Abstract: A system and method for predicting the defibrillation threshold energy of a defibrillation lead arrangement by determining the upper limit of vulnerability of the heart by shocking the heart at varying times during the T-wave at decreasing test shock energy levels until fibrillation is induced in the heart. The lowest energy level which fails to induce fibrillation is determined to be the upper limit of vulnerability and the defibrillation threshold is predicted to be an energy level incrementally higher in the range of about 5 Joules.