Defibrillator is a device which can deliver a controlled high voltage shock which lasts for a very short period. A heart which has stopped could be in either ventricular fibrillation or ventricular asystole. Rarely it could also be pulseless electrical activity during a cardiac arrest. A defibrillator is useful only in ventricular fibrillation. In ventricular fibrillation heart stops pumping because of a very rapid and disorganized electrical activity of the heart muscle. What a defibrillator does is to abolish this disorganized electrical activity (ventricular fibrillation) by giving a high voltage shock. When the abnormal activity is abolished, the natural pacemaker of the heart known as the sinus node has an opportunity to take control of the heart rhythm. If the sinus node is able to take over the control of the heart rhythm, the heart starts beating normal. This is possible only if too much time has not elapsed between the cardiac arrest and initiation of cardiopulmonary resuscitation.