Paroxysmal Atrioventricular Block Related to Hypopolarization and Spontaneous Diastolic Depolarization

Paroxysmal atrioventricular block: clinical experience with 20 patients

“…Using canine models of acute ischemic injury, Scherlag et al [2] and El-Sherif et al [20] found paroxysmal AV block to be tachycardiadependent, with sites of block at both the His-Purkinje and AV node level. The tachycardia-dependent, or phase 3 block occurs due to abnormally prolonged refractoriness in the His-Purkinje system and prolongation of action potential duration (repetitive concealed conduction) [18,20,23]. Bradycardia-dependent, or phase 4 block, has also been reported as a mechanism of paroxysmal AV block [3,14,21], and it has been explained by prolonged recovery, hypopolarization and spontaneous diastolic depolarization in the injured distal conduction system [3,23].…”

“…The tachycardia-dependent, or phase 3 block occurs due to abnormally prolonged refractoriness in the His-Purkinje system and prolongation of action potential duration (repetitive concealed conduction) [18,20,23]. Bradycardia-dependent, or phase 4 block, has also been reported as a mechanism of paroxysmal AV block [3,14,21], and it has been explained by prolonged recovery, hypopolarization and spontaneous diastolic depolarization in the injured distal conduction system [3,23]. Rosenbaum et al [23] described 8 clinical cases of paroxysmal AV block and reviewed 10 additional cases from the literature; 17 of the 18 patients had an intraventricular conduction defect (right bundle branch block in 15).…”

Paroxysmal atrioventricular block precipitated by an atrial premature beat. What is the mechanism?

“…In this situation there is a relatively narrow range of rates between the critical heart rates for the development phase 3 and phase 4 blocks where normal conduction can occur [5,6].…”

“…Lee et al [1] reported 68 cases of bradycardiadependent AV block (30 cases [group I] were from the investigators' two institutions and 38 cases were added from the literature [group II]) [2][3][4][5]. Paroxysmal AV block (PAVB) was defined as the abrupt occurrence of pause-dependent (phase 4) AV block in a patient with apparently normal 1:1 AV conduction between the episodes of PAVB [1].…”

Does Intermittent Accessory Pathway Block during Slow Sinus Rhythm Always Imply a Low Risk for Rapid AV Conduction of Preexcited Atrial Fibrillation?

“…ation block, occurs when an impulse reaches a conduction fiber during its repolarization phase. Phase 4 block, also referred to as deceleration block, occurs when an impulse reaches a conduction fiber during a time of slow diastolic depolarization (automaticity) [6,7]. The combination of phase 3 and phase 4 block results in the 'accordion effect' illustrated by figure 2.…”