Chapter 7 The Blood Vessels
A typical atheroma appears as an induration of the vessel’s wall. In an aorta opened at autopsy, it looks like a yellow bump or elevation of the intima. On cross-sectioning, it consists of a soft, lipid-rich center covered with a fibrotic capsule. Atheromas are often partially calcified.
Atheromas may remain unchanged for extended periods of time, and there is evidence that in certain circumstances they can even regress. However, in most cases atheromas will evolve further and show a number of secondary complications, including:
Lipoproteins are spherical particles composed of apoproteins, phospholipids, cholesterol and cholesteryl esthers, and triglycerides. The concentration of cholesterol in various lipoproteins varies, but approximately 70% of total blood cholesterol is contained in low-density lipoproteins (LDL).
In common parlance, HDL is called good lipoprotein, and LDL is called bad lipoprotein. Approximately 70% of total blood cholesterol is contained in LDL. A high concentration of LDL has been associated with an increased risk of atherosclerosis, whereas high levels of HDL reduce this risk. HDLs mobilize the cholesterol from the periphery and are instrumental in its transport to the liver for excretion from the body.
Hyperlipoproteinemias are classified into five groups on the basis of laboratory findings. Each group includes congenital and acquired hyperlipoproteinemias. The most common among these five groups are type II hyperlipoproteinemia, characterized by an increased concentration of LDL, and type IV hyperlipoproteinemia, characterized by an increased concentration of VLDL and low HDL.
Epidemiologic evidence indicates that hypertension promotes development of atherosclerosis, but the exact mechanism of this effect is not understood. In experimental animals, hypertension promotes the formation of smooth muscle cells and the insudation of lipid into the vessel wall. It may also cause endothelial cell injury. Injured endothelial cells may be more permeable to lipids. At the same time, they may also lose their capacity to control the proliferation of smooth muscle cells after vascular wall injury.
It should be noted that hypertension affects smaller blood vessels and arterioles as well. Even pulmonary hypertension is accompanied by the appearance of fatty streaks in lung arteries. In normal circumstances, the pulmonary artery is not affected by atherosclerosis, in part because of low blood pressure inside this blood vessel.
22 Besides lipoproteins, are there any other clinical laboratory findings that could predict development of atherosclerosis and some of its major complications?
Clinical studies have identified a number of laboratory findings that are abnormal in people suffering from coronary heart disease and other complications of advanced atherosclerosis. These tests may be used as predictors of coronary artery disease. The most promising tests under study are those measuring blood levels of: