Defibrillation is the only effective treatment for ventricular fibrillation (VF). Defibrillation works by generating an electric field in the heart that alters the transmembrane potential. To defibrillate, the strength of this electric field must be greater than 4 to 6 V/cm throughout the ventricles, depending on the shock waveform. This alteration of transmembrane potential must accomplish two things to defibrillate. First, it must stop the process maintaining VF. This usually means halting the wavefronts maintaining reentry. For VF that has been present for several minutes, this may also mean halting focal triggered activity. Second, it must not create new reentrant or focal sources that cause the heart quickly to refibrillate. This second requirement requires a stronger electric field than the first requirement. © 2013 by Nova Science Publishers, Inc. All rights reserved.