Beyond QRS Widening: Flecainide Toxicity as a Novel Cause of Pacemaker Failure: A Case Report

Background: Flecainide is a class Ic antiarrhythmic agent used for the management of atrial fibrillation. However, it has a narrow therapeutic index and can cause significant toxicity, particularly in patients with renal impairment. We present a case of flecainide intoxication in a pacemaker-dependent patient, resulting in massive pacemaker dysfunction. Case Report: A 62-year-old male with a history of atrial fibrillation and sick sinus syndrome, managed with flecainide and a dual-chamber pacemaker, presented to the emergency department following a syncopal episode. He had recently been diagnosed with renal impairment due to dehydration. Initial ECG revealed a markedly widened QRS complex and absence of sinus rhythm. Pacemaker interrogation showed significantly increased atrial and ventricular thresholds (2V and 5V, respectively) and reduced device longevity despite recent implantation. A sodium bicarbonate test resulted in significant clinical improvement, supporting the diagnosis of flecainide toxicity. Conclusions: Flecainide toxicity can lead to life-threatening pacemaker dysfunction, particularly in patients with renal impairment. Prompt recognition and treatment with sodium bicarbonate can reverse the toxic effects and restore pacemaker function.