Background: In the failing human heart contractile reserve during tachycardia is attenuated or absent. However, it is not known whether during tachycardia diminished inotropic reserve depends on the degree of ventricular dysfunction or lusitropic reserve is also diminished in patients with left ventricular (LV) dysfunction. Methods: We studied 18 patients with dilated cardiomyopathy or mildly depressed LV function and 13 subjects in a control group (ejection fraction 0.67 ± 0.09). The patients were classified into two groups based on whether their ejection fraction was less than or more than 0.40 (group 1, ejection fraction 0.27 ± 0.05; group 2, ejection fraction 0.49 ± 0.07). LV pressures were measured with a catheter-tip manometer during incremental right atrial pacing up to a heart rate of 150 beats/min. Results: With incremental pacing LV peak positive dP/dt rose progressively in both the normal group and in group 2, but the increase was less for group 2 than for the normal group; in group 1 the increase was slight or absent. In contrast, a significant and progressive decrease occurred in the time constant of LV relaxation in all three groups. Although their values remained significantly different at each heart rate, no intergroup differences in absolute or percent changes were present. Conclusions: These findings suggest that during tachycardia LV inotropic reserve may be diminished depending on the degree of ventricular dysfunction, and lusitropic reserve may be preserved in patients with depressed function despite an attenuated inotropic response. (Am Heart J 1998;135:584-91.)