Abstract
Atrial arrhythmias are frequent and potentially life-threatening late sequelae in adults with Fontan physiology. We present the case of a 36-year-old woman with an intracardiac lateral tunnel Fontan and prior Amplatzer Septal Occluder closure of a residual fenestration, who developed recurrent, drug-refractory supraventricular tachycardia. High-density electroanatomical mapping revealed a macro-reentrant flutter circuit within the conduit, successfully interrupted with linear ablation. Preprocedural cardiac CT guided a transbaffle puncture, performed inferior and right to the occluder, enabling common atrial access. Subsequent mapping identified an additional macro-reentrant flutter and a focal atrial tachycardia, both ablated with restoration of stable sinus rhythm. No arrhythmias were inducible post-procedure. This case underscores the feasibility and safety of catheter ablation in complex Fontan anatomy when guided by meticulous anatomical assessment, tailored access, and advanced mapping technologies.