Ratio of patient's height to thyromental distance improves prediction of difficult laryngoscopy

Several tests have been proposed to predict difficult laryngoscopy or intubation. The thyromental distance (TMD) is often used for these purposes but this measurement, used alone, is unreliable. This study tested the hypothesis that the ratio of the patient's height to TMD (ratio of height to TMD=RHTMD) would improve the accuracy of predicting difficult laryngoscopy compared with TMD alone. Two hundred and seventy patients were evaluated preoperatively using the TMD and RHTMD. The two tests were compared analyzing the area under the receiver operating characteristic curves (AUC). Difficult laryngoscopy occurred in 16 patients (5.9%). The AUC of RHTMD was significantly greater (P<0.007) when compared to TMD, indicating a more accurate prediction by the RHTMD. A ratio of 25 for the RHTMD was found to be the optimal cut-off value to predict difficult laryngoscopy. When the sensitivity of both tests was 0.81, the RHTMD had a significantly greater specificity (0.91) than the TMD (0.73). Based on our results, we recommend that the RHTMD should be used instead of the TMD.