A defibrillator is a device that delivers an electric shock to the heart to restore a normal heartbeat when a potentially fatal arrhythmia (abnormal heart rhythm) is happening in the hearts lower chambers (ventricles) . Defibrillation is most effective when a healthcare provider delivers the shock as soon as possible after an arrhythmia starts. Defibrillators are used for ventricular tachycardia (very fast heartbeat) without a pulse or ventricular fibrillation (quivering of the heart muscle), which is most often the cause of sudden cardiac arrest. Defibrillation briefly stops the moving heart muscle to allow the heart to generate an electrical impulse and start a normal rhythm. In essence, defibrillation restarts the heart. A defibrillator can save a life if its used for the correct arrhythmia and its done within 10 minutes after the abnormal rhythm starts. Defibrillators can be used by healthcare providers, but automated external defibrillators (AEDs) are also available for use by the general public in emergency situations.