What Is a Critical Mouth Opening for Macintosh Videolaryngoscopy? Results From a Prospective Observational Study

Although videolaryngoscopy is particularly helpful in patients with limited mouth opening, a severely restricted mouth opening is its hardest limitation. This analysis aimed to explore critical lower limits for interincisor distances that optimally predict (i) difficult Macintosh videolaryngoscopy or (ii) the risk of Macintosh videolaryngoscopy failure and may warrant consideration of alternative or awake techniques.

This analysis from a prospective observational study includes 192 adults at increased risk for difficult Macintosh videolaryngoscopy undergoing head and neck surgery managed with Macintosh videolaryngoscopy (C-MAC) as the first-choice technique. Interincisor distances were measured before and after anesthesia induction. Five clinical indicators (outcome measures) for difficult Macintosh videolaryngoscopy (multiple attempts, conversion to hyperangulated videolaryngoscopy, documentation of a difficult videolaryngoscopic intubation, hyperangulated videolaryngoscopy recommended for future anesthesia, anesthesia alert card issued) and 2 key indicators for the risk of Macintosh videolaryngoscopy failure (vocal cords not visible, awake flexible endoscopy recommended for future anesthesia) were systematically assessed. Mouth opening was grouped in 2 mm intervals; the Youden index was calculated for each of these intervals and the maximum Youden index ( Jmax ) was noted for each of the 7 indicator. For each mouth opening interval, the deviation from this maximum Youden index ( Jd-max ) and the mean Jd-max across all indicators were calculated to identify optimal thresholds.

The incidence of difficult videolaryngoscopic intubation was 56.8% (109/192). Optimal discrimination between easy and difficult Macintosh videolaryngoscopy (best mean Jd-max values) was derived from interincisor distances between 23 and 35 mm. The incidence of failed Macintosh videolaryngoscopy was 26.0% (50/192) in our study cohort and showed a clear inverse correlation with the interincisor distance. Discrimination between low and high risk of Macintosh videolaryngoscopy failure was optimal (best mean Jd-max values) between 21 and 23 mm. After anesthesia induction, interincisor distances increased in 31.8% (61/192), remained unchanged in 17.2% (33/192) but decreased in 51.0% (98/192) of the patients. The highest risk of reduction was found in individuals with degenerative cervical joint diseases (odds ratio [OR] = 6.43 [1.27-32.5]; P = .02), the lowest in those with facial fractures (OR = 0.27 [0.08-0.92]; P = .04) compared with a reference category.

Our findings suggest that among individuals with expected difficult airway management, interincisor distances below 21 to 23 mm indicate increased risk of Macintosh videolaryngoscopy failure and may warrant alternative or awake techniques. Caution should be exercised as the mouth opening frequently decreases after anesthesia induction. Results should not be extrapolated to routine intubations in noncomplex patients.