Defibrillator shocks may produce tissue damage due to high current density passing through the organs between the defibrillation electrodes. This is a particularly serious consequence if the damage is induced in cardiac tissue, which presumably will already be compromised by a disease process. Determination of current density in tissue is a difficult task and has not been accomplished to date. We propose in these series of investigations to determine current density in vivo, in dogs, when defibrillator shocks are applied. We will analyze this data using a technique of temperature measurement by thermistors with a very rapid response time. Since the heating is proportional to current and resistivity, this technique will allow us to determine current density during defibrillator shocks. Current density measurements will allow improved design of defibrillation electrodes to minimize damage of tissues. This will be accomplished by determination of optimal electrode shapes and configuration as well as optimal electrode-tissue interface materials.