Short thyromental distance: a predictor of difficult intubation or an indicator for small blade selection?

Background: Short thyromental distance (TMD; < 5 cm) has been correlated with difficult direct laryngoscopic intubation in adult patients. The authors hypothesized that a smaller Macintosh curved blade (No. 2 MCB) would improve the predicted difficult laryngoscopy in short-TMD patients over that with a standard Macintosh curved blade (No. 3 MCB).

Methods: In a preliminary study of 11 consenting adults (7 females and 4 males), American Society of Anesthesiologists physical status I and TMD < or = 5 cm, lateral neck radiographs were recorded during laryngoscopy with a No. 2 and No. 3 MCB in sequential fashion. In a prospective clinical study, laryngoscopy and tracheal intubation were evaluated in 83 adult patients with TMD < or = 5 cm by randomly assigning them to two groups for the blade used at first intubation. Those who failed intubation with the first blade were switched to the alternate blade. In total 100 laryngoscopies and intubations were performed: the No. 2 MCB (n = 50) and the No. 3 MCB (n = 50).

Results: Lateral neck radiographs recorded at the best laryngeal view revealed that the tip of the No. 2 MCB was proximal to the hyoid body with the No. 2 MCB and distal to it with the No. 3 MCB. The intubation distance (C5 to blade tip) on neck radiographs with the No. 2 MCB was significantly greater than it was with the No. 3 MCB for similar anterior jaw displacement. In the clinical study, the laryngoscopic grade with the No. 2 blade was considered easy (median, 2B), better than the grade with the No. 3 MCB (median, 3). When the No. 2 MCB was used, external laryngeal pressure improved the laryngoscopic grade (1, full glottic view) in 46% of patients. In contrast, when the No. 3 MCB was used, pressure improved the grade in only 10% of the patients. Intubation time with the No. 2 MCB was significantly (P < 0.05) less than it was in patients with No. 3 MCB. Overall, 14 patients who failed intubation with the No. 3 MCB were switched to the No. 2 MCB, and intubation was successful with an easy laryngoscopic grade. Three patients who failed intubation with the No. 2 MCB were switched to the No. 3 MCB.

Conclusions: The predicted difficult laryngoscopy and intubation with the use of the adult No. 3 MCB in standard adult patients with a TMD < or = 5 cm is significantly easier with use of the smaller No. 2 MCB.