Subject Review Exercise Limitation and Pulmonary Rehabilitation in Chronic Obstructive Pulmonary Disease
CHRISTOPHER O. OLOPADE, M.D.,* KENNETH C. BECK, Ph.D.; ROBERT W. VIGGIANO, M.D., BRUCE A. STAATS, M.D., Division of Thoracic Diseases and Internal Medicine
Impairment of exercise tolerance is a common problem in patients with severe chronic obstructive pulmonary disease. The cause of exercise intolerance in patients with severe chronic obstructive pulmonary disease is multifactorial and includes impaired lung mechanics, fatigue of inspiratory muscles, impaired gas exchange, right ventricular dysfunction, malnutrition, occult cardiac disease, deconditioning, and psychologic problems; however, impaired lung mechanics and gas exchange abnormalities seem to be the major limiting factors. Recently, the approach to management of pulmonary rehabilitation in patients with chronic obstructive pulmonary disease has changed because improvement in exercise tolerance has been demonstrated after pulmonary rehabilitation. Other adjunctive measures that have been shown to contribute to the observed improvement in exercise tolerance include administration of oxygen, nutritional support, cessation of smoking, and psychosocial support. The roles of ventilatory muscle endurance training, respiratory muscle rest therapy, nasally administered continuous positive airway pressure, and training of the muscles of the upper extremities are less clearly defined.
Patients with chronic obstructive pulmonary disease (COPD) often have poor tolerance of exercise.P The degree of exercise intolerance seems to reflect the severity of the underlying COPD.2 Other factors, however, such as muscle fatigue, malnutrition, and cardiac dysfunction, contribute to the difficulty with exercise in patients with COPD. In the 1950s and early 1960s, the accepted recommendation was to impose exercise limitations for patients with COPD. Because Barach and colleagues' noted progressive improvement in the ability to walk without dyspnea in patients with COPD who remained active (in contrast to their sedentary counterparts), they suggested that a physiologic response similar to a training effect in athletes may have been produced in such patients. The scientific basis for recommending exercise rehabilitation has only recently been established and, therefore, can only now be recommended. *Current address: University of Illinois, Chicago, Illinois. Address reprint requests to Dr. B. A. Staats, Division of Thoracic Diseases, Mayo Clinic, Rochester, MN 55905. Mayo Clin Proc 67:144-157,1992
Pierce and co-workers" attempted to understand the physiologic processes underlying the improved exercise tolerance in patients with COPD who were active. Nine patients with severe but stable COPD were enrolled in an exercise training program for 3 to 20 weeks after an initial period of acclimatization with treadmill exercise. After the training, the heart rate, respiratory rate, and minute ventilation substantially decreased and the exercise tolerance improved. These investigators, however, were unable to demonstrate any improvement in pulmonary function in these patients. They speculated that a physiologic training effect may have occurred.
EXERCISE LIMITATION During the past 2 decades, many investigators have attempted to explain the causes of improved exercise tolerance in active patients with COPD. The approach to rehabilitation in patients with COPD has also dramatically changed; exercise rehabilitation is now commonplace. The numerous proposed reasons for exercise intolerance are discussed in the following material. 144
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145
Impaired Lung Mechanics.-Impaired lung mechanics Impaired Gas Exchange-s-Gss exchangeis one of the major is one of the major factors that limit exercise performance in functions of the lung. The efficiency of gas exchange is patients with COPD. 5-8 In patients with severe COPD, expi- dependent on the balance between alveolar ventilation and ratory flow may be limited during tidal breathing. One of the pulmonary blood flow (perfusion). In a normal lung, regions strategies adopted by these patients to overcome limitation with the most ventilation tend to have the most perfusion, of expiratory flow (which can curtail exercise) is breathing at and, conversely, poorly ventilated areas receive minimal a higher end-expiratory lung volume to achieve higher expi- perfusion. One estimate of the degree of "matching" of ratory flows (Fig. I). This strategy increases the maximal ventilation to perfusion is the ratio of physiologic dead space exercise ventilation. The increase in end-expiratory lung to tidal volume (VDNT). The VDNT is optimal when perfuvolume is associated with expansion of both the thoracic sion is well matched to ventilation. At rest VDNT is norcage and the lung parenchyma-factors that, unfortunately, mally approximately O.3-that is, 30% of a given breath is necessitate an increase in the elastic work of breathing. In "wasted." Most of this dead-space ventilation is used to fill addition, at high lung volumes, the diaphragm flattens, and the conducting airways. During exercise, the VDNT dethe ribs become horizontally oriented; thus, both the inter- clines to 0.2 or less." In patients with COPD (especially costal muscles and the diaphragm are placed in unfavorable those with emphysema, because of the destruction of the gasmechanical positions (Fig. 2). The ensuing pattern of con- exchanging units), regions of appreciable ventilation-to-pertraction may cause the diaphragm to act like an expiratory fusion mismatch impair oxygenation of the blood and also muscle. This response is discernible on examination by elimination of carbon dioxide. At rest, the VDNT is ininward movement of the costal region (Hoover's sign). creased as a result of ventilation to poorly perfused alveoli Breathing at higher lung volumes decreases the patient's (so-called alveolar dead space). In contrast to the normal ability to increase tidal volume during exercise; hence, an decrease in VDNT that occurs during exercise, patients with increase in respiratory frequency is the major remaining COPD may experience further increases or lack of decline in means of increasing ventilation (ventilation equals frequen- VDNT. The high VDNT does not result in hypoxemia but cy of respiration multiplied by tidal volume). The rapid, increases the ventilatory requirements of an already overshallow pattern of breathing in patients with COPD contrasts taxed system. This result causes the ventilatory equivalent with that in normal persons, who typically have an increase for carbon dioxide to be increased; 13,14 together with a rein tidal volume before an increase in respiratory frequency duced maximal voluntary ventilation, these factors contribwith progressive exercise." Increased resistance to expira- ute to exercise limitation. In addition, hypoxemia may develop in many patients tory flow and decreased mechanical advantage of inspiratory muscles at high lung volumes result in an increase in the because of perfusion of poorly ventilated alveoli. Pulmowork or oxygen cost of breathing, which ultimately may lead nary capillary blood from these regions is not adequately to fatigue of the inspiratory muscles (diaphragm). oxygenated, and hypoxemia results when this blood is mixed Fatigue of Inspiratory Muscles (Diaphragm).-Belle- in the left atrium with normally oxygenated blood. Ventilamare and Grassino 10, 11 demonstrated the importance of the tion-to-perfusion mismatch causes an increase in the oxygen ratio of inspiration time to total breath duration and the ratio partial pressure difference between alveolar air and arterial of mean transdiaphragmatic pressure (abdominal pressure blood, which may become worse with exercise. The hyminus pleural pressure) generated with each inspiration to poxemia can be-but is not always-an exercise-limiting maximal transdiaphragmatic pressure as determinants of factor. 6 Depending on ventilatory drive, hypoxemia may diaphragmatic endurance in normal volunteers and patients further stimulate ventilation and hasten exercise limitation. with COPD. They found that when fatigue of the respiratory Right Ventricular Dysfunction and Pulmonary muscles developed, the tension time index (the product of Hypertension.