The most dangerous abnormal heart rhythm is called ventricular fibrillation. It happens when the walls of the lower parts of the heart are quivering instead of beating and the heart cannot pump blood. An abnormal heart rhythm is the most common cause of death during or after a heart attack. Doctors and paramedics can get a regular rhythm going again by using a machine that delivers an electric shock to your heart. But they must act within a few minutes of the abnormal heart rhythm starting.

In the meantime, other people can keep the blood flowing through your body if they know how to perform cardiopulmonary resuscitation (CPR).

The more people who know CPR, the better. If you learn CPR you may be able to help save someone's life. But you need training to do it safely and effectively. If you want to learn CPR, you can ask your doctor for information on local classes.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.