A cardiac arrest is the most extreme emergency and happens when the heart stops beating in a normal way, preventing the blood from pumping round the body.
Someone who is having a cardiac arrest will suddenly lose consciousness and will stop breathing normally. Unless immediately treated by CPR, the person will die in minutes.
A heart attack can lead to a cardiac arrest but they are not the same thing. A heart attack happens when there is a sudden interruption to the blood supply to the part of the heart muscle and it is starved of oxygen rich blood. It is likely to cause chest pain and damage to the heart.
Symptoms could include chest or arm pains, shortness of breath, vomiting or clamminess.