The effects of iv dobutamine and dopamine infusions were studied at six incremental doses (range 5 to 160 μg/kgċmin) in two groups of five dogs. Dobutamine decreased the systemic vascular resistance (SVR), without significant changes in mean arterial pressure (MAP), or pulmonary vascular resistance (PVR). Dopamine increased MAP, SVR, and PVR, except for a decrease at 10 μg/kgċmin. Both drugs produced dose-related increases in cardiac output and venous admixture; however, with dopamine the dose-response curve reached a plateau at doses greater than 40 μg/kgċmin. While the oxygen consumption (Vo2) increased progressively in both groups, the oxygen availability ratio (Do2/Vo2) and arteriovenous oxygen content difference (Cao2 — Cvo2) were maintained mainly by increased cardiac output in the dobutamine group and hemoglobin concentration in the dopamine group. Thirty minutes after termination of drug infusions, the Do2/Vo2 dropped, and Cao2 — Cvo2 increased significantly in both groups. These changes were mainly due to sustained high Vo2; however, in the dopamine group, a larger imbalance resulted from further decreases in cardiac output to levels below the control value.
