Prevention of pulmonary insufficiency through prophylactic use of PEEP and rapid respiratory rates

This study evaluated the effectiveness of prophylactic positive end-expiratory pressure (PEEP) rapid respiratory rates (RRR), and high tidal volume (HTV) in prevention of congestive atelectasis. Measurements of pulmonary hemodynamics, mechanics, gas exchange, functional residual capacity (FRC), pathology, and cinemicroscopy were performed in 45 anesthetized dogs subjected to hemorrhagic hypotension. Randomly, the animals received control ventilation, HTV (20 ml. per kilogram), RRR (32 breaths per minute), or PEEP (5 cm. of water). Carbon dioxide was added as needed to maintain normocapnia. Control and HTV animals showed characteristic changes of congestive atelectasis (capillary congestion, stasis, interstitial edema, periarterial hemorrhage, alveolar edema, and hemorrhage). These microscopic and cinemicroscopic changes were prevented by PEEP and RRR and correlated with decreased physiological shunting (PEEP 10 percent, RRR 13 percent, HTV 22 percent; p less than 0.01) in the postshock phase. PEEP increased FRC by 40 percent (p less than 0.02) and reduced the pulmonary artery--small pulmonary vein gradient (PA-SPV), suggesting a direct effect on the capillary bed. RRR did not affect FRC but minimized the SPV-LA gradient. This effect on the pulmonary venules theoretically could be mediated by stimulating lymphatic flow, thereby decreasing interstitial edema. Thus PEEP and RRR are beneficial when used prophylactically but may work by widely differing mechanisms.