Fig. 2. Polygraph record in one subject at a stable plane of general anesthesia showing the responses when mask pressure was rapidly reduced from a maintenance pressure (approximately 14 cm H2O) for five breaths and then returned to the maintenance pressure. This procedure was repeated at a range of positive or negative airway pressures, causing varying degrees of inspiratory flow limitation (a plateau in inspiratory flow while esophageal pressure decreased progressively, (A –C ); see also fig. 3), including a pressure sufficient to cause complete airway collapse (no inspiratory flow despite a changing esophageal pressure; (D )). Pm= mask pressure; Pnp= nasopharyngeal pressure; Pop= oropharyngeal pressure; Php= hypopharyngeal pressure; Pes= esophageal pressure.