COMPARATIVE CIRCULATORY EFFECTS OF ISOPROTERENOL AND DOPAMINE IN LAMBS WITH EXPERIMENTAL CYANOTIC HEART-DISEASE

To determine whether the hemodynamic responses to adrenergic agonists are altered during chronic hypoxemia secondary to an intracardiac right to left shunt, we studied seven lambs with surgically created pulmonic stenosis and atrial septal defect and nine controls during infusions of isoproterenol at 0.1 and 0.5-mu-g/kg/min and dopamine at 5 and 20-mu-g/kg/min. Isoproterenol increased heart rate by 89 +/- 17% in control but only 46 +/- 6% in experimental lambs (p < 0.05). However, because resting heart rate was higher in experimental lambs (213 +/- 7 versus 177 +/- 12 beats/min, p < 0.05), maximal heart rates were similar (310 +/- 7 versus 326 +/- 6 beats/min; NS). Cardiac output increased during isoproterenol from 219 +/- 20 to 425 +/- 54 mL/min/kg in experimental lambs (p < 0.05) and, similarly, from 180 +/- 20 to 425 +/- 71 in controls (p < 0.05) (experimental versus control; NS). Dopamine also increased cardiac output similarly in both groups, at both doses, but without changing heart rate. Isoproterenol did not alter aortic oxygen saturation and increased systemic oxygen transport more than oxygen consumption. In contrast, dopamine at both doses decreased aortic oxygen saturation in experimental lambs (rest, 71 +/- 2% versus dopamine, 59 +/- 2%; p < 0.05). With dopamine, the increase in systemic oxygen transport was equalled by an increase in oxygen consumption. Thus, circulatory responses to isoproterenol are similar in lambs with experimental cyanotic heart disease and controls, although higher resting heart rate in the experimental lambs reduces chronotropic reserve. Circulatory responses to dopamine are different, mainly due to the decrease in oxygen saturation in the experimental lambs. Inasmuch as the oxygen cost/benefit ratio of adrenergic agonists may be altered by chronic hypoxemia and associated hemodynamic abnormalities, further evaluation of this ratio in hypoxemic human infants receiving these agents is indicated.