-In the past, patients with COPD without the ratio of inspiration time to total breath duration and the coexisting ischemic heart disease or cor pulmonale were not ratio of mean transdiaphragmatic pressure with each inspira- thought to have exercise limitations because of cardiac dystion to maximal transdiaphragmatic pressure) was more than function. Nevertheless, studies conducted by Morrison and 0.15. 10, 11 Patients with severe COPD have limited ventila- associates" and Mahler and colleagues" with use of hemotory reserve even at rest and have been shown to have a dynamic monitoring and radionuclide studies during exerresting tension time index that approaches 0.15 (normal, cise have shown that right ventricular dysfunction does oc0,02),11 With the development of acute respiratory failure, cur in some patients during exercise. In these patients, patients may deteriorate from the borderline fatigue zone pulmonary artery pressure increased in the setting of normal into the definite fatigue zone (a tension time index that left ventricular function. The increased pulmonary artery exceeds 0,15),7 Similar findings have been noted during pressure returned to normal or near normal after exercise. In exercise in patients with COPD. patients with severe COPD, further pulmonary hypertension
146
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CHRONIC OBSTRUCTIVE PULMONARY DISEASE
10
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Fig. 1. Diagram of flow-volume loop dynamics in a normal person and a patient with chronic obstructive pulmonary disease (COPD). In patient with COPD, during tidal breathing expiratory flow abuts the expiratory limb of maximal expiratory flow-volume curve. During exercise (Ex), patient with COPD breathes at a slightly higher lung volume (leftward shift of flow-volume loop). This result contrasts with that in normal subject, who has a large ventilatory reserve both at rest and during exercise. TLC = total lung capacity.
often develops during exercise and may cause premature termination of exercise. Occult Cardiac Disease.-Dyspnea caused by left ventricular dysfunction and that caused by pulmonary disease are sometimes difficult to distinguish clinically. Left ventricular dysfunction is uncommon in patients with COPD in whom such dysfunction is clinically suspected. I? In an evaluation of the relationship between right and left ventricular function in patients with varied degrees of severity of COPD, however, Slutsky and colleagues" found that left ventricular dysfunction correlated with the decline in right ventricular function only in patients with severe COPD. The results of that study suggested that patients with severe COPD and pulmonary hypertension have worsening of left ventricular function· during exercise when right ventricular dysfunction is clearly evident. Altered Perception of Breathlessness.-An increased perception of breathlessness may contribute to exercise limitation in patients with COPD. Woodcock and co-workers'? postulated that the sensation of breathlessness may constitute a greater limitation to exercise than mechanical or cardiovascular factors. They administered a mild depressant of the central nervous system (dihydrocodeine) to patients with severe chronic limitation of airflow and demonstrated a de-
creased degree of breathlessness and increased exercise tol- . erance in comparison with the control situation. Altered Control of Ventilation.-In some patients with COPD, the control of breathing may be abnormal and may potentially contribute to retention of carbon dioxide. Assessing the ventilatory response to inhaled carbon dioxide is one method of determining the sensitivity of the respiratory drive. The ventilatory response, however, is nonspecific" because it is influenced by numerous factors, including abnormal pulmonary function and altered acid-base status, both of which are present in patients with COPD. The pressure generated in the first 100 ms when the inspiratory muscles contract during transient occlusion of the airway may be a better measure of respiratory drive." Some studies have shown that this pressure measurement is greater in patients with COPD than in normal subjects, an indication of increased respiratory drive. Some investigators believe that increased respiratory drive in the presence of abnormal pulmonary function is a mechanism for dyspnea in patients with COPD.22 Psychologic Factors.-Depression, anxiety, and inappropriate fear of exercise are prevalent in patients with COPD.23 Excitement or exertion results in increased respiratory rate and minute ventilation, leading to increased work of
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NORMAL
,,
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
147
EMPHYSEMA
"
"Zone of apposition
"Flat diaphragm
Fig. 2. Diagram depicting flattening of diaphragm and loss of region of apposition in patient with emphysema. Contraction of flattened diaphragm leads to inward movement of costal region, which is noticeable on physical examination (Hoover's sign). breathing and worsening of the sensation of breathlessness. A vicious cycle is thus created. Patients with COPDwho have incapacitating psychosocial problems (such as severe depression or anxiety) respond less favorably to rehabilitation programs than do patients without such problems.P' In addition, patients with well-established psychosocial skills exhibit positive responses to group psychotherapy." The contribution of psychologic factors to exercise limitation in COPD is considerable and must not be overlooked in a comprehensive rehabilitation program. Malnutrition.- The overall incidence of malnutrition in patients with COPD is unknown. In a group of hospitalized patients with COPD, however, the prevalence of nutritional depletion determined by anthropographic measurements and a body weight less than 90% of ideal was 50%.26.27 Poor nutritional status also correlates with suboptimal pulmonary function in patients with emphysemas-" and has been shown to be an independent risk factor for poor survival." Proposed causes of malnutrition include interference with gastrointestinal function by medications (such as theophylline and corticosteroids), vascular congestion of the gut caused by cor pulmonale, catabolic effects of corticosteroids, gastric distention from aerophagia, and high metabolic rate. Patients with emphysema, in particular, have a higher than normal metabolic rate as a result of their increased oxygen consumption during breathing." Dyspnea and arterial desaturation while eating may also cause decreased dietary intake in some patients." Ultimately, the malnutrition may interfere with ventilatory muscle strength and exercise per-
formance. In addition, malnutrition may predispose the patient with COPD to frequent infections of the respiratory tract and bacterial colonization of the airway through reduced secretory IgA, increased bacterial adherence, and decreased bacterial clearance.P:" Arora and Rochester" reported substantial decreases in respiratory .muscle strength and maximal voluntary ventilation that were proportional to the degree of weight loss in nutritionally depleted normal volunteers. Similar changes most likely occur in undernourished patients with COPD because of their high ventilatory requirements. Deconditioning.-Physical conditioning increases the ability to perform work. In a group of well-trained athletes, the imposition of strict bed rest or inactivity for 10 days to 2 months resulted in a deconditioned state characterized by the physiologic changes of a decrease in maximal oxygen uptake,":" an increased response of the heart rate and the blood pressure to exercise,36-38 a posture-dependent reduction in stroke volume.v-" and a reduction in size of the left ventricle." Exercise intolerance sometimes forces patients with severe COPD to adopt a sedentary life-style, which results in deconditioning. Depending on the degree of inactivity, physical deconditioning (in conjunction with the other factors previously discussed in this review) contributes considerably to exercise intolerance in patients with COPD.
