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 continuously checks the heart's rhythm. If the ICD detects a life-threatening rapid heart rhythm, it sends an electric shock to the heart to restore a normal rhythm. The device then goes back to its watchful mode.

An ICD is implanted under the skin in the chest. A wire threaded through a large vein connects the device to the heart.