Cardiovascular disease is a general diagnostic category consisting of several separate diseases of the heart and circulatory system. Cardiovascular diseases have been the major health problem and the leading cause of death in the United States for several decades. Despite impressive and sustained declines in the mortality rates from these diseases, the magnitude of the problem is still staggering. In 1997 alone, nearly 1 million people died of cardiovascular disease, which was about 40 percent of all deaths. The two most important components are coronary heart disease and cerebrovascular disease, with 460,390 dying of coronary heart disease and 158,060 dying of cerebrovascular disease in 1998. In 2000, it was estimated that cardiovascular diseases carried a direct heath expenditure cost of $186 billion and additional indirect costs of $190 billion, making these diseases a continuing major contributor to the escalating cost of health care in the United States.
These diseases have not always been the major health problem of the United States. In 1900 the five leading causes of death were: (1) pneumonia and influenza; (2) tuberculosis; (3) diarrhea, enteritis, and ulceration of the intestines; (4) diseases of the heart; and (5) intracranial lesions of vascular origin. These categories all had rates greater than 100 per 100,000 population. By 1940, only two disease categories still had rates greater than 100 per 100,000: diseases of the heart and cancer and other malignant tumors. The infectious diseases had been substantially reduced, but the "epidemic" of cardiovascular disease, especially coronary heart disease had begun. By 1963, the mortality rate from coronary heart disease reached its pear, and there has been a progressive and steady decline since then (see Figure 1). Despite the continued magnitude of the coronary heart disease problem, the focus recently has been on this dramatic reversal. Not only is the percentage of decline large (56% from 1963 to 1998), but this has greatly impacted the total number of deaths in the United States, leading to an increase in life expectancy. To illustrate the impact of this change, it is estimated that if the rate of coronary heart disease mortality had not changed from its peak in 1963, in the year 1998 an additional 684,000 Americans would have died from this cause.
From several studies around the world, several risk factors for cardiovascular disease have been identified. These risk factors can be grouped into two broad categories: unmodifiable factors (such as male gender, and family history of premature heart diseases) and potentially modifiable factors (such as cigarette smoking, high blood pressure, high blood-cholesterol level, physical inactivity,
diabetes, and obesity). These factors can be used to identify those in the population who are at especially high risk of developing cardiovascular disease.
Cigarette Smoking. Cigarette smoking has been established as a risk factor not only for lung cancer, emphysema, and bronchitis but also for coronary, cerebral, and peripheral vascular disease. This association has been seen in many countries, among widely diverse ethnic groups, in both sexes, and across various adult age groups. In addition, the risk increases with heavier cigarette use and the longer one has smoked. Equally important has been the observation that this increased risk falls rapidly over time when people quit smoking. For coronary heart disease, approximately 40 percent of the increased risk is removed within five years of quitting, although it takes several more years of nonsmoking to achieve the level associated with someone who has never smoked.
High Blood Pressure. High blood pressure is a powerful risk factor for cerebrovascular disease as well as for coronary heart disease. An estimated 50 million people have high blood pressure, defined as a level equal to or greater than 140 mmHg systolic pressure or 90 mmHg diastolic pressure, or as being on a regimen of antihypertensive medication. An important result of epidemiologic studies has been the observation that the relationship between blood pressure and cardiovascular risk is not only a positive one (higher blood pressure results in higher disease rates) but also a smooth one (there was no sharp breakpoint in the curve such that below a certain blood pressure level the risk remained constant or became nonexistent). Thus, the lower the blood pressure, within reasonable physiologic limits, the lower the level of risk. These observations prompted several important intervention trials, which have now clearly established the value of aggressively treating elevated blood pressure.
Blood Cholesterol Levels. A clear and positive relationship between blood cholesterol levels and subsequent coronary heart disease has repeatedly been demonstrated. Recent information has refined the nature of this association but not weakened it. Cholesterol in the plasma is transported by lipoproteins. The cholesterol level associated with the low-density lipoprotein (LDL) fraction is positively correlated with coronary heart disease, whereas the cholesterol associated with the high-density lipoprotein (HDL) is negatively correlated (the higher the level, the lower the risk). These observations have been verified in several different populations and have been shown to be independent of each other, as well as of other known risk factors. The evidence regarding HDL, although more recent than that for LDL, supports a powerful and independent role for HDL in lowering coronary heart disease risk and probably explains a significant portion of the difference in risk between men and women, with women having higher average levels of HDL than men.
Physical Inactivity. An association between a less active lifestyle and increased risk of coronary heart disease has been shown in multiple longitudinal and cross-sectional studies in such diverse groups as London transit workers, United States longshoremen, and United States college graduates. Traditionally, this risk factor was considered less important and less powerful than the three already mentioned. However, recent reviews of the total body of scientific evidence have led to the classification of this risk factor as one of the four major modifiable risk factors for coronary heart disease. Consequently, there are more consistent recommendations for an active lifestyle and recognition of its importance not only to health but also to disease prevention.
Obesity. Initial epidemiologic data identified obesity as an important risk factor for coronary heart disease. Subsequent analyses, however, suggested that obesity was not a primary risk factor, but rather acted indirectly through elevation of blood pressure and cholesterol levels. More recent analyses of the data from the Framingham Heart Study, with longer follow-up of people in the cohort, have once again suggested that obesity is indeed a primary risk factor that acts independently of those other factors. Clinically, the resolution of this issue of primary versus secondary causation is somewhat irrelevant. Weight reduction should lower the risk of coronary heart disease, whether it acts through a lowered blood pressure and/or cholesterol level or as a lowered risk factor itself.
Diabetes. Diabetes is a powerful and independent risk factor for cardiovascular disease, which remains the major cause of death in diabetic persons. An important remaining issue is whether and elevated blood-glucose level is responsible for the observed higher rate of cardiovascular disease and, if it is, whether lowering or, preferably, normalizing the glucose level will lower the risk. Regardless of the answers, for the present the important observation is that diabetic individuals are at higher risk of cardiovascular disease, and thus careful attention should be paid not just to the blood-glucose level and its control but also to the other risk factors that may coexist in a given patient and additionally elevate the risk.
Other risk factors for cardiovascular disease, such as homocysteine and LPa, have been identified in single or multiple studies, but further information is needed to establish them as independent, important prognostic factors.
WILLIAM T. FRIEDEWALD
(SEE ALSO: Atherosclerosis; Blood Lipids; Blood Pressure; Coronary Artery Disease; Diabetes Mellitus; Fats; Foods and Diets; HDL Cholesterol; LDL Cholesterol; Lifestyle; Lipoproteins; Physical Activity; Smoking Behavior; Smoking Cessation; Tobacco Control)
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