The most common cause of a Myocardial Infarction (MI) is a blood clot known as thrombosis. This forms inside a coronary artery, or one of its branches. This blocks the blood flow to a part of the heart.

Blood clots do not usually form in normal arteries. However, a clot may form if there is some atheroma within the lining of the artery. An Atheroma is similar to fatty patches or ‘plaques’ that develop, inside the lining of arteries. Plaques of atheroma may gradually form over a number of years, in one or more places within the coronary arteries. Each plaque has an outer firm shell with a soft inner fatty core.

A ‘crack’ develops in the outer shell of the atheroma plaque. This is called ‘plaque rupture’. This exposes the softer inner core of the plaque to blood. This can then trigger the clotting mechanism in the blood to form a blood clot. Therefore, a build up of atheroma is the base problem that leads to most cases of MI.

However; atheroma may develop in any section of the coronary arteries. ‘Clot busting’ drugs can break up the clot and undo the blockage. If given quickly enough this prevents damage to the heart muscle, or limits the extent of the damage. Aspirin is an example of an anti-clogging medication.

There are a number of other uncommon conditions that can block a coronary artery and cause an MI. For example, inflammation of the coronary arteries (rare); a stab wound to the heart; a blood clot forming elsewhere in the body (for example, in a heart chamber) and traveling to a coronary artery where it gets stuck. Cocaine abuse can cause a coronary artery to go into spasm. There may be complications from heart surgery or some other rare heart problems.