Paroxysmal Heart Block Following Septal Alcohol Ablation: What is the Mechanism?

A 79-year-old woman with hypertrophic obstructive cardiomyopathy and paroxysmal atrial fibrillation underwent alcohol septal ablation. She had a normal QRS complex at baseline, but immediately after the procedure, right bundle branch block (RBBB) and left posterior fascicular block developed (Figure 1). The next day, it changed to RBBB and left anterior fascicular block (Figure 2). The same morning, the patient felt severely lightheaded and became presyncopal while in bed, and that corresponded to an abnormal telemetry finding (Figure 3). As the telemetry tracing showed, the patient had initial sinus rhythm with normal PR interval, but RBBB and left anterior fascicular block. Subsequently, a premature junctional beat with a retrograde P wave (* in Figure 4) led to sinus rhythm with complete heart block without any escape beat for 9 seconds. The patient underwent a dual-chamber pacemaker implant the next day, and has done well.

Pause-dependent atrioventricular (AV) block, or phase 4 block, is uncommon and occurs in a diseased infra-Hisian conduction system.¹ At phase 4, or diastolic phase, diseased Purkinje cells exhibit spontaneous depolarization, a property only seen in pacemaker cells of the heart but not normal Purkinje cells. If a supraventricular or ventricular impulse comes at a later phase of this depolarization curve, it cannot reach the diseased His-Purkinje system, as the sodium channels are inactive (Figure 4). For the same reason, subsequent impulses cannot depolarize the diseased conduction system tissue, and asystole ensues. The impulse that can set up for the recurring complete heart block can be a premature atrial,² ventricular,³ or in this case, a junctional beat. The asystole typically persists until an appropriately timed escape beat or premature beat, which resets the transmembrane potential to maximum resting value to allow conduction again. 

Disclosure: Dr. Shen has no conflicts of interest to report regarding the content herein.   

1. Lee S, Wellens HJ, Josephson ME. Paroxysmal atrioventricular block. Heart Rhythm. 2009;6:1229-1234.
2. Atreya AR, Cook JR, Fox MT. Pause-dependent paroxysmal phase-4 atrioventricular block. BMJ Case Rep. 2015 Oct 16;2015.
3. Divakara Menon SM, Ribas CS, Ribas Meneclier CA, Morillo CA. Intermittent atrioventricular block: what is the mechanism? Heart Rhythm. 2012;9:154-155.