Exercise assessment and measurement of exercise capacity in patients with coronary heart disease

INTRODUCTION — Exercise capacity is positively associated with survival in people with coronary heart disease (CHD). This topic will cover the benefits of exercise in patients with CHD, exercise testing procedures, measurements obtained on exercise testing, contraindications to and indications for discontinuing exercise testing, and risk stratification of patients who are being prescribed exercise (with a focus on whom should be monitored during exercise testing).

The use of exercise testing in the diagnosis and prognosis of patients post-myocardial infarction (MI) and for risk stratification of patients with congestive heart failure are discussed separately. (See "Exercise capacity and VO2 in heart failure" and "Prognostic features of stress testing in patients with known or suspected coronary disease".)

Cardiac rehabilitation is discussed separately. (See "Cardiac rehabilitation: Indications, efficacy, and safety in patients with coronary heart disease", section on 'Exercise training'.)

BACKGROUND

Benefits of exercise — Epidemiologic studies have shown an inverse association between physical activity and coronary heart disease (CHD) incidence and mortality [1-3]. Among men with and without cardiovascular disease who were referred for treadmill exercise testing, peak exercise capacity measured in metabolic equivalents (METs) was the strongest predictor of the risk of death, during an average of 6.2 years follow-up [4]. For each 1-MET increase in exercise capacity, there was a 12 percent improvement in survival. Similarly, exercise capacity was shown to be an independent predictor of death in asymptomatic women [5]. (See "Cardiac rehabilitation: Indications, efficacy, and safety in patients with coronary heart disease", section on 'Exercise training'.)

Fitness also appears to be important, as a graded relation has been noted between the degree of fitness and the reduction in coronary risk (figure 1) [5-7].

Furthermore, exercise is a key component of cardiac rehabilitation following MI. (See "Cardiac rehabilitation: Indications, efficacy, and safety in patients with coronary heart disease".)

The benefits of aerobic exercise are mediated through hemodynamic and metabolic effects [8] (see "The benefits and risks of aerobic exercise"):

●Hemodynamic effects – Aerobic exercise increases physical work capacity, lowers resting and submaximal exercise heart rate, facilitates the loss of excess weight, and reduces blood pressure. Aerobic exercise also increases the dilating capacity of coronary arteries [9], and may promote regression or minimize progression of coronary lesions [10,11]. (See "Exercise in the treatment and prevention of hypertension" and "Coronary endothelial dysfunction: Clinical aspects".)

●Cardiometabolic improvements – Aerobic exercise improves glycemic control and induces favorable lipoprotein changes (a reduction in plasma triglycerides and an elevation in HDL-cholesterol are the most consistent changes) [12]. (See "Exercise guidance in adults with diabetes mellitus".)

●Muscle function – Chronic moderate exercise also improves muscle function. These changes include increases in the quantity of mitochondrial enzymes and in the number of "slow-twitch" muscle fibers, and the development of new muscle capillaries [13]. In addition, the action of insulin is improved with exercise training by an increase in the number of glucose transporters that facilitate diffusion of glucose into muscle cells [14,15]. Aerobic exercise also improves the peripheral muscle utilization and extraction of oxygen during exercise in old as well as young patients [16,17].

The frequency of anginal symptoms is also reduced by an exercise program. As a result, aerobic exercise is beneficial for both primary and secondary prevention of CHD. An exercise training prescription and program for a patient with known coronary disease should be designed according to the individual's health status and current level of physical activity. Prior to the design of such a specific program, it is helpful to perform exercise testing for assessment of risk, prognosis, and functional capacity. (See "Cardiac rehabilitation programs".)

Indications — In patients with known CHD, assessment of functional aerobic capacity can determine prognosis and serve as the basis for an exercise prescription. Also, among patients with uncomplicated MI (usually within three weeks) or following coronary artery revascularization, exercise testing can help evaluate functional aerobic capacity to help determine an exercise prescription [18]. Exercise capacity also provides information regarding prognosis.

Contraindications — Maximal exercise testing is absolutely contraindicated in the presence of active cardiovascular dysfunction such as an acute myocardial infarction, unstable angina, decompensated heart failure, or serious arrhythmias (table 1) [19,20]. Relative contraindications that can be overridden in certain circumstances with appropriate precautions include significant hypertension and moderate valvular disease (table 1).

The role of exercise testing and potential safety concerns in patients who present with possible acute coronary syndrome are discussed separately. (See "Evaluation of emergency department patients with chest pain at low or intermediate risk for acute coronary syndrome", section on 'Risk assessment'.)

Potential risks — Exercise testing is considered a relatively safe procedure when performed by appropriately trained clinicians [21]. When studies that included very high-risk patients (eg, history of life-threatening ventricular arrhythmias) were excluded, serious complications during exercise tests ranged from 0 to 35 events per 10,000 tests [21,22].

