Vascular Responses to Dopamine and Dobutamine in the Awake Calf During Constant Aortic Flow or Constant Aortic Pressure

We investigated vascular effects of dopamine and dobutamine infusions in awake calves implanted with the Penn State total artificial heart (TAH), This preparation uniquely permits independent servo-control of cardiac output (CO) and arterial blood pressure (BP). Thirty-two studies (22 with dopamine and 10 with dobutamine) were performed in four juvenile calves from 1 to 4 months after TAH implantation. Studies were performed in one of two TAH operating conditions(a) constant aortic flow, in which the CO is fixed and aortic BP varies with systemic vascular resistance (SVR), or (b) constant pressure, in which the CO varies to maintain a constant BP when SVR changes. During both constant flow and constant pressure studies, dopamine caused a dose-dependent increase and dobutamine caused a dose-dependent decrease in SVR. There was no difference in the SVR response between constant flow or constant pressure conditions at any dose of dopamine or dobutamine (p > 0.05). The infusion doses of dopamine required to rajse the SVR 20 and 50% during constant flow studies were 7.2 and 12.4 μg ± kg ± min, respectively. In constant pressure studies, these doses were 7.7 and 13.5 μg ± kg ± min. The infusion dose of dobutamine resulting in a 20% reduction in SVR was 27.2 μg ± kg ± min in constant flow studies 26.2 μg ± kg ± min in constant pressure studies. These data suggest that baroreflex and other indirect mechanisms are less important than direct vascular drug effects in this system. This in vivo preparation permits study of vascular effects of drugs because the vasculaiure is isolated functionally from the heartCentral nervous system (CNS) and hormonal control of vascular resistance remain intact but the link between heart function and the blood vessels is interrupted.