[Rapid ventricular pacing in dogs--the natural history and pathology]

Six dogs were rapidly paced for 4 weeks with a VVI pacemaker, and observed for 4 to 6 weeks after cessation of rapid pacing. Echocardiographic study revealed congestive heart failure at the end of rapid pacing associated with enlarged left ventricles and reduced left ventricular ejection fraction. Four weeks after cessation of rapid pacing, the left ventricular cavity and ejection fraction improved, but did not return to the normal pre-paced values. Six weeks after the cessation, cardiac function recovered, and pathological study of the resected hearts demonstrated normal myocardium without fibrotic or ischemic changes.