Pulmonary Vascular Pressure-Flow Characteristics: Effects of Dopamine before and after Pulmonary Embolism

We compared the general hemodynamic effects of dopamine and dobutamine in dogs with acute pulmonary hypertension complicated by a decrease in cardiac output (CO). The pulmonary hypertension was induced by injection of autologous blood clot. Emboli markedly increased mean pulmonary artery pressure (Ppa) and decreased CO (both p < 0.001). Both dopamine and dobutamine increased CO 50% (p < 0.05) and decreased pulmonary vascular resistance (PVR) (p < 0.05), calculated as (PAP - left ventricular end diastolic pressure)/CO. Mean PVR (mm Hg/L/min) decreased from 16.1 to 12.4 with dopamine and from 16. to 11.9 with dobutamine, both p < 0.05. Ventricular filling pressures were not affected. In another 12 dogs we investigated the effects of both drugs on pulmonary pressure-flow (P-Q) characteristics. P-Q characteristics were determined in dogs with normal Ppa values and in those with embolic pulmonary hypertension. The slope of the P-Q relationship defines the incremental vascular resistance and the extrapolated pressure intercept, the effective vascular outflow pressure. All P-Q relationships were described well by a linear equation. Despite significant systemic effects in both groups and despite a decrease in PVR with both drugs in embolized dogs, neither drug significantly affected pulmonary P-Q characteristics. The discrepancy between PVR and incremental resistance is explained by an incorrect assumption in PVR that the left ventricular filling pressure is the effective vascular outflow pressure. We conclude that both before and after the induction of pulmonary hypertension, both dopamine and dobutamine improve CO without affecting pulmonary vascular tone.