Coronary artery disease (CAD) or atherosclerotic heart disease  results from the buildup of cholesterol within the walls of the coronary arteries to form plaques. CAD covers a wide spectrum of conditions. At one end of the spectrum are asymptomatic individual with streak like deposits in the walls of the coronary arteries. These deposits do not obstruct the flow of blood, and coronary angiography performed at this stage may appear the arteries. The growth of plaques can be a slow, gradual process. Obstruction of less than 70 percent of the diameter of the vessel rarely cause symptoms. However, once the degree of obstruction increases to more than 70 percent of the vessel diameter, the reduction in blood flow (and the oxygen and nutrients carried in blood) to the heart muscle (myocardium) causes ischemia (a reversible dysfunction of the myocardial cell due to oxygen depletion). At this point patients develop symptoms of coronary artery disease, often during times of increased workload such as exercise. This condition is called angina pectoris or angina. When the reduction is severe and prolonged,  the myocardial cells die (also referred to as myocardial infarction).
Coronary plaques may also rupture, with the resulting hemorrhage and thrombus formation causing a sudden increase in the size of the plaque. The rapid reduction in blood flow can cause symptoms of ischemia to develop at minimal amounts of exertion or at rest, called acute coronary syndrome (also referred to as unstable angina). If the reduction is prolonged, a myocardial infarction (or heart attack) may result.
Abraham Salacata, MD, FACC