P5-16 CORRELATION BETWEEN THE OUTCOME OF CRYOTHERMAL BALLOON ISOLATION OF PULMONARY VEINS AND THE PATHOLOGIC FINDINGS AT THE VENO-ATRIAL JUNCTION IN DOGS Alvaro V. Sarabanda, MD, Susan B. Johnson, BS, Thomas J. Bunch, MD, Mark A. Milton, MD, Gregory K. Bruce, MD, Luiz R. Leite, MD and *Douglas L. Packer, MD. Mayo Clinic/St Marys, Rochester, MN. Background: Cryothermal balloon ablation is an emerging technique for pulmonary vein (PV) isolation. However, the correlation between the outcome of cryothermal balloon isolation of PVs and the pathologic find- ings at the veno-atrial junction is not known. Method: To evaluate the correlation between the outcome of cryothermal balloon isolation of PVs and the pathologic findings at the veno-atrial junction, 12 PVs were ablated in 6 dogs survived to 1 week. Cryothermal energy was delivered by means of a balloon catheter to the coldest achievable temperature for either 4 or 8 min. Before euthanizing the animals, electrical isolation of the PVs was reassessed with a circumferential decapolar catheter (Lasso). Post- mortem examination of the gross and histological specimens was performed to categorize the PV lesions in terms of 12 o’clock-face sectors and this was correlated with the key findings seen on Lasso catheter. Result: Successful electrical isolation was achieved in 10 out of 12 veins (83%). In all electrically isolated PVs, histological examination revealed a complete circumferential and transmural ostial lesion. Conversely, in the 2 unsuccessfully isolated LSPV, a noncircumferential lesion gap at the in- ferior aspect of the PV ostia was noted. In these, the position of the lesion discontinuity corresponded to the location of the remaining PV potentials as seen by the Lasso catheter. Conclusion: Excellent correlation was seen between the outcome of cryo- thermal balloon isolation of PVs as evaluated by the Lasso catheter and the pathologic findings at the veno-atrial junction. These data could help maximize successful PV isolation using this new technique. P5-17 ASSESSMENT OF PROARRHYTHMIC EFFECTS OF LINEAR ABLATION USING A NOVEL, INJECTED SALINE-ENHANCED RADIOFREQUENCY ENERGY DELIVERY SYSTEM Gregory K. Bruce, MD, Susan B. Johnson, BS, Thomas J. Bunch, MD and *Douglas L. Packer, MD. Mayo Clinic, Rochester, MN. Background: Techniques to increase the success of ablation of infarct-related ventricular tachycardia using large, transmural linear lesions are currently under investigation. However, the arrhythmogenic potential of these large lesions has not been evaluated. Methods: To assess the arrhythmic outcome of large lesion, linear ablation for modifying LV substrate, 4 dogs with normal ventricular function at baseline underwent epicardial ablation with a heated saline-injection, “vir- tual electrode” RF energy delivery system. Ablation was performed at 40 watts with 50 °C saline injection at a flow rate of 5 ml/min. Inducibility of VT by programmed stimulation was determined pre- and post-ablation. The upper limit of vulnerability (ULV) and the ventricular fibrillation threshold (VFT) were also determined pre- and post-ablation by a step-up shock strength method at -40msec, -20msec, 0, and +20msec relative to the peak of the T wave. Results: A mean of 6.3  1.0 lesions were made in each animal to create an ablation line from apex to base along the lateral LV wall. Programmed stimulation failed to induce any arrhythmia pre- or post-ablation. When examined according to T wave shock time, the ULV decreased post ablation at times -40, -20, and 0 msec, but increased modestly from pre-ablation at +20 msec. The VFT threshold showed little change post- ablation between -40 and 0 msec, but did increase above the pre-ablation VFT at +20msec. P-values for comparisons pre- and post-ablation were non-significant at all points. Conclusion: The zone of vulnerability between the VFT and ULV de- creases with linear ablation using heated saline-injection, “virtual elec- trode” RF ablation. Furthermore, the large lesion size is accomplished without increased proarrhythmic risk. P5-18 MECHANISMS FOR THE MAINTENANCE OF VENTRICULAR FIBRILLATION: THE NONUNIFORM DISPERSION OF REFRACTORINESS, RESTITUTION PROPERTIES, OR ANATOMIC HETEROGENEITIES? Hao Qin, MD, PhD, Jian Huang, MD, PhD, Jack M. Rogers, PhD, Gregory P. Walcott, MD, Dennis L. Rollins, William M. Smith, PhD and Raymond E. Ideker, MD, PhD. University of Alabama At Birmingham, Birmingham, AL. Introduction: The relative importance of nonuniform dispersion of refrac- toriness, steep restitution slopes, and anatomic heterogeneities in causing conduction block during ventricular fibrillation (VF) remains unknown. Methods and Results: In 6 open-chest pigs, ventricular refractoriness and restitution curves were estimated from activation recovery intervals (ARIs) calculated from 504 (21 x 24) unipolar electrode recordings 2 mm apart in a plaque sutured to the left ventricular (LV) free wall. A steady-state restitution protocol was performed twice at each of two pacing sites: the LV base and near the left anterior descending artery. VF was electrically induced four times and the incidence of conduction block at each electrode during the first 20 s was determined by an automated algorithm. The gradient of the ARI was calculated at each electrode to estimate the spatial dispersion of refractoriness. An exponential curve was fit to the restitution plots of ARIs versus the corresponding diastolic intervals for all pacing cycle lengths at each electrode. The locations of epicardial blood vessels were noted after the study. Spatial patterns of conduction block were significantly correlated between the four VF episodes in the same animal (r = 0.66  0.07, P  0.05). At the shortest pacing cycle length, the spatial distribution of ARIs, ARI gradients, and restitution slopes was not random but formed clusters of similar values. However, none of these variables was significantly correlated with the incidence of conduction block, even though ARI gradients  2 ms/mm were present between many clusters and 90% of restitution slopes were  1. Instead, conduction block tended to cluster along epicardial vessels. Conclusion: In the pig, anatomic heterogeneities may be more important than either the nonuniform dispersion of ventricular refractoriness or a steep restitution slope in causing conduction block during VF. P5-19 DYNAMICS OF REENTRANT ARRHYTHMIAS IN THE HUMAN VENTRICLES Kirsten H. W. J. Ten Tusscher, PhD, Rok Hren, PhD and Alexander V. Panfilov, PhD. Utrecht University, Utrecht, Netherlands and University of Ljubljana, Ljubljana, Slovenia. We study the dynamics of reentrant arrhythmias in an anatomically de- tailed model of the human ventricles in which single cell behavior is described using our recently developed human ventricular cell model. We show that for the standard parameter setting of our cell model, which corresponds to normal healthy human myocardium, premature stimulation leads to stable scroll wave formation. We find that scroll waves in the right ventricle are stationary, resulting in a monomorphic tachycardia ECG pattern, whereas scroll waves in the left ventricle meander, resulting in a S260 Heart Rhythm, Vol 2, No 5, May Supplement 2005