Prediction of difficult laryngoscopy in obstetric patients scheduled for Caesarean delivery

Background and objective: Failed incubation is an important cause of anaesthetic-related maternal mortality. The purpose of this study was to determine the ability to predict difficult visualization of the larynx from the following preoperative airway predictive indices, in isolation and combination: modified Mallampati test, the ratio of height to thyromental distance and the Upper-Lip-Bite test. Methods: We collected data on 400 consecutive parturients scheduled for elective Caesarean delivery under general anaesthesia requiring endotracheal intubation and then assessed all three factors before surgery. An experienced anaesthesiologist, not apprised of the recorded preoperative airway assessment, performed the laryngoscopy and grading (as per Cormack and Lehane's classification). Sensitivity, specificity and positive predictive value for each airway predictor in isolation and in combination were determined. Results: Difficult laryngoscopy (Grade 3 or 4) occurred in 35 patients (8.75%). The ratio of height to thyromental distance had a higher sensitivity, positive predictive value and fewer false negatives than the other variables tested. The ratio of height to thyromental distance of 21.24 provided the best cut-off point for predicting subsequent difficult laryngoscopy. The odds ratio (95% CI) of the ratio of height to thyromental distance, Mallampati class and the Upper-Lip-Bite test were 1.27.8 (44.8-364.5), 49.8 (20.3-121.8) and 6.6 (2.29-19.30), respectively. Conclusion: The ratio of height to thyromental distance may prove a useful screening test for predicting difficult laryngoscopy in obstetric population.