ATHEROSCLEROSIS



Atherosclerosis is a thickening and hardening of medium-size and larger arteries with narrowing of the arterial lumen by atherosclerotic plaques. Its cause is multifactorial. Preventable risk fac­tors, genetic susceptibility, local arterial and he­modynamic factors, and sex influence the devel­opment of atherosclerosis.

The fatty streak, consisting of lipids and lipoid proteins, located in the intima of the vessel with the overlying endothelium intact, is the earliest form of atherosclerosis. This yellow fatty streak seen in childhood is not necessarily a precursor of adult atherosclerosis and occurs in populations in which atherosclerosis is uncommon; it is presumably reversible at this stage. Around age 25, in populations in which atherosclerosis is com­mon, the fibrous plaque begins to develop. It is white, elevated, and may compromise the arterial lumen. Reversibility is questionable when fibrous tissue and intimal proliferation are present. In more advanced stages, deposition of fibrin and platelets and necrosis of tissue with growth of new vessels may occur. Cholesterol, calcification, and hemorrhage within the atherosclerotic plaque form complicated plaques. The intimal surface may ulcerate, thrombose, and occlude the vessel. Mechanical, chemical, or immunological injury that begins with the fatty streak may cause pro­gression of the atherosclerotic lesion. Different ar­teries appear to have different degrees of suscep­tibility to atherosclerotic lesions; the coronary arteries are particularly susceptible, mostly within the first 6 cm of origin. Plaques tend to occur at arterial bifurcations, possibly due to the turbulent flow in these areas.

Atherosclerotic lesions in the coronary arteries may be detected during life by coronary arteri­ography (Fig. 7-1). When a radiopaque contrast agent is injected into a coronary artery, athero­sclerotic plaques appear as narrowings in the col­umn of contrast as it travels down the artery. Nar­rowing of vessels is described as a per cent diameter narrowing. Lesions >50 per cent are probably hemodynamically significant, causing approximately 75 per cent narrowing of cross-sec­tional area, while lesions > 75 per cent are defi­nitely significant, producing 95 per cent cross-sectional narrowing. The gradation of obstruction at coronary angiography is approximate and often underestimates the actual degree of obstruction. Complete obstruction of a vessel at angiography is usually represented by a stump, the distal por­tion of the vessel often opacified via collateral cir­culation.