Dopamine-Induced Alterations in Left Ventricular Performance

Left ventricular responses to dopamine infusion of 1 to 5, 8, and 10 µg/kg per min for 30 minutes were studied in 14 anesthetized intact dogs. Cardiac output (dye dilution) and stroke volume increased progressively while the end diastolic volume (thermodilution) remained constant, signifying more complete ventricular ejection and resulting in a progressive decline in end systolic volume. Heart rate remained unchanged as did aortic mean pressure, but in the presence of a greatly increased systemic blood flow, the calculated peripheral vascular resistance declined progressively. A linear dose-dependent increase in both systolic ejection rate and circumferential shortening rate suggested directionally similar changes in myocardial fiber shortening. Isovolumic contraction was characterized by a marked increase in the peak velocity of left ventricular pressure rise and presumably in the velocity of force generation without change in the maximum pre-ejection force. These results indicate that dopamine increased myocardial contractility. At these infusion doses dopamine differs from norepinephrine and isoproterenol in causing little change in systemic blood pressure or heart rate in the intact dog. The steady heart rate, stable blood pressure, and constant end diastolic volume suggest that the inotropic response to dopamine administration was due to a direct myocardial action which was otherwise unmodified by the preload or afterload of ventricular contraction.