The implantable cardioverter defibrillator is designed to slow down a fast heart rate using electrical pulses or shocks to help control life-threatening arrhythmias (usually ventricular tachycardia or ventricular fibrillation) that can cause sudden cardiac arrest (SCA).

The device monitors the heart rhythm continually and if it detects an irregular rhythm, eg: the heartbeat is too fast, it will try to restore the heartbeat to normal by delivering low-energy electrical signals to the heart. If the low-energy pulses do not restore the normal heart rhythm, the ICD can send a shock to the heart muscle to defibrillate it or stop the cycle of rapid twitching. This interrupts the pattern of the rhythm disorder and allows the heart to resume its normal rhythm. An ICD also has backup pacemaker function.

An ICD is implanted in a similar way to pacemakers, it is a battery powered small device placed under the skin in the chest area and attached to one or two leads (thin, coated wires), which are placed in or on the heart muscle.

A newer generation of ICD is called the subcutaneous (S-ICD) which does not need to be implanted into the heart.