The mechanism of intermittent bundle branch block: relationship to prolonged recovery, hypopolarization and spontaneous diastolic depolarization.

Fourteen cases of intermittent bundle branch block (BBB) are reported, in which Mock was shown to occur both at rapid and slow rates, with an intermediate normal conduction range. Tachycardia-dependent BBB was related to prolonged recovery and was termed "phase-3 block;" bradycardia-dependent BBB was related to hypopolarization plus spontaneous diastolic depolarization (SDD), and was termed "phase4 Mock." Two critical rates do exist; one, separating phase3 Mock from normal conduction; the other, separating normal conduction from phase-4 block. The range of normal conduction seems to be inversely related to the amount of injury. It may be extremely narrow when recovery is greatly prolonged, and/or when hypopolarization is severe, and/or when SDD is enhanced. This may simulate supernormality. In other patients, or in the same patient, the normal conduction range may widen up to several seconds, at the expense of both phase3 and phase-4 Mock (accordion-like effect). Deeply inverted T waves correlated well with phase-3 and phase-4 block. Bayley's theory of conventional myocardial injury applies remarkably well to the analysis of injury of the intraventricular conducting fascicles. The only difference is that SDD plays an important additional role in the conduction system.