Effects of Exercise on Cardiovascular Health

Definitions

Numerous terms are used in the literature in reference to exercise and physical activity. In this chapter, we use these terms as they are commonly defined. Physical activity is any bodily movement produced by contraction of skeletal muscle that increases energy expenditure above the basal level. Physical activity is generally categorized by mode, intensity, and purpose. Leisure activities are considered to be physical activities performed by a person that are not required as essential activities of daily living and are performed at the discretion of the person. Leisure activities include sports participation, exercise conditioning or training, and recreational activities, such as going for a walk, dancing, and gardening. Exercise is a subcategory of physical activity that is “planned, structured, and repetitive” and generally has a purpose to improve or maintain some aspect of physical fitness. Physical fitness is defined in many ways, but an accepted definition is “the ability to carry out daily tasks with vigor and alertness, without undue fatigue and with ample energy to enjoy leisure-time pursuits and meet unforeseen emergencies.” There are many components to fitness, both performance and health related. Health-related fitness consists of cardiorespiratory fitness, muscle strength and endurance, body composition, flexibility, and balance. Because of the explosion of technology and mobile communications resulting in >1 billion smartphones and >165 million tablets shipped annually worldwide, there are now novel, evolving platforms for healthcare monitoring and delivery, and a new vocabulary related to this technology. The WHO defines mobile health (mHealth) as “medical and public health practice supported by mobile devices such as mobile phones, patient monitoring devices, personal digital assistants and other wireless devices.”

Primary Prevention

High levels of sedentary activity in adults are prevalent according to the 2015 National Health Interview Survey. One in three adults does not engage in leisure-time physical activity. Inactivity increases with age, in women, with Hispanic and black adults providing a large potential pool of the public to target for activity interventions to reduce CVD risk. There is a strong inverse relationship between physical activity and the risk of coronary disease and death. Across studies, there is an estimated 30% risk reduction in all-cause mortality, comparing the most physically active subjects with the least active subjects. Similar CV benefit from fitness also exists in both sexes and across different races and ethnic groups ( Fig. 4.1 ). The inverse dose–response relation for total volume of physical activity is curvilinear, meaning that those with the lowest physical activity levels have the largest risk reduction with increased physical activity. Studies in men support a role for physical activity in reducing the risk of mortality. In nonsmoking, retired men, ages 61 to 81 years, who had other risk factors controlled, the distance walked daily at baseline inversely predicted the risk for all-cause mortality during a 12-year follow-up. Of 10,269 Harvard alumni born between 1893 and 1932, those individuals who began moderately vigorous sports between 1960 and 1977 had a reduced risk of all-cause and CHD-related death over an average of 9 years of observation compared with those who did not increase sports participation. This finding was independent of the effects of lower BP or lifestyle behaviors related to low cardiac risk, such as cessation of smoking and maintenance of lean body mass. Data on the leisure-time physical activity levels of men participating in the Multiple Risk Factor Intervention Trial (MRFIT) supported a reduction of risk for all-cause and CHD-related fatalities when leisure time was spent doing moderate or high (compared with low) levels of physical activity. The effect was retained when confounding factors, including baseline risk factors and MRFIT intervention group assignments, were controlled. Mortality rates for the high and moderate physical activity groups were similar. The Lipid Research Clinics Mortality Follow-up Study found that men with a lower level of physical fitness, as indicated by heart rate (HR) during phase 2 (submaximal exercise) of the Bruce Treadmill Test, were at significantly higher risk for death due to CV causes within 8.5 years compared with men who were physically fit.

The same benefits from physical activity accrue for women. In women, higher physical activity level was related to an improved health outcome in several longitudinal studies. The Iowa Women's Health Study observed 40,417 postmenopausal women for 7 years; moderate and vigorous exercise were associated with a reduced risk of death. This reduction of risk was present for all-cause mortality and specifically for deaths resulting from CV and respiratory causes.

Women who increase their frequency of activity from rarely or never to ≥4 times per week also have a reduced risk of death. The Women's Health Initiative (73,743 postmenopausal women) and the Nurses' Health Study (72,488 women aged 40–65 years) assigned subjects into quintiles based on energy expenditure. Age-adjusted risk decreased incrementally from the lowest to the highest energy expenditure group, was statistically significant when other CV risk factors were controlled, and was similar in white and black women. In addition, energy expenditure from vigorous exercise or walking and time spent walking were linked to a lowered risk for the development of CHD. This inverse relation between CHD risk and activity level was observed in groups of women with other high-risk factors, including smokers and women with high cholesterol levels, although it was not observed in hypertensive women. In one study of postmenopausal women, the odds ratios for nonfatal MI, adjusted for confounding factors, decreased across the second, third, and fourth highest quartiles of energy expenditure compared with the lowest quartile. Exercise equivalent to 30 to 45 minutes of walking 3 days/week decreased the risk for MI by 50%.

Studies show that in black and white men and women, lack of exercise is associated with a higher risk of 5-year all-cause mortality, independent of age, male sex, low income, BP, or a number of CV measures (left ventricular [LV] ejection fraction, abnormal ECG) or other physiological measures (e.g., glucose level, creatinine level). A community-based study of older adults (aged 65 years or older) with no history of heart disease showed that walking at least 4 hours weekly significantly reduced the risk of hospitalization due to CV disease events during the subsequent 4 to 5 years.

