that high inflation pressures are avoided, otherwise leakage occurs past the cuff. This reduces ventilation and may cause gastric inflation.
The patient’s reflexes must be suppressed to a level similar to that required for the insertion of an oropharyngeal airway to prevent coughing or laryngospasm.
• The cuff is deflated (Fig. a) and the mask lightly lubricated.
• A head tilt is performed, the patient’s mouth opened fully and the tip of the mask inserted along the hard palate with the open side facing but not touching the tongue (Fig. b).
• The mask is further inserted, using the index finger to provide support for the tube (Fig. c). Eventually, resistance will be felt at the point where the tip of the mask lies at the upper oesophageal sphincter (Fig. d).
• The cuff is now fully inflated using an air-filled syringe attached to the valve at the end of the pilot tube (Fig. e).
• The laryngeal mask is secured either by a length of bandage or adhesive strapping attached to the protruding tube.
• A ‘bite block’ may be inserted to reduce the risk of damage to the LMA at recovery.
The LMA is reusable, provided that it is sterilized between each patient. There are now four additional types of LMAs available:
• A version with a reinforced tube to prevent kinking (Fig. b).
• The Proseal LMA (Fig. c): this has an additional posterior cuff to improve the seal around the larynx and reduce leak when the patient is ventilated.
It also has a secondary tube to allow drainage of gastric contents.
• The anaesthetist is not required to hold it in position, avoiding fatigue and allowing any other problems to be dealt with.
• It significantly reduces the risk of aspiration of regurgitated gastric contents, but does not eliminate it completely.
Its use is relatively contraindicated where there is an increased risk of regurgitation, for example in emergency cases, pregnancy and patients with a hiatus hernia.
The LMA has proved to be a valuable aid in those patients who are difficult to intubate, as it can usually be inserted to facilitate oxygenation while additional help or equipment is obtained (see below).