The laryngeal mask is, in fact, the only gravity-enhanced ventilation device. Gravity and loss of tone defeat the face mask. Conversely, administration of propofol, which induces a deep loss of upper-airway tone, is far and away the most commonly used medication for laryngeal mask anesthesia. The loss of tone in a supine position is ideal for creating the laryngeal mask seal around the bowl of the device. The soft tissues of the upper airway (base of tongue, epiglottis, and perilaryngeal structures) collapse backward onto the bowl of the LMA as muscular tone about the mandible is abolished.
In addition to the ergonomic challenges of doing effective mask ventilation—trying to create a face seal while pressing against the face but pulling up on the mandible simultaneously—there are many physiologic reasons why mask ventilation in a flat position is bad for oxygenation and also why it has a high risk for regurgitation. As a face mask is squeezed over the mouth, the oropharynx gets pressurized (see Figure 1). The goal is to have air only go into the lungs as opposed to the collapsed esophagus. Unfortunately, as pressure increases, especially at about 20 cmHg, air will enter the esophagus and subsequently the stomach. This insufflation of the stomach then leads to regurgitation of stomach contents back up the esophagus to the perilaryngeal area of the hypopharynx (at the top of the esophagus). Regurgitation risk is dramatically increased by having the patient’s stomach and head on the same level or, in an obese patient, the stomach higher than the mouth when the patient is supine. As the stomach gets insufflated, gravity promotes regurgitation of stomach contents into the upper airway.
The LMA, in contradistinction to a face mask, provides some isolation of the esophagus and larynx. The tip of an LMA-type device wedges into the upper esophagus. It’s not a complete isolation like a cuffed tube in the trachea, but by “corking” the top of the esophagus, there is some protection from gas entering the esophagus. Additionally, the bowl of the LMA is sitting directly under the laryngeal inlet, so the amount of pressure needed to get oxygen into the lungs is less than what is used typically with a face mask, having to start from outside the mouth and flow around the tongue.