Modern management of congestive heart failure (CHF) employs inotropic drugs, vasodilators, and diuretics. Although pharmacologic classification of drugs is possible in animals, identification of predominant hemodynamic mechanisms in humans is more complex, because many effects of vasodilators and inotropic drugs are similar. We compared the effects of a vasodilator, prazosin, and two agents with both inotropic and vasodilatory properties, amrinone and pirbuterol, on cardiac index (CI), mean aortic pressure, left ventricular stroke work index (LVSWI), LV filling pressure (LVFP), systemic vascular resistance, LV ejection fraction (LVEF), and myocardial O2 consumption (MVO2) in 34 patients with advanced CHF. We concluded that (1) a rise in CI and LVEF, together with a fall in LVFP, does not necessarily indicate an inotropic effect; (2)both CI and LVEF may be increased by an inotropic mechanism in advanced CHF without a rise in MVO2; and (3) a drug-induced rise in LVSWI with stable or lower LVFP suggests an inotropic mechanism of action.