Introduction: Bidirectional conduction block at the subeustachian isthmus predicts long-term efficacy of atrial flutter ablation. Limited data are available on the incidence and outcome of minor conduction changes such as unidirectional or incomplete block. This prospective study sought to systematically assess discrete acute and long-term alterations of bidirectional conduction prior to a complete conduction block.
Methods and results: In 41 patients with type I atrial flutter, pulse propagation through the subeustachian isthmus during low lateral and proximal coronary sinus pacing was documented and analyzed following each consecutive radiofrequency (RF) application. In cases of altered conduction properties and noninducibility of atrial flutter, patients were followed-up for 12 months. Three sets of results were found. First, following RF application, 23 patients presented a progressive conduction delay prior to a complete conduction block. Second, RF application did not always affect counterclockwise and clockwise conduction simultaneously or to the same extent. In 13 patients, an initial alteration of counterclockwise conduction was present before an alteration of clockwise conduction; in 5 patients, clockwise conduction was primarily affected. Third, the recurrence rate of typical atrial flutter was 9% (2/22) in patients with a complete bidirectional conduction block, 54% (7/13) in patients with unidirectional conduction block, and 100% (6/6) in patients with sole bidirectional conduction delay.
Conclusion: In 50% of the patients, consecutive RF applications resulted primarily in a progressive conduction delay rather than a sudden conduction block. Since counterclockwise and clockwise conduction were not always affected simultaneously or to the same extent, lateral as well as septal pacing is recommended for improvement of bidirectional conduction block. Normalization of primarily altered conduction and, therefore, recurrence of atrial flutter are high in all patients without bidirectional block.