The prescription of exercise intensity in patients with chronic obstructive pulmonary disease (COPD) is not well defined (1,5,11,17). Specific questions concerning the prescription of exercise for COPD patients include what is the optimal exercise intensity and what is the best method for prescribing exercise intensity. The American College of Sports Medicine (ACSM) recommends four methods for prescribing exercise intensity in COPD patients. These include exercise at 50% of peak oxygen consumption (O2peak), exercise at or above the anaerobic threshold, exercise at near-maximal intensity, and the use of ratings of dyspnea (2). If 50% of O2peak is used as the method of exercise prescription, it then becomes necessary to determine how this intensity will be monitored during the exercise sessions. One method that has been suggested is to use a certain percentage of peak heart rate (%HRpeak). Because of the linear relationship between heart rate and oxygen consumption (O2), the heart rate measured during exercise is a method of estimating O2, and, therefore, can be used as an indicator of exercise intensity (18). More specifically, the assessment of exercise intensity, as reflected by %O2peak, can be indirectly measured as a function of %HRpeak. According to the ACSM, 62, 70, 85, and 90% of HRpeak represent 50, 60, 80, and 85% of O2peak, respectively, for the general population (2).
Swain et al. (20) recently examined the relationship between %HRpeak and %O2peak in apparently healthy men and women. The investigation by Swain et al. differed from previous studies examining this relationship by the development of a regression equation that used %HRpeak as the dependent variable and %O2peak as the independent variable. Furthermore, Swain et al. performed linear regressions for each participant and then formulated a mean regression equation from all these values as opposed to using all subject data to develop a single equation. These investigators found that the %HRpeak obtained by their subjects at a given %O2peak were significantly greater than those suggested by the ACSM (2). Additionally, Swain et al. found that the subject’s fitness level affected the %HRpeak and %O2peak relationship. Because of the effect that fitness level has on this relationship, Swain et al. (20) suggested that this relationship be studied in individuals with a low functional capacity.
Based on the recommendation of Swain et al. and because of the fact that the relationship between %HRpeak and %O2peak has profound implications concerning exercise prescription in COPD patients, the purpose of this investigation was to examine the %HRpeak and %O2peak relationship in these patients. Additionally, because fitness level has been shown to affect this relationship, a secondary purpose was to examine the effects of 3 months of exercise training in these patients on this relationship.
The study population which included 125 ambulatory persons was comprised of 70 men and 55 women between the ages of 55 and 80. These patients were a subset of patients from a single center two arm randomized clinical trial investigating the effect of regular exercise on physical function and disability in COPD patients—Reconditioning Exercise and Chronic Obstructive Pulmonary Disease Trial, REACT. All patients met the following criteria: 1) an expiratory airflow limitation such that forced expiratory volume in one second/forced vital capacity (FEV1/FVC) was less than 70% and the forced expiratory volume in one second (FEV1) was greater than 20% of predicted; 2) no unstable cardiac disease or other medical problem which would limit participation in an exercise program; 3) not actively participating in regular exercise program or pulmonary program nor had the subject participated in either for the preceding 6 months; and 4) the ability to walk continuously for 6 min. All subjects were informed about the study and of their rights as subjects and signed an informed consent approved by the university’s institutional review board. Predicted FEV1.0 values were from Knudson et al. (14). Descriptive data of the subjects are presented in Table 1.
All patients were tested before entry into REACT, participated in a 12-wk supervised exercise training program, and then were tested again after the exercise training program. Patients initially reported to Wake Forest University on three separate occasions for screening and testing visits. During the first visit, patients signed the informed consent, performed spirometry and lung volume testing, and completed questionnaires related to the primary outcomes of REACT. During the second visit, the patients performed a maximal graded exercise test on a treadmill. The third screening visit consisted of patients completing additional questionnaires and physical function tests also related to the outcomes of REACT.
After the screening and testing visits, the patients began 12 wk of exercise training. The exercise training program consisted of walking, upper body strength training, and stretching exercises. Classes met 3 d a week and patients exercised for approximately 1 h each day. Trained exercise specialists supervised each exercise session. Each patient completed 30–40 min of aerobic exercise followed by 15–20 min of upper-extremity strength-training exercises. Participants reported to the exercise facility and were seated for 5 min before beginning exercise. During this time, oxygen saturation, heart rate, and blood pressure were measured. Each patient walked for 3 min to warm up, then proceeded to walk for 30 min on an indoor track. Participants were instructed to walk at a rating of perceived dyspnea (RPD) of 3–4 (moderate-somewhat hard) based on the Borg categorical scale (6). At 20 min into exercise, each subject’s heart rate, oxygen saturation and RPD were recorded. After 30 min of walking, each patient cooled down. Participants then performed upper-extremity strength-training exercises. Two sets of eight repetitions of biceps curls, triceps extension, shoulder flexion, shoulder abduction, and shoulder elevations were performed.
