This information is for people with heart failure. It tells you about having a defibrillator put in your chest. It is based on the best and most up-to-date research.
Does it work?
Probably. Having a defibrillator put in your chest reduces your chances of dying from an abnormal heartbeat if you have heart failure.
What is it?
An implantable cardiac defibrillator is a small device that can be put in your body to help correct abnormal heart rhythms.
The device is about the size of a pack of cards. You'll have an operation to put the defibrillator under the skin in the upper part of your chest. The device has a generator and leads. The generator holds the battery and an electric circuit that checks the electrical signals given out by your heart. The leads are connected to your heart.
The leads feed the electrical signals from your heart back to the generator where they are checked. If your heart goes into an abnormal rhythm, the generator uses an electric impulse to shock it back into a normal rhythm.
Implantable cardiac defibrillators were first used in 1980. Each device lasts for about five to seven years. 
To read more about the operation to put in a defibrillator, see Having a defibrillator fittedin our section on operations and tests.
Instead of a defibrillator, some people with heart failure have a pacemaker fitted. They're similar, but instead of shocking your heart, a pacemaker gives out a constant signal to keep your heart beating at the right speed. Some pacemakers aim to help the left and right sides of your heart beat together. To read more, see Having a pacemaker put in.
Some devices combine a pacemaker and a defibrillator.
How can it help?
Having a defibrillator fitted can help you live longer if you're at risk of getting a dangerously fast heartbeat.   You're about half as likely to die suddenly of heart problems if you have a defibrillator put in. 
How does it work?
If you have a cardiac arrest, you need treatment straight away to restart your heart. If you don't have treatment, you will die.
An electric pulse can shock your heart into beating normally again. You may have seen this in medical dramas on television, with actors using electric paddles to shock someone's heart into beating again.
A defibrillator that's put in your chest works in the same way as the paddles. It shocks your heart into beating normally if you suddenly get an abnormal heartbeat. The idea is that a small shock given early should stop you having a cardiac arrest.