Upper septal transformation of verapamil-sensitive idiopathic left ventricular tachycardia during catheter ablation: Wolf in sheep׳s clothing?☆

Introduction Verapamil-sensitive fascicular ventricular tachycardia (VT) is the commonest form of idiopathic left ventricular tachycardia (ILVT). The characteristic diagnostic features of fascicular ILVT include: (i) induction with atrial pacing, (ii) right bundle branch block (RBBB) with left-axis deviation morphology during VT, (iii) absence of structural heart disease, and (iv) verapamil sensitivity. Fascicular ILVTs are classified according to the QRS morphology into 3 subtypes: (i) left posterior fascicular (LPF) VT, exhibiting an RBBB with left-axis deviation electrocardiogram (ECG) pattern (90% of all fascicular ILVT); (ii) left anterior fascicular VT, exhibiting an RBBB with right-axis deviation ECG pattern (10% of all fascicular ILVT); and (iii) upper septal (US) fascicular VT, exhibiting a narrow QRS pattern with normal or right-axis deviation (very rare; o1% of all ILVT). The US variant of ILVT can be mistaken for supraventricular tachycardia (SVT), and in certain situations it can be incessant, resulting in tachycardia-induced cardiomyopathy. In a multicenter registry of 193 verapamil-sensitive ILVT subjects, 12 subjects (6.2%) were found to have the US variant, of whom 6 (50%) had previously undergone catheter ablation for the common LPF variant. We present an interesting and hitherto unreported case of fascicular ILVT