Cardiovascular Epidemiology and Risk Prediction Models

Hypertension

The American Heart Association (AHA) defines ideal blood pressure as <120/80 mm Hg. Observational cohort studies have consistently demonstrated increased risk for stroke, heart attack, heart failure, and cardiovascular mortality across all age groups at blood pressures above this level. On average, every increase in the systolic blood pressure by 20 mm Hg or diastolic blood pressure by 10 mm Hg is associated with a doubling in the risk of death caused by stroke, coronary heart disease, or other vascular disease. Using JNC 7 guidelines, hypertension was defined as a systolic blood pressure greater than 140 mm Hg, a diastolic blood pressure greater than 90 mm Hg, or the use of blood pressure–lowering medications. In the fall of 2017, the AHA and ACC redefined hypertension as blood pressure greater than 130/80 mm Hg.

Using statistics that reflect JNC 7 guidelines, there are an estimated 85.7 million adults in the United States with hypertension. The prevalence of hypertension increases with age and varies by race. African Americans have substantially higher rates of hypertension than age-matched white Americans. Unfortunately, only 76% of those with hypertension are on antihypertensive medication, and only 54% have their blood pressure under adequate control.

Hyperlipidemia or Dyslipidemia

Higher levels of total cholesterol, low-density lipoprotein (LDL) cholesterol, and non–high-density lipoprotein (HDL) cholesterol are associated with increased risks for atherosclerotic cardiovascular disease (ASCVD). There is approximately a 50% higher ASCVD risk for every 40 mg/dL increase in total cholesterol. The association appears log-linear across higher levels of total cholesterol. Thus, total cholesterol and the atherogenic fractions LDL cholesterol and non-HDL cholesterol have become targets for therapy with lifestyle modification and pharmacotherapy. Conversely, across usual levels of HDL cholesterol, there is an inverse association with ASCVD risk.

Hyperlipidemia is commonly defined as total cholesterol levels >230 mg/dL. Within the United States, an estimated 28.5 million adults, or 11.9% of the population, are considered to have hyperlipidemia. Over the last 14 years, the prevalence of hyperlipidemia has decreased by approximately 7%. The most recent estimates of mean total cholesterol, LDL cholesterol, HDL cholesterol, and triglyceride levels in the United States are 196, 113, 53, and 103.5 mg/dL, respectively.

Tobacco Use

Cigarette smoking increases ASCVD risk twofold to fourfold. Cessation of tobacco products is associated with rapid changes in physiology and substantial reductions in CVD risk. Therefore, tobacco use has been a major target of public health campaigns. Because of these large efforts, tobacco use has been declining over the last 50 years from prevalence rates of 51% in men and 34% in women in 1965 to 16.7% in men and 13.7% in women in 2017.

Although the overall trend in tobacco use is promising, certain minority groups, including sexual and gender minorities, individuals with low socioeconomic status, disabled persons, and individuals with psychiatric illness, have not experienced the same decrease in prevalence rates that are seen in the overall population. Furthermore, recent increases in electronic cigarette use, particularly in adolescent populations, could result in increased tobacco use in younger age groups.

Diabetes

The AHA defines an ideal fasting glucose as <100 mg/dL. Currently, only 56% of adults in the United States meet this criterion. Diabetes is associated with a twofold to threefold increase in risk for coronary heart disease, stroke, peripheral artery disease, heart failure, and atrial fibrillation. It is also associated with a 6- to 8-year shorter life expectancy than is seen in nondiabetics.

The National Heart, Blood, and Lung Institute defines diabetes as a fasting blood sugar >125 mg/dL, and it defines prediabetes as a fasting blood sugar between 100 and 125 mg/dL. In a recent representative survey of the United States population, 23.4 million adults have diabetes and 7.6 million are not aware of having diabetes. Furthermore, 81.6 million have prediabetes. Of the cases of diabetes identified, 90% to 95% are classified as type 2 diabetes. In part because of the obesity epidemic, the prevalence of diabetes has been increasing over the last 10 to 15 years. African Americans and Hispanic Americans have substantially higher rates of diabetes than white Americans.

Obesity, Diet, and Physical Activity

The AHA defines an ideal body weight as a body mass index (BMI) of 18.5 to 25 kg/m ² . Obesity is defined as a BMI >30 kg/m ² , and overweight, which also confers an increased risk for CVD, is defined as a BMI between 25 and 30 kg/m ² . Obesity is a risk factor for CVD, including ASCVD, heart failure, stroke, venous thromboembolism, and atrial fibrillation, and a risk factor for other CVD risk factors, including dyslipidemia, hypertension, and diabetes. Obesity rates have slowly increased over the past several decades, with most recent prevalence rates reported in 2013 to 2014 as 37.7%. Women have a higher prevalence of both obesity and class III obesity, defined as BMI >40 kg/m ² , with prevalence rates of 40.4% and 9.9%, respectively. There is substantial regional variation in obesity prevalence rates, with higher levels observed in the Midwest and southeastern United States.

A central determinant of the obesity epidemic is caloric excess and physical inactivity. The AHA defines a healthy diet as one that is rich in fresh fruits, vegetables, whole grains, low-fat dairy, seafood, legumes, and nuts. This diet has consistently been found to reduce blood pressure, improve lipid fractions, and reduce risks for heart attack and stroke. Conversely, diets that are high in saturated fats and salt, and low in fruits and vegetables are associated with adverse changes in blood cholesterol and blood pressure, and likely increase ASCVD risk. Current estimates suggest that only 1.5% of adults consume an ideal healthy diet and that 678,000 deaths per year are attributable to a suboptimal diet.

Like obesity and dietary indiscretion, physical inactivity increases risks for CVD and CVD risk factors. Currently, the AHA recommends that adults perform at least 150 minutes of moderate intensity exercise per week or 75 minutes of vigorous activity a week plus 2 days of muscle strengthening. Individuals who meet these recommendations have been found to have a 30% to 40% lower risk for diabetes, a 30% to 40% lower mortality risk, and a 20% to 30% lower risk for coronary heart disease. Forty-four percent of adults meet the criteria specified by the AHA; however, 30% of adults do not engage in any physical activity. Women, older adults, African Americans, and Hispanics meet these requirements less frequently than other sex, race, and ethnic groups.

Population and High-Risk Approaches to Cardiovascular Disease Risk Reduction

Cardiovascular risk has a bell-shaped distribution within the population. A high-risk approach targets individuals at the highest risk with aggressive risk reduction. This type of approach is currently recommended to determine which patients should receive statin therapy for primary prevention of ASCVD events. Because the relative benefits of statin medications are consistent across absolute levels of risk, the high-risk approach will result in the greatest absolute benefit in individuals at highest absolute risk.

In contrast, a population-level approach attempts to reduce risk factor levels in the population as a whole through optimization of population mean risk factor levels. Although this may seem counterintuitive, this approach often results in greater numbers of prevented events because there are much larger numbers of individuals around the mean value in a bell-shaped distribution. Therefore, most events occur within this portion of the population. To continue with the example of impact of cholesterol on ASCVD events, the risk of an individual for ASCVD with mean cholesterol levels is low because of the large denominator of people close to the mean. However, the risk for this segment of the population is still high. Therefore, even a modest decrease in the mean cholesterol concentrations would translate into a large number of prevented CVD events ( Fig. 5.1 ).

High-risk and population-level approaches are not mutually exclusive: in fact, they are complimentary. Thus, CVD epidemiological efforts are aimed at reducing population-level burdens of risk factors and at identifying individuals at higher risk for aggressive primary prevention interventions.