Evaluation of thyromental height, ratio of height to thyromental distance and thyromental distance as predictors of difficult laryngoscopy (and difficult intubation) in adult patients

Unexpected difficult laryngoscopy (DL) or intubationor failed tracheal intubation is an importantaetiological factor in anaesthesia related morbidity and mortality. Severalanthropometric measurements have been proposed to identify potentially difficultairways. Amongst these, the thyromental distance (TMD) has been widely used inthe preoperative airway evaluation. The sensitivity and specificity of TMD as apredictor for difficult laryngoscopy is low and its role as a predictive testis debatable.  As TMD will vary withpatient size, the ratio of height to TMD (RHTMD) may have a better predictivevalue than TMD alone, as the TMD cut-off value will be adjusted for patientsize.

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

The studyhypothesis is that the height between the anterior borders of the mentum andthyroid cartilage, the thyromental height, can be a predictor of difficultlaryngoscopy. The research question is to evaluate and compare the thyromentalheight, ratio of height to thyromental distance and thyromental distance forprediction of difficult laryngoscopy in adult patients undergoingelective surgery under general anaesthesia. The primary outcome measure iscalculation of the prediction index (cut off value) and validity indices for thethyromental height.