Alternation of right bundle branch block with beats showing normal conduction and with a less complete right bundle branch block pattern.

We report the case of an 80-year-old man with a preexistent right bundle branch block (RBBB) who underwent dobutamine stress echocardiography. As the sinus cycle length gradually decreased during the stress test, he first developed a 2:1 RBBB pattern alternating with beats showing normal conduction and then an alternating 2:1 less complete and more complete RBBB pattern. An elucidation of the potential mechanism of the latter very rare phenomenon is provided.