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

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We determined whether the solvent drag reflection coefficient ( σ f) for total proteins of a canine perfused left lower lung lobe (LLL) preparation decreases at elevated venous pressures ( P v). We found that σ f (estimated using the hematocrit-protein technique) remained constant at all P v's (30–95 mm Hg) evaluated. These results were unanticipated, since previous studies reported increases in protein permeability at P v'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 P v (85 mm Hg) on σ f of a canine perfused forelimb preparation and found σ f to be reduced. This difference in response suggests that the normal σ f's observed in the LLL were not due to high P v 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 σ f's observed in the LLL at high P v'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 σ f at high P v. These results indicated that it is not possible to easily distinguish between a normal and a damaged pulmonary vasculature when σ f is measured at high P v. We suggest that the normal estimates of σ f obtained at high P v in the LLL result from an increased fraction of the transvascular flow occurring through pathways that exclude macromolecules.