Exercise and the Heart

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Chapter 44

Exercise and the Heart

Eric H. Awtry, MD and Gary J. Balady, MD

1. What is the difference between physical activity and exercise?

Physical activity refers to the contraction of skeletal muscle that produces bodily movement and requires energy. Exercise is physical activity that is planned and is performed with the goal of attaining or maintaining physical fitness. Physical fitness is a set of traits that allows an individual to perform physical activity.

2. What is the difference between isometric and isotonic exercise?

Isotonic muscle contraction produces limb movement without a change in muscle tension, whereas isometric muscle contraction produces muscle tension without a change in limb movement. Most physical activities involve a combination of both forms of muscle contraction, although one form usually predominates. Isotonic exercise (also referred to as aerobic, dynamic, or endurance exercise) involves high-repetition movements against low resistance and includes such activities as walking, running, swimming, and cycling. Isometric exercise (also referred to as resistance exercise or strength training) consists of low-repetition movements against high resistance and includes such activities as weight lifting and body building.

3. What is the training effect?

Regular isotonic exercise results in improved exercise capacity, whereas regular resistance exercise results in increased strength. These changes allow an individual to exercise at a higher intensity and for a longer duration while attaining a lower heart rate (HR) for a given submaximal level of exercise. This is referred to as the training effect.

4. What are the acute cardiovascular changes that occur with exercise?

Isotonic exercise results in an increase in HR and stroke volume that produces a four- to sixfold increase in cardiac output in healthy individuals. The increase in HR relates both to a withdrawal of vagal tone and an increase in sympathetic tone. HR gradually rises during exercise to a maximal level that can be predicted by the following formula:

Stroke volume increases by 20% to 50% as a result of both increased venous return from exercising muscles and more complete left ventricular emptying (owing to enhanced myocardial contractility and to decreased peripheral vascular resistance due to vasodilation in exercising muscle). Vascular beds other than in the heart, brain, and exercising muscle undergo vasoconstriction during exercise. This, in conjunction with the increase in cardiac output, results in a rise in systolic blood pressure. The diastolic blood pressure remains unchanged or falls slightly.

Isometric exercise results in a moderate increase in cardiac output, predominantly as a result of an increase in heart rate. Contracting muscle produces a rise in peripheral vascular resistance and may result in an increase in both systolic and diastolic blood pressure.

5. What are the chronic cardiovascular changes that occur with exercise?

The increase in cardiac output associated with isotonic exercise creates a volume load that results in left ventricular dilation with minimal increase in wall thickness. The vasoconstriction and increased afterload associated with isometric exercise produces a pressure load that results in left ventricular hypertrophy without dilation.

6. How is exercise intensity defined?

Exercise intensity is defined by the amount of energy required for the performance of the physical activity per unit of time. This can be measured directly using respiratory gas analysis to quantify oxygen uptake during exercise or can be approximated using standard regression models to estimate energy expenditure per a given work rate of exercise. Exercise intensity can also be expressed in terms of resting oxygen requirement (metabolic equivalents [METs]), where one MET equals the amount of oxygen consumed by a resting, awake individual, and is equivalent to 3.5 mL O₂/kg of body weight/min. Light exercise denotes those activities requiring less than 3 METs, moderate activity denotes activities requiring 3 to 6 METs, and vigorous activity denotes activities requiring more than 6 METs.

7. How much exercise is necessary to maintain cardiovascular fitness?

Current guidelines from the American Heart Association (AHA) and the American College of Sports Medicine (ACSM) recommend that all healthy adults perform moderate-intensity aerobic exercise (e.g., brisk walking) for at least 30 minutes on at least 5 days of the week, or perform vigorous-intensity aerobic exercise (e.g., jogging) for at least 20 minutes on at least 3 days of the week. In addition, resistance exercise should be performed on at least 2 days of the week. Individuals who wish to improve their level of fitness or achieve substantial weight loss may need to perform significantly greater amounts of exercise. Importantly, exercise need not be performed all at one sitting; 10- or 15-minute periods of exercise can be accumulated throughout the day and applied to the daily exercising goal.

