CONCEALED RIGHT BUNDLE BRANCH BLOCK IN PRESENCE OF TYPE B VENTRICULAR PRE-EXCITATION

A 10-year-old boy is presented in whom complete RBBB is masked by Type B WPW syndrome. The spontaneous occurrence of this phenomenon parallels what has been demonstrated in the catheterization laboratory by stimulating the ventricular septum and producing RBBB only when Type B WPW is absent. This finding supports the contention that pre-excitation is conducted by means of a separate accessory conduction pathway connecting the right atrium and right ventricle. The presence of simultaneous Type B pre-excitation and right intraventricular block reported in patients with Ebstein's disease is explained as resulting from peripheral conduction defects rather than central bundle branch block. © 1969.