Continuous versus pulsatile pulmonary hemodynamics in canine oleic acid lung injury

Pulmonary hypertension occurs commonly in the acute respiratory distress syndrome (ARDS), but associated right ventricular failure is relatively rare. We tested the hypothesis that this apparent contradiction is explained by a peripheral location of the increased pulmonary vascular resistance (Rpva). Experimental ARDS was induced in eight dogs by injection of oleic acid (0.07 ml/kg). Changes in Rpva were evaluated by measurements of pulmonary artery pressure (Ppa) at several levels of flow (Q), which was altered by manipulation of venous return. The analysis of Ppa decay curves after arterial balloon occlusion was used to partition Rpva into arterial and venous segments. Right ventricular afterload was evaluated by determination of pulmonary vascular impedance (Zpva), which was calculated from spectral analysis of Ppa and Q waves. Oleic acid lung injury was associated with an increase in both the slope and the extrapolated pressure intercept of Ppa/Q plots, no change in the partitioning of Rpva, no change in time-domain indices in wave reflection or in pulmonary arterial compliance, and a decrease in both the characteristic impedance and pulsatile component of total right ventricular hydraulic load. We conclude that the site of increased Rpva in oleic acid lung injury is the smallest pulmonary arterioles, which, together with a decreased characteristic impedance, contributes to minimize right ventricular afterload.