Electrocardiographic abnormalities in Ebstein's anomaly. Deductions based on comparison with endomyocardial fibrosis.

Probable causes of ECG abnormalities in Ebstein's anomaly were investigated by comparing tracings from 18 young patients with the disease (group I) and 20 age-matched patients with a morphologically similar cardiopathy: right ventricular endomyocardial fibrosis (group II). Tall p waves (greater than or equal to 2.5 mm) occurred in about a third of the patients in each group and were attributable to right atriomegaly. 8 patients, 4 from each group, had prolonged P-R intervals (greater than or equal to 17 s) resulting from increased P-R segment (7 cases) and prolonged P-wave duration (4 cases). Right bundle branch block was, however, more prevalent in group I (44%) than in group II (5%), and is thought to result mainly from a paucity of conduction fibres in the atrialized right ventricle in Ebstein's anomaly and partly, in group II especially, from septal fibrosis. In both groups R-wave deflections in V3R and V1 were reduced, probably because of a clockwise cardiac rotation and paucity of right ventricular muscle mass.