A cardiac arrest is also known as cardiopulmonary arrest or circulatory arrest.
It happens when the heart stops pumping blood around the body because of a disturbance in the normal heart rhythm.
The most common life threatening disturbance of the heart rhythm is called ventricular fibrillation.
Ventricular fibrillation occurs when the electrical activity of the heart becomes so chaotic that the heart stops pumping and quivers or ‘fibrillates' instead.
This is a cardiac arrest. It can sometimes be corrected by giving an electric shock through the chest wall, using a device called a defibrillator.
This is often successful in restoring a normal heartbeat and afterwards the person can do just as well as if they had not had the cardiac arrest.
A cardiac arrest is different from a heart attack
If you have a heart attack, you do not always experience the life threatening rhythms that lead to a cardiac arrest. There are many causes of cardiac arrest and it is not always because of an existing heart condition.
If a person has a cardiac arrest, they lose consciousness almost at once.
There are also no other signs of life such as breathing. This is the most extreme emergency.
Unless someone starts cardiopulmonary resuscitation (CPR) within three to four minutes, the person may suffer permanent damage to the brain and other organs.
Rescue breathing (inflating the lungs by using mouth-to-mouth resuscitation), and
Chest compression (pumping the heart by external cardiac massage), to keep the breathing and circulation going until the ambulance arrives.
Ambulance staff are trained in advanced resuscitation and all emergency ambulances carry a defibrillator.
Source - British Heart Foundation