The Circulatory Effects of Electrically Induced Changes in Ventricular Rate at Rest and during Exercise in Complete Heart Block *

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Abstract
Fourteen patients with complete heart block (13 acquired and 1 congenital) were studied at rest and during exercise in the supine position, both at idioventricular and various electrically stimulated heart rates. Cardiac output at rest was abnormally low, despite an increased stroke volume, and the response of cardiac output to exercise was greatly impaired. Pulmonary arterial and wedge pressures were abnormally high at rest and during exercise. At faster, electrically stimulated rates, the cardiac output at rest rose to normal levels and there was an improvement in the response of cardiac output to exercise. The optimal rate at rest usually lay between 70 and 83 beats/minute. The resting oxygen uptake was normal at idioventricular rates but increased significantly when the heart rate rose. The hemodynamic abnormalities in these patients appear to be due to a combination of bradycardia and impaired myo-cardial contractility.