Regional lung volume trajectories during expiratory flow in dogs

Parenchymal markers were placed in the left caudal lobes of anesthetized dogs, and the three-dimensional positions of the markers were tracked by biplane videofluoroscopy during expiration from total lung capacity (TLC) to ~30% TLC at steady expiratory flows of 0.3-7% TLC/s. Regional volumes of samples of parenchyma with volumes at TLC of 1-5 cm³ were obtained by computing the volumes of tetrahedra defined by taking the markers, four at a time, as apices of the tetrahedra. Regional volume (Vr), as a fraction of volume at TLC, was plotted against average volume (VL), as a fraction of volume at TLC, and Vr was fit by a quadratic function of VL. The initial slopes of the plots, dVr/dVL at VL = 1, varied by ±26% from the mean slope of 1. The curvatures were highly correlated with the slopes, and the Vr vs. VL plots formed a nest of 'onion skins.' The initial slopes were weakly correlated with the vertical position of the centroid of the tetrahedron, with the slope increasing by ~20%/cm, on average, in the dorsal direction in the supine dog. The vertical gradient in Vr accounted for ~30% of the total variability; small-scale heterogeneity contributed the remaining 70%. These results confirm earlier quasistatic measurements of nonuniform Vr and provide the first data on curvature of Vr trajectories. The mechanisms that cause the nongravitational component of Vr variability and the curvatures of the trajectories are unknown.