TESTING PROCEDURES — During a maximal exercise test, work rate increases on an ergometric device (ie, speed and grade on a treadmill or resistance on a bicycle ergometer) until the individual reaches his physiologic limit (V_O2max) [19,23], which is determined by the maximal cardiac output and peripheral extraction of oxygen. Maximal exercise testing allows the individual to reach his physiologic limit. This is observed when the V_O2 does not rise despite a further increase in work rate or an individual reaches or exceeds his age-predicted maximal heart rate (ie, 220 minus age in years).

Maximal testing is the most accurate measure of functional capacity, although it requires a highly motivated patient and carries some degree of risk. Submaximal exercise testing implies that the test is terminated at a predetermined point. Examples include a predischarge post-myocardial infarction (MI) exercise test to 70 percent of age-predicted maximal heart rate or a symptom-limited exercise test prior to the initiation of cardiac rehabilitation.

Various protocols (table 2) are available for both maximal and submaximal exercise testing (treadmill, bicycle ergometer, step tests, distance runs, walking tests). The decision to select a given protocol is based upon body size, level of physical fitness, functional ability, and available equipment. In order for measurements of time or rate-pressure response to be directly related to the actual cardiac work, the subject must approach a steady-state. This implies that continued exercise at the same intensity would be associated with the same heart and cardiac output. This usually requires two to three minutes of exercise at each stage. (See "Exercise ECG testing: Performing the test and interpreting the ECG results", section on 'Common exercise protocols'.)

Clinical exercise testing for the purpose of determining functional (exercise) capacity and serving as the basis for an exercise prescription does not involve expired gas analysis. However, heart rate response to graded exercise and the amount of work performed in metabolic equivalents (METs; or stage of exercise protocol achieved) are typically used to estimate exercise capacity and are surrogates for physiologic parameters (eg, V_O2, anaerobic threshold, etc). Percentage of expected METs determines if an individual is at, above, or below average for age and sex. The exercise test is used to guide the appropriate training intensity according to measured parameters: heart rate reserve (40 to 80 percent of peak), percent of peak exercise heart rate achieved (65 to 80 percent), and rating of perceived exertion (11 to 16 on a 6-to-20 RPE scale). (See "Cardiac rehabilitation programs", section on 'Exercise training'.)

MEASUREMENTS

Maximal heart rate — Maximal heart rate can be estimated by the following equation:

It must be recognized that the standard deviation for this equation is 10 to 15 beats/minute. Given that this calculation was derived by studies in men, another study determined that the above formula overestimates the maximum heart rate in women [24]. Therefore, it is most accurate to directly measure the maximal heart rate at highest exercise intensity on the maximal exercise test, particularly in cardiac patients. (See "Exercise ECG testing: Performing the test and interpreting the ECG results", section on 'Test endpoints'.)

Peak exercise capacity — Peak exercise capacity is defined as "the maximum ability of the cardiovascular system to deliver oxygen to exercising skeletal muscle and of the exercising muscle to extract oxygen from the blood" [25]. As a result, exercise tolerance is determined by three factors: pulmonary gas exchange; cardiac performance; and skeletal muscle metabolism. (See "Exercise physiology".)

Exercise capacity can be quantitated clinically by measurement of oxygen uptake (V_O2), carbon dioxide production (V_CO2), and minute ventilation [26]. These parameters are measured during exercise with rapidly responding gas analyzers capable of breath-by-breath determination of O₂ and CO₂ concentrations [27,28]. The maximal oxygen uptake (V_O2max) eventually reaches a plateau despite increasing workload (figure 2). Not surprisingly, V_O2max has a strong linear correlation with heart rate, cardiac output, and skeletal muscle blood flow (figure 3A-B) [29].

It is convenient to express oxygen uptake in multiples of resting requirement. The metabolic equivalent (MET) is a unit of resting oxygen uptake. Rather than using each patient's own value, one MET is designated as the average value (3.5 mL O₂ uptake/kg per min). Maximum values occur between age 15 and 30 and then progressively decrease with age; however, they can be increased with aerobic exercise training. Values of approximately 12 METs are seen with maximal exercise in moderately active young men, while distance runners can have values as high as 18 to 24 METs. A mean exercise capacity of 10 METs has been observed in nonathletic, healthy, middle-aged men.

Anaerobic threshold — The anaerobic threshold (AT) is another index used to estimate exercise capacity. It is defined as the point at which minute ventilation increases disproportionately relative to V_O2, a response that is generally seen at 60 to 70 percent of V_O2max (figure 2). The AT is a reflection of the increase in lactic acid production by working muscles. It can be used to distinguish between noncardiac (pulmonary or musculoskeletal) and cardiac causes of exercise limitation, since patients who fatigue prior to reaching AT are likely to have a noncardiac problem [26]. If the AT occurs at less than 40 percent of predicted V_O2max, it is taken as evidence of cardiovascular impairment. (See "Exercise physiology".)

