Antidromic Atrioventricular Reentrant Tachycardia Dependent on a Unidirectional Left Anterior Accessory Pathway Mimicking Peri-mitral Ventricular Tachycardia: Successful Ablation via a Transseptal Approach.

Antidromic atrioventricular reentrant tachycardia (aAVRT) is rare compared to orthodromic atrioventricular reentrant tachycardia (oAVRT). An aAVRT that is dependent on a unidirectional, decremental accessory pathway (AP) is even rarer. Idiopathic ventricular tachycardias (iVT) that have benign prognoses and respond well to medical therapy can be confused with aAVRTs dependent on APs having ventricular insertion sites close to the iVT focus and have a real risk of sudden death. The preferred approach of ablation for such tachycardias with anterograde conduction only is a retrograde aortic approach, which facilitates the mapping of the earliest ventricular activation during atrial pacing or tachycardia from the ventricular side. This, however, necessitates access to the arterial system with accompanying complications. We describe herein the case of a wide complex tachycardia, which was treated initially as VT with intravenous lidocaine. The baseline electrocardiogram (ECG) did not show preexcitation. An electrophysiology study (EPS) revealed a left anterior AP that conducted anterograde only. AVRT was easily inducible at a cycle length of 290 ms. Successful ablation was undertaken via the transseptal approach without recurrence.