Bundle branch block on alternate beats: by what mechanism?

In a patient with right bundle branch block occurring on alternate beats during regular sinus rhythm, the conduction disturbance disappeared during hyperventilation induced increase in heart rate, and reappeared with slight slowing of the sinus rate due to carotid sinus massage. The following mechanisms are potentially involved in the electrogenesis of bundle branch block alternans with regular RR intervals: a) phase-3 2:1 bidirectional block; b) phase-3 antegrade block with retrograde concealed activation of the involved bundle branch and subsequent "supernormal" conduction; and c) phase-4 antegrade block with transseptal retrograde concealed invasion of the affected bundle branch by impulses traversing the unimpaired bundle branch. Analysis of the tracing excluded both mechanisms a and b and favored bradycardia-dependent right bundle branch block as a key to explain the alternate pattern of intraventricular conduction.