EXERCISE REHABILITATION In recent years, exercise training has become the focus of many cardiac rehabilitation programs. Cardiovascular and
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CHRONIC OBSTRUCTIVE PULMONARY DISEASE
peripheral muscle training responses have been achieved through rehabilitation after both myocardial infarction and a cardiac surgical procedure.P:" In contrast to the success in training cardiac patients, numerous studies with various designs during the past 25 years have shown that exercise training only modestly improves tolerance in patients with COPD.4,42-54 The results of some of these studies are summarized in Table 1. Few studies showed improvement in pulmonary function; those that did often involved patients recuperating from an acute illness immediately before enrolling in the rehabilitation program." A more recent investigation, however, suggested that a physiologic training effect mediates much of the improved exercise tolerance after rehabilitation." Casaburi and colleagues" showed that patients with moderately severe COPD attained training responses similar to those in normal subjects at a high rate of work. They also demonstrated reductions in serum lactate levels and minute ventilation after 8 weeks of training. Their study challenges the previous notion that patients with COPD cannot tolerate enough exertion for lactic acidosis to develop. The mechanism that improves exercise tolerance is unclear. Measurements of pulmonary function rarely show improvement; therefore, the possibilities include improved aerobic capacity (an actual training response), increased motivation, desensitization to the sensation of dyspnea, and improved muscular efficiency during exercise (lower oxygen cost of exercise). The effects of exercise training can be measured in several ways, including increases in maximal oxygen uptake, decreases in perception of work as assessed on the rating of perceived exertion scale, and increases in the duration and distance walked in a standardized protocol. Improved Aerobic Capacity.-The maximal oxygen uptake is perhaps the best indicator of aerobic capacity." During training, normal subjects undergo. both cardiovascular and peripheral conditioning.v-" The cardiovascular changes include decreased response of the heart rate to exercise and increased stroke volume; the peripheral changes include increased concentration of muscle enzymes, increased capillary density, and increased mitochondrial size and number. To achieve this training ·effect, the person must exercise at approximately 50% of maximal oxygen uptake for 20 to 30 minutes three to four times weekly for at least 3 weeks." Patients with severe COPD are usually limited by symptoms before they achieve the level of exercise that would ensure a physiologic training effect." Belman and Kendreganv trained two groups of patients with COPD (seven trained their legs and eight trained their arms) for 6 weeks in an attempt to demonstrate peripheral changes from the exercise. Muscle biopsies were performed in both trained and nontrained muscles. Those investigators found no increase in muscle enzymes despite improved exercise tolerance and no evidence of the typical cardiovascular
Mayo CliD Proc, February 1992, Vol 67
response to training. This study suggested that a critical level of exercise must be attained before training can occur and that this level is not reached in patients with COPD. Casaburi and colleagues55 did not perform muscle biopsies in their study group; however, they showed that, even at low rates of work, lactic acidosis can develop in patients with severe COPD and that a "training effect" (defined as a decreased production of lactate and a corresponding decreased minute ventilation for the same level of exercise) occurred in most of their patients during high-level exercise training. Increased Motivation.-Improved motivation, a heightened sense of well-being, and decreased dyspnea are noted in most patients enrolled in an exercise rehabilitation program. An analog scale (Fig. 3) is commonly used to evaluate breathlessness and relative exercise intensity." The scale for rating of perceived exertion provides a reproducible means of quantifying subjective exercise intensity and has been shown to correlate well with minute ventilation." This scale is placed within easy view during an exercise session, and the patient is asked to rate the level of exertion or degree of breathlessness. Exercise is considered maximal when the patient reports a perceived rate of exertion of 19 or 20. Desensitization to Dyspnea.-The added sense of security derived from the presence of physicians, nurses, and therapists seems to reassure the patients undergoing rehabilitation and encourage them to exercise longer without panic, Patients are taught that dyspnea is not a sensation to be feared. Better motivation related to a heightened sense of well-being has been shown to lead to improved exercise tolerance." Unfortunately, like motivation, desensitization to the sensation of dyspnea cannot be measured objectively for assessment of its contribution to improved endurance. Improved Technique of Exercise.-Some studies" have shown improved efficiency in treadmill walking with repeated attempts. Sinclair and Ingram" observed a decrease in the number of steps taken in a given distance during a 12minute walk. More efficient performance of an exercise task decreases the oxygen uptake necessary for exercise and concomitantly reduces the ventilatory requirement.
COMPOSITION OF A PULMONARY REHABILITATION PROGRAM Even with the widening application of pulmonary rehabilitation for patients with COPD, many questions about what constitutes an optimal program remain unanswered. Should every patient with COPD have a complete exercise evaluation before enrollment? What type of exercise is most appropriate? How intense should the exercise program be? What should be the duration of a rehabilitation program? Should the program be hospital-, home-, or outpatient-based rehabilitation?
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Table l.-Summary of Selected Reports of Exercise Training in Patients With Chronic Obstructive Pulmonary Disease* Improvement Reference Pierce et aI,4 1964
No. of Patients Controls 9
0
Mean FEV l (L) 0.97
Petty et al,53 1969 Mungall & Hainsworth.'? 1980
182
0
0.94
10
0
1.54
Sinclair & Ingram." 1980
33
"With controls"
1.1
Cockcroft et al," 1981
39
Randomized control
1.52 (1.32 for controls)
Busch & Mcf'lements.f 1988
20
10
0.84
379'll
0
0.46-0.72
Foster et al,"? 1988
Training program
PFT
3-20 wk of treadmill walking2-5 min 5-10 times/day at speeds slower than maximal attainable 12 mo of comprehensive training 3 12-wk periods-before, during, and after supervised training for 12 min/day For 10 mo, daily l2-min walks and stair-climbing, supervised once/wk For 6 wk, swimming, rowing, and walking at rehabilitation center; remainder at home. Control subjects-6 wk of rehabilitation after 4 mo as controls 18 wk of supervised daily mobility, strengthening, and endurance exercises 4 wk of comprehensive training at rehabilitation center (retrospective analysis)
No
Metabolic data' .l-HR(24%), .l-RR(40%),
Exercise tolerance Yes
l(TE (40%)
No
No
Yest
No
No
Yes
No
No
Yes*
No
tvo, in
Yes§
No
No
Yes?'
