Typical Atrial Flutter Ablation: Conduction Across the Posterior Region of the Inferior Vena Cava Orifice May Mimic Unidirectional Isthmus Block

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Abstract
The aim of this study was to map the low right atrium before and after radiofrequency ablation of the inferior vena cava-tricuspid annulus (IVC-TA) isthmus in patients with typical atrial flutter (AFI) to better understand the electrophysiologic meaning of incomplete or unidirectional block following the ablation procedure and its relationship with AFI recurrence. We performed atrial mapping in 12 patients using a "basket" catheter in the IVC orifice, Halo catheter in the right atrium, and multipolar catheters in the coronary sinus (CS) and His region. In patients in sinus rhythm, atrial activation was analyzed during pacing from the CS and low lateral right atrium (LLRA) before and after ablation. Atrial activation propagated across the isthmus and posterior region of the IVC orifice simultaneously before ablation. Mapping during AF1 in four patients showed that the crista terminalis was a site of functional block. After ablation, evaluation of Halo catheter recordings in three patients showed apparent unidirectional counterclockwise block, whereas analysis of basket catheter recordings demonstrated complete bidirectional block. The apparent conduction over the isthmus during pacing from proximal CS was due to conduction along the posterior part of the IVC orifice, which activated the LLRA despite complete isthmus block. Our results demonstrate that limited endocardial mapping may yield a pattern compatible with unidirectional block in the IVC-TA isthmus, although bidirectional block is present at this anatomic level.