Rhythm control strategies in patients with atrial fibrillation (AF) can bring many clinical benefits. However, there is still uncertainty regarding selection of the optimal rhythm control strategy for persistent AF. Chronicity, substrate alteration, and underlying bradyarrhythmias could influence the clinical outcomes. Current guidelines do not provide a distinct recommendation for electrical cardioversion in patients with AF with a slow ventricular response (SVR). We present two cases of sudden cardiac arrest due to sustained ventricular tachycardia/fibrillation after electrical cardioversion of persistent AF with SVR.