Yes
iFEV j , iPImax
Yes
exercise group
*FEV I = forced vital capacity or forced expiratory volu,mein the first second; H~ = heart rate; PFT = pulmonary function tests; PImax = maximal inspiratory pressure; RR = respiratory rate; VE = minute ventilation; VOz = oxygen uptake. [Of 138 patients, 91 had subjective improvement at 6 mo; 95 of 113 had objective increase in walking distance at 6 mo. Number of hospital days declined from 868 to 542; number of hospitalized patients decreased. *In exercise group, 24% had an increase in walking distance. No improvement was noted in control subjects. §Control group also had improvement after 6 wk of rehabilitation. Improvement was maintained 7 mo after training. #Exercise tolerance increased 3% in exercise group but decreased 28% in control group. 'lI0f the 379 patients, 62 were excluded. Patients were from acute-care facilities.
Exercise Evaluation.-In patients with COPD, the goals of formal exercise testing before entry into a rehabilitation program are to evaluate exercise capacity, to assess the safety of exercise, and to search for hypoxemia and other factors that might limit exercise. Formal exercise testing in the pulmonary laboratory usually includes measurements of expired gases for determination of oxygen uptake and output of carbon dioxide, ventilation, electrocardiographic findings, oxygen saturation, and blood gases. Patients with poor exercise capacity usually have a minute ventilation that approximates the maximal voluntary ventilation and will be unable to attain their predicted maximal oxygen uptake. Subsequently, the simpler exercise tests (which save both time and money) can be used for monitoring progress and follow-up. Such tests include a 6- or 12-minute walk with or without electrocardiographic monitoring and an incremental treadmill test that allows measurement of oxygen saturation, electrocardiographic findings, and the rate of perceived exertion.
Type of Exercise. Treadmill Walking.-Treadmill walking has proved effective in several studies of patients with cardiac disease.62•63 It has also been successfully used in patients with COPD. Some disadvantages include safety issues and the need to develop the skill of walking on a treadmill. As mentioned previously, the efficiency of treadmill walking usually improves with successive attempts.v" Such improvement makes it difficult to obtain a suitable baseline value before initiation of exercise training. For research studies, an accommodation period should probably be implemented before the "initial" measurements. Nontreadmill Walking.-Routine walking is simple and natural. It specifically trains a patient in an exercise mode that is useful for everyday activities. Walking can easily be performed outside the hospital or clinic setting. In inclement weather, indoor shopping malls and schools have frequently. been used for walking programs. Quantification of exercise is possible by timing the duration and measuring the total
150
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
PERCEIVED EXERTION RATING
6 7 8 9 10 11
Very, very light Very light Fairly light
12
13 14 15 16 17 18 19 20
Somewhat hard Hard Very hard Very, very hard
Fig. 3. Perceived exertion rating of subjective exercise intensity. Scale ranges from perception of "very, very light" exercise to "very, very hard" exercise.
distance walked during each session, such as a 12-minute walk.f Stationary Cycling.-Many exercise programs include stationary cycling because it is safe and readily available; however, it may be unfamiliar to some patients. Moreover, some doubt exists about whether the benefits of cycling are comparable to those of other types of exercise that more closely imitate the activities of daily living, such as walking." Arm Exercise.-In patients with coexistent orthopedic or neurologic disorders of the lower extremities, arm exercise may be useful. It can be designed to train the accessory muscles of respiration." Stretching Exercises.-Stretching exercises should be done with slow progression to greater ranges of motion. Such activities can improve mobility and help maintain flexibility. Intensity of Exercise.-The use of a target heart rate during exercise training is most beneficial in normal subjects and in patients with coronary artery disease. The main objectives are to have the patient achieve a heart rate during exercise that is within a safe range and, concurrently, to attain a training threshold. The usefulness of a target heart rate for patients with severe COPD is probably irrelevant because they are limited by ventilatory, not cardiovascular, function." The predicted target heart rate in most patients
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with severe COPD is higher than their actual heart rate at maximally tolerated exercise. Moreover, improved aerobic capacity is not essential for improved tolerance (as opposed to training) in patients with COPD. The recommendations proposed by the American College of Sports Medicine for exercise training in patients with COPD are shown in Table 2. Patients with mild COPD can probably exercise to an intensity that would result in an actual training effect. More severely limited patients (those with a forced expiratory volume during the first second of less than 60% of that predicted) are advised to undergo breathing training, to receive oxygen supplementation as needed, and to have the exercise intensity tailored to their tolerance." Guidelines based on a rating of perceived exertion have been advocated; patients are advised to exercise to a specific level of breathlessness. Gradually increasing the level of exercise, a method that is simple and easy to implement, has been shown to be valuable for improving endurance.v-" Belman'" recommended a program of gradually increasing nontreadmill exercise; the initial emphasis is on duration rather than intensity of exercise. After the subject is able to exercise for 45 to 60 minutes, the exercise intensity is increased. Improvements in exercise duration give the patient enough independence to participate in shopping and other activities away from home. Such accomplishments, which are taken for granted by normal persons, represent major milestones to previously homebound patients with COPO. Duration ofa Rehabilitation Program.-In rehabilitation programs, improved exercise tolerance has been substantiated as early as 4 weeks after onset." Persistence of the improvement has also been shown up to 7 months after discontinuation of the exercise training." Although no "ideal" duration has been established, 4 to 8 weeks is a common duration for many exercise programs.