After 12 wk of exercise training, the subjects were tested again using the same testing procedures as were performed before the training program.
Pulmonary function tests.
Pulmonary function tests were administered to the patients during their initial screening visit. A Medical Graphics (St. Paul, MN) 1085 body plethysmograph was used to perform the tests. The patients were instructed not to use bronchodilators during the 8 h before the test. Calibrations of the pneumotachograph and body plethysmograph were performed according to the manufacturer’s specifications. Spirometry and lung volume measurements were performed using the guidelines of the American Thoracic Society (3)
Graded exercise tests.
The patients performed an incremental graded exercise test (GXT) using a Quinton 4000 (Seattle, WA) electrocardiograph and a Q55XT treadmill. The test was administered using a modified Naughton protocol similar to that of Berry et al. (4). The only difference between the protocol used in this investigation and that of Berry et al. (4) was that all stages of the GXT in this investigation were 2 min in duration. In the protocol of Berry et al. (4) the second stage was 4 min in duration. Before the start of the tests, patients received standardized instructions and were told that they should continue to exercise as long as possible. Indications for terminating the test were according to ACSM guidelines. Patients received standardized encouragement during each of the graded exercise tests. Once the test had been terminated, the reason for the test termination was recorded. Throughout the GXT, the patients’ ventilatory and gas exchange responses were measured on a breath-by-breath basis using a computerized system (Medical Graphics, CPX-D system). Gas exchange and minute ventilation were calculated on a breath by breath basis and reported as minute values. Data used to calculate the minute values were collected for the entire 60-s period. If 60 s of data were not available for the last minute of the test, a minimum of 30 s of data had to be collected. These data were then used to represent the last minute. During the tests, patients breathed through a rubber mouthpiece connected to a disposable Pitot tube flow meter. Expiratory airflow was measured using the Pitot tube flow meter, and fractions of O2 and CO2 were determined using rapid responding analyzers. Calibrations of the pneumotachograph and gas analyzers were performed before each test according to the manufacturer’s specifications. All gases used in calibration were certified standard gases which had been verified via Haldane chemical analysis. The values for O2 and CO2 concentrations and volume of expired air were used to calculate O2. Heart rate was monitored continuously using a Quinton Q4000 electrocardiograph. Electrode configuration for all testing was the Mason-Likar placement such that it allowed standard 12-lead electrocardiogram tracings to be obtained during all testing. Heart rates were obtained from the electrocardiogram tracing and were recorded during the final 10 s of each stage of the exercise test. The GXT was terminated according to ACSM guidelines.
Individual regression equations were computed for each patient for both the baseline and the follow-up GXT. Oxygen consumption was the independent variable and was expressed as %O2peak. O2peak was defined as the highest O2 value attained during the GXT. Data from the 2nd minute of each stage and the final minute of the GXT were used in computing the regression equations. Heart rate was the dependent variable and was expressed as %HRpeak. From each individual patient’s baseline and follow-up equations, % HRpeak was then predicted at 50, 60, 80, and 85% of O2peak. Single sample t-tests were then used to compare the predicted %HRpeak at baseline and follow-up to the values of 62, 70, 85, and 90% of HRpeak. This single sample t-test comparison was done with these values since the ACSM states that 62, 70, 85, and 90% of HRpeak is equivalent to 50, 60, 80, and 85% of O2peak, respectively. The slopes and intercepts from the regression equations of all baseline tests were then averaged, and these average values are reported as the representative baseline equation. Similarly, the slopes and intercepts from the regression equations of all follow-up tests were averaged, and these average values are reported as the representative follow-up equation. The correlation coefficients for each patient’s individual regression equation were computed both at baseline and follow-up. Additionally, paired sample t-tests were used to compare differences between the slopes and differences between the intercepts of the baseline and follow-up equations. Significance was set at the 0.05 level for all tests.
Descriptive data along with the results of pulmonary function tests and results from the graded exercise tests are shown in Table 1. No significant differences were found between baseline and follow-up when comparing weight or pulmonary function data. Examination of the results from the GXT show participants went significantly longer at follow-up as compared with baseline. This was the only GXT variable significant when comparing baseline and follow-up values. The majority of patients reported dyspnea as the primary reason for terminating the GXT at both baseline and follow-up. The second most frequently reported reason for terminating the GXT at both baseline and follow-up was leg fatigue.