8. What is the effect of exercise on cardiac risk factors?

Exercise has beneficial effects on hypertension, diabetes, hyperlipidemia, and obesity. In addition, exercise has favorable effects on endothelial function, thrombosis, inflammation, and autonomic tone (Table 44-1).

TABLE 44-1

BENEFICIAL EFFECTS OF ENDURANCE EXERCISE ON ATHEROSCLEROTIC RISK FACTORS

FACTOR	EFFECT OF EXERCISE
Hypertension	Modest ↓ in both SBP (approximately 4 mm Hg) and DBP (approximately 3 mm Hg)
Diabetes	↑ Insulin sensitivity, ↓ hepatic glucose production, preferential use of glucose over fatty acids by exercising muscle
Hyperlipidemia	Significant ↓ in TG, modest ↑ in HDL, minimal change in LDL
Obesity	Modest weight loss (2-3 kg), ↓ in body fat necessary to maintain weight loss
Thrombosis	↓ Fibrinogen, ↓ platelet activation
Endothelial function	Improved vasodilation, possibly through ↑ NO synthesis
Autonomic tone	↑ Vagal tone, ↓ sympathetic tone
Inflammation	↓ Inflammatory markers (CRP, TNF-α, IL-6)

CRP, C-reactive protein, DBP, diastolic blood pressure; HDL, high-density lipoprotein, IL, interleukin; LDL, low-density lipoprotein, NO, nitric oxide, SBP, systolic blood pressure; TG, triglycerides, TNF, tumor necrosis factor.

9. Do endurance training and resistance training have similar benefits?

Endurance and resistance training have some similar effects and some complementary effects. Both improve insulin resistance, bone mineral density, and body composition. Endurance training improves peak measured exercise capacity, whereas resistance training improves muscle strength. Endurance training expends a greater amount of calories during exercise, whereas resistance training increases resting energy expenditure because of increased muscle mass.

10. What is the effect of exercise on mortality?

Observational studies demonstrate an inverse linear relationship between the amount of physical activity performed and all-cause mortality. This is true for healthy individuals and for those with chronic diseases including diabetes and cardiovascular disease. A person who adheres to current exercise recommendations may benefit from a 30% to 50% reduction in all-cause mortality, when compared with inactive individuals; however, this benefit has not yet been demonstrated in adequately powered randomized controlled trials. In addition, exercise capacity (as measured in METs) is a strong predictor of the risk of death in patients with and without cardiovascular disease. Greater exercise capacity is associated with longer survival.

11. Is it ever too late to obtain the benefits of exercise?

There does not appear to be an age limit after which exercise confers no benefit, and available data suggest that exercise is associated with a reduction in mortality even in the elderly. Additionally, individuals who are inactive but subsequently become physically active have a decreased risk of cardiovascular events and a lower mortality than those persons who remain inactive. Exercise attenuates, but does not prevent, the gradual decline in exercise capacity that occurs with aging; however, it does increase a person’s maximal exercise capacity at any given age. Importantly, exercise that is performed at a young age but is not continued throughout adulthood does not appear to improve long-term survival.

