Background: In typical counterclockwise atrial flutter (AFL), the route of impulse propagation during transient entrainment when pacing from the coronary sinus (CS) is expected to be similar to that when pacing from the same CS site during sinus rhythm (SR) (after ablation) provided there is block at the cavotricuspid isthmus (CTI) and activation of the lateral wall of the right atrium (RA) proceeded orthodromically during transient en- trainment. Methods: A total of 23 patients with typical AFL [cycle length, (CL) 24227 ms] underwent CTI ablation during AFL with catheters at the CS and lateral RA (20-poles). CS pacing was performed at a CL 20 ms less than AFL CL before ablation (n=23), during SR with conduction through the CTI (n=8) and after CTI block (n=23). Results: Transient entrainment with orthodromic activation of all recorded RA sites was obtained in all 23 patients. The Table shows conduction time from CS to inferolateral RA (CS-RA) during transient entrainment (T- entr), during pacing in SR (T-PSR) with conduction through the CTI (group 1 of measures), and with CTI block (group 2 of measures). T-entr T-PSR group 1 group 2 Group 1 vs. Group 2 Corr T- entr and group 1 Corr T- entr and group 2 CS-RA (ms) 197  24 129  30 184  23 P0.001 r=0.269, p=0,519 r=0.863, p0.001 Difference between T-entr and T-PSR was  35 ms in all of group 2. A “time entrainment (TE) index” expressing the difference between T-entr and T-PSR was calculated for each T-PSR measurement: [(T-entr) - (T-PSR)]  100 / (T-entr). TE index was 3415% in group 1 and 66% in group 2 (p0,001). ROC analysis for the TE index showed an area under the curve of 0.973 with a cut-off point of 18% for group 1 and 18% for group 2, having 100% sensitivity and 88% specificity for CTI block. Conclusions: comparison of conduction times during transient entrain- ment from the CS and during pacing from the same site and rate in SR can help to establish whether clockwise CTI block has been achieved in patients with typical AFL. P3-77 ANTICOAGULATION IN PATIENTS UNDERGOING LEFT ATRIAL ABLATION FOR ATRIAL FIBRILLATION K. Chandrasekaran, MD, M Eyman Mortada, MD, Imran Sheikh, MD, Girish Narayan, MD, Vikram Nangia, MD, Ryan Cooley, MD, Atul Bhatia, MD, Masood Akhtar, MD and Jasbir Sra, MD. Aurora Sinai/St. Luke’s Medical Centers, University of Wisconsin Medical School-Milwaukee Clinical Campus, Milwaukee, WI. Catheter ablation for atrial fibrillation (AF) can increase the risk of left atrial (LA) thrombi and stroke. Optimal anticoagulation pre and post procedure has not been determined, though many authors suggest low molecular weight heparin, the administration of which can be difficult and expensive. We report the role of administering warfarin and ASA without low molecular weight heparin in patients undergoing AF ablation. Methods: A total of 207 consecutive patients underwent radiofrequency ablation (RFA) for AF. Transseptal puncture was done after ruling out clot in the LA. After first transseptal puncture, the sheath was flushed with heparin (5000 units/ml) and a Cordis Webster 7F ablation catheter was inserted into the LA. After second transseptal puncture, a bolus of 80 units/kg of heparin was given, followed by an infusion of 1000 units/hr to maintain ACT around 300 secs. Mapping of LA and pulmonary veins (PVs) was done. PVs were isolated and cleared of PV potentials. Linear lesions were made in the LA connecting the superior veins on the roof and between the posterior mitral annulus and the left inferior pulmonary vein. Patients were taken off warfarin but started on ASA 325 mg daily 3 days prior to the procedure and restarted on warfarin the day of the procedure. Both medications were continued for 4 to 6 weeks post RFA. Thirty-seven patients who showed smoke in the LA on TEE were also given low molecular weight heparin post procedure until INR was therapeutic. Results: Thirty-two patients had persistent and 175 had paroxysmal AF. Direct current cardioversion (DCCV) was done on 87 patients. Total number of DCCV done was 150 (range, 1 to 6 times per patient; mean, 2). Two patients had TIA on the sixth and eighth day, respectively, post RFA with subtherapeutic INRs. They recovered totally in 24 hours. Two patients developed small groin hematomas. There was no evidence of pulmonary vein stenosis in these patients. Conclusion: In patients without demonstrable clot or smoke in the LA, starting ASA 3 days prior to ablation and warfarin immediate post RFA, without low molecular weight heparin, with meticulous anticoagulation during the procedure, appears to be a safe mode of anticoagulation. P3-78 FRACTIONATED ATRIAL POTENTIALS: MARKERS OF THE ORIGIN OF FOCAL ATRIAL TACHYCARDIA Martin J. Schalij, MD and Natasja M. S. De Groot, MD, PhD. Leiden University Medical Center, Leiden, The Netherlands. Background: Focal atrial tachycardia (FAT) originates from areas with poor cell-to-cell coupling, giving rise to fractionation of extra-cellular potentials. Due to cellular uncoupling, electrotonic inhibition of rapid firing foci does not occur and FAT may arise. This study analysed characteristics of atrial potentials in relation to its distance to the site of earliest activity during FAT. Methods: 3-D activation/voltage maps obtained from patients (N=15, 6 male, age 4014 yrs referred for ablation of a FAT were analysed. All bipolar atrial potentials (BP) were categorized according to the number of deflections. Double and fractionated potentials contained respectively 2 or more deflections. The peak-to-peak amplitude and the time interval be- tween the first and last deflection (fractionation duration) of each BP was measured. The BP with the earliest local activation time with respect to the surface p wave was defined as the site of origin of the focus. The distance to this site of earliest activity was measured for each BP in order to study the occurrence of fractionated potentials at the FAT origin in relation to the remainder of the atria. Results: Eighteen different AT (CL 346109) ms were analysed. The incidence of single potentials in the right atrium was 5916%. The oc- currence of double and fractionated potentials within 2 cm of the site of earliest activity (‘focal area’) was higher compared to the remainder of the atria (6722% versus 3414%, p0.001). Comparing characteristics of atrial potentials in the focal area and the remainder of the atria, focal area potentials were characterized by a longer fractionation duration (49 ms versus 34 ms, p0.001) and a lower peak-to-peak amplitude (focal area: 0.510.43 [0.12-1.7] mV versus remainder of the atria: 0.940.69 [0.22- 2.58] mV, p=0.03). Conclusion: Significant differences in fractionation, fractionation duration and peak-to-peak-amplitude of atrial potentials between the focal area and the remainder of the atria exist. The results of this study support the hypothesis that a certain degree of cellular uncoupling is required for the development of a FAT thereby providing further insight into the aetiology of FAT. P3-79 RADIOFRQUENCY CATHETER ABLATION IN THE AORTIC ROOT OF THE ORIGIN IN THE LEFT VENTRICULAR OUTFLOW TRACT IN PEDIATRIC PATIENTS WITH VENTRICULAR ARRHYTHMIAS Segey A. Termosesov, MD, Rustem S. H. Garipov, MD, Ilya L. Ilich, MD and Maria A. Shkolnikova, MD, PhD. Research Institute of Pediatry and Pediatry Surgery, Moscow, Russian Federation. Radiofrequency catheter ablation (RFCA) now is one of the main methods for treating different types of drug resistant arrhythmias. The aim of this S203 Poster 3