Organised atrial fibrillation simulating atrial flutter.

We present a case of drug-refractory paroxysmal atrial fibrillation in which episodes of AF spontaneously alternated with a regular arrhythmia resembling atrial flutter. Isthmus-dependent atrial flutter was initially diagnosed and the patient was treated with cavotricuspid isthmus ablation. Two months later the patient returned with recurrent arrhythmias. On the 12-lead ECG there was again evidence of atrial flutter with both typical and atypical characteristics. On electrophysiological study, careful mapping revealed that the regular rhythm actually represented organised slow atrial fibrillation. Circumferential pulmonary vein ablation resulted in the abolition of both arrhythmias.