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A two-dimensional finite element model of the canine heart and thorax developed to examine different aspects of the distribution of current through cardiac tissue during defibrillation is discussed. This model allows comparison of the various electrode configurations for the implantable cardioverter/defibrillator. Since the electrical criteria for predicting defibrillation thresholds are not known, defibrillation energy in dogs was measured to determine the voltages to apply to the model for calculating current distribution. Analyzing isopotential contours, current lines, power distributions, current lines, power distributions, current density histograms, and cumulative current distributions allowed the critical fraction and threshold current density for defibrillation to be estimated, various electrode configurations to be compared, and the sensitivity of the defibrillation threshold to electrode position, patch size, and tissue conductivity to be assessed. It was found that blood can shunt defibrillation current away from the myocardium, that myocardial tissue conductivity strongly affects the current distributions, and that epicardial patch size is more important than subcutaneous patch size.