Variations in the critical cycle length inducing rate dependent bundle branch block.

In five patients with tachycardic RDBBB (rate dependent bundle branch block), variations in the critical CL (cycle length) inducing RDBBB were investigated. In one patient, independent of changes in the preceding CL, the critical CL inducing RDBBB was invariable, extending over two years. In the other patients, the critical CL inducing RDBBB varied within a comparatively short period. However, when RDBBB was maintained for only a few cycles, the critical CL allowing reversion to normal conduction was always longer than the critical CL inducing RDBBB here, in all patients except one. In one patient such a distinct difference in CL could not be disclosed because the critical CL inducing RDBBB varied from cycle to cycle. These observations suggest that independent of variations in the refractory period of the blocked bundle branch, the critical CL allowing reversion to normal conduction was considerably longer than this refractory period in all patients. The possible mechanisms by which RDBBB could be maintained are discussed. The most probable mechanism appears to be "true" unidirectional block in the affected bundle branch, in which the refractory period for the retrogradely conducted impulse is shorter than that for the anterogradely conducted impulse.