‌"Do you know the difference between a heart attack and cardiac arrest?" ‌A heart attack, also known as a myocardial infarction, occurs when the heart's blood supply is suddenly blocked. ‌Intense chest pain and difficulty breathing may be the initial warning signs you link with a heart attack. If you suspect someone is having a heart attack, you should dial 999 immediately. A person may experience several symptoms before a sudden cardiac arrest occurs.