Defibrillators are used to treat the most common cardiac rhythm seen in cardiac arrest. Added in edit: It looks like both sides of the things you ve heard are partly right, but incomplete. Cardiac arrest means that the heart stops functioning. There may be some electrical impulses, but whether there is electrical activity or not, the heart is not pumping blood. Ventricular fibrillation is one type of abnormal heart rhythm that causes the heart to stop functioning-stop pumping blood- mainly because the electrical impulses are so chaotic that the heart muscle can not beat well enough to circulate blood. Ventricular Fibrillation is the most frequently seen electrical rhythm in sudden cardiac arrest. The sooner V-fib is treated the better chance there is that a normal rhythm can be restored and the heart can begin beating well enough to circulate blood again. This is why people are being urged to learn CPR and also be trained to use and know the location of Automatic External Defibrillators (AEDs). Defibrillation is the only effective treatment for ventricular fibrillation. Defibrillation delivers a precisely measured electrical shock to the heart, the shock will often allow the heart to resume a normal electrical rhythm and begin functioning normally again. Defibrillation can also be used for certain types of Ventricular Tachycardia-which is another dangerous electrical rhythm of the heart that can lead to sudden cardiac arrest. Automatic External Defibrillators are designed to be used by trained lay-people. This device recognizes cardiac rhythms and will deliver a shock if it is appropriate. To me, full arrest implies that the heart is not pumping blood, so there is no pulse, and that the person is not breathing. I suppose that people speaking of full arrest could be referring to asystole, which means there is no cardiac electrical activity at all, and so the heart is not beating. Defibrillation alone is unlikely to treat asystole, however, asystole usually comes after ventricular fibrillation...in other words ventricular fibrillation occurs first then if it is not treated and normal heart function restored then all electrical activity in the heart stops (asystole)-so having AEDs available is still a good goal