The epidemic of obesity in the United States has significantly affected the development of CHD, hypertension, diabetes, and other atherosclerosis risk factors. In 2011 to 2014, it was estimated that approximately 69% of the adult population aged older than 20 years was overweight or obese with a body mass index (BMI) of ≥25 kg/m ² . The prevalence of obesity differs across racial/ethnic and socioeconomic groups. Native Americans, African Americans, Hispanics, and Pacific Islanders have significantly higher BMIs compared with whites and Asian Americans. There is also a significant sex–ethnicity interaction. African American women have a much higher prevalence of obesity (BMI >30 kg/m ² ) (57%) compared with Hispanic (46%) and white (38%) women. This holds true for men as well, although the prevalence is lower (38%, 39%, and 34%, respectively). The total estimated costs in 2008 related to obesity were $147 billion USD. There is a dose–response relationship between physical activity and weight loss, but in general, successful weight loss and maintenance is a complex issue, which includes caloric restriction, in addition to increased physical activity. Several studies have shown that anthropometric measures (BMI, waist circumference, waist-to-hip ratio) are associated with CHD risk factors and/or adverse events. The increased risk is partially explained by the milieu of insulin resistance, inflammation, and other atherosclerotic risk factors associated with obesity. Although weight loss is important and improves CV risk factors, the direct benefit of weight reduction alone on CV risk is not clear. However, physical activity reduces CV risk. A study of women being evaluated for suspected myocardial ischemia found that measures of increased BMI, waist circumference, waist-to-hip ratio, and waist-to-height ratio were not independently associated with coronary artery disease (CAD) or adverse CV events. Lower levels of self-reported physical fitness scores were associated with higher prevalence of CHD risk factors and angiographic CAD, and higher risk of adverse events during follow-up, independent of other risk factors. This supports the findings that fitness may be more important than overweight or obesity in women and men.

Secondary Prevention

Recent studies have conclusively demonstrated that exercise and fitness are as beneficial for patients with an established diagnosis of CHD as for those who do not have known CHD ( Fig. 4.2 ). In subjects with higher levels of physical activity, there is a 20% to 35% lower risk for CVD, CHD, and stroke compared with those with the lowest levels of activity. In a large study of men with established heart disease, regular light to moderate activity (such as 4 hours/week of moderate to heavy gardening or 40 min/day of walking) was associated with reduced risk of all-cause and CV mortality compared with a sedentary lifestyle. Another large study assessed health status and physical fitness in men during two medical examinations scheduled approximately 5 years apart. Men who were unfit at both examinations (baseline and 5 years later) had the highest subsequent 5-year death rate (122/10,000 man-years). The death rate was substantially lower in initially unfit men who improved their fitness (68/10,000 man-years) and was lowest in the group who maintained their fitness from the first to the second examination (40/10,000 man-years). The mortality risk decreased approximately 8% for each minute that the maximal treadmill exercise time at the second examination exceeded the baseline treadmill time. These results were retained when subjects were stratified by health status, demonstrating that unhealthy and initially healthy individuals benefited from exercise fitness.

Exercise intervention experiments have documented better health and survival even in patients who have experienced an MI. In one randomized study, patients were enrolled in a rehabilitation program of three 30-minute periods of exercise weekly, whereas other patients—matched by age, sex, coronary risk factors, site and level of cardiac damage, and acute-phase complications—served as control subjects. At 9 years after the initial MI, the rate of death caused by acute MI and the frequency of angina pectoris were lower in the treatment group. In the National Exercise and Heart Disease Project, male post-MI patients were randomly assigned to a 3-year program of supervised regular vigorous exercise (jogging, cycling, or swimming) or to regular care not involving an exercise program. Subjects were reevaluated at 3, 5, 10, 15, and 19 years to determine total and CV-related mortality. A moderate advantage of the treatment versus control condition in reducing the risk of all-cause and CV death was seen at the first follow-up time point but diminished and eventually reversed as the time since baseline increased. This may indicate that the benefits of an intensive exercise program are time-limited or may be related to several other factors (see later discussion). Each metabolic equivalent unit by which the work capacity of the participant increased from the outset to the completion of the 3-year program resulted in an incremental reduction in total and CV-related mortality, which suggested that increasing exercise fitness did promote survival. Failure to observe a long-term benefit in the treatment group versus the control group might have resulted from crossover between the two groups during the protracted follow-up period, improvements in medical therapy (routine use of statins), and/or revascularization approaches.

A large meta-analysis of 10 randomized clinical trials of post-MI patients showed that cardiac rehabilitation (CR) with exercise reduced all-cause mortality by 24% and CV death by 25% versus that of control subjects. However, the risk of nonfatal recurrent MI did not differ between groups.

Exercise training plays an important role in post-MI rehabilitation. Significant increases in functional capacity (10%–60%) and reductions of myocardial work at standardized exercise workloads (10%–25%) have been observed after 12 weeks of post-MI CR. The Exercise in Left Ventricular Dysfunction Trial demonstrated that exercise training after an MI might also improve ventricular remodeling and LV function. The American Heart Association (AHA) guidelines on physical activity in secondary prevention after MI, bypass surgery, or clinical ischemia recommend that the maximal benefit occurs when an exercise–CR program is initiated at supervised facilities where symptoms, HR, and BP can be monitored. A symptom-limited exercise test is essential for all participants before starting an exercise program.