Epithelial and endothelial flux after bypass in dogs: effect of positive end-expiratory pressure.

Cardiopulmonary bypass (CPB) causes lung injury that occasionally progresses to the adult respiratory distress syndrome (ARDS). We measured the effect of 10 cmH2O of positive end-expiratory pressure (PEEP) on small solute and protein flux in dogs 1 wk before and 2 h after the completion of CPB. As an index of alveolar epithelial permeability, the clearance from lung to blood of inhaled technetium-99m-labeled diethylenetriaminepentaacetic acid (99mTc-DTPA) was measured. To assess microvascular endothelial integrity, the rate of accumulation in the lung interstitium of intravascular 113mIn-transferrin was measured. The clearance half time (t 1/2) for 99mTc-DTPA in the study dogs declined from 18.8 +/- 1.9 min (mean +/- SE) at base line to 9.4 +/- 2.0 min during PEEP (P less than 0.05). Two hours after CPB, the t 1/2 was 8.1 +/- 1.6 min at base line and unchanged during PEEP. The 113mIn-transferrin rate of accumulation was unchanged by PEEP before CPB. After CPB, the index was 3.25 +/- 0.95 slope/min X 10(-3) (P less than 0.05). Of the five dogs with a significant slope, four showed a decrease in microvascular flux during PEEP, although for the group the mean change in slope was not significant (P = 0.10). We conclude that the application of PEEP does not increase 99mTc-DTPA clearance in lungs already injured by CPB, and may actually decrease the apparent microvascular protein flux in some cases.