Neck circumference to thyromental distance ratio: a new predictor of difficult intubation in obese patients

This study was performed to assess whether intubation is more difficult in obese patients and to assess the ability of a new index: the ratio of the neck circumference to thyromental distance (NC/TM), to predict difficult intubation in obese patients. The incidence of difficult tracheal intubation in 123 obese (BMI≥27.5 kg m−2) and 125 non-obese patients was compared. Difficult intubation was determined using the intubation difficulty scale (IDS≥5). The NC/TM ratio was calculated and its ability to predict difficult intubation in obese patients was compared with that of established predictors including high BMI, the Mallampati score, the Wilson score, NC, width of mouth opening, sternomental distance, TM, and a previous history of difficult intubation. Difficult intubation was more frequent in obese patients than in non-obese patients (13.8% vs 4.8%; P=0.016). Multivariate analysis revealed that the Mallampati score, the Wilson score, and NC/TM independently predicted difficult intubation in obese patients. Among these three indices, NC/TM showed the highest sensitivity and a negative predictive value, and largest area under the curve on an ROC curve. Difficult intubation was more common in obese patients and the NC/TM was a better method for predicting difficult intubation than other established indices.