Myocardial Infarction (MI) is commonly known as a heart attack. It means interruption of blood supply to a portion of the heart, causing heart cells to die.
An acute myocardial infarction begins with the occlusion of a coronary artery. The region of myocardium that is vascularized by that coronary artery is deprived of oxygen and, unless blood flow resumes shortly, it will die.
Complete, but transient blockade of blood flow to the myocardium, even though it does not lead to cell death, may lead to a patern of ECG changes similar to that seen during the acute phase of myocardium infarction.
The areas of infarction are regional, because blood flow is regional.
The heart muscle requires a constant supply of oxygen-rich blood to nourish it. The coronary arteries provide the heart with this critical blood supply. That is the reason why in coronary artery disease, those arteries become narrow and blood cannot flow as well as they should, which can finally cause a heart attack. Fatty matter, calcium, proteins, and inflammatory cells build up within the arteries to form plaques of different sizes. The plaque deposits are hard on the outside and soft inside.
When the plaque is hard, the outer shell cracks (plaque rupture), platelets (disc-shaped particles in the blood that aid clotting) come to the area, and blood clots form around the plaque. If a blood clot totally blocks the artery, the heart muscle becomes ‘starved’ for oxygen. Within a short time, death of heart muscle cells occurs, causing permanent damage. This is a heart attack.
While it is unusual, a heart attack can also be caused by a spasm of a coronary artery. During a coronary spasm, the coronary arteries restrict or spasm on and off, reducing blood supply to the heart muscle (ischemia). It may occur at rest and can even occur in people without significant coronary artery disease. Ischemia without cell death due to a "fixed" degree of occlusion is often associated with changes in the ECG, typically ST segment and T-wave changes.
Each coronary artery supplies blood to a region of heart muscle. The amount of damage to the heart muscle depends on the size of the area supplied by the blocked artery and the time between injury and treatment.
Healing of the heart muscle begins soon after a heart attack and takes about eight weeks. Just like a skin wound, the heart's wound heals and a scar forms in the damaged area. But, the new scar tissue does not contract or pump as well as healthy heart muscle tissue. So, the heart's pumping ability is lessened after a heart attack. The amount of lost pumping ability depends on the size and location of the scar.
References: "Medical Physiology. A cellular and Molecular Approach", Walter F. Boron, Emile L. Boulpaep
http://www.webmd.boots.com/heart-disease/default.htm
http://www.webmd.boots.com/heart-disease/guide/heart-disease-heart-attacks
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