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ABSTRACT: Inotropic and lusitropic reserves of failing hearts were investigated using a stepwise intravenous dobutamine loading test in patients with chronic heart failure. The forty-one patients enrolled in this study were classified into 4 groups according to the left ventricular end-systolic volume indices (LVESVI). The left ventricular size of Group 1 was within the normal range (LVESVI=37.0±3.5ml/m^2), and this group served as the control. LVESVI of Group 2 was mildly dilated (53.9±6.4ml/m^2), that of Group 3 was moderately dilated (77.3±4.7ml/m^2), and Group 4 was severely dilated (120.0±34.9ml/m^2). Left ventricular contractility and relaxation properties were estimated by left ventricular maximum positive dP/dt and minimum negative dP/dt. At the basal condition, both positive dP/dt and the absolute value of negative dP/dt were low in patients with chronic heart failure in accordance with LVESVI. Acceleration of positive dP/dt by dobutamine infusion was more attenuated in Group 4 than that of Group 1. However, responses of negative dP/dt were identical among the 4 groups. Thus, there was a dissociation between inotropic and lusitropic reserves of human failing hearts for β-adrenergic stimulation. These findings imply that a lusitropic reserve is preserved in the failing left ventricles in spite of an impairment to the inotropic reserve.