A tale of two QRS: Misleading irregular tachycardia in an ICD carrier

We report a 52-year-old man with coronary artery disease, reduced left ventricular ejection fraction, and a cardiac resynchronization therapy-defibrillator (CRT-D), admitted to the emergency department for palpitations. Surface ECG showed a wide-complex tachycardia with a 2:1 ventricular-to-atrial (V:A) relationship. Device interrogation revealed episodes detected in the ventricular tachycardia monitor zone, according to the atrioventricular discrimination algorithms. Detailed analysis of surface ECG and device intracardiac electrograms was performed, followed by an electrophysiological study and catheter ablation of the responsible circuit of the tachycardia. This case illustrates the diagnostic complexity and differential diagnosis of tachycardias presenting with a 2:1 V:A relationship and highlights the importance of integrating surface ECG, device tracings, and electrophysiological findings to correctly identify the underlying mechanism, to avoid misclassification by device algorithms, and to treat the arrhythmia effectively.