---Arteriosclerotic Heart Disease------------Dock has shown that males have thicker sary for the preservation of the ground coronary intimal layers, even at birth, and substance. that both sexes have from birth very much Acute infections and cholesterol feedthicker intimas in the epicardial part of the coronary artery than in any arteries of com- ing both lower systemic ascorbic acid. 路The former may explain the atheromatosis parable size elsewhere in the body. of youth seen after acute infections, and It is believed that intimal thickening, the latter may possibly be the way in whether innate, as in the epicardial branches, which hypercholesterolemia produces ground or acquired, as in syphilitic mesaortitis, pre- substance injury. disposes to rapid local atherosclerosis. This The local stress in atherosclerosis is the may explain why men who show about the mechanical stretching of the arterial wall. same rate of atheroma formation in the abdominal aorta and iliac arteries as women, Just how it causes ascorbic acid depletion form atheromas much more rapidly in the is obscure, but it is a factor constantly present to a greater or lesser degree. coronary arteries. (2) Defects in the internal elastic membrane-The internal elastic membrane may be an important barrier to the passage of lipid. Early in life it begins to split with increasing duplication, especially near the branching of vessels. The frequency of breaks in the membrane beneath atheroma, and the fact that atheroma are sharply limited to the intima as long as the membrane remains intact suggests that defects in the membrane may play a role in atherogenesis. (3) Changes in the intimal ground sublance-Willis has shown that experimental scurvy in guinea pigs produces lesions of the arterial intima indistinguishable from the lesions which have been described in early human atherosclerosis. Cholesterol feeding results in similar lesions but with concurrent hypercholesterolemia and lipid deposits in the reticuloendothelial system which have no counterpart in man except in such conditions as primary xanthomatosis, nephrosis, myxoedema and diabetes. Thus atherosclerosis of scurvy closely simulates the human form of the disease. It is thought that ascorbic acid is essential for the maintenance of the ground substance of the arterial intima. Lesions, therefore, are localized only at points where the sum of the systemic and local depletion exceeds the critical point neces-
32
( 4) Mechanical stress - The factor of importance in mechanical stress 路 is the degree of stretching of the artery. Burton has shown that the degree of stretching is related to the blood pressure, surrounding tissue pressure, radius of the artery and curvature of a curved artery. Those portions of the coronary arteries which lie in the subepicardial tissues are exposed during systole both to the maximum blood pressure in the root of the aorta, and resistance from their myocardial branches which are being compressed by the contracting heart muscle. The result is that during a part of the cardiac cycle the pressure in the larger coronary arteries is unusually high. The surrounding tissue support is relatively lax. These factors may explain the fact that atherosclerosis of coronary arteries is usually, if not always confined to the epicardial part of their course and never involves the penetrating myocardial branches to any significant degree. (.5) Intimal hemorrhage- Paterson first pointed out the importance of intimal hemorrhage into an atheromatous plaque. Normally the intima receives its nourishment by diffusion from the arterial lumen and from the vasa vasorum which penetrate into the outer two-thirds of the media. In atherosclerosis, dilated capillaries invade the intima from the vasa
U.W.O.
MEDICAL JOURNAL