The common cause of an MI is a blood clot (thrombosis) that forms inside a coronary artery, or one of its branches and 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. Atheroma is the technical term for fatty patches or ’plaques’ (This is similar to water pipes that get ’furred up’). Plaques of atheroma may gradually form over a number of years in one or more places in the coronary arteries. Each plaque has an outer firm shell with a soft inner fatty core.
What happens is that a ’crack’ (’plaque rupture’) develops in the outer shell of the atheroma plaque. This exposes the softer inner core of the plaque to blood and can trigger the clotting mechanism in the blood to form a blood clot. Therefore, a build up of atheroma is the root problem that leads to most cases of MI.