Defibrillation works by carefully sending a strong electric current to the heart, which "resets" the heart, and hopefully returns a normal, coordinated heart rhythm. It needs to depolarise (take away the existing electric charge) from a large part of the heart muscle, which stops the arrhythmia and allows the normal heartbeat to return. Defibrillators can work outside the body with the use of an external defibrillator or an automated external defibrillator, or inside the body with an implantable cardioverter-defibrillator.