Blood pressure — Systolic BP increases linearly with work rate to approximately 200 mmHg in normotensives and higher in hypertensives. The diastolic BP does not change with work rate in normotensives but does increase in hypertensives. A fall in systolic BP with rising work rate may be due to aortic valve obstruction, impaired left ventricular function, or global myocardial ischemia. The blood pressure should be allowed to decrease gradually following exercise. A cool-down period of a low-exercise workload is necessary to prevent postexercise hypotension due to excessive venous pooling.

Rating of perceived exertion — The rating of perceived exertion (RPE or Borg scale) is a subjective measure of exercise intensity and is a good indicator of relative fatigue (table 3). It can be used in conjunction with the heart rate to determine the consistency of effort from one study to the next in a given patient. The RPE can also be used by the unsupervised patient to control the intensity of exercise. An alternative in elderly patients is the "talk test"; patients are instructed to exercise only to an intensity that permits them to continue talking to an exercising companion. (See "Cardiac rehabilitation programs", section on 'Summary'.)

Other measurements — Other measurements obtained during exercise testing include:

●The double product (heart rate multiplied by systolic BP) – This measurement parallels myocardial oxygen demand.

●12-lead ECG (during and after exercise) – These tests can detect dysrhythmias and ischemic changes (eg, ST-segment shifts, perhaps alterations in conduction).

●The respiratory exchange ratio (V_CO2/V_O2) – This measurement rises above 1 beyond the ventilatory threshold.

●The O₂ pulse (V_O2/HR) – This is related to stroke volume.

●The VE/MVV (minute ventilation divided by maximum voluntary ventilation) – This is a measure of breathing reserve.

INDICATIONS FOR DISCONTINUATION — There are both absolute and relative indications for discontinuation of an exercise test other than the desired endpoint of attainment of the maximal heart rate (table 4) [19,20]. Absolute indications include the patient's request and acute cardiac events such as suspicion of a myocardial infarction, moderate to severe angina, hypotension, signs of poor perfusion, severe shortness of breath, and serious arrhythmias. Relative indications include electrocardiographic changes, increasing chest pain, severe fatigue, dyspnea, significant hypertension and less serious arrhythmias.

INDIVIDUALIZING EXERCISE AND MONITORING — Exercise testing as a measure of functional capacity is useful in clinical decisions regarding the appropriateness for exercise training and the type and intensity of exercise to be prescribed. The following risk stratification developed by the American Heart Association is useful in determining the degree of medical monitoring or supervision required during exercise training [19].

●Class A individuals are apparently healthy and have no clinical evidence of increased cardiovascular risk of exercise. These patients can exercise without medical supervision.

●Class B individuals have established coronary heart disease (CHD) that is clinically stable. These patients are at low risk of cardiovascular complications of vigorous exercise. They require supervision during prescription sessions (6 to 12 medical sessions) and can exercise without supervision once they understand how to monitor activity.

●Class C individuals are at moderate or high risk of cardiac complications during exercise by virtue of a history of multiple myocardial infarctions or cardiac arrest, NYHA class III or IV (table 5), exercise capacity of less than six metabolic equivalents (METs), and/or significant ischemia on the exercise test. These patients require supervised exercise with continuous ECG monitoring until the safety of the exercise program has been established.

●Class D patients are those with unstable disease who require restriction of activity and for whom exercise is contraindicated.

Outpatient cardiac rehabilitation is typically appropriate for patients who belong to class B or C. (See "Cardiac rehabilitation programs".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Chronic coronary syndrome" and "Society guideline links: Lifestyle management and cardiac rehabilitation".)

SUMMARY

●Background – Epidemiologic studies have shown an inverse association between physical activity and coronary heart disease (CHD) incidence and mortality. (See 'Benefits of exercise' above.)

●Benefits of exercise – Aerobic exercise increases physical work capacity, lowers heart rate at rest and following exercise, facilitates weight loss, reduces blood pressure, improves glycemic control, and induces favorable changes in lipids. Chronic moderate exercise improves muscle function and reduces the frequency of anginal symptoms. (See 'Benefits of exercise' above.)

●Indications – In patients with known CHD, assessment of functional aerobic capacity can determine prognosis and serve as the basis for an exercise prescription. Also, among patients with uncomplicated myocardial infarction (MI; usually within three weeks) or following coronary artery revascularization, exercise testing can help evaluate functional aerobic capacity to help determine an exercise prescription. (See 'Indications' above.)

●Contraindications – While exercise testing is generally safe, there are absolute and relative contraindications (table 1) to exercise testing. (See 'Contraindications' above.)

●Measurements – Key measurements obtained on exercise testing include maximal heart rate, peak exercise capacity, anerobic threshold, blood pressure (BP), and rating of perceived exertion, among others. (See 'Measurements' above.)

●Individualizing exercise prescriptions – Exercise testing is useful in determining the type and intensity of exercise that is appropriate for individual patients. (See 'Individualizing exercise and monitoring' above.)