Background: Atrial flutter (AFL) is a large re-entrant circuit located in the right atrium. Anti-arrhythmic drugs (AADs) can provoke AFL with 1:1 atrioventricular conduction (AVC) to cause hemodynamic collapse. We elucidated the characteristics of patients with AFL exhibiting spontaneous 1:1 AVC. Methods: Fifteen patients (1:1 AFL group; 11 males, 52.4±13.7 years old) who documented AFL with 1:1 AVC were enrolled and compared to 77 patients without 1:1 AVC (Control group; 71 males, 68.1±10.9 years old). Results: The use of AADs was greater in the 1:1 AFL group than in the control group (60.0 vs. 14.3%, p < 0.001). AFL cycle length during maximum AVC was significantly longer in the 1:1 AFL group than in the control group (274.7 ± 37.0 vs. 220.4 ± 26.2 msec, p < 0.001). Among 1:1 AVC group, 9 patients had AADs and AFL cycle length was significantly longer during 1:1 AVC as compared with 2:1 AVC documented the other day (284.4 ± 41.3 vs. 233.3 ± 26.0 msec, p <0.001), suggesting enhancement effect of the AADs during 1:1 AVC. Remaining 6 patients who did not take AADs, 2 patients showed enlargement of the tricuspid annulus and 3 patients developed 1:1 AVC during exercise. Conclusions: In addition to the enhancement of AAD effect, prolonged AFL cycle length associated with enlargement of the tricuspid annulus and shortened refractory period of the AV node might increase the risk of 1:1 AVC during AFL. Keywords: atrial flutter, atrial flutter cycle length, tricuspid annulus. Atrioventricular node, atrioventricular conduction, anti-arrhythmic drug

Introduction: Early recurrence (ER) of atrial fibrillation (AF) is defined as the recurrence of atrial tachyarrhythmias within 3 months after AF ablation, however, this definition is based on data from the era of radiofrequency catheter ablation (RFCA), without contact force (CF) technology. We investigated the significance of ER as a risk factor for late recurrence (LR) in paroxysmal AF (PAF) patients treated with CF and non-CF-guided ablation. Methods and Results: We studied 395 patients with PAF who underwent RFCA. Of these, 97 patients underwent RFCA without CF technology (Non-CF group) and 298 underwent with CF technology (CF group). Over a 2-year post-ablation follow-up period, LR occurred in 54 of 97 (55.7%) patients in the Non-CF group, and in 105 of 298 (35.2%) patients in the CF group. ER had a more significant relationship with LR in the CF than in the Non-CF group, and all patients in the CF group with ER in the 3rd month developed LR. Conclusion: ER in PAF patients who have undergone CF-guided ablation have a greater risk of LR than those who have undergone non-CF-guided ablation. ER in the 3rd month after CF-guided ablation may indicate an absolute risk of LR. Blanking period could be defined as 2 months in the CF era.