Ratio of Height to Sternomental Displacement as a Predictor for Difficult Airway: A Prospective Observational Study

Introduction: Unanticipated difficult airway is still a cause of morbidity and mortality. Various parameters are used to predict difficult airways. Recent studies have found that measuring the Sternomental Displacement (SMDD) can help establish a difficult airway. Aim: To find whether the Ratio of Height to Sternomental Displacement (RHSMDD) could be used as a predictor for Difficult Laryngoscopy (DL) and intubation. Secondary objectives included comparing RHSMDD with routine assessment parameters like Modified Mallampati Score (MMT), SMD, Thyromental Distance (TMD), and Inter Incisor Distance (IID). Materials and Methods: A prospective observational study was conducted at Malabar Medical College, Calicut, Kerala, India among 120 adult patients undergoing elective surgeries under general anaesthesia. Airway parameters like MMT, IID, TMD, SMD, Sternomental Displacement (SMDD), RHSMDD, Ratio of Height to Sternomental Distance (RHSMD), and Ratio of Height to Thyromental Distance (RHTMD) were measured preoperatively and associated with Cormack Lehane's laryngoscopic grading and Intubation Difficulty Scale (IDS) value. A comparison of airway parameters with laryngoscopy and intubation was done by the Mann -Whitney U test. Receiver Operating Characteristics (ROC) curves were constructed, and optimal cut-off values for significant quantitative indices were calculated. Sensitivity, specificity, Positive Predictive Value (PPV), and Negative Predictive Value (NPV) were also calculated and compared. Results: The incidence of DL was 27.5%, and Difficult Intubation (DI) was 10.8%. Age (p-value=0.013), weight (p-value=<0.001), and height (p-value=0.019) showed a significant association with DL. It was found that only RHTMD was statistically significantly higher in the DI group (p=0.044). All other parameters did not show statistical significance in either group. The calculated cutoff value for RHTMD was >= 18.45 cm. The highest specificity (96.3%) and NPV (90.43%) were observed for Body Mass Index (BMI) alone as a predictor of DI. The highest sensitivity (93.75%) was observed for TMD alone in predicting DL. The combination of MMT+RHSMD yielded the highest sensitivity and NPV for DI. Conclusion: RHTMD showed significance in DI and remains a better predictor. RHSMDD cannot be used as a predictor for a difficult airway. The combination of parameters demonstrated high sensitivity and NPV, suggesting they can be combined with modern ultrasound airway measurement for more accuracy.