A new cut-off point of thyromental distance for prediction of difficult airway

Difficult intubation, often unexpected, remains a primary concern for the anesthesiologist. None of the bedside tests have proven to be efficacious and highly predictive. The prupose of this study was to correlate the full range of thyromental distance with both the Cormack grade and the Mallampati score.

One hundred and sixty patients undergoing general anesthesia were included in the study. Patients with abnormal atlanto-occipital extension, small chin size and mouth opening were excluded from the study. The thyromental distance, the Mallampati score, and the Cormack grade were determined in all patients. The correlation between the thyromental distance and each of the Cormack grade and the Mallampati score was determined, as well as the correlation between the Mallampati score and the Cormack grade. The cut-off value of the tyromental distance that discriminated best between patients with easy glottic visualization (Cormack grade 1,2) vs. difficult visualization (Cormack grade 3,4) was derived.

A significant and high correlation was observed between the thyromental distance and the Cormack grade (r = -0.72). In contrast, a lower correlation was observed between the thyromental distance and the Mallampati score (r = -0.41) and the Mallampati score and the Cormack grade (r = 0.47). A cut-off value for the TMD of 4 cm was the best in discriminating between patients with difficult and easy glottic visualization. The combination of TMD < or = 4 cm and Mallampati score of 3,4 predicted the patients with the most difficult glottic visualization.

Patients with a thyromental distance greater than 4 cm can be easily intubated. In contrast, when the TMD < or = 4 cm, tracheal intubation is difficult in 48% of patients if the Mallampati score is 1,2 and in 79% if the Mallampati score is 3 or 4.