Bundle branch block on alternate beats during atrial fibrillation.

This article reports a case of tachycardia-dependent right bundle branch block (RBBB) occurring during atrial fibrillation. In some sections of the recording, an alternans occurs between complexes with a complete RBBB pattern and complexes showing normal intraventricular conduction or incomplete RBBB. Alternans is frequently observed during phases of fast and nearly regular rhythm, but it occurs even in the presence of a markedly irregular ventricular response. The RBBB alternans associated with short and regular RR intervals is likely to represent a manifestation of 2:1 bundle branch supernormal conduction, whereas alternans occurring with irregular cycles expresses a complex interaction between the RR cycle length and some mechanisms affecting intraventricular conduction, such as tachycardia-dependent bundle branch block, supernormal conduction and concealed retrograde activation of the anterogradely blocked bundle branch (the so-called "linking" phenomenon).