Changing coronary sinus activation during catheter ablation of isthmus-dependent right atrial flutter: what is the mechanism?

A 69-year-old woman was referred for cavotricuspid isthmus ablation due to typical isthmus-dependent right atrial flutter. During ablation, intracardiac activation sequence in coronary sinus (CS) changed without altering the cycle length or the P-wave morphology. This suggests that interatrial connection via CS was blocked and left atrium (LA) was activated from Bachmann's bundle or/and connections between fossa ovalis and LA.