Why do ICDs shock the heart?

If your heart device detects a fast and unstable heart rhythm, it delivers a therapy shock to your heart. This therapy is called defibrillation. It is like the treatment provided by an external defibrillator, which uses paddles on the outside of the body to shock the heart. However, your heart device delivers much less electricity because the therapy is applied directly to the heart through the leads. Your heart device responds automatically with lifesaving defibrillation during the first crucial seconds of a fast heart rhythm. This response is important because an emergency team may be minutes away or unavailable. The therapy shock usually stops the abnormal electrical impulses that are causing your heart to beat too fast. After each therapy shock, your heart device monitors your heart for a normal rhythm. If the rhythm is still too fast, it delivers another therapy shock. When a normal heart rhythm is restored, the device delivers no more therapy.