Coronary Heart Disease
When the incredible complexity of the human system is considered along with the vast number of possible sources of illness and death, it seems incongruous that the life of so many depends finally on the health of two small arteries; but the fact is undeniable. Disease of the coronary arteries has become the single greatest threat to life in industrialized countries throughout the world. In the United States, for example, more than 650,000 deaths a year—or one-third of all deaths—are directly attributable to this one disease.
As the sole blood supply to the heart musculature (myocardium), the coronary arteries assume extreme importance. Any significant interference with blood flow through these vessels can impair the entire function of the myocardium, with dire consequences including sudden death. Before describing the clinical aspects of coronary disease it is pertinent first to consider the coronary arteries and the basic disease process that affects them.
THE CORONARY ARTERIES
The two coronary arteries, the left and right, arise from the aorta just above the aortic valve. The left coronary artery then divides into two large branches: the left anterior descending artery and the left circumflex artery.
Each artery supplies a different area of the heart. Briefly, the left anterior descending artery supplies most of the anterior wall of the left ventricle, the anterior portion of the interventricular septum, as well as the anterior wall of the right ventricle. The left circumflex artery supplies the lateral aspect of the left ventricle and the left atrium. The right coronary artery supplies the right atrium and the right ventricle, along with the posterior portions of the left ventricle and interventricular septum. The arteries lie on the outer surface of the ventricles and give offininierous branches that penetrate all parts of the heart. The terminal branches of the arteries have many interconnections, forming an extensive vascular network throughout the myocardium.
The function of the coronary arteries is to bring oxygen-carrying blood to the myocardium, oxygen being an essential ingredient in producing the energy the heart requires to contract. As a pump that works incessantly (contracting more than 100,000 times a day), the myocardium has very great oxygen needs. This constant demand call be met only by an adequate coronary blood flow. Indeed, 250 cc of blood per minute—or 36,000 liters per day—pass through the coronary arteries to oxygenate the myocardium under normal conditions. The oxygen demands of the myocardium increase greatly with exercise or emotional stress. Since the heart utilizes nearly all of its available oxygen supply even with normal activity and has a very limited oxygen reserve, these additional needs can only be satisfied by an increase in coronary blood flow.
The primary disease affecting the coronary arteries is atherosclerosis, a process in which fatty substances (particularly cholesterol) deposit as plaques along the inner lining of the vessels and narrow the passages. If the narrowing reaches a stage where the blood flow through the arteries is insufficient to meet the oxygen demands of the myocardium, then coronary heart disease (CHID) is said to exist.
Coronary atherosclerosis usually develops gradually over a period of years. However, the process begins at an early age so that by adulthood most men (and women, to a lesser degree) have some evidence of atherosclerosis in the coronary arteries. Autopsy studies have shown, for example, that among young American soldiers (average age of 22 years) killed in action during the Korean war nearly 80% had definite signs of coronary atherosclerosis. It is essential to realize, however, that the critical determinant of coronary heart disease is not the mere presence of atherosclerosis but rather the extent of arterial narrowing and the reduction in blood flow the lesions produce. Atherosclerosis can be categorized into four grades according to the degree of arterial obstruction. Grade I atherosclerosis indicates that the diameter (lurnen) of the artery is reduced by no more than 25%; grade 2 represents a 50% reduction, grade 3 a 75% reduction, and grade 4 complete (100%) obstruction of the vessel . An obstruction of at least 75% is necessary to produce a significant reduction in coronary blood flow; lesser degrees of narrowing can usually be tolerated without affecting myocardial function. Obstruction may occur in any (or all) of the coronary arteries, but involvement of the left anterior descending artery is particularly dangerous. This vessel supplies a much larger portion of the total myocardial mass than the right coronary and left circumflex arteries and therefore has the greatest blood flow. Even more serious is obstruction of the left main coronary artery. Significant narrowing of this short (2 cm) vessel causes a reduction in blood flow through both the left anterior descending and left circumflex arteries and therefore compromises the blood supply to nearly all of the left ventricle. Fortunately, obstruction of the left main coronary artery is the least common lesion of the coronary circulation, occurring in only 5-10% of patients with symptoms of CHD.
The site and extent of arterial narrowing call be determined by means of coronary aneriograplv, a technique that permits the arteries to be visualized by x-rays. The procedure involves the insertion of a catheter into the root of the aorta (by way of a peripheral artery) and the injection of a radiopaque dye through the openings (ostia) of the two coronary arteries. As the dye is being injected a rapid series of x-ray films or photographs (cineangiograms) are taken to outline the entire arterial tree; significant lesions can readily be detected in this way.
to be continued…..”CAUSES OF CORONARY HEART DISEASE“.