DOES PLASMA-PROTEIN DEPLETION INCREASE LUNG LIQUID CONDUCTANCE

Lung liquid conductance (K-f) is calculated as the quotient of lung ly mph flow divided by net filtration pressure (Pnf), where Pnf is the ba lance of osmotic and hydrostatic pressures in the lung microcirculatio n. In protein depletion, lymph flow rises with little change in Pnf, s uggesting that calculated K-f also rises. However, several previous re ports have concluded that protein depletion causes little change in K- f, leaving open the question of how lung lymph flow can rise in protei n depletion with little change in Pnf. To address this, we measured K- f in sheep following two kinds of protein depletion: batch plasmaphere sis (BP; n = 5) and thoracic duct drainage (TD; n = 5). Both methods l owered plasma protein concentrations by 30%, and raised lung lymph flo ws by 55%. Lung microvascular hydrostatic pressures and plasma-to-lymp h osmotic pressure gradients both changed by 1 to 2 mm Hg. With BP, ca lculated K-f rose from 0.26 +/- 0.09 at baseline to 0.50 +/- 0.20 on D ay 1, and to 0.39 +/- 0.27 ml/mm Hg/30 min on Day 2 (p less than or eq ual to 0.05). With TD, calculated K-f rose from 0.28 +/- 0.13 at basel ine to 0.43 +/- 0.19 on Day 1, and to 0.43 +/- 0.19 ml/mm Hg/30 min on Day 2 (p less than or equal to 0.05). Calculated K-f rose because fil tration increased even though the hydrostatic and osmotic driving forc es responsible for filtration changed little. This is puzzling because it suggests that lymph flow rose with little or no change in the forc es affecting filtration. Our findings contradict several previous repo rts that concluded that protein depletion produces little or no change in calculated K-f.