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If you have a cardiac arrest, you will blackout (become unconscious). If you were standing up, you will fall to the ground. This is because too little blood is getting to your brain and the rest of your body.

Before a cardiac arrest you may:

• Feel sick to your stomach
• Feel dizzy
• Get pains in your chest
• Find it hard to breathe.

To confirm that someone is having a cardiac arrest, a doctor or paramedic will check if they have a pulse. They may also look at the rhythm of their heart using an electrocardiogram (ECG). The ECG shows the electrical activity in the heart as a line on a moving graph or screen. Doctors look at the size, shape and spacing of the line to see what's happening to the heartbeat.^{1 2}

When there is no heartbeat (or pulse), no blood pressure and no electrical activity in the heart, the ECG line is flat. This is called asystole. It usually means that the heart hasn't had any oxygen for some time. But it's still sometimes possible to get a normal heartbeat back. If asystole continues for five to 10 minutes, a person is usually declared dead.

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.