Ultrasound quantification of anterior soft tissue thickness fails to predict difficult laryngoscopy in obese patients

Morbid obesity is associated with difficult laryngoscopy and intubation. In the general population, bedside indices for predicting difficult intubation (i.e. Mallampati classification, thyromental distance, sternomental distance, mouth-opening and Wilson risk score) have poor-to-moderate sensitivity (20-62%) and moderate-to-fair specificity (82-97%). In the obese population, although the risk of difficult intubation after a positive Mallampati test is 34%, it is still not sufficient to be used as a single predictive test. An abundance of pretracheal soft tissue anterior to the vocal cords, as quantified by ultrasound, was a better predictor of difficult laryngoscopy than body mass index (BMI) in Israeli patients. Obesity is a growing problem in the United States: therefore we sought to confirm this finding in the obese population in the United States. We used ultrasound to quantify the neck soft tissue, from the skin to the anterior aspect of the trachea at the vocal cords, in 64 obese patients (BMI >35). We assessed thyromental distance, mouth-opening, jaw movement, limited neck mobility, modified Mallampati score, abnormal upper teeth, neck circumference, confirmed obstructive sleep apnoea, BMI, age, race and gender as predictors. Twenty patients were classified as difficult laryngoscopy; they were older (47+/-9 vs 42+/-1 years; P=0.048; mean+/-SD) and had less soft pretracheal tissue (20.4+/-3.0 vs 22.3+/-3.8 mm; P=0.049) than did easy laryngoscopy patients. Multivariate regression indicated that none of the factors was an independent predictor of difficult laryngoscopy. We conclude that the thickness of pretracheal soft tissue at the level of the vocal cords is not a good predictor of difficult laryngoscopy in obese patients in the United States.