Home-, Hospital-, or Outpatient-Based Exercise Rehabilitation.-Ample evidence has shown that exercise training can be done successfully in the home or in an outpatient setting, provided an initial screening test has been performed to ensure the safety of exercise. 43•68 The advantages of a hospital-based outpatient rehabilitation program include facilities for patient education, nutritional and psychologic counseling, and respiratory therapy. 53 After 4 to 8 weeks of a hospital-based pulmonary rehabilitation program, the exercise training may be continued at home under the supervision of a health-care professional or a knowledgeable family member. Self-monitoring of improvement can be done by using the 6- or 12-minute walking test. Successful home-based programs involve close follow-up'? and high motivation. Reimbursement for Pulmonary Rehabilitation.-Reimbursement for pulmonary rehabilitation varies from state
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Table 2.-Recommendations Proposed by the American CoUegeof Sports Medicine for Exercise Training in Patients With Chronic Obstructive Pulmonary Disease*
Degree of impairment
Results ofPFr
Mild
FVC 60-80% FEV] 60-80%
Moderate or severe
FVC
CHRISTOPHER O. OLOPADE, M.D.,* KENNETH C. BECK, Ph.D.; ROBERT W. VIGGIANO, M.D., BRUCE A. STAATS, M.D., Division of Thoracic Diseases and Internal Medicine
Impairment of exercise tolerance is a common problem in patients with severe chronic obstructive pulmonary disease. The cause of exercise intolerance in patients with severe chronic obstructive pulmonary disease is multifactorial and includes impaired lung mechanics, fatigue of inspiratory muscles, impaired gas exchange, right ventricular dysfunction, malnutrition, occult cardiac disease, deconditioning, and psychologic problems; however, impaired lung mechanics and gas exchange abnormalities seem to be the major limiting factors. Recently, the approach to management of pulmonary rehabilitation in patients with chronic obstructive pulmonary disease has changed because improvement in exercise tolerance has been demonstrated after pulmonary rehabilitation. Other adjunctive measures that have been shown to contribute to the observed improvement in exercise tolerance include administration of oxygen, nutritional support, cessation of smoking, and psychosocial support. The roles of ventilatory muscle endurance training, respiratory muscle rest therapy, nasally administered continuous positive airway pressure, and training of the muscles of the upper extremities are less clearly defined.
Patients with chronic obstructive pulmonary disease (COPD) often have poor tolerance of exercise.P The degree of exercise intolerance seems to reflect the severity of the underlying COPD.2 Other factors, however, such as muscle fatigue, malnutrition, and cardiac dysfunction, contribute to the difficulty with exercise in patients with COPD. In the 1950s and early 1960s, the accepted recommendation was to impose exercise limitations for patients with COPD. Because Barach and colleagues' noted progressive improvement in the ability to walk without dyspnea in patients with COPD who remained active (in contrast to their sedentary counterparts), they suggested that a physiologic response similar to a training effect in athletes may have been produced in such patients. The scientific basis for recommending exercise rehabilitation has only recently been established and, therefore, can only now be recommended. *Current address: University of Illinois, Chicago, Illinois. Address reprint requests to Dr. B. A. Staats, Division of Thoracic Diseases, Mayo Clinic, Rochester, MN 55905. Mayo Clin Proc 67:144-157,1992
Pierce and co-workers" attempted to understand the physiologic processes underlying the improved exercise tolerance in patients with COPD who were active. Nine patients with severe but stable COPD were enrolled in an exercise training program for 3 to 20 weeks after an initial period of acclimatization with treadmill exercise. After the training, the heart rate, respiratory rate, and minute ventilation substantially decreased and the exercise tolerance improved. These investigators, however, were unable to demonstrate any improvement in pulmonary function in these patients. They speculated that a physiologic training effect may have occurred.
EXERCISE LIMITATION During the past 2 decades, many investigators have attempted to explain the causes of improved exercise tolerance in active patients with COPD. The approach to rehabilitation in patients with COPD has also dramatically changed; exercise rehabilitation is now commonplace. The numerous proposed reasons for exercise intolerance are discussed in the following material. 144
Mayo Clio Proc, February 1992, Vol 67
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Impaired Lung Mechanics.-Impaired lung mechanics Impaired Gas Exchange-s-Gss exchangeis one of the major is one of the major factors that limit exercise performance in functions of the lung. The efficiency of gas exchange is patients with COPD. 5-8 In patients with severe COPD, expi- dependent on the balance between alveolar ventilation and ratory flow may be limited during tidal breathing. One of the pulmonary blood flow (perfusion). In a normal lung, regions strategies adopted by these patients to overcome limitation with the most ventilation tend to have the most perfusion, of expiratory flow (which can curtail exercise) is breathing at and, conversely, poorly ventilated areas receive minimal a higher end-expiratory lung volume to achieve higher expi- perfusion. One estimate of the degree of "matching" of ratory flows (Fig. I). This strategy increases the maximal ventilation to perfusion is the ratio of physiologic dead space exercise ventilation. The increase in end-expiratory lung to tidal volume (VDNT). The VDNT is optimal when perfuvolume is associated with expansion of both the thoracic sion is well matched to ventilation. At rest VDNT is norcage and the lung parenchyma-factors that, unfortunately, mally approximately O.3-that is, 30% of a given breath is necessitate an increase in the elastic work of breathing. In "wasted." Most of this dead-space ventilation is used to fill addition, at high lung volumes, the diaphragm flattens, and the conducting airways. During exercise, the VDNT dethe ribs become horizontally oriented; thus, both the inter- clines to 0.2 or less." In patients with COPD (especially costal muscles and the diaphragm are placed in unfavorable those with emphysema, because of the destruction of the gasmechanical positions (Fig. 2). The ensuing pattern of con- exchanging units), regions of appreciable ventilation-to-pertraction may cause the diaphragm to act like an expiratory fusion mismatch impair oxygenation of the blood and also muscle. This response is discernible on examination by elimination of carbon dioxide. At rest, the VDNT is ininward movement of the costal region (Hoover's sign). creased as a result of ventilation to poorly perfused alveoli Breathing at higher lung volumes decreases the patient's (so-called alveolar dead space). In contrast to the normal ability to increase tidal volume during exercise; hence, an decrease in VDNT that occurs during exercise, patients with increase in respiratory frequency is the major remaining COPD may experience further increases or lack of decline in means of increasing ventilation (ventilation equals frequen- VDNT. The high VDNT does not result in hypoxemia but cy of respiration multiplied by tidal volume). The rapid, increases the ventilatory requirements of an already overshallow pattern of breathing in patients with COPD contrasts taxed system. This result causes the ventilatory equivalent with that in normal persons, who typically have an increase for carbon dioxide to be increased; 13,14 together with a rein tidal volume before an increase in respiratory frequency duced maximal voluntary ventilation, these factors contribwith progressive exercise." Increased resistance to expira- ute to exercise limitation. In addition, hypoxemia may develop in many patients tory flow and decreased mechanical advantage of inspiratory muscles at high lung volumes result in an increase in the because of perfusion of poorly ventilated alveoli. Pulmowork or oxygen cost of breathing, which ultimately may lead nary capillary blood from these regions is not adequately to fatigue of the inspiratory muscles (diaphragm). oxygenated, and hypoxemia results when this blood is mixed Fatigue of Inspiratory Muscles (Diaphragm).