The mean (±SD) attendance rate, as calculated from the number of exercise sessions attended and expressed as a percent of the total possible number of sessions that could have been attended during the 12-wk exercise intervention, was 88.1 ± 12.0. The mean (±SD) training ratings of dyspnea over the 12-wk period was 3.1 ± 0.4 units. The mean (±SD) training heart rate over the 12 wk period was 87.5 ± 10.7% HRpeak.
Comparisons of %HRpeak as suggested by ACSM and as found at baseline and follow-up at 50, 60, 80, and 85% O2peak, respectively, are shown in Table 2. Single sample t-tests comparing the baseline %HRpeak with the suggested ACSM %HRpeak values revealed significant differences between the two values at 50, 60, and 80% O2peak. There was no significant difference between the baseline %HRpeak and the suggested ACSM %HRpeak values at 85% O2peak. Single sample t-tests comparing the follow-up %HRpeak with the suggested ACSM %HRpeak values revealed significant differences between the two values at 50, 60, and 80% O2peak. There was no significant difference between the follow-up %HRpeak and the suggested ACSM %HRpeak values at 85% O2peak. Matched pairs t-tests revealed no significant differences between the baseline %HRpeak and follow-up %HRpeak at 50, 60, 80 or 85% O2peak.
Averaging the slopes and intercepts from the patient’s individual baseline and follow-up regression equations produced the two regression lines illustrated in Figure 1. The mean (±SD) correlation coefficient from all patients’ regression equations at baseline and follow-up were 0.967 ± 0.005 and 0.955 ± 0.008, respectively. In addition to these two regression lines, the regression line from the relationship between %HRpeak and %O2peak suggested by ACSM is also shown in this figure. Matched t-tests showed that neither the intercepts nor the slopes were significantly different when comparing them at baseline and follow-up. Since there were no differences between these values, we pooled the data from the baseline and follow-up tests to develop the following equation to describe the relationship between %HRpeak and %O2peak for COPD patients (values shown for the slope and intercept are mean ± SEM):
The results from this investigation show that the relationship between %HRpeak and %O2peak described by the ACSM is not applicable to patients with COPD. Data obtained from the patients before and after the 3-month exercise intervention show that the patients averaged 70 and 71% of HRpeak when at 50% of O2peak and averaged 76 and 77% of HRpeak when at 60% of O2peak. These heart rates are significantly higher than the 62% and 70% suggested by the ACSM. Additionally, the data obtained from the patients before and after the exercise intervention indicate that at 80% and 85% of O2peak the values of %HRpeak more closely approximate the corresponding values suggested by the ACSM. It should be noted, however, that this smaller difference at higher intensities is expected, because the %HRpeak values from the ACSM regression and the %HRpeak values from average regressions from this study both approach their maximums. These findings are similar to those of Swain et al. (20). They found that 70, 76, 89, and 92% of HRpeak corresponded to 50, 60, 80, and 85% of O2peak. Both studies show the same level of disagreement with the ACSM values.
In a large number of laboratories, O2peak is not measured during treadmill testing due to the expense involved. As a result, an exercise prescription is often based on the %HRpeak and %O2peak relationship described by the ACSM. As previously stated, the relationship between %HRpeak and %O2peak, as described by the ACSM, does not accurately apply to patients with COPD, especially at low exercise intensity levels. Consequently, an important clinical finding from this investigation is that both heart rate and oxygen consumption should be measured during graded exercise testing if exercise is to be accurately prescribed in patients with COPD based on the relationship between %HRpeak and %O2peak. If heart rate and oxygen consumption cannot be measured during graded exercise testing, an alternate approach would be to use the equation we have developed to prescribe the exercise.
One method suggested for prescribing exercise intensity in patients with COPD is to exercise at an intensity equal to 50% of O2peak (2). Because of the linear relationship between %HRpeak and %O2peak, it is suggested that a given percent of HRpeak can be used to monitor the exercise intensity. Our results suggest that the relationship between %HRpeak and %O2peak described by ACSM cannot be used with COPD patients and will result in errors in the exercise prescription if this approach is used. For example, if we had a patient whose HRpeak was 160, according to the ACSM %HRpeak and %O2peak relationship this person should be exercising at a heart rate of 100 beats per min if s/he wished to exercise at 50% of his/her O2peak. The results from this investigation indicate that 100 beats per min may be too low of a heart rate. Based on the results of the equation we developed from our baseline data, this patient should be exercising at a heart rate of 113 beats per min if s/he wishes to be at 50% of his/her O2peak. According to our results, if this patient were exercising at a heart rate of 100 beats per min, s/he would be exercising at approximately 35% of his/her O2peak not the prescribed 50%.
