Uneven driving pressures and redistribution of intrapulmonary blood flow

Pulmonary arterial pressure (Ppa) and airway pressure (Paw) change equally when Paw varies widely in normal lungs under zone 2 conditions. To evaluate the mechanism underlying the decrease in Ppa at a given Paw in edematous lungs, we measured the quantity of right-to-left shunting of blood (Qs/QT) with sulfur hexafluoride (SF6) using in situ perfused dog lungs from five control animals, five animals with bronchial occlusion, and eight animals with oleic acid-induced pulmonary edema. Under base-line conditions when positive end-expiratory pressure (PEEP) was increased from 5 to 15 cmH2O, Ppa increased because Paw was fully transmitted to Ppa. After bronchial occlusion at 5 cmH2O PEEP, observed QS/QT was less than predicted (P less than 0.01) on the basis of regional conductances, but at 15 cmH2O observed and predicted QS/QT did not differ (P greater than 0.5). In five of eight dogs with oleic acid-induced pulmonary edema. QS/QT increased when PEEP was increased. We conclude that pressure barriers that create nonuniform driving pressures cause a decrease in Ppa and a redistribution of blood flow.