Mahaim fiber-mediated tachycardia.

We present the case of a previously healthy 42-year-old man who attended the emergency department due to a sudden onset of rapid and regular palpitations. The ECG showed 190 bpm, wide QRS with left bundle branch block tachycardia. He was started on amiodarone with progression to 230 bpm, wide QRS tachycardia with multiple morphologies, followed by spontaneous conversion to sinus rhythm, normal PR interval and rS pattern in LIII. The echocardiogram was negative for structural heart disease. The electrophysiological study demonstrated the presence of an accessory pathway with anterograde decremental conduction and no retrograde conduction. Both episodes of clinical tachycardia were induced. A diagnosis of Mahaim fiber-mediated antidromic atrioventricular reentrant tachycardia and pre-excited atrial fibrillation was made. Mapping was performed with detection of an M potential (His-like) at the lateral region of the tricuspid ring followed by radiofrequency ablation with immediate success criteria. Post-ablation there was a change to a qR pattern in LIII. At 12-months follow-up there was no recurrence of the tachycardia.