Direct versus indirect laryngoscopy using a Macintosh video laryngoscope: a mannequin study comparing applied forces

Upper airway injury and sympathetic activation may be related to the forces applied during laryngoscopy. We compared the applied forces during laryngoscopy using direct and indirect visualization of a standardized mannequin glottis.

Force transducers were applied to the concave surface of a GlideScope T-MAC Macintosh-style video laryngoscope that can also be used as a conventional direct-view laryngoscope. Thirty-four anesthesiologists performed four laryngoscopies (two direct and two indirect views) on an Ambu mannequin in a randomized sequence. During each laryngoscopy, participants were instructed to obtain views corresponding to > 80% and 50% of the glottic opening aperture. Peak and impulse forces were measured for each view.

To achieve a 50% glottic opening view, the top 10^th percentile force was higher with direct vs indirect laryngoscopy in terms of peak (difference, 9.1 newton; 99% confidence interval [CI], 7.4 to 13.9) and impulse (difference, 56.4 newton·sec; 99% CI, 49.0 to 81.7) forces. To achieve >80% view of the glottic opening, median force was higher with direct vs indirect laryngoscopy in terms of peak (difference, 3.6 newton; 99% CI, 1.6 to 7.3) and impulse (difference, 20.4 newton·sec; 99% CI, 11.7 to 35.1) forces.

In this mannequin study, lower forces applied during indirect vs direct laryngoscopy may reflect an advantage of video laryngoscopy, but additional studies using patients are required to confirm the clinical implications of these findings.