not in AFL underwent arrhythmia induction with a burst pacing protocol(25 impulses with stepwise reduction of pacing cycle length from 300down to 200ms). EP testing was performed in the standard fashion using a20-pole Halo catheter in the right atrium and 2 quadri-polar catheterspositioned into the coronary sinus and various locations in the right atrium.AFL was characterized as CTI-dependent or CTI-independent based onactivation and entrainment mapping.Results: 41 out of 42 pts with persistent AFL had CTI-dependent AFL,while only 2 of 16 pts with paroxysmal AFL (p�0.0001) had CTI-dependent AFL. 37 out of 40 pts who presented to the EP lab in AFL hadCTI-dependent AFL, but only 2 of 16 who presented to the EP lab inanother rhythm had inducible CTI-dependent AFL (p�0.0001). Otherinduced rhythms included CTI-independent AFL (n�4) and atrial fibrilla-tion (n�6) in these pts. Isthmus dependence of AFL was not predicted byhistory of atrial fibrillation, structural heart disease, hypertension, or pre-vious cardiac surgery. There was no significant difference in age betweenpts with CTI-dependent and CTI-independent AFL (63�12 and 60�13years).Conclusion: The presence of persistent rather than paroxysmal AFL andpresentation to the EP lab in AFL are strong predictors of cavotricuspidisthmus dependent AFL. Paroxysmal AFL is rarely CTI-dependent. Thesefindings have important implications for referral of patients for catheterablation of AFL.

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NOVEL VOLTAGE CHARACTERISTICS OF CAVOTRICUSPIDISTHMUS AND MITRAL ISTHMUS: IMPLICATION FORCATHETER ABLATION OF RIGHT AND LEFT ATRIAL FLUTTERShih-Lin Chang, MD, Ching-Tai Tai, MD, Yenn-Jiang Lin,MD, Mary Gertrude Y. Ong, MD and Shih-Ann Chen, MD.Taipei Veterans General Hospital, Taipei, Taiwan Republicof China.

Background: The voltage of a bipolar electrogram may reflect the thicknessof atrial musculature, and the thicker myocardium may be associated withgaps in the ablation line. The aim of this study was to investigate thevoltage characteristics of cavotricuspid and mitral isthmus.Methods and Results: 24 consecutive patients (mean age 61�15 years)with atrial fibrillation referred for catheter ablation were enrolled in thisstudy. After the biatrial geometry was created by the NavX mapping andnavigation system, a total of 188�10 and 190�15 points were tagged tocreate the contact bipolar voltage map of the right and left atrium, respec-tively. The cavotricuspid isthmus was equally divided into two areas; neartricuspid annulus (TA) or inferior vena cava (IVC). The mitral isthmus wasequally divided into two areas; near mitral annulus (MA) or inferiorpulmonary veins (PVs); it was also equally divided into the lateral andmedial aspect of mitral isthmus. The peak to peak voltage was measured.(See Table)Conclusion: The cavotricuspid isthmus has a higher voltage than mitralisthmus. The voltage near TA is lower than IVC side, and the voltage nearMA is higher than PVs side; thus the possible gap after ablation of isthmuswould be near the Eustachian ridge of cavotricuspid isthmus and near themitral annulus of mitral isthmus. Furthermore, lateral mitral isthmus maybe easier for ablation than medial mitral isthmus.

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ACUTE AND CHRONIC EFFICACY AND SAFETY OF CATHETERCRYOABLATION OF THE CAVO-TRICUSPID ISTHMUS FORTREATMENT OF HUMAN TYPE 1 ATRIAL FLUTTER*Gregory K. Feld, MD, James P. Daubert, MD, Raul Weiss,MD, *William Miles, MD, Arjun Sharma, MD, William

Pelkey, PhD and Ruchir Sehra, MD for the CryoablationAtrial Flutter Efficacy (CAFE) Trial Investigators, San Diego,CA.

