Introduction: The canine chronic, complete atrioventricular block (CAVB) model is characterized by a high susceptibility to dofetilide-induced Torsade de Pointes (TdP).Altered activation due to a changed origin for impulse formation (idioventricular rhythm) is an important contributor to the ventricular remodeling (QT: +24%), including interventricular dispersion (LV APD-RV APD) and associated TdP susceptibility (76%).Maintaining a constant physiological activation with high septal pacing reduces electrical remodeling (QT: +11%), and susceptibility to TdP (44%, JACC 2007).In the present study, it was investigated whether right ventricular apex (RVA) pacing together with bradycardia is able to reduce LV proarrhythrnic remodeling, and TdP.It was hypothesized that the largest changes would occur in the area of earliest activation (RVA), and the smallest at distant sites (LV), thereby reducing interventricular dispersion.Methods: After AVB induction, dogs (n=8) were paced (VVI) from the RVA at the lowest captured ventricular rate (median 53 bpm).At VVI 1000 ms, electrophysiological parameters were measured acutely after AVB, after at least 3 weeks remodeling and during the following dofetilide (25 μg/kg/5) challenge.Analysis included measurement of beat-to-beat variability of repolarization duration (BVR), because this parameter is supposed to have a higher predictive value for drug-induced TdP than QT.Results: LV monophasic action potential duration (APD), RV APD and BVR (from 1.1+0.6 to CAVB: 0.8+0.2 ms)were not increased significantly after remodeling, while interventricular dispersion was reduced.Only QT showed a small increase: 342±28 to 382±28, p<0.05, +12%.Surprisingly, dofetilide induced TdP in 6 out of 8 dogs (75%),which was associated with a large increase (unmasking) of all proarrhythmic electrophysiological parameters (BVR:3.4+2.6 ms, p<0.05).Conclusions: 1) altered activation due to RVA pacing in combination with bradycardia causes pro-arrhythmic remodeling and 2) precautious use of baseline electrophysiological parameters to predict TdP is still warranted.