Mahaim Tachycardia Mimicking Ventricular Tachycardia

BACKGROUND: Atypical accessory pathways can closely mimic the electrocardiographic features of idiopathic ventricular tachycardia from the tricuspid annulus. CASE SUMMARY: A 39-year-old man with a history of previous unsuccessful radiofrequency ablation for a presumed diagnosis of idiopathic ventricular tachycardia presented with symptomatic wide QRS complex tachycardia. A subsequent redo procedure using high-density mapping revealed the true arrhythmia mechanism to be an atypical Mahaim pathway with no discernible atrial insertion. DISCUSSION: The misdiagnosis of wide QRS complex tachycardia often results in failed ablation attempts. High-density mapping plays a pivotal role in redefining the arrhythmia substrate, uncovering the true mechanism, and guiding effective ablation; it should be recommended, especially in redo procedures. TAKE-HOME MESSAGES: The origin of some atypical Mahaim pathways can be distinctly subvalvular, rather than purely annular. High-density mapping is essential for a comprehensive evaluation of the subvalvular apparatus. Identification of a broad earliest activation site at specific anatomical locations should prompt a more rigorous investigation into the underlying electrophysiologic mechanism.