Dynamic Activation Changes during Complex Atypical Atrial Flutter in a Previously Untreated Atrium.

Atypical atrial flutter (AFL) is a macro-reentrant atrial tachycardia that most frequently develops after atrial fibrillation ablation or cardiac surgery. Although ablation-related circuit modification is recognized, the occurrence of multiple sequential AFL transformations within a single procedure, particularly in a previously untreated atrium, is exceedingly rare.A 76-year-old woman with hypertension and paroxysmal atrial fibrillation, without prior cardiac surgery or catheter ablation, presented with drug-refractory AFL. Electrophysiological study initially demonstrated an activation characteristic suggestive of CTI-related AFL. Following stepwise ablation, the arrhythmia sequential left atrial activation pattern changes involved posterior wall-related activation pattern AFL, peri-right superior pulmonary vein AFL, and mitral isthmus-related AFL. Despite apparent endocardial conduction block, the tachycardia persisted with dynamic coronary sinus (CS) activation reversal. The distal CS musculature appeared to participate in the remaining tachycardia conduction pathway. Epicardial ablation within the CS resulted in immediate termination, and no atrial tachyarrhythmias were inducible thereafter.This case illustrates a rare example of sequential transformation among dynamic atypical AFL activation changes in a previously untreated atrium, ultimately suggesting likely participation of the CS musculature in the remaining tachycardia conduction pathway. Multiple LA macro-reentry mechanisms, dynamic CS activation reversals, and eventual reliance on a CS epicardial pathway created a diagnostic and therapeutic challenge. Recognizing the possibility of epicardial CS involvement is crucial when typical ablation strategies fail to terminate atypical AFL.