Effect of extreme elevations in venous pressure on reflection coefficient in the lung

We determined whether the solvent drag reflection coefficient (sigma f) for total proteins of a canine perfused left lower lung lobe (LLL) preparation decreases at elevated venous pressures (Pv). We found that sigma f (estimated using the hematocrit-protein technique) remained constant at all Pv's (30-95 mm Hg) evaluated. These results were unanticipated, since previous studies reported increases in protein permeability at Pv's within this range. We conducted two additional studies to better understand the basis for these observations. In the first, we evaluated the effect of high Pv (85 mm Hg) on sigma f of a canine perfused forelimb preparation and found sigma f to be reduced. This difference in response suggests that the normal sigma f's observed in the LLL were not due to high Pv per se, but rather that there is some intrinsic difference between the pulmonary and the systemic circulations that accounts for the difference. The second study was designed to determine whether the normal sigma f's observed in the LLL at high Pv's provide meaningful information about pulmonary vascular endothelial permeability. We damaged LLL's with alloxan, oleic acid, or HCl and obtained near normal estimates of sigma f at high Pv. These results indicated that it is not possible to easily distinguish between a normal and a damaged pulmonary vasculature when sigma f is measured at high Pv. We suggest that the normal estimates of sigma f obtained at high Pv in the LLL results from an increased fraction of the transvascular flow occurring through pathways that exclude macromolecules.