Heterogeneous lung emptying during forced expiration.

Several lines of evidence suggest that the healthy mammalian lung empties homogeneously during a maximally forced deflation. Nonetheless, such behavior would appear to be implausible if for no other reason than that airway structure is known to be substantially heterogeneous among parallel pathways of gas conduction. To resolve this paradox we reexamined the degree to which lung emptying is homogeneous, and considered mechanisms that might control differential regional emptying. Twelve excised canine lungs were studied. Regional alveolar pressure relative to pleural pressure was used as an index of regional lung volume. By use of a capsule technique, alveolar pressure was measured simultaneously in each of six regions during flow-limited deflations; flow from the lung was measured plethysmographically. The standard deviation of interregional pressure differences, which was taken as an index of nonuniformity, was 0.0, 0.74, 0.64, and 0.90 cmH2O at mean recoil pressures of 30, 8.4, 4.5, and 2.1 cmH2O (0, 25, 50, and 75% expired vital capacity), indicating that interregional pressure differences increased more rapidly earlier in the deflation. When we examined the time rate of change of regional alveolar pressure as an index of regional flow, we observed an intricate pattern of differential regional behavior that was inapparent in the maximum expiratory flow-volume (MEFV) curve. The most plausible interpretation of these findings is that regions of the healthy excised canine lung empty heterogeneously to a small degree, but in an interdependent compensatory pattern that is inapparent in the configuration of the maximum expiratory flow-volume curve.