Incomplete right bundle-branch block. An electrocardiographic enigma and possible misnomer.

Incomplete right bundle-branch block (IRBBB) usually is thought to be associated with abnormalities of the peripheral Purkinje system. This paper discusses the results of a study of six dogs from the same family and three nonrelated dogs with congenital IRBBB. Cardiac catheterization data were normal, and no evidence or history of cardiac disease was found before or after death. The depolarization sequence of ventricular epicardial activation was determined, and delays in right ventricular (RV) epicardial activation times were observed. Multipoint intramural electrodes were used to study intramural activation of the ventricular septum and free walls. The "electrical thickness" near the base of the RV mass was more than double that of the normal RV mass (9 vs 4 mm). Conduction along the right bundle branch (RBB) was analyzed during cardiopulmonary bypass, and electrograms recorded simultaneously from six sites along the RBB demonstrated that conduction velocity down the bundle was normal. The normal time of activation of the endocardial RV Purkinje fibers demonstrated that conduction in the right peripheral Purkinje system also was normal. Therefore, IRBBB in these dogs did not result from conduction abnormalities within the RV specialized conduction system. Interestingly, six of these dogs were members of the F1 generation from a mating of a female beagle with pulmonary stenosis and a male beagle with ventricular septal defect. Both defects as well as IRBBB were observed in the F2 generation. The present findings suggest that IRBBB may be a developmental variation in thickness of the RV free wall rather than an abnormality of the RV conduction system in cases without apparent heart disease. The developmental variant appears to have a genetic basis.