Edema clearance in isolated sheep lungs

Edema may be cleared from the lung by lymphatic drainage, transudation across the visceral pleural, vascular reabsorption, and movement into the mediastinum. To determine the quantity and mechanisms of edema clearance associated with spontaneous edema formation in isolated sheep lungs, we perfused six lungs for 180 min with blood (100 ml · kg^-1 · min^-1) at subatmospheric left atrial pressure (Pla) from a weighed reservoir. In six other lungs, Pla was increased to 20 mmHg at 30-75 min to further augment edema. Fluid drainage from the lung was fractionated into blood and water components by serial measurements of drainage and perfusate hematocrit. Changes in weight of circulating intravascular blood and extravascular lung water (EVLW) were also directly measured by dye dilution and standard gravimetric techniques, respectively. From these measurements, we calculated that 3.04 ± 0.53 g/g blood-free dry lung of water filtered into the extravascular space during perfusion. Of this amount, 42% was reabsorbed into the pulmonary vasculature; 18% drained from the lung via lymphatics, visceral pleura, and mediastinum; and 40% was retained in the lung. Compared with low Pla lungs, transient elevation of Pla increased lung hemorrhage and the final change in reservoir weight, but the quantity and clearance of cumulative filtered water and the final values of EVLW and wet-to-dry weight ratio (WW/DW) were not altered. These results suggest that 1) significant edema clearance occurred in isolated sheep lungs, primarily by vascular reabsorption, and 2) measurements of EVLW and WW/DW underestimated injury in the presence of lung hemorrhage and significant edema clearance.