An implantable cardioverter-defibrillator (ICD) is a small device that uses electrical pulses or shocks to help control abnormal heart rhythms, especially ones that can be life-threatening. An ICD is also known as an automatic implantable cardioverter-defibrillator (AICD).

An ICD is implanted under the skin in the chest. It's attached to one or two wires (called leads). Most of the time, these leads go into the heart through a vein. Some ICDs have a lead that is placed under the skin so that it lies near your heart.

An ICD is always checking your heart rate and rhythm. If the ICD detects a life-threatening rapid heart rhythm, it tries to slow the rhythm back to normal using electrical pulses. If the dangerous rhythm does not stop, the ICD sends an electric shock to the heart to restore a normal rhythm. The device then goes back to its watchful mode. If your heart is beating too slowly, some ICDs can act as a pacemaker and send mild electrical pulses to bring your heart rate back up to normal.