Defibrillation is the act of defibrillating the heart to treat the heart when it is not pushing blood because the heart muscle is not working in any pattern. Electricity causes a muscle to contract, this is how nerves cause muscles to work. The heart is a large muscle, and certain parts need to contract before other parts in order to push blood through the heart and out to the rest of body correctly. Sometimes the heart muscle doesn't contract in the correct way or at the correct speed, this is called an "arrhythmia" (out of rhythm). Fibrillation is a type of arrhythmia when the whole heart is quivering, not organized to make heart beats of any kind. CPR (Cardiopulmonary resuscitation) can get blood to the body when the heart is fibrillating, but the heart still needs to be restarted. Sometimes CPR will do that as well, but often a defibrillator is needed. Defibrillation is done with a defibrillator.