ABSTRACTS S106 Heart, Lung and Circulation Abstracts 2010;19S:S1–S268 247 Influence of Transmurality and Pacing Location on Bipolar and Unipolar Electrogram Characteristics J. Pouliopoulos ∗ , W. Chik, K. Byth, P. Kovoor, A. Thia- galingam Westmead Hospital, University Sydney, NSW, 2145, Australia Introduction: Unipolar and bipolar electrograms (egm) are utilized to identify arrhythmogenic substrate. It is unclear how each modality is influenced spatially by distant stimulation. We quantified the effect of increas- ing distance from the source of propagation on local electrogram amplitude; and determined if transmural electrophysiological gradients exist with respect to prop- agation and stimulation depth. Methods: Mapping was performed on 5 sheep. Deploy- ment of >50 quadripolar transmural needles in the LV were located in Cartesian space using Ensite. Contact electro- grams from all needles were recorded during multisite bipolar pacing from epicardial then endocardial elec- trodes. Analysis was performed to determine stimulus distance to local activation time, peak negative voltage (V -P ), and peak–peak voltage (V P-P ). Each sheep was ana- lyzed using repeated ANOVA. Results: For unipolar egms, increasing distance from pacing sites led to significant (p < 0.01) attenuation of amplitude. Independent of pacing depth, a significant (p ≤ 0.01) transmural electrophysiological gradient was observed. These effects were not significant for bipolar egms. However, propagation was earlier at the epicardium than endocardial layer by 1.0 ± 2.0 ms for unipolar and 1.1 ± 2.8 ms for bipolar egms (all p< 0.01). Conclusion: Electrogram amplitude is inversely pro- portional to propagation distance for unipolar modalities only, which affected V -P > V P–P . Conduction proceeds preferentially via the epicardium during stimulation and is believed to contribute to a transmural unipolar egm ampli- tude gradient. doi:10.1016/j.hlc.2010.06.914 248 Lead Abandonment in situ is a Reasonable Alternative to Lead Extraction in Selected Patients with ICDs M. Hudaverdi ∗ , H. Samardhi, A. Sharma, S. Rateesh, G. Peters, M. Santos, R. Denman The Prince Charles Hospital, Australia Background: The management of nonfunctioning leads at the time of ICD implantation remains controversial. Options include lead extraction versus lead abandonment in situ. Potential complications from lead abandonment include sensing malfunction with potential inappropriate shocks, high defibrillation thresholds, venous throm- boembolism and venous obstruction. These risks must be weighed against the potentially fatal risks of lead extraction. Methods: We performed a retrospective analysis of The Prince Charles Hospital Pacing & ICD database who had at least 1 right ventricular (RV) lead abandoned in situ. Results: 61 patients with at least 1 RV lead abandoned in situ since 2001 were identified (mean age 61 ± 17 years, 77% male). A total of 61 RV leads were abandoned: 35 ICD and 27 pacemaker leads. Indications for lead abandon- ment included: upgrade to ICD (43%), lead failure (38%), and elective replacement of leads at high risk of failure (19%). During a median follow up of 911 ± 890 days the following events occurred: no episode of inappropriate device function due to lead–lead interaction; no docu- mented venous thromboembolism or obstruction; 3 cases of late device infection; 1 case of early exit block and one fidelis lead fracture in newly implanted leads requiring revision. No deaths occurred during the follow up period that were attributable to lead or device malfunction. Conclusions: In selected patients lead abandonment in situ continues to be a reasonable management option and can be considered an alternative to lead extraction. doi:10.1016/j.hlc.2010.06.915 249 Left Atrial and Pulmonary Vein Dimensions are Depen- dent on ECG-Gating of Computed Tomography in Sinus Rhythm and Atrial Fibrillation L. Wilson ∗ , C. Wong, P. Kuklik, A. Brooks, N. Kyriacou, D. Lau, M. Alasady, H. Lim, H. Abed, G.Young, P. Sanders Cardiovascular Research Centre, Department of Cardiology, Royal Adelaide Hospital and the Disciplines of Medicine and Physiology, University of Adelaide, Australia Introduction: 3D mapping is now common during AF ablation. However, cardiac motion and changes in size throughout the cardiac cycle may affect catheter location or cause mismatch during image integration. This study aims to analyse changes in LA–PV dimensions using ECG- gated CT. Methods: Forty-two patients (25 M, 58 ± 11 years) under- went CT with images processed using retrospective ECG-gating of 35% (systole), 75% (diastole) and non-gated