12. Is it safe for patients with known coronary artery disease (CAD) to exercise?

Yes. Exercise is not only safe in patients with known CAD, but it confers multiple benefits. Meta-analyses demonstrate that patients with CAD who are enrolled in cardiac rehabilitation (CR) programs have a 20% lower risk of death and a 25% lower risk of cardiac death compared with patients who are not enrolled in an exercise program. In addition, in patients following percutaneous coronary intervention (PCI), participation in CR is associated with a 43% reduction in all-cause mortality. Furthermore, patients who undergo exercise training have improvement in various cardiac risk factors, have less angina and less ischemia at a given level of exertion, and have increased exercise capacity. Importantly, there is a direct relationship between the number of CR sessions attended and the magnitude of the benefit obtained. In a recent analysis of 30,161 persons in the Medicare database who had suffered a recent myocardial infarction (MI), acute coronary syndrome, or had undergone recent coronary artery bypass surgery, those who attended all 36 CR sessions had an incrementally lower risk of both death and MI than patients who attended 24 sessions (14% and 12% reduction, respectively), 12 sessions (22% and 23% reduction, respectively), or only one session (47% and 31% reduction, respectively). Unfortunately, only a minority of patients attend the full course of CR; addressing barriers to participation in these programs is paramount to their success.

13. How long after MI can a patient begin an exercise program?

Patients who are clinically stable after MI can begin an exercise program as part of inpatient CR within 1 to 2 days of their infarction. Initial activity may be limited to range-of-motion exercises, but is rapidly increased to assisted walking. Activity is then gradually increased so that most patients can independently perform activities of daily living at the time of hospital discharge. Current AHA/American College of Cardiology (ACC) guidelines suggest that after MI all stable patients should be referred to formal outpatient CR programs. This is especially true for patients with multiple cardiac risk factors and for moderate- or high-risk patients (e.g., patients with residual CAD, patients with depressed left ventricular [LV] systolic function) for whom a supervised exercise program is appropriate. Stable patients can usually enroll in these programs 2 to 3 weeks after MI.

14. Is exercise safe for patients with heart failure?

Yes, providing that the patient is compensated and not congested. The hemodynamic effects associated with isotonic exercise (increased stroke volume, decreased systemic vascular resistance [SVR]) are beneficial in patients with depressed LV systolic function. In a recent large trial of patients with stable heart failure, exercise training resulted in improved exercise capacity, reduced heart failure symptoms, and improved quality of life scores. Importantly, these benefits were not associated with an increase in adverse cardiac events, including no significant increase in device discharges in patients with implantable cardioverter-defibrillators. Although isometric exercise was previously avoided in patients with heart failure, recent data suggest that light to moderate levels of resistance training are well tolerated by patients with heart failure and may yield similar benefits as in healthy individuals.

15. Does exercise benefit patients who are limited by leg claudication?

Yes. Supervised exercise programs (usually walking) are currently recommended as first-line therapy for the treatment of intermittent claudication. Recent studies demonstrate that exercise training in patients with claudication results in improved exercise capacity, reduction in claudication symptoms, and improved quality of life scores. These benefits may be of a similar magnitude to those achievable with surgical or percutaneous peripheral revascularization. Furthermore, in patients with peripheral arterial disease (PAD) but without claudication, exercise training improves exercise capacity and leg strength. Patients with symptomatic PAD should be encouraged to exercise to the point of mild to moderate claudication, and then to rest; exercise should be resumed when symptoms have resolved. Because PAD is strongly associated with CAD, an aggressive risk-factor modification regimen is an essential adjunct to exercise training in patients with PAD.

16. What is an exercise prescription?

An exercise prescription is a recommended exercise regimen that is individualized for a particular patient and takes into account the patient’s physical abilities, cardiac status, and medical comorbidities. The exercise prescription has four components: intensity, duration, frequency, and modality.

17. How is an exercise prescription developed after MI?

Upon entry into a CR program, patients undergo formal exercise testing to quantify their exercise capacity, assess for inducible ischemia, and derive an exercise intensity that is both safe and effective. Exercise intensity is generally prescribed using a range of HRs derived from the exercise test that represents 50% to 85% of heart rate reserve, where heart rate reserve is the difference between resting HR and peak exercise HR. For patients who develop ischemia during the exercise test (manifested by symptoms or electrocardiogram [ECG] changes), the peak training HR should be set at 10 beats/min below the rate at which the ischemia occurred. Exercise should then be performed for a minimum duration of 20 to 30 minutes and a frequency of 3 to 5 days per week. Modalities for endurance training commonly include walking and/or jogging (treadmill), rowing, cycling, and stair climbing.

