Myocardial infarction also known as heart attack is an irreversible damage (death) to the heart muscles due to loss of blood supply following narrowing and occlusion of a coronary artery. The coronary arteries are the arteries which supply the heart muscles (myocardium). The right and left main coronary arteries are the first branches of the aorta, carrying oxygenated blood from the left ventricle. These 2 arteries divide into many branches which supply every part of the heart. Unlike many other parts of the body, the heart does not have an alternate source of oxygenated blood. This is why the coronary arteries are called end arteries. Narrowing of these arteries therefore puts the heart tissue at risk of ischemic injury (Ischemic heart disease -IHD). Also, unlike other parts of the body which get continuous blood supply, the muscle layer of the heart only gets blood supply in pulses. This is because each time the heart muscles contract (systole), they squeeze the arteries running through them and temporarily stop blood flow, when they relax (diastole), the arteries become patent again and blood flows to supply the muscles and inner lining of the heart. [An average heart contracts and relaxes about 70 times each minute]. This intermittent oxygen supply contributes to the susceptibility of the hard working heart muscles to hypoxic damage. Fortunately, the heart muscles are capable of extracting more oxygen from blood than any other tissue in the body. Also, the blood flow to the heart increases almost proportionately to increase in workload and corresponding increase in oxygen consumption, by dilatation of the coronary arteries. These measures normally prevent hypoxia and tissue damage.
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| Patrick J. Lynch, medical illustrator |
Coronary arteries may become narrowed or blocked by plaques (in coronary artery disease), blood clot building over a ruptured plaque or rarely, loose clot (embolus) formed within a heart with damaged valve or wall and carried in blood into the aorta. Narrowing can also occur due to spasm of the artery. A plaque is mainly composed of fat (in form of cholesterol), calcium and fibrous tissue. Plaques take years to develop. They are commonly seen with ageing but may occur in younger people with high cholesterol levels, diabetes mellitus or hypertension. The presence of plaques in any artery is called atherosclerosis- meaning hardened artery. Apart from narrowing the lumen, the plaque also hardens the walls of the artery preventing it from dilating to supply more blood in response to increased oxygen demand.
However, even in the presence of a plaque, symptoms of ischemia do not occur until about 70% of the lumen of the artery is occluded. The individual experiences squeezing chest pain (angina) brought on by any increase in workload on the heart such as with exertion (e.g. climbing stairs, physical exercise), in cold weather, emotional situations etc., and relieved by rest. If the plaque ruptures, a raw surface is exposed on the arterial wall and this stimulates the formation of a blood clot much like a bleeding injury anywhere in the body. This further narrows the lumen; worsening the angina which may now occur even at rest. Sometimes, the clot or thrombus can grow large enough to completely occlude the artery, totally depriving the part of the heart muscle (supplied by this artery) of oxygenated blood. This is what is referred to as a heart attack.
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See Heart attack (2)
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