Superoxide dismutase with heparin prevents increased lung vascular permeability during air emboli in sheep

We studied the effects of bovine superoxide dismutase on the increased lung microvascular permeability to fluid and protein during air emboli in unanesthetized sheep. We measured pulmonary arterial and left atrial pressures, cardiac output, lung lymph flow, and lymph and plasma protein concentrations. In air emboli experiments we continuously infused the same dose of air bubbles 1 mm in diameter into the right atrium of each sheep to increase pulmonary vascular resistance to 2-4 times base-line values. We did experiments in pairs so that each sheep served as its own control. We found an increase in protein-rich lymph flow from the lung during embolization in untreated sheep, indicating an increase in microvascular permeability to both fluid and protein. When we pretreated the sheep with heparin (300 U/kg) and infused them with intravenous bovine superoxide dismutase (1 mg X kg-1 X h-1 beginning before the air infusion), we found that vascular pressures and pulmonary vascular resistance were not different from untreated sheep but that the expected increases in transvascular fluid and protein flow during emboli were significantly decreased (P less than 0.0005). Heparin alone did not significantly attenuate the increased microvascular permeability but we found that it greatly enhanced the effectiveness of superoxide dismutase in preserving microvascular functional integrity during air emboli. We conclude that superoxide anion, probably produced and released by leukocytes, is a central factor in the microvascular injury that results in increased permeability in the lungs of sheep during air microembolization.