18. Should patients with heart disease perform resistance training exercise?

Yes. Resistance training is now a standard part of a comprehensive exercise regimen for patients with cardiovascular disease, and appears to be particularly beneficial in the elderly, patients with stable heart failure, and those with diabetes. The improvement in muscular strength and endurance that accompanies resistance training aids in the performance of activities of daily living and may facilitate return to the workplace. Resistance training is prescribed at an intensity of 10 to 15 repetitions per set, using a load that is based on the maximum load (ML) that the patient can lift a single time (upper body: 30% to 40% ML; lower body: 50 to 60% ML). Patients should perform 1 to 3 sets of 8 to 10 different upper- and lower-body exercises at a frequency of 2 to 3 times per week. Common modalities for resistance training include free weights, weight machines, wall pulleys, elastic bands, and calisthenics.

19. What is aerobic interval training?

Standard exercise training involves a period of continuous moderate intensity exercise. Aerobic interval training (AIT) refers to a regimen that alternates 3- to 4-minute periods of high-intensity exercise (90% to 95% peak exercise HR) with similar periods of moderate intensity exercise (60% to 70% peak exercise HR). When compared to a standard exercise regimen, AIT appears to result in greater improvement in measurements of cardiovascular fitness in patients with heart failure and following coronary artery bypass graft (CABG). Nonetheless, at this time, the limited safety and efficacy data with this regimen precludes the routine recommendation for AIT in patients with cardiac disease.

20. What are the cardiovascular risks of exercise?

Overall, the risk of adverse cardiovascular events related to exercise in healthy individuals is extremely low and varies depending on an individual’s age, gender, level of physical fitness, and medical condition. Most adverse events relate to structural or congenital heart disease in young athletes (e.g., hypertrophic cardiomyopathy, coronary anomalies, right ventricular dysplasia) or CAD in older individuals. The performance of vigorous exercise is associated with a transient increase in MI and sudden cardiac death, especially in sedentary individuals with underlying CAD.

21. Should patients be screened before enrolling in an exercise program?

The AHA recommends screening of high school and college athletes by obtaining a personal and family history and performing a cardiovascular physical examination before competing in sports, and every 2 to 4 years thereafter. Further testing is not suggested in the absence of abnormal findings. In general, healthy individuals without symptoms of cardiovascular disease can embark on low- to moderate-intensity exercise programs without preexercise screening. However, preexercise stress testing should be considered in asymptomatic men older than age 45 years and asymptomatic women older than age 55 years (particularly those with multiple cardiovascular risk factors or diabetes) who plan to participate in vigorous exercise, in patients with established CAD, and in those who have exercise-related symptoms that suggest the possibility of CAD.

22. What are the contraindications to participation in an exercise program?

Absolute contraindications to exercise include unstable coronary heart disease, decompensated heart failure, symptomatic valvular stenosis, severe systemic hypertension (blood pressure more than 180/110 mm Hg), and uncontrolled arrhythmias. Detailed recommendations regarding athletic competition are provided in the 36th Bethesda Conference on Eligibility Recommendations for Competitive Athletes with Cardiovascular Abnormalities.

Bibliography, Suggested Readings, and Websites

1. Maron, B.J., Zipes, D.P. 36th Bethesda Conference: eligibility recommendations for competitive athletes with cardiovascular abnormalities. J Am Coll Cardiol. 2005;45:2–64.

2. American Association of Cardiovascular and Pulmonary Rehabilitation. The AACVPR website. Available at http://www.aacvpr.org. Accessed March 13, 2013

3. American College of Sports Medicine. The ACSM website. Available at http://www.acsm.org. Accessed March 13, 2013

4. American Diabetes Association. Physical activity/exercise and diabetes. Diabetes Care. 2006;29:1433–1438.

5. American Heart Association. The AHA website (search Exercise). Available at http://www.heart.org. Accessed March 13, 2013

6. Awtry, E.A., Balady, G.J. Exercise and physical activity. In Topol E.J., ed.: Textbook of cardiovascular medicine, ed 3, Philadelphia: Lippincott Williams and Wilkins, 2007.

