Alterations in pulmonary fluid balance induced by positive end-expiratory pressure

We examined the consequences of positive end-expiratory pressure ventilation (PEEP) on pulmonary lymph flow in normal lungs and in lungs with alloxan-induced vascular injury. Studies were made in anesthetized sheep prepared with a lung lymph fistula. Prior to lung injury, the sheep were subjected to a protocol consisting of ventilation with zero end-expiratory pressure (ZEEP) for 1 h followed by ventilation with 20 cm H2O PEEP for 1 h. Subsequently, each animal received an infusion of alloxan (0.75 mg/kg i.v.) during ZEEP ventilation to induce lung vascular injury. ZEEP ventilation was maintained for 1 h and was followed by 1 h of PEEP ventilation. Alloxan infusion more than doubled lymph flow and lymph protein flux from baseline. PEEP ventilation did not affect lymph flow and lymph protein flux of normal lungs. However, in the alloxan-injured lung, PEEP ventilation caused significant decreases in lymph flow and lymph protein flux. Although PEEP ventilation redistributed regional pulmonary blood flow, as determined by radiolabelled microspheres, changes were identical in the normal and the alloxan-injured lung. The results indicate that PEEP ventilation does not alter fluid balance in the normal lung. However, PEEP ventilation depresses lymphatic flow in the injured lung, which may increase extravascular lung water content.