Hemodynamic responses in halothane-anesthetized horses given infusions of dopamine or dobutamine

The hemodynamic changes induced by constant infusions of dopamine or dobutamine (each 3, 5, and 10 micrograms/kg/min) were observed in halothane-anesthetized horses. Left ventricular dp/dt and cardiac output were increased in horses given dobutamine at dosage of 3 micrograms/kg/min and in those given either of the drugs at dosages of 5 and 10 micrograms/kg/min. Concomitant increases in systemic arterial blood pressure occurred at lower infusion dosage rates of dobutamine than those of dopamine and were modulated by dosage-related changes in peripheral vascular resistance that were different between the 2 drugs. Total peripheral vascular resistance was unchanged by the 3 micrograms/kg/min dosage of dopamine, significantly (P less than 0.05) decreased at the 5 micrograms/kg/min dosage of dopamine, and returned to base line in horses when given the largest dosage. Dobutamine had no effect on total peripheral vascular resistance. Heart rate was significantly (P less than 0.05) decreased by the 2 smaller dosages of dobutamine, but returned to base line at the 10 micrograms/kg/min dosage. Heart rates did not change significantly in response to dopamine over the range of the dosages. Sinus tachycardia with atrioventricular conduction block was noted in some horses given either of the drugs. The results of the present study indicate that the hemodynamic responses in horses given dopamine or dobutamine infusions are similar to those in other species, and are the net result of the different adrenergic pharmacologic profiles of the 2 drugs combined with autonomic nervous system reflexes.