Finite element analysis of cardiac defibrillation current distributions

We have developed a two-dimensional finite element model of the canine heart and thorax to examine different aspects of the distribution of current through cardiac tissue during defibrillation. This model allows us to compare various electrode configurations for the implantable cardioverter/defibrillator. Since we do not yet know the electrical criteria to apply for predicting defibrillation thresholds, such as the minimum current density required for defibrillation or the critical mass if indeed such quantities are applicable, we measured defibrillation energy in dogs to determine the voltages to apply to the model for calculating current distributions. By analyzing isopotential contours, current lines, power distributions, current density histograms, and cumulative current distributions, we estimated the critical fraction and threshold current density for defibrillation, compared various electrode configurations, and assessed the sensitivity of the defibrillation threshold to electrode position, patch size, and tissue conductivity. We found that blood can shunt defibrillation current away from the myocardium, particularly in configurations using a two-electrode catheter, that myocardial tissue conductivity strongly affects the current distributions, and that epicardial patch size is more important that subcutaneous patch size. Our results are consistent with successful defibrillation requiring that 80 +/- 5% of the heart must be rendered inexcitable by a current density of 35 +/- 5 mA/cm2 or greater. This two-dimensional, isotropic model has allowed us to analyze some of the determinants of defibrillation, but more detailed interpretation of experimental data may require the extension of the model to three dimensions.