What is a heart attack? When the blood supply to the heart is slowed or stopped because of a blockage, a heart attack occurs. Atherosclerosis, the narrowing of coronary arteries due to plaque buildup, causes more than 90% of heart attacks. What is cardiac arrest? Cardiac arrest – or sudden cardiac arrest, sudden cardiac death – is a medical emergency. When a person stops breathing and the heart stops beating, then a person is experiencing cardiac arrest. (Cardiac arrest is not a heart attack, which is when the blood supply to the heart is slowed or stopped because of a blockage.)