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

Abstract

A 65 year-old woman presented with two epi-sodes of syncope. She developed paroxysmal atrio-ventricular (AV) block when an ECG was being re-corded (Fig. 1). She developed at least one othersimilar episode. Then, she developed complete AVblock with a stable escape ventricular rhythm ofabout 30 bpm during which an atrial premature beatwas conducted (Fig. 2). Isoproterenol was admi-nistered which increased the ventricular rate to47 bpm. The patient remained stable and laterreceived a permanent dual chamber pacemaker.Lee et al. [1] reported 68 cases of bradycardia-dependent AV block (30 cases [group I] were fromthe investigators’ two institutions and 38 caseswere added from the literature [group II]) [2–5].Paroxysmal AV block (PAVB) was defined as theabrupt occurrence of pause-dependent (phase 4) AVblock in a patient with apparently normal 1:1 AVconduction between the episodes of PAVB [1]. Mostpatients present with syncope or near-syncope.There were 19 of 68 patients (28%) with a normalECG as in our patient (Fig. 1). Thus, a normal ECGdoes not rule out PAVB. This form of AV block canbe precipitated by atrial or ventricular prematurebeats, His bundle extrasystoles, sinus slowing ei-ther spontaneous or induced by carotid sinus mas-sage and rarely after supraventricular tachycardiaby overdrive suppression upon termination. In