Fig. 4. Representative data from one subject showing the relation between maximal inspiratory airflow during flow-limited breaths and mask pressure (P^m) at each of the three levels of general anesthesia. Each point represents the average from breaths three to five of each reduction in P^m. As the depth of general anesthesia decreased (from an end-tidal isoflurane concentration of 1.2% to 0.8% to 0.4%), the critical pressure required to collapse the airway (P^crit) decreased from 1.55 to 0.75 to 0.02 cm H²O, respectively. Dashed line and triangles = 1.2%; solid line and squares = 0.8%; dotted line and circles = 0.4%.