Electrophysiological studies in patients with rate-related intermittent left bundle-branch block.

Two patients who developed left bundle-branch block with increase in heart rate are presented. Atrial pacing during His bundle recording studies showed differing responses in the ventricular specialized conduction system of the two patients. In one, the onset of left bundle-branch block was associated with no change in the previously normal HVinterval (so msec), while in the second the HV interval doubled to 8o msec during the development of left bundle-branch block. These two responses are discussed in the light of the electrophysiology of the ventricular specialized conduction system, and suggest that certain patients with rate-related left bundle-branch block have trifascicular disease.