-Belle- in the left atrium with normally oxygenated blood. Ventilamare and Grassino 10, 11 demonstrated the importance of the tion-to-perfusion mismatch causes an increase in the oxygen ratio of inspiration time to total breath duration and the ratio partial pressure difference between alveolar air and arterial of mean transdiaphragmatic pressure (abdominal pressure blood, which may become worse with exercise. The hyminus pleural pressure) generated with each inspiration to poxemia can be-but is not always-an exercise-limiting maximal transdiaphragmatic pressure as determinants of factor. 6 Depending on ventilatory drive, hypoxemia may diaphragmatic endurance in normal volunteers and patients further stimulate ventilation and hasten exercise limitation. with COPD. They found that when fatigue of the respiratory Right Ventricular Dysfunction and Pulmonary muscles developed, the tension time index (the product of Hypertension.-In the past, patients with COPD without the ratio of inspiration time to total breath duration and the coexisting ischemic heart disease or cor pulmonale were not ratio of mean transdiaphragmatic pressure with each inspira- thought to have exercise limitations because of cardiac dystion to maximal transdiaphragmatic pressure) was more than function. Nevertheless, studies conducted by Morrison and 0.15. 10, 11 Patients with severe COPD have limited ventila- associates" and Mahler and colleagues" with use of hemotory reserve even at rest and have been shown to have a dynamic monitoring and radionuclide studies during exerresting tension time index that approaches 0.15 (normal, cise have shown that right ventricular dysfunction does oc0,02),11 With the development of acute respiratory failure, cur in some patients during exercise. In these patients, patients may deteriorate from the borderline fatigue zone pulmonary artery pressure increased in the setting of normal into the definite fatigue zone (a tension time index that left ventricular function. The increased pulmonary artery exceeds 0,15),7 Similar findings have been noted during pressure returned to normal or near normal after exercise. In exercise in patients with COPD. patients with severe COPD, further pulmonary hypertension
146
Mayo Clin Proc, February 1992, Vol 67
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
10
8 6
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Fig. 1. Diagram of flow-volume loop dynamics in a normal person and a patient with chronic obstructive pulmonary disease (COPD). In patient with COPD, during tidal breathing expiratory flow abuts the expiratory limb of maximal expiratory flow-volume curve. During exercise (Ex), patient with COPD breathes at a slightly higher lung volume (leftward shift of flow-volume loop). This result contrasts with that in normal subject, who has a large ventilatory reserve both at rest and during exercise. TLC = total lung capacity.
often develops during exercise and may cause premature termination of exercise. Occult Cardiac Disease.-Dyspnea caused by left ventricular dysfunction and that caused by pulmonary disease are sometimes difficult to distinguish clinically. Left ventricular dysfunction is uncommon in patients with COPD in whom such dysfunction is clinically suspected. I? In an evaluation of the relationship between right and left ventricular function in patients with varied degrees of severity of COPD, however, Slutsky and colleagues" found that left ventricular dysfunction correlated with the decline in right ventricular function only in patients with severe COPD. The results of that study suggested that patients with severe COPD and pulmonary hypertension have worsening of left ventricular function· during exercise when right ventricular dysfunction is clearly evident. Altered Perception of Breathlessness.-An increased perception of breathlessness may contribute to exercise limitation in patients with COPD. Woodcock and co-workers'? postulated that the sensation of breathlessness may constitute a greater limitation to exercise than mechanical or cardiovascular factors. They administered a mild depressant of the central nervous system (dihydrocodeine) to patients with severe chronic limitation of airflow and demonstrated a de-
creased degree of breathlessness and increased exercise tol- . erance in comparison with the control situation. Altered Control of Ventilation.-In some patients with COPD, the control of breathing may be abnormal and may potentially contribute to retention of carbon dioxide. Assessing the ventilatory response to inhaled carbon dioxide is one method of determining the sensitivity of the respiratory drive. The ventilatory response, however, is nonspecific" because it is influenced by numerous factors, including abnormal pulmonary function and altered acid-base status, both of which are present in patients with COPD. The pressure generated in the first 100 ms when the inspiratory muscles contract during transient occlusion of the airway may be a better measure of respiratory drive." Some studies have shown that this pressure measurement is greater in patients with COPD than in normal subjects, an indication of increased respiratory drive. Some investigators believe that increased respiratory drive in the presence of abnormal pulmonary function is a mechanism for dyspnea in patients with COPD.22 Psychologic Factors.-Depression, anxiety, and inappropriate fear of exercise are prevalent in patients with COPD.23 Excitement or exertion results in increased respiratory rate and minute ventilation, leading to increased work of
Mayo Clin Proc, February 1992, Vol 67
NORMAL
,,
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
147
EMPHYSEMA
"
"Zone of apposition
"Flat diaphragm
Fig. 2. Diagram depicting flattening of diaphragm and loss of region of apposition in patient with emphysema. Contraction of flattened diaphragm leads to inward movement of costal region, which is noticeable on physical examination (Hoover's sign). breathing and worsening of the sensation of breathlessness. A vicious cycle is thus created. Patients with COPDwho have incapacitating psychosocial problems (such as severe depression or anxiety) respond less favorably to rehabilitation programs than do patients without such problems.P' In addition, patients with well-established psychosocial skills exhibit positive responses to group psychotherapy." The contribution of psychologic factors to exercise limitation in COPD is considerable and must not be overlooked in a comprehensive rehabilitation program. Malnutrition.- The overall incidence of malnutrition in patients with COPD is unknown. In a group of hospitalized patients with COPD, however, the prevalence of nutritional depletion determined by anthropographic measurements and a body weight less than 90% of ideal was 50%.26.27 Poor nutritional status also correlates with suboptimal pulmonary function in patients with emphysemas-" and has been shown to be an independent risk factor for poor survival." Proposed causes of malnutrition include interference with gastrointestinal function by medications (such as theophylline and corticosteroids), vascular congestion of the gut caused by cor pulmonale, catabolic effects of corticosteroids, gastric distention from aerophagia, and high metabolic rate. Patients with emphysema, in particular, have a higher than normal metabolic rate as a result of their increased oxygen consumption during breathing." Dyspnea and arterial desaturation while eating may also cause decreased dietary intake in some patients." Ultimately, the malnutrition may interfere with ventilatory muscle strength and exercise per-
formance. In addition, malnutrition may predispose the patient with COPD to frequent infections of the respiratory tract and bacterial colonization of the airway through reduced secretory IgA, increased bacterial adherence, and decreased bacterial clearance.P:" Arora and Rochester" reported substantial decreases in respiratory .muscle strength and maximal voluntary ventilation that were proportional to the degree of weight loss in nutritionally depleted normal volunteers. Similar changes most likely occur in undernourished patients with COPD because of their high ventilatory requirements. Deconditioning.-Physical conditioning increases the ability to perform work. In a group of well-trained athletes, the imposition of strict bed rest or inactivity for 10 days to 2 months resulted in a deconditioned state characterized by the physiologic changes of a decrease in maximal oxygen uptake,":" an increased response of the heart rate and the blood pressure to exercise,36-38 a posture-dependent reduction in stroke volume.v-" and a reduction in size of the left ventricle." Exercise intolerance sometimes forces patients with severe COPD to adopt a sedentary life-style, which results in deconditioning. Depending on the degree of inactivity, physical deconditioning (in conjunction with the other factors previously discussed in this review) contributes considerably to exercise intolerance in patients with COPD.
