Role of microvascular pressure in reactive oxygen-induced lung edema.

O2 radicals are important in the pathogenesis of acute lung injury. The purpose of this investigation was to determine the role that microvascular pressure plays in edema induced by reactive O2 species generated by xanthine oxidase. In isolated rat lungs perfused with Krebs buffer plus 4% albumin, 5 mM glucose, and 2 mM xanthine at constant flow (13 ml/min), addition of xanthine oxidase (0.02 U/ml) caused a progressive increase in both pulmonary arterial and microvascular pressure (double occlusion method), which preceded the onset of edema. Both the pressure rise and edema formation were blocked by catalase, suggesting that vascular injury was related to H2O2 production. Lungs not exposed to free radicals that had microvascular pressure elevated to match that of the xanthine oxidase-perfused lungs showed only a small, reversible (nonedematous) weight gain. Lungs exposed to xanthine oxidase but perfused at constant microvascular pressure (5 Torr, similar to control lungs) showed a significant delay in protein-rich edema formation. These data indicate that reactive O2 metabolites induced lung injury, which is accompanied by increased microvascular pressure. Although the rise in microvascular pressure was shown not to be essential for edema formation, it does play a role in acceleration of the rate of transvascular fluid loss.