Rhabdomyoma of the heart producing right bundle branch block.

A 4-day-old female had an uneventful prenatal course and delivery. Immediate cyanosis improved with oxygen therapy. Cardiomegaly on chest radiograph, right bundle branch block on electrocardiogram, and apparent increased tricuspid valve excursion on echocardiography led to a diagnosis of Ebstein's anomaly. Death occurred unexpectedly. Autopsy demonstrated a rhabdomyoma of the interventricular septum, which had produced obstruction to right ventricular outflow and had interrupted the right bundle branch of the conducting system. It appears that the massive size of the septal tumor led to its misdiagnosis on the echocardiogram.