Ventricular fibrillation induced by atrial threshold search: a case report

Background: Implantable cardioverter defibrillators (ICDs) have been proven to reduce the risk of sudden cardiac death from ventricular tachyarrhythmias. However, ICDs can sometimes induce malignant arrhythmias. We describe a case of ventricular fibrillation triggered by an automatic atrial threshold search. Case summary: A 72-year-old man presented after a syncopal episode. His medical history included moderate aortic regurgitation and a symptomatic second-degree atrioventricular (AV) block, for which he received a dual-chamber pacemaker in 2013, later upgraded to a CRT-D due to pacing-induced heart failure. ICD interrogation revealed an episode of ventricular fibrillation terminated by a shock. The arrhythmia started directly after an atrial threshold search. Extensive work-up did not reveal a cause of the arrhythmia, therefore, we considered it most likely that the atrial threshold test triggered the ventricular fibrillation. Atrial Capture Management was disabled, and the patient was discharged. No further ventricular arrhythmias or ICD therapies were observed. Discussion: Automatic threshold measurement algorithms are intended to ensure effective myocardial capture and enhance safety but can sometimes inadvertently cause arrhythmias. The underlying mechanism in this case may be related to the switch from biventricular to right ventricular (RV)-only pacing during Atrial Capture Management, which increases dispersion in repolarization, facilitating early afterdepolarizations and triggering polymorphic tachycardias. Notably, newer ICD models mitigate this risk by maintaining biventricular pacing during this test. This case underscores the need for careful programming and monitoring of ICD algorithms.