Coronary arteries, which supply the heart with oxygenated blood, can slowly narrow due to a build-up of plaque leading to angina. In a heart attack, there is a sudden plaque rupture leading to blood clot formation, blocking the artery.
In spite of the treatment given, there is still a 10% mortality rate in patients with a heart attack. This is usually caused by abnormal heart rhythm (ventricular fibrillation) due to electrical instability of the heart or heart failure due to a massive heart attack. Occasionally, the heart muscle can rupture after a heart attack and this is usually fatal.