Your heart needs to pump blood around your body constantly. |If it does not, cells die within a few minutes. This is why a heart attack is a medical emergency. The heart can start to twitch irregularly – a state of fibrillation - not pumping any blood.
Some blood can be made to flow using heart massage, but the heart must be started as soon as possible.
As the name suggests, defibrillation stop fibrillation, the useless trembling that a person's heart muscles can adopt during a cardiac arrest. A defibrillator works by using a high-voltage (200-1000 volts) to pass an electric current through the heart in an attempt to 'reset' it into pumping normally. The patient's heart receives roughly 300 joules of electrical energy (about as much as a 100 watt incandescent lamp uses in three seconds).
The kind of defibrillator you see on TV consists of an electric supply unit and two metal electrodes called paddles that are pressed very firmly to the patient's chest. The paddles are place either side of the heart. In order for the electric current to flow properly, and to reduce the risk of skin burns, the electrodes have to be applied close together. They must also make good electrical contact with the skin, so a solid or liquid conducting gel is usually applied to the patient's chest.