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(Chest. 2004;126:766-773.)
© 2004 American College of Chest Physicians

Physiologic Responses to Incremental and Self-Paced Exercise in COPD*

A Comparison of Three Tests

Sian E. Turner, BSc; Peter R. Eastwood, PhD; Nola M. Cecins, MSc; David R. Hillman, MD and Sue C. Jenkins, PhD

* From the School of Physiotherapy (Ms. Turner, Ms. Cecins, and Dr. Jenkins), Curtin University of Technology; the School of Anatomy and Human Biology (Dr. Eastwood), University of Western Australia; and the Department of Pulmonary Physiology (Dr. Hillman), Sir Charles Gairdner Hospital, Perth, WA, Australia.

Correspondence to: Sue C. Jenkins, PhD, Physiotherapy Department, Sir Charles Gairdner Hospital, Hospital Avenue, Nedlands, WA, Australia 6009; e-mail: S.Jenkins{at}curtin.edu.au


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Objectives: To investigate cardiorespiratory and dyspnea responses to incremental and self-paced exercise tests in patients with COPD.

Design: A prospective within-subject design was used.

Patients: Twenty stable subjects (15 men), with a mean (± SD) age of 64.0 ± 7.5 years and moderate-to-severe COPD (ie, mean FEV1, 0.8 ± 0.3 L and 28.9 ± 7.9% predicted) were studied.

Methods: Each subject completed a 6-min walk test (6MWT), an incremental shuttle walking test (ISWT), and a cycle ergometer test (CET), within a 2-week period. The tests were performed at least 24 h apart. Standardized encouragement was utilized in each test with the aim of maximizing performance. Heart rate (HR) and dyspnea were measured each minute throughout the tests, and pulse oximetric saturation (SpO2) was measured before and immediately after each test. The distances walked in the 6MWT and ISWT were compared to peak oxygen uptake (O2) values from the CET.

Results: HR increased linearly with increasing workload during the CET and ISWT, but increased alinearly with a disproportionate increase early in the 6MWT. In contrast, dyspnea scores increased linearly during the 6MWT, but increased alinearly with a disproportionate increase late during the CET and ISWT. Peak HR and dyspnea were not significantly different between tests. SpO2 was significantly lower at the end of both walking tests compared to that at the end of the CET (p < 0.001). The distance walked in both the ISWT and 6MWT were related to peak O2 values on the CET (for both tests, r = 0.73; p < 0.001).

Conclusions: The patterns of response in HR and dyspnea seen during the 6MWT suggest that patients with COPD titrate exertion against dyspnea to achieve a peak tolerable intensity. This strategy is not possible in an externally paced ISWT or CET. However, it is a limited strategy, with performance converging at higher workloads. Similar peak exercise responses were achieved in the 6MWT, ISWT, and CET. Greater oxygen desaturation was observed during the field walking tests, suggesting that both the ISWT and 6MWT are more sensitive than the CET in detecting exercise-induced hypoxemia and in assessing ambulatory oxygen therapy needs.

Key Words: COPD • cycle ergometer test • dyspnea • heart rate • incremental shuttle walking test • oxygen saturation • 6-min walk test


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Assessment of exercise capacity is important in the management of patients with COPD to determine the severity of disease and its changes with time or treatment. While laboratory-based exercise tests such as the incremental cycle ergometer test (CET) or treadmill test are the "gold standard" for this purpose,12 they are expensive and may be difficult to access. For these reasons, field walking tests, such as the 6-min walk test (6MWT) and the incremental shuttle walking test (ISWT), have become an important part of assessment during pulmonary rehabilitation.34 Distances walked during field walking tests have been used to indicate the level of disability, to prescribe the intensity of a walking program, and to assess outcome following rehabilitation.56

The 6MWT and ISWT protocols are very different. The 6MWT is self-paced and can be continuous or intermittent, depending on whether the subject rests during the test. In contrast, the ISWT is externally paced and requires an incremental increase in walking speed each minute, to a point at which the test is terminated because of breathlessness or the inability to sustain the walking speed. It is likely that the 6MWT and ISWT provide different physiologic information,789 and that they are suitable for different groups of individuals. For example, a test that is incremental in nature is advantageous when assessing the outcomes of an exercise program due to the ability to compare pretraining and posttraining responses at an equivalent exercise intensity. However, the 6MWT may be better tolerated in patients with more severe COPD due to its self-paced protocol and the capacity to rest.410

Previous studies have suggested that similar peak heart rates (HRs) are achieved by subjects with moderate-to-severe COPD during the ISWT compared to during an incremental CET,11 and in subjects with end-stage lung disease during a 6MWT compared to during a CET.12 However, in a subject population that included subjects with less severe disease, lower peak HRs have been reported13 in the 6MWT compared to that in the incremental CET. Studies comparing the ISWT and the 6MWT show that peak exercise responses are similar between each test in subjects with moderate-to-severe COPD,1415 but are lower in the 6MWT in a population that included subjects with mild COPD.16 Because walking is a familiar activity, some have argued that field walking tests in general may better reflect the ability of patients to undertake physically demanding activities of daily living than performance in a laboratory-based exercise test.17 Perhaps related to this, some studies1118192021 have suggested that hypoxemia, another important clinical end point, is more profound following incremental or constant load walking tests than following bicycle exercise in some subjects with COPD.

