Inhomogeneity during deflation of excised canine lungs. III. Single-breathe O₂ tests

Both interregional and intraregional mechanisms may cause changes in N₂ concentration of expired gas during the phases of the single-breath O₂ test (SBO₂) that follow dead-space washout. To evaluate the possible importance of each mechanism, we performed the SBO₂ in excised canine lungs that were first suspended in air and then immersed in stable foams that simulated the vertical gradient of pleural pressure. The lungs were deflated at constant submaximal flows. The slope of phase III diminished with increasing expiratory flow and increased with foam immersion. The onset of phase IV depended on flow, and a terminal decrease in N₂ concentration (phase V) was often observed. Simultaneously measured estimates of regional flows and volumes (J. Appl. Physiol. 65: 1764-1774, 1988) were used to further interpret these results. The onset of phase IV at flows greater than quasi-static signified the onset of flow limitation of dependent regions. The onset of phase V corresponded to flow limitation of nondependent regions.