Despite a bedside examination of the airway that predicts ease of tracheal intubation, DL can sometimes prove more difficult than expected. A large meta-analysis found that DL resulted in a Cormack-Lehane grade “III” or “IV” view in 5.8% of patients. Among obese patients, this increased to 15.8%. Another study found that more than two attempts at DL were required in 1.8% of adult nonobstetrical patients. Finally, in a prospective study looking at an intubation difficulty score (>5), at least moderate difficulty was encountered in 7.7% of patients, requiring two or more techniques in 9% of patients.

Bedside screening designed to predict a difficult tracheal intubation include the Mallampati oropharyngeal classification, thyromental distance, sternomental distance, and mouth opening. Individually they yield poor to moderate sensitivity (20%-62%) and moderate to fair specificity (82%-97%). The most useful predicative test appears to be a combination of the Mallampati classification and thyromental distance. The Mallampati classification estimates the size of the tongue relative to the oral cavity, hence the likelihood of an unobstructed laryngeal view. Thyromental distance is an indicator of mandibular space, reflecting capacitance for tongue displacement. These tests have been DL validated and may not be applicable to other techniques. Backward upward rightward laryngeal pressure (BURP) and the head elevation laryngoscopy position (HELP) may increase laryngeal exposure.

Videolaryngoscopy has been demonstrated to produce superior laryngeal views as compared to DL. In a large study where the GlideScope (GVL) (Verathon, Bothell, WA) was used in 2,004 of 71,570 attempted intubations, 81% of whom had features predictive of a difficult intubation by DL an overall success rate of 97% was observed. When used in patients with and without predictors of difficult DL the success rates were 96% and 98%, respectively. When videolaryngoscopy was used after failed DL or FIS, the respective success rates were 94% (n = 239) and 8 of 10 cases. Many of the failures (35%) occurred despite good laryngeal exposure. Videolaryngoscopy appears to provide better laryngeal exposure, often proving successful in situations where the line-of-sight required of DL fails, but intubation may take longer to complete and demands additional eye-hand coordination.