Author(s): Mohan Jagannatha1, Kailash Prabhudev2

Preoperative evaluation is important in the detection of patients at risk for difficult tracheal intubation. Thyromental distance (TMD) is often used for these purposes, but its value as an indicator for difficult intubation is questionable, as it varies with patient size and body proportions. The purpose of the present study was to evaluate and compare the accuracies of the Ratio of Patient’s Height to TMD (ratio of height to TMD=RHTMD), with TMD and Modified Mallampati classification (MP) in the prediction of difficult tracheal intubation. OBJECTIVE: This study is an attempt in finding an airway index by making simple measurements to anticipate difficult airway and compare RHTMD with TMD and MP classification for predicting difficult airway. METHODS: 170 apparently normal ASA I & II patients who were undergoing elective surgeries under General Anaesthesia (GA) were included in the study. The MP, TMD and RHTMD were determined in each patient preoperatively and Cormack – Lehane (CL) grading was assessed during laryngoscopy. TMD ≤6.5cm, RHTMD >25 and MP class III & IV were considered difficult intubation; these values were compared with CL grading. CL grade III & IV were considered as difficult intubation. The optimal predictive value was chosen using a receiver operating characteristic (ROC) curve. The areas under the ROC curves (AUC) of TMD and RHTMD were compared to determine the performance of the different predictive tests used. The sensitivity, specificity, and positive and negative predictive values of each of the predictive tests were calculated according to standard formulae. RESULTS: Difficult intubation occurred in 6 out of 170 patients (3.5%) in the study. The sensitivity of Modified Mallampati classification was 33.3% and specificity was 90.8%. The test has a positive predictive value of 11.7%, negative predictive value of 97.3% and overall accuracy of 88.8%. The sensitivity of TMD was 33.3% and specificity was 79.2%. The test has a positive predictive value of 5.5%, negative predictive value of 97% and overall accuracy of 77.6%. The sensitivity of RHTMD was 83.3% and specificity was 97.5%. The test has a positive predictive value of 55.5%, negative predictive value of 99.3% and overall accuracy of 97%. In our study the Area Under Curve (AUC) of RHTMD was 0.87, it is significantly higher than TMD (0.184) and MP test (0.726), indicating a more accurate prediction of RHTMD.A p value was calculated based on AUC, it shows that RHTMD had a higher and significantly better predictive value than MP classification and TMD with p value of 0.028. CONCLUSION: The Ratio of Height to Thyromental Distance has a better predictive value than Thyromental Distance and Modified mallampati classification. Ratio of Height to Thyromental Distance may represent a useful means of achieving a faster, simpler and more accurate predictor of difficult airway.

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