Continuous airway pressure with oxygen minimizes the metabolic lesion of 'pump lung'

Water distribution and energy status of the lung were measured in ten rabbits at two hours of hypothermic cardiopulmonary bypass (CPB) with left heart venting and incision of the parietal pleurae. During CPB, half the animals had their airways open to room air at ambient atmospheric pressure (ZEEP), and the remainder had their lungs inflated (CPAP) at a pressure of 5 cms H2O with the oxygen-enriched (70-75 per cent) gas mixture exiting from the disc oxygenator. In both the ZEEP and CPAP groups, there was more than doubling of the pulmonary extravascular sodium-free water (intracellular) space and reciprocal reduction of the pulmonary extravascular sodium (extracellular) space, compared with 12 control animals not undergoing CPB. In the ZEEP group, there was an 18-fold increase in the pulmonary lactate/pyruvate (L/P) ratio compared with controls, and the pulmonary energy charge (E.C.) was significantly less than in controls (0.74 +/- 0.02 vs. 0.89 +/- 0.01). In the CPAP group the pulmonary L/P ratio was 2 1/2 times control values and the pulmonary E.C. was virtually identical with that of the control group. These data suggest that CPB results in a shift of fluid from the pulmonary interstitium into the pulmonary intracellular compartment with no net increase in total pulmonary extravascular water. The data also suggest that pulmonary deflation during CPB results in a significant pulmonary energy deficit which can be prevented by keeping the lung inflated with an oxygen-enriched gas mixture during CPB.