Contribution of the Closure of Pulmonary Units to Impaired Oxygenation during Anesthesia

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Abstract
Associations between airway closure, alveolar-arterial O2 tension difference (A-aDO2), and positive end-expiratory pressure (PEEP) were investigated in anesthetized, paralyzed, artificially ventilated patients. The difference between closing capacity (CC) and functional residual capacity (FRC) was measured with a modified standard technique using a bolus of N2 to detect airway closure in denitrogenated patients. AT FIO2 [fractional inspiratory oxygen] = 0.4 during anesthesia before application of PEEP, A-aDO2 was larger than expected in comparable conscious subjects and increased at .apprx. 1 mmHg/yr of age. CC was below FRC in young subjects but above FRC in older subjects, the 2 coinciding at .apprx. age 43 yr. During anesthesia both A-aDO2 and CC-FRC increased with age. The proximity and point of coincidence of CC and FRC suggested that CC is reduced during anesthesia. In patients whose CC exceeded FRC, imposition of PEEP estimated to be sufficient to elevate FRC above CC decreased A-aDO2 to a level comparable to that in patients exhibiting airway closure below FRC without PEEP. Patients in whom CC was initially below FRC failed to improve oxygenation with PEEP. At least half of the decrease in A-aDO2 associated with application of PEEP persisted for 20-30 min after withdrawal of PEEP, although the withdrawal resulted in an immediate recurrence of airway closure above FRC. Evidently, closure of pulmonary units operates in some circumstances to contribute to pulmonary dysfunction in anesthetized patients but is neither the only nor necessarily the most important such mechanism.