Paroxysmal Atrioventricular Block Related to Hypopolarization and Spontaneous Diastolic Depolarization

Eight cases of paroxysmal atrioventricular block (PAVB), and ten similar cases from the literature were analyzed. Atrioventricular block was shown to occur both at rapid and slow rates, with an intermediate normal conduction range. The tachycardia-dependent AY block was related to prolonged recovery and was termed “phase-3 block;” the bradycardia-dependent AV block was related to hypopolarization plus spontaneous diastolic depolarization and was termed “phase-4 block.” However, the PAVB truly began and was maintained when a single P wave fell on the phase-4 block range. Therefore, this form of PAVB can be termed “phase-4 PAVB.” Normal AV conduction was reestablished only after a ventricular escape beat, when a subsequent P wave fell on the normal conduction range. This indicated that the escape beat activated the injured region, starting the whole process anew. In phase-4 block, as well as in phase-3 block, retrograde conduction tends to be preserved at a time when orthograde conduction has already failed. Diffuse involvement of the intraventricular conduction system was documented in most cases. The critically injured region was most likely intranodal in location. Other explanations, such as supernormal conduction, vagal effects, and Wedensky effect and facilitation, are discussed.