Aerobic and hemodynamic performance, at rest and during exercise, has been explored in patients with either atrial synchronous ventricular or respiratory-dependent ventricular pacing. In patients with chronic AV block and normal left ventricle, the cardiac output, VO2 at maximal exercise, and work tolerance increased significantly with rate-responsive pacing in comparison with fixed ventricular pacing. Chronic AV block and left ventricle dysfunction in patients with respiratory-dependent ventricular pacing improved maximal cardiac output, VO2 anaerobic threshold, and physical work capacity. The benefit of rate-responsive pacing was found to be significantly greater in patients with left ventricle dysfunction than in patients with normal left ventricle function.