7. Balady, G.J., Ades, P.A. Exercise and Sports Cardiology. In: Bonow R.O., Mann D.L., Zipes D.P., Libby P., eds. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. ed 9. Philadelphia: Saunders; 2012:1784–1792.

8. Balady, G.J., Ades, P.A., Bittner, V.A., et al. Referral, enrollment, and delivery of cardiac rehabilitation/secondary prevention programs at clinical centers and beyond: a presidential advisory from the American Heart Association. Circulation. 2011;124:2951–2960.

9. Balady, G.J., Williams, M.A., Ades, P.A., et al. Core components of cardiac rehabilitation/secondary prevention programs: 2007 update. A statement from the American Heart Association. Circulation. 2007;115:2675–2682.

10. Downing, J., Balady, G.J. The role of exercise training in heart failure. J Am Coll Cardiol. 2011;58:561–569.

11. Hamburg, N.M., Balady, G.J. Exercise rehabilitation in peripheral artery disease: functional impact and mechanisms of benefits. Circulation. 2011;123:87–97.

12. Hammill, B.G., Curtis, L.H., Schulman, K.A., et al. 2010 Relationship between cardiac rehabilitation and long-term risks of death and myocardial infarction among elderly Medicare beneficiaries. Circulation. 2010;121:63–70.

13. Haskell, W.L., Lee, I.-M., Pate, R.P., et al. Physical activity and public health: updated recommendation for adults from the American College of Sports Medicine and the American Heart Association. Circulation. 2007;116:1081–1093.

14. Maron, B.J., Thompson, P.D., Ackerman, M.J., et al. Recommendations and considerations related to preparticipation screening for cardiovascular abnormalities in competitive athletes: 2007 update. A scientific statement from the American Heart Association Council on Nutrition, Physical Activity, and Metabolism. Circulation. 2007;115:1643–1655.

15. Murphy, T.P., Cutlip, D.E., Regensteiner, J.G., et al. Supervised exercise versus primary stenting for claudication resulting from aortoiliac peripheral artery disease: six-month outcomes from the Claudication: Exercise Versus Endoluminal Revascularization (CLEVER) study. Circulation. 2012;125:130–139.

16. O’Connor, C.M., Whellan, D.J., Lee, K.L., et al. Efficacy and safety of exercise training in patients with chronic heart failure: HF-ACTION randomized controlled trial. JAMA. 2009;301:1439–1450.

17. Thompson, P.D. Exercise prescription and proscription for patients with coronary artery disease. Circulation. 2005;112:2354–2363.

18. Thompson, P.D., Franklin, B.A., Balady, G.J., et al. Exercise and acute cardiovascular events: placing the risks into perspective: a scientific statement from the American Heart Association Council on Nutrition, Physical Activity, and Metabolism and the Council on Clinical Cardiology. Circulation. 2007;115:2358–2368.

19. Thompson W.R., ed. American College of Sports Medicine Guidelines for Exercise Testing and Prescription, ed 8, Philadelphia: Lippincott, Williams and Wilkins, 2010.

20. Williams, M.A., Haskell, W.L., Ades, P.A., et al. Resistance exercise in individuals with and without cardiovascular disease: 2007 update. A scientific statement from the American Heart Association Council on Clinical Cardiology and Council on Nutrition, Physical Activity, and Metabolism. Circulation. 2007;116:572–584.

21. Wisløff, U., Støylen, A., Loennechen, J.P., et al. Superior cardiovascular effect of aerobic interval training versus moderate continuous training in heart failure patients: a randomized study. Circulation. 2007;115:3086–3094.

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