Fig. 3. Upper airway critical closing pressure (Pcrit) and airway pressure associated with beginning of flow limitation in awake healthy volunteers at baseline before neuromuscular blockade, with impaired neuromuscular transmission and a target train-of-four (TOF) ratio of 0.5 and 0.8, and after recovery of the TOF ratio to unity. Data are means (± SD) from 60 measurements in 15 volunteers. Upper airway closing pressure ( black bars ) significantly increased during partial neuromuscular blockade and was still abnormal, even with recovery of the TOF ratio to unity. With neuromuscular transmission intact at baseline, evidence of flow limitation ( gray bars ) was first observed at an average pressure of –12 cm H2O. With partial neuromuscular blockade at a TOF ratio of 0.5 and 0.8, flow limitation occurred at significantly less negative values of mask pressure, i.e., airway integrity is impaired. * P < 0.05 versus baseline.