A leftward shift in the %HRpeak and %O2peak relationship after exercise training has been shown in other studies (i.e., there is a higher %HRpeak at a given %O2peak following training) (10,20). In contrast to these previous findings, we found the relationship to remain stable following training. Although our finding may appear contradictory to previous findings, it can easily be explained by the fact that COPD patients may not show an increase in O2peak after exercise training (7). The shift in the %HRpeak and %O2peak relationship to the left with training in normal healthy individuals is due to the fact that training increases O2peak. Subsequently, trained individuals will be working at a lower percent of their O2peak at a given submaximal workload. This is partially compensated by the fact that trained individuals will also have a lower heart rate at this submaximal workload; however, the overall effect is trained individuals have a higher %HRpeak at a given %O2peak as compared with untrained individuals (20). Given the fact that our COPD patients did not improve their O2peak after exercise training, they were not working at a lower percentage of their O2peak at a given submaximal workload.
As previously mentioned, O2peak may or may not increase after exercise training in COPD patients. Casaburi (7) reviewed 18 training studies with COPD patients and found that O2peak improved in 10 of the studies but failed to increase in the remaining 8. Casaburi was unable to find any disparities among the various studies to help explain the differences. The results of this investigation may provide some help in explaining the differences among the various studies. If exercise was prescribed at a certain percent of O2peak and was monitored by heart rate, as described above, this could lead to an under-prescription of exercise intensity. As a result of this under-prescription, the patients would not achieve a physiological training effect, i.e., O2peak would not increase after training. Unfortunately, details on the method of prescribing the exercise intensity among these studies are not well defined and, thus, do not allow for a direct comparison. Recently, Vallet et al. (21) demonstrated that those COPD patients that train at a higher intensity achieve increases in O2peak as compared to patients that train at lower intensities. These investigators compared the effects of training at an intensity equal to the gas exchange threshold versus an intensity equal to 50% of the maximal heart rate reserve. Their results show that those patients who trained at the gas exchange threshold had significant increases in O2peak after exercise training. Those that trained at 50% of maximal heart rate reserve did not. Although the intensity of training may be a factor influencing whether or not patients experience increases in O2peak, Lacasse et al. (16) have questioned the importance of maximum exercise capacity. In a carefully conducted meta-analysis of randomized controlled trials in pulmonary rehabilitation, these authors concluded that pulmonary rehabilitation improves important domains of health-related quality of life in patients with COPD. However, the value of improvements in oxygen consumption was not clearly defined by their results. Subsequently, these authors cautioned that the measurement of maximal exercise capacity should not be substituted for measures of quality of life (15).
There are several different methods suggested by the ACSM for the prescription of exercise intensity in COPD patients. An alternate approach to prescribing exercise at 50% of O2peak is to the use dyspnea levels to monitor exercise intensity. Horowitz and colleagues (12) have shown that COPD patients are able to regulate and monitor their exercise intensity using dyspnea ratings obtained during an incremental exercise test. This same group of investigators has also found that patients with COPD can use dyspnea ratings obtained from a cycle ergometry test to regulate their exercise intensity during treadmill exercise as long as the exercise intensity is sufficiently high (13). Additionally, Franco and colleagues (9) have shown that during high-intensity exercise (i.e., 77% O2peak), dyspnea ratings are similar when comparing exercise at steady state and during an incremental exercise test. However, these same investigators found that during low-intensity exercise (i.e., 50% O2peak), there was a small statistically significant difference between the dyspnea levels during steady state and incremental exercise. Collectively, these results support the use of dyspnea ratings for the prescription of exercise intensity in COPD patients.
Other approaches to prescribing exercise intensity for COPD patients include having them exercise at or above the anaerobic threshold or at near maximal levels of intensity (2,8,9). The rationale behind training at or above the anaerobic threshold is that minute ventilation can be reduced after the training, thus allowing for an increase in ability of these patients to perform heavy work (8). Because many COPD patients are limited by their ventilatory system and not the cardiovascular system, it has been thought that these subjects should be able to exercise at levels approaching those achieved at maximum levels during graded exercise tests (2,19). Although the two above mentioned approaches to exercise prescription have been suggested, there are potential problems such as compliance and injuries when exercise is prescribed at such high intensities. Subsequently, there is still no clear consensus as to the optimal method of prescribing exercise intensity in COPD patients. Additional studies are needed to compare and contrast the different approaches and to determine the best method.
This work was supported by National Heart Lung and Blood Institute Grant HL53755 and National Institute of Aging Grant AG10484.
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