Cardiac arrest is caused by an abnormal heart rhythm which stops the heart pumping blood round the body. This can happen during a heart attack, but can also occur as a result of various other heart conditions. If a normal heart rhythm cannot be reinstated within in a few minutes, the person will not survive. It is sometimes be possible to keep victims alive with CPR – cardiopulmonary resuscitation – but typically a normal heart rhythm can only be restored by using a heart defibrillator. This is why many aircraft carry defibrillators.