The analysis thus far has concerned risk factors that are associated with a number of diseases besides coronary heart disease. Risk factors specific to coronary heart disease are based on theories of the vascular changes that reduce blood flow to the heart muscle, with each theory being associated with different types of treatments.

Coronary Atherosclerosis and Coronary Thrombosis

All theories of the etiology of coronary heart disease seek to explain its defining characteristic, the diminution of blood flow to the heart muscle. Early in the twentieth century, the primary cause was considered to be a thrombus (clot) in one of the coronary arteries. A thrombus can form on the inner wall of the coronary artery and block or reduce blood flow at that point or it can form elsewhere, break loose, and flow in the bloodstream until it becomes lodged in the lumen (hollow center) of a coronary artery. The obstruction can produce sudden death, myocardial infarction, angina pectoris, or other forms of coronary heart disease.

The coronary thrombosis theory focused on the obstruction because it considered subsequent events like myocardial infarctions to be consequences. However, evidence soon accumulated that coronary heart disease could develop without the presence of a fresh thrombus and that thrombosis did not produce coronary heart disease when collateral circulation had developed in other coronary arteries.

Beginning in the 1940s, coronary atherosclerosis became accepted as the primary cause of the growing coronary heart disease epidemic. Atherosclerosis is the buildup inside an artery wall of an atheroma, a gelatinous plaque composed of cholesterol and other blood components. Atherosclerosis diminishes blood flow by making the artery wall less elastic and reducing the size of the lumen. Coronary atherosclerosis was found in many cases of coronary heart disease and was most common in the elderly, who had the highest rates of the disease.