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?

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