Airway Closure and Closing Pressure During Mechanical Ventilation

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Abstract
Human subjects [6] 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 [functional residual capacity] by multiple breath N2 washout. Airway closing pressure was taken to be the transpulmonary pressure at which closure commenced. Airway closure occurred within a normal breath in 2 of 6 subjects breathing spontaneously, but in all during mechanical ventilation. Closing capacity was the same in awake and anesthetized states, while FRC was reduced by 0.41 when anesthesia was instituted. Transpulmonary pressure at FRC was on average 1.5 cm H2O (0.15 kPa [kilo Pascal unit]) and airway closing pressure 4.5 cm H2O (0.44 kPa) greater during anesthesia than in the awake state. Lung compliance, calculated during a vital capacity maneuver and during a tidal breath, was lower with anesthesia. Apparently 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. Airway closure occurs at the same lung volume in the anesthetized compared to the non-anesthetized subject.