EXERCISE REHABILITATION In recent years, exercise training has become the focus of many cardiac rehabilitation programs. Cardiovascular and
148
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
peripheral muscle training responses have been achieved through rehabilitation after both myocardial infarction and a cardiac surgical procedure.P:" In contrast to the success in training cardiac patients, numerous studies with various designs during the past 25 years have shown that exercise training only modestly improves tolerance in patients with COPD.4,42-54 The results of some of these studies are summarized in Table 1. Few studies showed improvement in pulmonary function; those that did often involved patients recuperating from an acute illness immediately before enrolling in the rehabilitation program." A more recent investigation, however, suggested that a physiologic training effect mediates much of the improved exercise tolerance after rehabilitation." Casaburi and colleagues" showed that patients with moderately severe COPD attained training responses similar to those in normal subjects at a high rate of work. They also demonstrated reductions in serum lactate levels and minute ventilation after 8 weeks of training. Their study challenges the previous notion that patients with COPD cannot tolerate enough exertion for lactic acidosis to develop. The mechanism that improves exercise tolerance is unclear. Measurements of pulmonary function rarely show improvement; therefore, the possibilities include improved aerobic capacity (an actual training response), increased motivation, desensitization to the sensation of dyspnea, and improved muscular efficiency during exercise (lower oxygen cost of exercise). The effects of exercise training can be measured in several ways, including increases in maximal oxygen uptake, decreases in perception of work as assessed on the rating of perceived exertion scale, and increases in the duration and distance walked in a standardized protocol. Improved Aerobic Capacity.-The maximal oxygen uptake is perhaps the best indicator of aerobic capacity." During training, normal subjects undergo. both cardiovascular and peripheral conditioning.v-" The cardiovascular changes include decreased response of the heart rate to exercise and increased stroke volume; the peripheral changes include increased concentration of muscle enzymes, increased capillary density, and increased mitochondrial size and number. To achieve this training ·effect, the person must exercise at approximately 50% of maximal oxygen uptake for 20 to 30 minutes three to four times weekly for at least 3 weeks." Patients with severe COPD are usually limited by symptoms before they achieve the level of exercise that would ensure a physiologic training effect." Belman and Kendreganv trained two groups of patients with COPD (seven trained their legs and eight trained their arms) for 6 weeks in an attempt to demonstrate peripheral changes from the exercise. Muscle biopsies were performed in both trained and nontrained muscles. Those investigators found no increase in muscle enzymes despite improved exercise tolerance and no evidence of the typical cardiovascular
Mayo CliD Proc, February 1992, Vol 67
response to training. This study suggested that a critical level of exercise must be attained before training can occur and that this level is not reached in patients with COPD. Casaburi and colleagues55 did not perform muscle biopsies in their study group; however, they showed that, even at low rates of work, lactic acidosis can develop in patients with severe COPD and that a "training effect" (defined as a decreased production of lactate and a corresponding decreased minute ventilation for the same level of exercise) occurred in most of their patients during high-level exercise training. Increased Motivation.-Improved motivation, a heightened sense of well-being, and decreased dyspnea are noted in most patients enrolled in an exercise rehabilitation program. An analog scale (Fig. 3) is commonly used to evaluate breathlessness and relative exercise intensity." The scale for rating of perceived exertion provides a reproducible means of quantifying subjective exercise intensity and has been shown to correlate well with minute ventilation." This scale is placed within easy view during an exercise session, and the patient is asked to rate the level of exertion or degree of breathlessness. Exercise is considered maximal when the patient reports a perceived rate of exertion of 19 or 20. Desensitization to Dyspnea.-The added sense of security derived from the presence of physicians, nurses, and therapists seems to reassure the patients undergoing rehabilitation and encourage them to exercise longer without panic, Patients are taught that dyspnea is not a sensation to be feared. Better motivation related to a heightened sense of well-being has been shown to lead to improved exercise tolerance." Unfortunately, like motivation, desensitization to the sensation of dyspnea cannot be measured objectively for assessment of its contribution to improved endurance. Improved Technique of Exercise.-Some studies" have shown improved efficiency in treadmill walking with repeated attempts. Sinclair and Ingram" observed a decrease in the number of steps taken in a given distance during a 12minute walk. More efficient performance of an exercise task decreases the oxygen uptake necessary for exercise and concomitantly reduces the ventilatory requirement.
COMPOSITION OF A PULMONARY REHABILITATION PROGRAM Even with the widening application of pulmonary rehabilitation for patients with COPD, many questions about what constitutes an optimal program remain unanswered. Should every patient with COPD have a complete exercise evaluation before enrollment? What type of exercise is most appropriate? How intense should the exercise program be? What should be the duration of a rehabilitation program? Should the program be hospital-, home-, or outpatient-based rehabilitation?
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149
Table l.-Summary of Selected Reports of Exercise Training in Patients With Chronic Obstructive Pulmonary Disease* Improvement Reference Pierce et aI,4 1964
No. of Patients Controls 9
0
Mean FEV l (L) 0.97
Petty et al,53 1969 Mungall & Hainsworth.'? 1980
182
0
0.94
10
0
1.54
Sinclair & Ingram." 1980
33
"With controls"
1.1
Cockcroft et al," 1981
39
Randomized control
1.52 (1.32 for controls)
Busch & Mcf'lements.f 1988
20
10
0.84
379'll
0
0.46-0.72
Foster et al,"? 1988
Training program
PFT
3-20 wk of treadmill walking2-5 min 5-10 times/day at speeds slower than maximal attainable 12 mo of comprehensive training 3 12-wk periods-before, during, and after supervised training for 12 min/day For 10 mo, daily l2-min walks and stair-climbing, supervised once/wk For 6 wk, swimming, rowing, and walking at rehabilitation center; remainder at home. Control subjects-6 wk of rehabilitation after 4 mo as controls 18 wk of supervised daily mobility, strengthening, and endurance exercises 4 wk of comprehensive training at rehabilitation center (retrospective analysis)
No
Metabolic data' .l-HR(24%), .l-RR(40%),
Exercise tolerance Yes
l(TE (40%)
No
No
Yest
No
No
Yes
No
No
Yes*
No
tvo, in
Yes§
No
No
Yes?'
