The mechanisms of cardiac defibrillation, the most effective clinical procedure for termination of lethal cardiac arrhythmias, are not well understood. Shocks establish an electric field in the heart, which, in turn, induces changes in the transmembrane potential leading to defibrillation. The complexity of cardiac structure makes it diffcult to explain, based on experiments alone, how exactly the electric field affects transmembrane potential. In an attempt to provide additional insight into the issue, in this computational study we explore the effects of strong electric shocks on myocardium in fibrillation. Cardiac tissue is re..