Functional Interplay Between Upper Loop Reentry and Clockwise Cavotricuspid Isthmus-Dependent Flutter in a Non-surgical Right Atrium: A Case of Dual-Loop Macroreentry.

Typical atrial flutter is a common macroreentrant arrhythmia dependent on the cavotricuspid isthmus (CTI). However, complex right atrial substrates can support multiple concurrent reentrant circuits even in the absence of prior cardiac surgery. We present the case of a 68-year-old man with persistent clockwise atrial flutter and no history of surgical intervention or prior ablation. High-density electroanatomical mapping and entrainment maneuvers identified a dual-loop macroreentrant mechanism involving both a clockwise CTI-dependent circuit and an upper loop reentry circuit. Initial ablation of the superior loop resulted in a significant prolongation of the tachycardia cycle length from 264 ms to 308 ms without termination, unmasking the slower CTI-dependent component. Restoration of sinus rhythm was achieved only after targeted ablation of fragmented potentials at the CTI-inferior vena cava junction and completion of a bidirectional CTI block. This case highlights the functional interplay between different right atrial loops facilitated by functional barriers like the crista terminalis in a non-surgical atrium. It underscores the importance of systematic mapping and the recognition that CTI ablation alone may be insufficient when multiple loops coexist.