Apparent Unmasking of Left Anterior Fascicular Ventricular Tachycardia After Prior Left Posterior Fascicular Ablation: Possible Alternative Exit From a Shared Septal Purkinje Substrate

INTRODUCTION: Fascicular ventricular tachycardia (FVT) is a Purkinje-mediated ventricular tachycardia classically associated with verapamil sensitivity and reentry within the left ventricular conduction network. Although catheter ablation is usually effective, recurrent tachycardia with a different fascicular QRS axis after prior ablation raises important mechanistic questions. Contemporary models suggest that the septal fascicular network may provide a shared, interconnected substrate with multiple potential fascicular exits. CASE REPORT: We report a 39-year-old man with complete heart block, pacemaker dependence, and prior ablation for left posterior fascicular ventricular tachycardia (LPFVT), who presented with recurrent incessant ventricular tachycardia and severe left ventricular dysfunction (left ventricular ejection fraction, 15%). The presenting arrhythmia demonstrated right bundle branch block morphology with a right inferior axis, consistent with left anterior fascicular ventricular tachycardia (LAFVT). Electrophysiology study revealed a 1:1 relationship between conduction-system activation and ventricular activation, with the earliest Purkinje potential 60 ms pre-QRS near the proximal septum, approximately 1.5 cm below the His bundle. Radiofrequency ablation at this site resulted in transient acceleration, rapid tachycardia termination, and noninducibility. The patient had complete arrhythmia suppression and recovery of left ventricular function to 50%-55%. DISCUSSION: This case is best interpreted as apparent unmasking of an alternative fascicular exit from a shared septal Purkinje substrate after prior distal LPF ablation, rather than definitive proof of simple circuit transformation or rerouting. The septal fascicular network model provides a physiologically plausible framework for the emergence of a new fascicular VT morphology after ablation. However, alternative mechanisms remain plausible, including focal Purkinje automaticity, bundle branch/interfascicular reentry, and scar-related Purkinje VT. CONCLUSION: Recurrent FVT after prior distal fascicular ablation may reflect dynamic expression of a shared septal Purkinje network with alternative exits. Careful proximal septal and fascicular mapping is important, while mechanistic conclusions should remain appropriately cautious when definitive circuit evidence is lacking.