https://www.selleckchem.com/products/odq.html 05) VT risk compared to epicardial pacing when pacing in proximity to scar (0.2cm). Endocardial pacing location does not significantly affect VT risk, but epicardial pacing at 0.2cm compared to 3.5cm from scar increases (P less then 0.05) it. Inverting the transmural APD gradient reverses this trend. Idealised models predict that propagation in the direction opposite to APD gradient decreases VT risk. CONCLUSION Endocardial pacing is less arrhythmogenic than epicardial pacing when pacing proximal to scar and is less susceptible to pacing location relative to scar. The physiological repolarization sequence during endocardial pacing mechanistically explains reduced VT risk compared to epicardial pacing. BACKGROUND Little is known about the long-term outcomes and predictors of success for High Power Short Duration(HPSD) Contact Force(CF) atrial fibrillation(AF) ablations. OBJECTIVES To determine long-term freedom from AF and predictors of freedom from AF for 50W 5-15 second CF ablation. METHODS We examined 4-year outcomes and predictors of freedom from AF after AF ablation for 1250 consecutive patients undergoing HPSD CF ablations. RESULTS The demographic were age=66.6±10.5, female=30.9%, LA size=4.26±0.66 cm, paroxysmal AF=35.7%, persistent AF=56.6%, longstanding AF=7.7%. The initial ablation times were procedure 114.2±45.9 min, fluoroscopy 15.5±11.5 min, total RF 20.6±7.7 minutes. We used TactiCath™ in 47.7%, SmartTouch® in 52.3% and posterior wall isolation(PWI) in 34%. The 4-year freedom from AF after multiple ablations were paroxysmal AF 87.0%, persistent AF 71.9% and longstanding AF 64.9%. Single procedure success was 74.9% for TactiCath™ and 64.7% for SmartTouch®(P less then 0.001) and was 73.0% for no PWI vs 58.9% for PWI(P= less then 0.0001). PWI did not change outcomes for paroxysmal AF, but had worse outcomes for non-paroxysmal AF. Multivariate analysis showed 6 independent predictors of worse outcome after initi