An implantable cardioverter-defibrillator (ICD) is a small battery powered device, much like a pacemaker. When implanted it can monitor your heart beat and help reduce the risk of cardiac arrest. It helps detects abnormal heartbeats and sends a jolt of electricity to stop it. An irregular heartbeat is called an arrhythmia.

How it works

The ICD monitors your heart rhythm and rate all the time. It measures whether your heart is beating too fast or slow. Then it sends electric shock based on the amount programmed by your doctor.
It works by sending pacing signals until the heart goes back to a normal rhythm.
It works by sending a mild shock to the heart, a process called cardioversion.
It works by sending a more intense shock called defibrillation if the cardioversion does not work.



Implanting and ICD is a minor procedure. Just like you would implant a pacemaker an ICD requires electrode wires to be attached to the right chambers of the heart. The wire is connected to the vein that leads to your heart and to the ICD. The ICD is implanted under your skin into a "pocket" created by the doctor. It can be placed in several locations, under your collarbone, outside of your ribcage, or in your stomach area. Unlike pacemakers, ICD's are often permanent and are used for sudden arrhythmia and not to correct it. The ICD will deliver enough energy to defibrillate the heart. Once the doctor has checked the ICD the battery should be tested anytime you attend the hospital and should last you 7 years.

Human diagram of an ICD implant


Epicardial vs. Endocardial ICD's

A 1995 study of ICD's determined that both epicardial (outer layer of the heart) ICD's and endocardial (inner layer of the heart) ICD's both had the same success rate of 98% in terminating spontaneously occurring arrhythmia's.

ICD's vs. Pacemakers

Although ICD's are functionally similar to pacemakers, they have distinct differences in their purposes. Pacemakers are used to keep the heart beating at a normal rate, usually from abnormally slow heartbeat rates. It uses continuous, low energy electrical pulses to achieve this task.
ICD's are used for the specific purpose of preventing life threatening arrhythmia's. They remain dormant in the patients body until it detects the occurrence of an arrhythmia, in which it will then deliver a high energy pulse to prevent the patient from going into cardiac arrest. For that reason, ICD's are considered to have a more urgent purpose than pacemakers.

Risks of ICD's

The most common risks associated with ICD's include:
  • Risk of infection where the device was implanted
  • Allergic reactions to the medications administered to implant the ICD
  • Damage to the vein(s) where the ICD leads are placed
  • Blood leakage from the heart valve that the ICD is associated with
  • Unnecessary pulses delivered to the heart that can be life threatening

Web Resources

Related Terminology

Ventricular fibrillation
Brugada syndrome