Angular change in the line of vision to the larynx: implications for determining the laryngoscopic view

We measured the angular change from the line of vision to the larynx around the upper incisors under defined laryngoscopic forces and investigated its association with the laryngoscopic view.

Laryngoscopy was performed under general anesthesia with muscle paralysis in male patients with a difficult laryngoscopy (DLG, n = 11) and in male patients matched for age and body mass index with an easy laryngoscopy (ELG, n = 11). A Macintosh blade #3 was used for the procedure. The line of vision was marked on lateral photographs during laryngoscopy by simultaneously delineating two straight lines: a line from the upper incisors to the lowest surface of the laryngoscope blade and a line from the upper incisors to the thyroid notch. The angle difference, defined as the angle between those two lines, was measured at laryngoscopic forces of 10-50 N.

The angle difference was significantly greater in the DLG than in the ELG at 50 N [median, 18.0° (range, 16.5-21.0°) vs 12.0° (12.0-13.5°), respectively; P < 0.001] and at lower forces (10-40 N; P ≤ 0.001). A higher Cormack-Lehane grade was associated with a greater angle difference at 50 N (P < 0.001).

Compared with ELG, DLG is associated with a larger angle difference, i.e., a larger gap between the underside of the blade and the thyroid notch at all laryngoscopic forces (10-50 N). The concept of angle difference, based on the angular change in the line of vision around the upper incisors, may provide a new approach to understanding DLG. This study was registered with the Clinical Research Information Service, registration number KCT0000433.