BRU, P., ET AL.: Resumption of Right Atrial Isthmus Conduction Following Atrial Flutter Radiofrequency Ablation. Right atrial isthmus block is currently accepted as a success criterion of atrial flutter ablation. An electrophysiological study performed days after the ablation procedure may show recovery of con duction across the isthmus in some patients, followed by arrhythmia recurrence. However, few data are available on the time course of this recovery and on the monitoring of isthmus conduction at the end of the ablation procedure as a means of increasing the success rate of the procedure. Radiofrequency (RF) catheter ablation was performed in 28 men and 7 women (mean age = 65 ± 11 years] presenting with com mon or clockwise atrial flutter (AFL) resistant to 2.9 ± 1.8 antiarrhythmic drugs. Underlying heart disease was present in 13 patients. The ablation procedure was performed with an 8-mm-tip catheter, by several 45-second applications at a target temperature of 65°C, directed to the isthmus between tricuspid annulus and inferior vena cava. Bidirectional isthmus block (BDB) was created with 4-24 RF applications in all but one patient. Special attention was paid to exclude incomplete block by meticulous mapping dur ing pacing at the coronary sinus os and at the low lateral right atrium every 5 minutes for 20 minutes there after. Conduction recovered across the isthmus in 5 patients at 10, 10,12,15, and 16 minutes, respectively, and further RF applications were needed to obtain stable block. At a follow-up of 17 ±10 months, AFL oc curred in the patient without, and in one patient with BDB. Thirty-three of the 34 patients (97%) with per sistent BDB remained free of arrhythmia recurrence. This study showed that conduction resumed across the isthmus within 20 minutes, after AFL ablation in 15% of the patients. The long-term results of the pro cedure can be optimized by ascertaining the persistence of BDB during that period of time. (PACE 2000; 23[Pt. Π]:1908 23[Pt. Π]: -1910 atrial flutter, radiofrequency ablation, isthmus conduction block