Fentanyl, a synthetic opioid, is considered to minimally effect left ventricular performance and is a frequently administered analgesic for patients with decreased ventricular function. Some patients, such those with severe heart failure or shock or those weaned from cardiopulmonary bypass, may also require catecholamine therapy, commonly epinephrine or phenylephrine infusion. Investigators have separately evaluated the hemodynamic effects of fentanyl or these catecholamine therapies, and the effect of fentanyl on circulating catecholamine level. However, the effect of fentanyl on the cardiovascular response to exogenous catecholamine therapy has not been fully evaluated. Therefore, whether the cardiovascular effects of catecholamine infusion in subjects treated with fentanyl can be extrapolated from effects of these infusions in the absence of fentanyl is not known. We previously evaluated differences in the cardiovascular effects of epinephrine or phenylephrine when given with or without fentanyl. With no catecholamine infusion, fentanyl had no effect on cardiovascular function. However, in animals treated with epinephrine infusion, fentanyl depressed ventricular function as demonstrated by decreased cardiovascular function with no decrease in circulating epinephrine level. Fentanyl also inhibited the MAP (and MPAP) response to phenylephrine again with n decrease in endogenous catecholamine levels. We are now investigating the interaction of fentanyl and isoproterenol, a more specific beta-adrenergic agonist than epinephrine, in order to isolate the effects of fentanyl to suppression of beta-receptor function.