A heart attack is a circulation problem. A blocked artery cuts off the blood supply to part of the heart muscle, causing damage. The person is usually conscious, often in severe chest pain, and the heart normally keeps beating. Call 999 immediately, but a heart attack is not cardiac arrest.
Cardiac arrest is an electrical problem. The heart’s rhythm collapses so chaotically — usually into ventricular fibrillation — that it can no longer pump blood at all. The person becomes unresponsive and stops breathing normally. Without CPR and defibrillation within minutes, it is fatal.
A heart attack can trigger a cardiac arrest, but they are distinct emergencies. Both require an immediate 999 call. If the person is unresponsive and not breathing normally, start CPR straight away — do not wait to be sure which one it is.