V-on-A Tachycardia Misclassified as Ventricular Tachycardia by Atrial Blanking

BACKGROUND: Dual-chamber implantable cardioverter-defibrillator (ICD) discrimination algorithms using the atrial-to-ventricular event ratio predictably misclassify supraventricular tachycardias with a very short ventriculoatrial (VA) interval as ventricular tachycardia (VT). CASE SUMMARY: A 75-year-old man with nonischemic cardiomyopathy and a dual-chamber ICD had 74 episodes of regular 1:1 tachycardia labeled as nonsustained VT/ventricular fibrillation; some episodes triggered antitachycardia pacing. Stored electrograms were analyzed using 4 questions: how the tachycardia initiated, its response to ventricular overdrive pacing, the direction of cycle-length wobble, and how it terminated. These findings excluded VT and identified typical slow-fast atrioventricular nodal re-entrant tachycardia, confirmed on electrophysiology study and treated with slow-pathway modification. DISCUSSION: Tachycardias with short VA intervals may produce a V-on-A pattern with atrial blanking or undercounting, causing ratio-based ICD algorithms to favor VT. TAKE-HOME MESSAGE: A 4-question electrogram approach can distinguish short-VA supraventricular tachycardia from VT, prevent inappropriate therapy, and guide definitive electrophysiologic evaluation.