Paroxysmal atrioventricular block: Are phase 3 and phase 4 block mechanisms or misnomers?

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Paroxysmal atrioventricular block (PAVB), defined as the sudden and unexpected repetitive block of the atrial impulse on its way to the ventricles, is an important etiology of syncope primarily caused by the delayed emergence of an adequate escape rhythm.1 The majority of cases of PAVB develop in conjunction with increased rate of the atrial input to the atrioventricular (AV) conduction system, that is, TD-PAVB.2,3 On the other hand, there are published examples in which PAVB was initiated by a supraventricular pause.1,4–6 These cases have been labeled as either “bradycardia- dependent PAVB” or “phase 4 PAVB.” However, as demonstrated below, they are more accurately described as “pause-dependent PAVB” (PD-PAVB). In an unpublished series of 42 cases seen by one of us (NE-S), tachycardia-dependent PAVB (TD-PAVB) occurred in 37 patients (88%), while PD-PAVB occurred in the remaining five patients (12%). In many of these patients, the presenting symptoms were presyncope or syncope, depending on the duration of ventricular asystole. Yet in a few patients the early emergence of a stable escape rhythm prevented the development of significant symptoms. Although PAVB is a serious arrhythmia, its association with sudden cardiac death in some cases is difficult to ascertain. In a study of 22 patients with syncope, an implantable loop recorder revealed episodes of PAVB in association with syncope in 17 patients.7 None of these patients suffered injury attributable to syncopal relapse, and a majority of patients eventually received a permanent pacemaker. However, in some patients, syncope associated with PAVB can result in serious injury.