Delay and block of cardiac impulse caused by enhanced phase-4 depolarization in the His-Purkinje system.

The underlying mechanism of bradycardia-dependent bundle-branch and paroxysmal atrioventricular block appears to be enhancement of phase-4 depolarization in a branch or in a natural or acquired monofascicular pathway. Clinical records of these forms of impaired conduction occurring in the bundle-branches, with either longer or shorter cardiac cycle lengths, are presented and analysed. These also include the combination of Mobitz typw I atrioventricular block with variable degrees of bundle-branch block, as a representative example of narrow ventricular escape beats firing in the zone where prominent diastolic depolarization is present.