Representative response to a reduction of airway pressure and airway occlusion during light sevoflurane anesthesia. (A) During breaths 1 to 3, continuous positive airway pressure (CPAP) was applied and no flow limitations were detectable. Subsequently, pressure was reduced to atmospheric level (vertical dotted line) causing flow limitations (#) and genioglossus electromyography activity to increase. (B) The inspiratory limb was occluded, each inspiratory effort produced a decrease in mask (P_MASK) and epiglottic pressure (P_EPI). While P_EPI progressively decreased to a minimum value, P_MASK separated from P_EPI during breaths 7 to 9 as the result of upper airway collapse. The inflection points (upper airway closing pressure [P_CLOSE]) of the P_MASK trace were defined as the points at which airway collapse occurred (red shade). (C) Superimposition of P_MASK and P_EPI. GG-EMG = genioglossus muscle electromyography; GG-EMG MTA = genioglossus muscle electromyography moving time average.