During the just concluded ASA national conference in Chicago, a study was presented that pointed to potential causes of failure in LMA insertion and ventilation. Satya-Krishna Ramachandran, MD and Michael Mathis, MD, both out of University of Michigan, observed 15,795 patients from 2006 to 2009 who had a laryngeal mask airway placed. The LMA was considered a failure if it had to be removed from the patient and an endotracheal tube placed instead. They found that 1.1% of LMA usage failed.
The researchers discovered four factors that could lead to the LMA not functioning properly. They are an elevated BMI, poor dentition, male sex, and the operating table turned away from the anesthesiologist. It's understandable that patients who are obese or have bad or missing teeth have higher rates of LMA failure. The table being turned 90 to 180 degrees away from the anesthesiologist increases the likelihood that the LMA could get dislodged by the surgeon or his assistants. I'm not exactly sure why being male increases the incidence of LMA failure. Maybe the larger oropharynx leads to more air leak around the LMA compromising ventilation.
We were always taught that LMA's shouldn't be placed in patients who are morbidly obese. They potentially have a full stomach despite hours of fasting and worse ventilatory efforts. In fact, lawsuits have been filed because of severe consequences of LMA failure in the obese patient. However I usually find the highest predictor of LMA failure are in patients who are edentulous. When a patient has no teeth, the soft tissue in the oropharynx just collapses, making it difficult for the LMA to seat properly. I asked our LMA rep one time about this. He said he's heard this complaint before and suggested I use the LMA Supreme, a model with a flatter tube and higher angulation. Of course it is also more expensive. When I tried it on my next edentulous patient, it didn't work all that much better. It was harder to place due to the sharper angulation, and it still didn't sit that well in the oropharynx. So the search goes on for the perfect LMA and ventilatory device.