1099 Prospective Observations in the Clinical and Electrophysiological Characteristics of Intra-Isthmus Reentry YANFEI YANG, M.D., NIRAJ VARMA, M.D., NITISH BADHWAR, M.D., RONN E. TANEL, M.D., SIRISHA SUNDARA, M.D., RANDALL J. LEE, M.D., Ph.D., BYRON K. LEE, M.D., ZIAN H. TSENG, M.D., GREGORY M. MARCUS, M.D., ALBERT M. KIM, M.D., Ph.D., JEFFREY E. OLGIN, M.D., and MELVIN M. SCHEINMAN, M.D. From the University of California–San Francisco, San Francisco, California; and Loyola University Medical Center, Chicago, Illinois, USA ECG and EGM of IIR. Introduction: Intra-isthmus reentry (IIR) is a circuit within the cavotricuspid isthmus (CTI). The purpose of this study is to prospectively define the electrogram and surface ECG characteristics of IIR, and its clinical implications. Methods and Results: Fourteen patients underwent electrophysiological studies and were found to have IIR. Detailed electrogram mapping of the CTI was available in all, electroanatomic mapping (EAM) in 8 of 14 (57%) patients. In all, entrainment mapping during tachycardia proved reentry, and showed that the anteroinferior CTI was out of the circuit and the septal CTI was in the circuit in 12 of 14 patients, whereas in 2, the circuit was confined within the mid and/or anteroinferior CTI. Fractionated potentials (FPs) spanning 34–71% of the tachycardia cycle length were recorded within the CTI in all, and double potentials were inscribed in 10 of 14 (71%). Analysis of the tricuspid annulus electrograms showed spontaneous shifts from a counterclockwise (CCW) to clockwise or fusion patterns. Surface ECGs showed either typical CCW pattern (12 patients) or atypical patterns (3 patients). The EAMs showed a focal pattern in 3, a CCW pattern in 5. The successful ablation site always occurred at the area with maximal FP duration. Over the same period, 33 of 384 (9%) patients who underwent ablation for CTI-dependent flutter had prior successful CTI ablation, 7 of 33 (21%) were found to have IIR during the redo procedure. Conclusions: (1) Electrogram and ECG patterns of IIR frequently show atypical flutter. (2) IIR was successfully ablated in an area of the CTI associated with maximal duration of FPs. (3) IIR is a significant cause of “recurrent flutter” in patients with prior CTI ablation. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1099-1106, October 2010) atrial flutter, catheter ablation, cavotricuspid isthmus, entrainment mapping, electroanatomic mapping Introduction We have recently described a microreentrant atrial flutter (AFL) circuit localized to the septal portion of the cavotricus- pid isthmus (CTI). 1 This intra-isthmus reentrant (IIR) circuit was thought to involve the septal isthmus as well as the os of coronary sinus (CS). Our initial report of this arrhyth- mia was largely retrospective. This report is a prospective study from two medical centers designed to define detailed electrogram characteristics as well as to correlate the sur- face ECG with respective endocardial activation sequence. Finally, we sought to define the clinical implications of this arrhythmia. Presented in abstract form at 29 and 30th Annual Scientific Sessions of Heart Rhythm Society (2008 and 2009). No disclosures. Address for correspondence: Melvin M. Scheinman, M.D., Cardiac Electro- physiology, University of California–San Francisco, 500 Parnassus Avenue, MU East 4S, Box 1354, San Francisco, CA 94143-1354, USA. Fax: +415- 476-6260; E-mail: scheinman@medicine.ucsf.edu Manuscript received: 18 December 2009; Revised manuscript received: 11 March 2010; Accepted for publication: 12 March 2010. doi: 10.1111/j.1540-8167.2010.01778.x Material and Methods Patients A total of 384 patients with CTI-dependent AFL were re- ferred to University of California, San Francisco and Loyola University Medical Center for electrophysiology study (EPS) and AFL ablation from 2004 to 2009. Among them, 33 pa- tients had prior CTI ablation for typical CTI-dependent AFL. None of the patients described in our retrospective study 1 were included in this study. Fourteen patients were found to have IIR. There were 12 males and 2 females, aged from 19 to 82 years (aver- age 60 ± 18 years). All patients had 12-lead ECGs showing AFL. Among 14 patients, 10 had an ECG compatible with a counterclockwise (CCW) AFL, whereas 2 had an ECG showing an atypical AFL pattern, and 2 had AFL with 1:1 or 2:1 AV conduction, which did not allow for clear analyses of flutter wave morphology. Four of 14 patients (29%) had a past history of structural heart disease; 3 patients (21%) had a history of hypertension, whereas 4 patients had a history of atrial fibrillation, and among them 2 had either a prior pulmonary vein isolation or “MAZE” procedure. Thirteen patients had an echocardiogram prior to the study, which showed left atrial (LA) enlargement (defined as: LA volume for male >58 mL, female >52 mL; and/or LA diameter for male >4.0 cm, female >3.8 cm) in 10 of 13 (77%) patients and right atrial (RA) enlargement in 5 (36%). Eleven of