Process by which a coronary artery is blocked, causing cardiac infarction (death of part of the heart muscle). When a coronary artery is affected by atherosclerosis its internal diameter decreases and the interior surface is roughened; blood cells and platelets can then stick to the surface, causing thrombosis. As soon as the clot is large enough to block the artery completely part of the heart muscle receives no more oxygen and dies off (infarct). The precise reason for a clot to start to form is not known; it is possible that the size of the calcified patch, exertion and stress are factors. It is sometimes possible to disperse a clot by injecting anticoagulants, but only if this is done within a few hours of the occurrence of the clot and the infarction. This procedure can reduce the number of heart muscle cells affected, but cannot restore tissue which has died.