Collapsibility of the Upper Airway during Anesthesia with Isoflurane

Background: The unprotected upper airway tends to obstruct during general anesthesia, yet its mechanical properties have not been studied in detail during this condition.

Methods: To study its collapsibility, pressure-flow relationships of the upper airway were obtained at three levels of anesthesia (end-tidal isoflurane = 1.2%, 0.8%, and 0.4%) in 16 subjects while supine and spontaneously breathing on nasal continuous positive airway pressure. At each level of anesthesia, mask pressure was transiently reduced from a pressure sufficient to abolish inspiratory flow limitation (11.8 +/- 2.7 cm H2O) to pressures resulting in variable degrees of flow limitation. The relation between mask pressure and maximal inspiratory flow was determined, and the critical pressure at which the airway occluded was recorded. The site of collapse was determined from simultaneous measurements of nasopharyngeal, oropharyngeal, and hypopharyngeal and esophageal pressures.

Results: The airway remained hypotonic (minimal or absent intramuscular genioglossus electromyogram activity) throughout each study. During flow-limited breaths, inspiratory flow decreased linearly with decreasing mask pressure (r2 = 0.86 +/- 0.17), consistent with Starling resistor behavior. At end-tidal isoflurane of 1.2%, critical pressure was 1.1 +/- 3.5 cm H2O; at 0.4% it decreased to -0.2 +/- 3.6 cm H2O (P < 0.05), indicating decreased airway collapsibility. This decrease was associated with a decrease in end-expiratory esophageal pressure of 0.6 +/- 0.9 cm H2O (P < 0.05), suggesting an increased lung volume. Collapse occurred in the retropalatal region in 14 subjects and in the retrolingual region in 2 subjects, and did not change with anesthetic depth.