Abstract
Six horses were anesthetized with isoflurane and positioned in dorsal recumbency. End expiratory isoflurane concentration was maintained at 1.7±0.1%. Four doses of dobutamine (0.5, 1.0, 2.0 and 4.0μg/kg/min) were intravenously infused and hemodynamic responses, hematocrit and serum protein were measured. Cardiac output was significantly increased by dobutamine infusions at doses of 2.0 and 4.0μg/kg/min. Stroke volume was also increased dose dependently from 0.5 to 2.0μg/kg/min of dobutamine, however the value was not elevated at 4.0μg/kg/min. Heart rate increased at doses of 2.0 and 4.0μg/kg/min of dobutamine infusions. Mean heart rate was elevated to 71.2±16 bpm by 4.0 μg/kg/min of dobutamine. Accordingly, increased cardiac output at a dose of 4.0μg/kg/min was presumed to be due to the increases in heart rate. Systemic vascular resistance increased at doses of 0.5 and 1.0μg/kg/min, but the values at doses of 2.0 and 4.0μg/kg/min were lower than those at smaller doses of dobutamine. Mean arterial blood pressure significantly and dose dependently increased by dobutamine infusion. Therefore, mechanisms for the increases of mean arterial blood pressure by dobutamine at smaller doses (0.5, 1.0μg/kg/min) and larger doses (2.0, 4.0μg/kg/min) were different. Increases in systemic vascular resistance would be the main cause at smaller doses, but those of cardiac output at larger doses. Hematocrit significantly and dose dependently increased by dobutamine infusions, but only slight increase in serum total protein was observed.
