Overperfusion, hypoxia, and increased pressure cause only hydrostatic pulmonary edema in anesthetized sheep

Overperfusion (high pressure and flow through a restricted microvascular bed) has been suggested as the mechanism for both microembolic and high altitude pulmonary edema. In eighteen anesthetized, ventilated sheep, we measured pulmonary hemodynamics, lung lymph flow, and lymph:plasma protein concentration ratio. After a 2-hour stable baseline, we resected 65% of lung mass (right lung and left upper lobe) and gave whole blood transfusions to maintain cardiac output. During overperfusion of the left lower lobe, lymph flow increased moderately (5.8 +/- 2.3 to 7.7 +/- 3.8 ml/hr) and lymph:plasma protein concentration decreased (0.73 +/- 0.08 to 0.64 +/- 0.08). After a 2-hour stable period, we decreased inspired oxygen in 10 sheep (Pao2 = 40 +/- 3 mm Hg). With added alveolar hypoxia, pulmonary artery pressure increased modestly, but lymph flow and the lymph:plasma protein concentration ratio did not change. In eight sheep (four hypoxic, four normoxic), we raised left atrial pressure approximately 12 cm H2O for 2 hours. Lymph flow rose (10.8 +/- 3.8 ml/h) and lymph:plasma protein concentration decreased further (0.52 +/- 0.07). At each step, lymph:plasma protein concentration decreased, as predicted for the calculated rise in microvascular pressure. There was no evidence that overperfusion, with or without alveolar hypoxia, increased lung endothelial barrier protein permeability.