Background: Catheter cryoablation (CCA) is an effective alternative toradiofrequency catheter ablation (RFCA). Since type 1 atrial flutter (AFL)is one of the commonest arrhythmias treated by RFCA today, we studiedthe safety and effectiveness of CCA as an alternative.Methods: CCA was performed with a 10Fr catheter (CryoCor, Inc) in 121patients (pts) with AFL (93 male and 18 female), with mean age 62�11yrs,mean LVEF 54�11%, of whom 72 (60%) also had atrial fibrillation. Allpts underwent EP study with induction of AFL. Right atrial (RA) activationsequence was confirmed by multi-electrode or computerized 3D activationmapping, with cavo-tricuspid isthmus (CTI) dependence confirmed bypacing demonstrating concealed entrainment. CCA ablation of the CTI wasperformed with multiple 2.5 minute freezes until bi-directional block wasdemonstrated during pacing from the low lateral RA and coronary sinusostium. Patients were examined in followup at 1, 3, 6, and 12 months(mos), and underwent weekly and symptomatic event monitoring. Acuteprocedural success was defined as CTI block persisting 30 minutes after thefinal ablation.Results: Acute success was achieved in 111 of 121 pts (91.7%). Totalprocedure time was 198�74 minutes (min), ablation time including 30 minwaiting period was 141�63 min, and fluoroscopy time was 62�34 min. Atotal of 27.4�15.2 freezes were delivered to achieve CTI block, withaverage and nadir temperatures of -81.7�3.5° C and -85.5�3.7° C, re-spectively. There were 6 pts (4.96%) with procedure related adverseevents, including 2 with groin hematomas, 1 with thigh numbness, 1 witha femoral vein pseudoaneurysm, 1 with chest wall pain, and 1 withcomplete AV block requiring permanent pacemaker. Of 72 pts completing3 mos followup to date, 63 (87.5%) remain free of AFL recurrence.Conclusion: This large pivotal study demonstrated that the acute andchronic efficacy and safety of CCA is comparable to that of RFCA fortreatment of type 1 AFL.

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CARDIAC TISSUE TEMPERATURE MEASUREMENT USING MRIPierre Jaıs, MD, PhD, Bruno Quesson, PhD, Baudouin Denisde Senneville, PhD, *Gwenael Herigault, Chrit Moonen,PhD, Prashanthan Sanders, MBBS, Meleze Hocini, MD,Frederic Sacher, MD, Jacques Clementy, MD and MichelHaıssaguerre, MD. Hopital Haut Leveque - UniversiteBordeaux 2, Bordeaux-Pessac, France and ImagerieMoleculaire et fonctionnelle - CNRS Bordeaux 2, Bordeaux,France.

Background: RF ablation should ideally be controlled by tissue tempera-ture to achieve the best safety and efficacy. This study investigates the roleof magnetic resonance (MR) in cardiac temperature monitoring.Methods and Results: Healthy volunteers (n�4) were studied in a 1.5Tscanner (Philips Intera). A single slice short axis of the left ventricle (LV)was considered for thermometry. The phase sensitive MR imaging se-quence was a combination of a transient field echo and a segmented EPItechnique (TFEPI) to acquire during end diastole a complete image within1 to 4 cardiac cycles. Cardiac triggering was used and respiratory motionwas compensated using a navigator. A rectangular field of view (65%) wasused with a matrix of 128, resulting in a pixel size of 2.2 mm. Slicethickness was set to 7 mm and blood signal suppressed. Several dynamicswere acquired over a period of time of �2 minutes to estimate the temporalstability of cardiac MR thermometry. Each individual image was processedoffline to calculate apparent temperature changes from phase difference.The stability of MR thermometry was estimated in each pixel from thecalculation of temperature standard deviation (SD) over time. Additionalimage processing was performed to correct for possible remaining dis-placements and susceptibility related effects by applying an image regis-tration algorithm on magnitude images. LV tissue temperature maps wereobtained in all with a temperature SD lower than 5°C in 87�3% of the

S238 Heart Rhythm, Vol 2, No 5, May Supplement 2005