Does plasma protein depletion increase lung liquid conductance?

Abstract

Lung liquid conductance (Kf) is calculated as the quotient of lung lymph flow divided by net filtration pressure (Pnf), where Pnf is the balance of osmotic and hydrostatic pressures in the lung microcirculation. In protein depletion, lymph flow rises with little change in Pnf, suggesting that calculated Kf also rises. However, several previous reports have concluded that protein depletion causes little change in Kf, leaving open the question of how lung lymph flow can rise in protein depletion with little change in Pnf. To address this, we measured Kf in sheep following two kinds of protein depletion: batch plasmapheresis (BP; n = 5) and thoracic duct drainage (TD; n = 5). Both methods lowered plasma protein concentrations by 30%, and raised lung lymph flows by 55%. Lung microvascular hydrostatic pressures and plasma-to-lymph osmotic pressure gradients both changed by 1 to 2 mm Hg. With BP, calculated Kf rose from 0.26 +/- 0.09 at baseline to 0.50 +/- 0.20 on Day 1, and to 0.39 +/- 0.27 ml/mm Hg/30 min on Day 2 (p < or = 0.05). With TD, calculated Kf rose from 0.28 +/- 0.13 at baseline to 0.43 +/- 0.19 on Day 1, and to 0.43 +/- 0.19 ml/mm Hg/30 min on Day 2 (p < or = 0.05). Calculated Kf rose because filtration increased even though the hydrostatic and osmotic driving forces responsible for filtration changed little. This is puzzling because it suggests that lymph flow rose with little or no change in the forces affecting filtration. Our findings contradict several previous reports that concluded that protein depletion produces little or no change in calculated Kf.