Introduction
Atrial fibrillation (AF) is the most common arrhythmia, and the incidence and prevalence are increasing.1 Recently, the benefits of rhythm control strategies in AF patients has been spotlighted to a greater extent.2 For successful rhythm control and prevention of complications, appropriate patient selection is essential. In particular, the rhythm control strategy in AF patients with slow ventricular response (SVR), who might have underlying sick-sinus syndrome (SSS) or atrioventricular (AV) conduction disturbance, requires greater attention. In these patients, pacemaker implantation is an established treatment modality when the HR is less than 40 bpm. The management of symptomatic AF patients with SVR, in whom heart rate (HR) is in the gray zone between 40 and 60 bpm, is uncertain. There is no definitive recommendation for electrical cardioversion in patients with AF with SVR, and the outcome of rhythm control is unknown for these populations. We present two cases of sudden cardiac arrest with sustained ventricular tachycardia/fibrillation (VT/VF) after electrical cardioversion of persistent AF with SVR.