Typical atrial flutter ablation: Conduction across the posterior region ofthe inferior vena cava orifice may mimic unidirectional isthmus block

Atrial Flutter Mapping. Introduction: The aim of this study was to map the low right atrium before and after radiofrequency ablation of the inferior v ena cava-tricuspid annulus (IVC-TA) isthmus in patients with typical atrial butter (AFI) to better understand the electrophysiologic meaning of incomp lete or unidirectional block following the ablation procedure and its relat ionship with AFI recurrence. Methods and Results: We performed atrial mapping in 12 patients using a "ba sket" catheter in the IVC orifice, Halo catheter in the right atrium, and m ultipolar catheters in the coronary sinus (CS) and His region. In patients in sinus rhythm, atrial activation was analyzed during pacing from the CS a nd low lateral right atrium (LLRA) before and after ablation. Atrial activa tion propagated across the isthmus and posterior region of the IVC orifice simultaneously before ablation. Mapping during AFI 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 un idirectional counterclockwise block, whereas analysis of basket catheter re cordings demonstrated complete bidirectional block. The apparent conduction over the isthmus during pacing from proximal CS was due to conduction alon g the posterior part of the IVC orifice, which activated the LLRA despite c omplete isthmus block. Conclusion: Our results demonstrate that limited endocardial mapping may yi eld a pattern compatible with unidirectional block in the IVC-TA isthmus, a lthough bidirectional block is present at this anatomic level.