A case of alternating bundle branch block in combination with intra-Hisian block.

We describe a 66-year-old woman who had an alternating bundle branch block consisting of coexisting occurrence of right bundle branch block (RBBB) and left bundle branch block (LBBB) combined with Mobitz type II atrioventricular block (AVB). A prolonged PQ interval was associated with the RBBB pattern whereas it was not apparent in the LBBB pattern. Electrophysiologic study revealed that the LBBB pattern was combined with a double His bundle potential. On the other hand, the RBBB pattern was combined with a markedly prolonged HV interval with a low voltage monophasic His bundle potential, which we speculated was the former part of the split His bundle potential seen during the LBBB pattern. A combination of the longitudinal dissociation in the His bundle and the gap phenomenon at the intra-Hisian block portion may account for this observation.