Mouth-opening increases upper-airway collapsibility without changing resistance during midazolam sedation

Sedative doses of anesthetic agents affect upper-airway function. Oral-maxillofacial surgery is frequently performed on sedated patients whose mouths must be as open as possible if the procedures are to be accomplished successfully. We examined upper-airway pressure-flow relationships in closed mouths, mouths opened moderately, and mouths opened maximally to test the hypothesis that mouth-opening compromises upper-airway patency during midazolam sedation. From these relationships, upper-airway critical pressure (Pcrit) and upstream resistance (Rua) were derived. Maximal mouth-opening increased Pcrit to -3.6 ± 2.9 cm H₂O compared with -8.7 ± 2.8 (p = 0.002) for closed mouths and -7.2 ± 4.1 (p = 0.038) for mouths opened moderately. In contrast, Rua was similar in all three conditions (18.4 ± 6.6 vs. 17.7 ± 7.6 vs. 21.5 ± 11.6 cm H₂O/L/sec). Moreover, maximum mouth-opening produced an inspiratory airflow limitation at atmosphere that was eliminated when nasal pressure was adjusted to 4.3 ± 2.7 cm H₂O. We conclude that maximal mouth-opening increases upper-airway collapsibility, which contributes to upper-airway obstruction at atmosphere during midazolam sedation.