Mechanism of conversion of atypical right atrial flutter to atrial fibrillation.

The purpose of this study was to explore the mechanisms of conversion from atypical atrial flutter (AFL) to atrial fibrillation (AF), and the long-term results of cavotricuspid isthmus ablation in these patients. We retrospectively reviewed the records of 221 patients with typical AFL referred to our hospital for ablation. A total of 25 patients had atypical AFL, and cavotricuspid isthmus ablation was performed in 23 with isthmus-dependent atypical AFL, as well as in 180 patients with typical counterclockwise and/or clockwise AFL. In all, 13 spontaneous transitions from atypical AFL to AF were documented in 11 of 17 patients. Before AF, a pattern of lower loop reentry was observed in 11 of 13 patients (85%) and upper loop reentry in 3 (1 had both). Multiple early breaks along the tricuspid annulus during AFL were noted in 6 of 13 patients (46%). Among the 13 transitions, discrete atrial premature complexes before AF were found in 5 patients with lower loop reentry and in 1 with upper loop reentry (46%). In the remaining patients, a more rapid atrial rhythm was involved in the development of AF with a pulmonary venous focus in 2. In some cases, additional "breaks" in the functional line of block occurred before the development of AF. There was a significant increased incidence of AF (68%) in those with atypical AFL compared with those with typical AFL (38%) (p = 0.004). After a mean follow-up of 28 +/- 9 months for the atypical group and 18 +/- 11 months for the typical group, the AF recurrence rate was similar (57% vs 48%, p = 0.4). Discrete atrial premature complexes or atrial tachycardia may initiate AF either directly or by producing further breaks in lines of functional block. Bidirectional cavotricuspid isthmus block is associated with cure or control of AF in approximately 50% of patients with AFL.