Yes
iFEV j , iPImax
Yes
exercise group
*FEV I = forced vital capacity or forced expiratory volu,mein the first second; H~ = heart rate; PFT = pulmonary function tests; PImax = maximal inspiratory pressure; RR = respiratory rate; VE = minute ventilation; VOz = oxygen uptake. [Of 138 patients, 91 had subjective improvement at 6 mo; 95 of 113 had objective increase in walking distance at 6 mo. Number of hospital days declined from 868 to 542; number of hospitalized patients decreased. *In exercise group, 24% had an increase in walking distance. No improvement was noted in control subjects. §Control group also had improvement after 6 wk of rehabilitation. Improvement was maintained 7 mo after training. #Exercise tolerance increased 3% in exercise group but decreased 28% in control group. 'lI0f the 379 patients, 62 were excluded. Patients were from acute-care facilities.
Exercise Evaluation.-In patients with COPD, the goals of formal exercise testing before entry into a rehabilitation program are to evaluate exercise capacity, to assess the safety of exercise, and to search for hypoxemia and other factors that might limit exercise. Formal exercise testing in the pulmonary laboratory usually includes measurements of expired gases for determination of oxygen uptake and output of carbon dioxide, ventilation, electrocardiographic findings, oxygen saturation, and blood gases. Patients with poor exercise capacity usually have a minute ventilation that approximates the maximal voluntary ventilation and will be unable to attain their predicted maximal oxygen uptake. Subsequently, the simpler exercise tests (which save both time and money) can be used for monitoring progress and follow-up. Such tests include a 6- or 12-minute walk with or without electrocardiographic monitoring and an incremental treadmill test that allows measurement of oxygen saturation, electrocardiographic findings, and the rate of perceived exertion.
Type of Exercise. Treadmill Walking.-Treadmill walking has proved effective in several studies of patients with cardiac disease.62•63 It has also been successfully used in patients with COPD. Some disadvantages include safety issues and the need to develop the skill of walking on a treadmill. As mentioned previously, the efficiency of treadmill walking usually improves with successive attempts.v" Such improvement makes it difficult to obtain a suitable baseline value before initiation of exercise training. For research studies, an accommodation period should probably be implemented before the "initial" measurements. Nontreadmill Walking.-Routine walking is simple and natural. It specifically trains a patient in an exercise mode that is useful for everyday activities. Walking can easily be performed outside the hospital or clinic setting. In inclement weather, indoor shopping malls and schools have frequently. been used for walking programs. Quantification of exercise is possible by timing the duration and measuring the total
150
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
PERCEIVED EXERTION RATING
6 7 8 9 10 11
Very, very light Very light Fairly light
12
13 14 15 16 17 18 19 20
Somewhat hard Hard Very hard Very, very hard
Fig. 3. Perceived exertion rating of subjective exercise intensity. Scale ranges from perception of "very, very light" exercise to "very, very hard" exercise.
distance walked during each session, such as a 12-minute walk.f Stationary Cycling.-Many exercise programs include stationary cycling because it is safe and readily available; however, it may be unfamiliar to some patients. Moreover, some doubt exists about whether the benefits of cycling are comparable to those of other types of exercise that more closely imitate the activities of daily living, such as walking." Arm Exercise.-In patients with coexistent orthopedic or neurologic disorders of the lower extremities, arm exercise may be useful. It can be designed to train the accessory muscles of respiration." Stretching Exercises.-Stretching exercises should be done with slow progression to greater ranges of motion. Such activities can improve mobility and help maintain flexibility. Intensity of Exercise.-The use of a target heart rate during exercise training is most beneficial in normal subjects and in patients with coronary artery disease. The main objectives are to have the patient achieve a heart rate during exercise that is within a safe range and, concurrently, to attain a training threshold. The usefulness of a target heart rate for patients with severe COPD is probably irrelevant because they are limited by ventilatory, not cardiovascular, function." The predicted target heart rate in most patients
Mayo Clin Proc, February 1992, Vol 67
with severe COPD is higher than their actual heart rate at maximally tolerated exercise. Moreover, improved aerobic capacity is not essential for improved tolerance (as opposed to training) in patients with COPD. The recommendations proposed by the American College of Sports Medicine for exercise training in patients with COPD are shown in Table 2. Patients with mild COPD can probably exercise to an intensity that would result in an actual training effect. More severely limited patients (those with a forced expiratory volume during the first second of less than 60% of that predicted) are advised to undergo breathing training, to receive oxygen supplementation as needed, and to have the exercise intensity tailored to their tolerance." Guidelines based on a rating of perceived exertion have been advocated; patients are advised to exercise to a specific level of breathlessness. Gradually increasing the level of exercise, a method that is simple and easy to implement, has been shown to be valuable for improving endurance.v-" Belman'" recommended a program of gradually increasing nontreadmill exercise; the initial emphasis is on duration rather than intensity of exercise. After the subject is able to exercise for 45 to 60 minutes, the exercise intensity is increased. Improvements in exercise duration give the patient enough independence to participate in shopping and other activities away from home. Such accomplishments, which are taken for granted by normal persons, represent major milestones to previously homebound patients with COPO. Duration ofa Rehabilitation Program.-In rehabilitation programs, improved exercise tolerance has been substantiated as early as 4 weeks after onset." Persistence of the improvement has also been shown up to 7 months after discontinuation of the exercise training." Although no "ideal" duration has been established, 4 to 8 weeks is a common duration for many exercise programs.
Home-, Hospital-, or Outpatient-Based Exercise Rehabilitation.-Ample evidence has shown that exercise training can be done successfully in the home or in an outpatient setting, provided an initial screening test has been performed to ensure the safety of exercise. 43•68 The advantages of a hospital-based outpatient rehabilitation program include facilities for patient education, nutritional and psychologic counseling, and respiratory therapy. 53 After 4 to 8 weeks of a hospital-based pulmonary rehabilitation program, the exercise training may be continued at home under the supervision of a health-care professional or a knowledgeable family member. Self-monitoring of improvement can be done by using the 6- or 12-minute walking test. Successful home-based programs involve close follow-up'? and high motivation. Reimbursement for Pulmonary Rehabilitation.-Reimbursement for pulmonary rehabilitation varies from state
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151
Table 2.-Recommendations Proposed by the American CoUegeof Sports Medicine for Exercise Training in Patients With Chronic Obstructive Pulmonary Disease*
Degree of impairment
Results ofPFr
Mild
FVC 60-80% FEV] 60-80%
Moderate or severe
FVC
