Effect of a hydrostatic pleural pressure gradient on mechanical behavior of lung lobes

Using 133Xe, the vertical distribution of regional volume (Vr) was measured in three regions of excised canine lobes both in air and when completely submerged in saline at 40, 60, 70, and 80% lobar vital capacity (VC). The estimated pleural pressure gradient, derived from values of Vr, distance between regions, and the lobar pressure-volume (PV) curve, underestimated the true gradient by 45%. Conversely, the gradient of Vr was substantially less than predicted. From the mean depth of each region below the waterline, pleural, and hence transpulmonary, pressure (PL) was computed. The values of Vr-PL for each region at 40, 60, and 80% lung volume (VL) were related to the lobar PV curve. Slopes of lines joining initial VL-PL points on the lobar PV curve to corresponding Vr-PL points in submerged lobes represent an effective regional compliance of a lobe undergoing deformation. With one exception this was less than the corresponding homogeneous compliance, indicating a stiffening of the lobe during deformation. Slopes of lines joining Vr-PL points of each region at the three lobar volumes represent effective regional compliance of a deformed lobe undergoing volume change. This was not significantly different from the homogeneous compliance. However, effective compliance can only be an approximate indicator of the forces required for a given volume change due to the inadequacy of PL to represent the unequal stress components induced by lobe deformation.