Unexpected difficult laryngoscopy (DL) or intubation or failed tracheal intubation is an important aetiological factor in anaesthesia related morbidity and mortality. Several anthropometric measurements have been proposed to identify potentially difficult airways. Amongst these, the thyromental distance (TMD) has been widely used in the preoperative airway evaluation. The sensitivity and specificity of TMD as a predictor for difficult laryngoscopy is low and its role as a predictive test is debatable.  As TMD will vary with patient size, the ratio of height to TMD (RHTMD) may have a better predictive value than TMD alone, as the TMD cut-off value will be adjusted for patient size.   

The thyromental height (the height between the anterior borders of the mentum and thyroid cartilage with the patient in supine position and mouth closed) has been suggested as predictor of difficult laryngoscopy.  

The study hypothesis is that the height between the anterior borders of the mentum and thyroid cartilage, the thyromental height, can be a predictor of difficult laryngoscopy. The research question is to evaluate and compare the thyromental height, ratio of height to thyromental distance and thyromental distance for prediction of difficult laryngoscopy in adult patients undergoing elective surgery under general anaesthesia. The primary outcome measure is calculation of the prediction index (cut off value) and validity indices for the thyromental height.