Interdependence of regional expiratory flows limits alveolar pressure differences

Wilson et al. (J. Appl. Physiol. 59:1924-28, 1985) have asserted that interdependence of regional expiratory flows could cause differences of interregional alveolar pressures to relax to time-independent limits during forced deflation. To test the hypothesis that such limiting differences do arise, we examined regional alveolar pressures during complete and partial maximally forced deflations of six excised canine lungs. Alveolar pressures were monitored using alveolar capsules on each of six lobes during forced deflations initiated at transpulmonary pressures of 30, 20, 15, and 10 cmH2O. In all lungs and in all maneuvers, interregional heterogeneity of alveolar pressure increased rapidly early in the deflation but much less so or not at all later in the deflation. When we compared complete with partial forced deflations, 16 of 24 maneuvers in six lungs showed clear evidence that as deflation progressed the degree of heterogeneity at isovolumic points became independent of the transpulmonary pressure from which the deflation was initiated. That is, alveolar pressures relaxed to limiting interregional differences that did not depend on time elapsed from the onset of the deflation. These data offer strong evidence of the existence of limiting differences. Such behavior implies that the sequence of regional emptying is controlled by a competition of opposing influences: nonuniformities of airway and parenchymal properties promoting nonuniformity of emptying vs. interdependence of regional expiratory flows promoting uniformity. As nonuniformity of regional pressures grows so do those factors that oppose that nonuniformity. These data underscore the insensitivity of maximum expiratory flow-volume curve configuration to the underlying inhomogeneous pattern of regional lung emptying.