Airway closure and closing pressure during mechanical ventilation.

Six subjects without clinical evidence of lung disease were investigated for airway closure and airway closing pressure before and during fentanyl-thiopentone anesthesia with mechanical ventilation. Airway closure was measured by single breath and FRC by multiple breath nitrogen washout. Airway closing pressure was taken to be the transpulmonary pressure at which airway closure commenced. Airway closure occurred within a normal breath in two out of six subjects breathing spontaneously, but in all during mechanical ventilation. Closing capacity was the same in both the awake and anesthetized states while FRC was reduced by 0.41 when anesthesia was instituted. Transpulmonary pressure FRC was on average 1.5 cmH2O (0.15 kPa) and airway closing pressure 4.5 cmH2O (0.44 kPa) greater during anesthesia than in the awake state. Compliance of the lung, calculated both during a vital capacity maneuver and during a tidal breath, was lower with anesthesia. The results of this study suggest that the airways are less stable during mechanical ventilation. But, since lung compliance is lower during anesthesia, a higher transpulmonary pressure is required to maintain a given lung volume. Hence, airway closure occurs at the same lung volume in the anesthetized compared to the non-anesthetized subject.