Ratio of height to thyromental distance (RHTMD) is a relatively new predictor of difficult visualization of larynx (DVL). Studies on Indian population are limited and none is carried out in the northeastern part of India.

The primary aim was to assess the performance of RHTMD as a predictor of DVL and compare its predictive performance with modified Mallampati test. The secondary aim was to compare the performance of RHTMD with sterno-mental distance, inter-incisor distance and upper-lip bite test, as a predictor of DVL. We also aimed to evaluate the performance of RHTMD as a predictor of Intubation difficulty scale (IDS) > 5.

In this prospective, observer blinded, single center study, adult patients of either sex scheduled to receive general anaesthesia with tracheal intubation were enrolled after permission from institutional committee. RHTMD, modified Mallampati test, sterno-mental distance, inter-incisor distance and upper-lip bite test wereassessed pre-operatively by a single examiner trained in the airway indices but not involved in laryngoscopy. On the day of surgery, laryngoscopy was performed by a trainee anaesthesiologist with minimum two years of experience who was blinded to the airway indices noted by the examiner preoperatively. The Cormack-Lehane grading and subsequent difficulty in intubation as per Intubation difficult scale was noted. RHTMD≥23.5, inter-incisor distance ≤ 4 cm, sterno-mental distance ≤12.5 cmand upper lip bite test class 3 were considered as predictor of difficult laryngoscopy.

The incidence of DVL and difficult intubation in our study was 6.57% (4.85 %-8.82%, 95% CI) and 1.37% (0.68%–2.7%, 95% CI) respectively. The sensitivity of RHTMD, modified Mallampati test, sterno-mental distance, inter-incisor distance and upper-lip bite test were 65.12%, 53.49%, 18.6%, 30.23% and 72.09% respectively. Specificity of RHTMD, modified Mallampati test, sterno-mental distance, inter-incisor distance and upper-lip bite test was 90.51%, 90.34%, 97.22%, 98.20%, and 89.69% respectively. Area under the ROC curve was highest for RHTMD (0.881). Area under the ROC curve for modified Mallampati test, sterno-mental distance, inter-incisor distance and upper-lip bite test was 0.742, 0.621, 0.757 and 0.815 respectively. RHTMD had a sensitivity, specificity, and AUC for predicting difficult intubation of 66.67%, 87.60%, and 0.908 respectively.

We primarily conclude that RHTMD has a good predictive performance and is similar to modified Mallampati test as a predictor of difficult visualization of larynx. We secondarily conclude that RHTMD is a better predictor of difficult visualization of larynx compared tosterno-mental distance, inter-incisor distance and upper-lip bite test. It is also an excellent predictor of IDS> 5.