Wide complex tachycardia with two QRS morphologies: what is the mechanism?

Clinical introduction

A man in his 50s presented to the emergency department with palpitations. He had a background of arrhythmic right ventricular cardiomyopathy. He had previously undergone the placement of a dual-chamber implantable cardioverter-defibrillator for atrioventricular block and ventricular tachycardia (VT). The device was programmed in a dual-chamber rate-modulated pacing mode with a lower rate of 80 beats per minute (bpm), an upper track/sensor rate of 120 bpm, paced atrioventricular delay (P-AVD) and sensed atrioventricular delay of 190 and 150 ms, respectively, ventricular safety pacing on and mode switch on. His regular medications included flecainide and bisoprolol.

A 12-lead ECG was obtained demonstrating a ventricular rate of 133 bpm (figure 1A). Following the infusion of intravenous magnesium and amiodarone, the …