Eighteen patients with right ventricular anomalous muscle bundles associated with ventricular septal defect and one patient with isolated anomalous muscle bundle were studied. The clinical differentiation of these patients from those with isolated ventricular septal defects or tetralogy of Fallot was difficult. Eleven patients were asymptomatic at the time of diagnosis; eight had various degrees of dyspnea on exertion, with three of these having cyanosis. A physiologically split second sound

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... h a pulmonic component of normal intensity and a diffuse thrill and holosystolic murmur maximal at the lower left sternal border suggest the diagnosis. Chest X-rays and electrocardiograms usually show evidence of right ventricular hypertrophy, but may suggest left ventricular hypertrophy in the presence of a large left-to-right shunt. However, the diagnosis is best made by cardiac catheterization with measurement of a pressure gradient in the body of the right ventricle and by selective angiography with the demonstration of a wedge-shaped filling defect in the right ventricle. Progressive obstruction by the muscle bundle occurred in four of seven patients who had repeated catheterization associated with the development of electrocardiographic changes of right ventricular hypertrophy. To date, 11 patients have had surgery; one died before surgery; and surgery is being planned for the remaining patients.