Both CRT-Ds and ICDs are implanted under the skin, in the patient’s upper chest area. The devices have wires called leads that attach to the heart and regulate the rhythm of the muscle. Defibrillators help patients with bradycardia – a slow heartbeat – by pacing the heartbeat and those with tachycardia – a fast heartbeat – by delivering shocks that reset heartbeats to normal.
Source: wiktionary