Hence, it is by no means clear whether these tests are associated with similar levels of cardiopulmonary stress and, therefore, whether maximal performance is comparable between them. The validity of field walking tests, and their merit relative to each other, is contingent on such information. To this end, a comparison of maximal HR and dyspnea between field tests and the current "gold standard" of CET is important in helping to determine the degree to which maximal cardiac and ventilatory stress have been achieved. However, examining the evolution of changes during the performance of the tests could further clarify these issues as it allows an examination of the mechanisms by which the maxima were achieved under the essentially different conditions of field testing vs laboratory testing, and self-pacing vs imposed increments in workload.

To date, most studies of field walking tests of exercise capacity have examined only the end point of each test, the maximal distance walked. Furthermore, while some have considered the pattern of respiratory and cardiovascular responses occurring with the tests in COPD subjects,182223 to our knowledge, no study has compared serial HRs and dyspnea responses developing during the 6MWT, ISWT, and a laboratory-based CET in the same individuals. Hence, the aims of this study were to examine submaximal and maximal physiologic responses during exercise testing to determine the validity of commonly used field tests of exercise capacity relative to CET and their relative merit in assessing exercise capacity in patients with moderate-to-severe COPD. We hypothesized that there would be no significant difference in peak HR or dyspnea scores among the three exercise tests in a population that included only subjects with moderate-to-severe disease. We used protocols that encouraged maximum effort but understood that submaximal responses would differ in accord with the self-paced or imposed incremental nature of the test.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Subjects
Twenty stable subjects (15 men) with moderate-to-severe COPD were studied. These subjects were drawn from patients who had been referred to the outpatient pulmonary rehabilitation program at Sir Charles Gairdner Hospital. Demographic and lung function data of the subjects are summarized in Table 1 . All but two subjects had an FEV1 of < 40% predicted. Nineteen subjects were ex-smokers, and one subject was a current smoker. No subject had received therapy with oral corticosteroids for at least 3 months prior to the study, and all subjects continued with a stable regimen of medications throughout the study. Subjects were excluded if any of the following criteria applied: age > 75 years; required oxygen therapy; presence of symptomatic cardiovascular conditions limiting exercise capacity; use of medications affecting exercise responses; diabetes mellitus; musculoskeletal conditions likely to influence exercise performance; the inability to understand written or spoken English; and impaired hearing or vision affecting a subject’s ability to complete the required tests. The study was approved by the Human Research Ethics Committees of Sir Charles Gairdner Hospital and Curtin University of Technology. Written, informed consent was obtained from all subjects.


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  		Table 1.. Demographic and Lung Function Data for the 20 Subjects (15 Men)*

 
Study Design
A prospective within-subject design was used, with subjects completing the CET, 6MWT, and ISWT within a 2-week period and each test conducted > 24 h apart. The number of subjects with prior experience of the tests was 6, 10, and 11, respectively, for the CET, 6MWT, and ISWT. Twelve of the subjects were naive to pulmonary rehabilitation, and the remaining 8 subjects had completed an 8-week rehabilitation program at least 12 months prior to commencement of the study. The order of the exercise tests was randomized, and the time of day at which tests were conducted was maintained at a constant time. Subjects were asked to refrain from ingesting food or drinks containing caffeine and from smoking for 2 h prior to each test. Supplementary oxygen was not administered during any of the tests.

Exercise Protocols
Incremental Cycle Ergometry: The CET was performed on an electronically braked cycle ergometer (Ergometrics 900; Medi-prax; Munich, Germany). An integrated breath-by-breath gas analysis system (Benchmark Exercise System; PK Morgan; Gillingham, UK) was used to provide a direct measure of oxygen uptake (O2), carbon dioxide output, and minute ventilation (E). The calibration of the gas analyzers and flow transducer was performed prior to each test. Subjects performed only one CET, as good test-retest reliability for peak workload, peak O2, and peak HR obtained from a single CET has previously been demonstrated in subjects with COPD.2425

The test required 4 min of seated rest on the ergometer, during which time baseline measures were collected. Subjects then were instructed to begin pedaling at 60 to 75 revolutions per minute against an initial workload of 20 W. Workload was increased by an increment of 8 W each minute until the subject was unable to maintain the pedaling frequency or voluntarily ceased pedaling. Subjects were given standard verbal encouragement throughout the test. Peak values for all variables were obtained by averaging data over the last 20 s of the maximum completed workload.

Self-Paced Walking Test: The 6MWT was performed over a 45-m course in a level enclosed corridor with chairs placed at both ends of the course. The protocol was modified from that developed by Butland et al26 to include standardized instructions prior to the test and the verbal encouragement "do your best" at the start of each minute throughout the test. In addition, each minute subjects were informed of the elapsed time, and, if a subject rested, the verbal encouragement "begin walking as soon as you feel able" was provided at 15-s intervals. To allow for any potential learning effect, subjects unfamiliar with the 6MWT completed three tests,27 each separated by 30 min of rest.

Incremental Walking Test: The ISWT was conducted using the protocol described by Singh et al.16 Subjects were required to walk around a 10-m course marked by cones placed 9 m apart, thus allowing 0.5 m for turning at each end. Walking speed was regulated by prerecorded signals on a cassette tape. The test started at an initial speed of 0.5 m/s, and the speed was increased each minute by 0.17 m/s. Standard taped instructions were played to the subject prior to the test. The calibration of the tape was checked each week during the study. The investigator ensured that subjects understood the instructions prior to the test. Verbal encouragement was confined to "increase your speed now" immediately following the triple bleep indicating the increase in walking speed. At the first failure to maintain speed, at each level, the instruction "you’re not going fast enough, try and make up the speed this time" was provided. To allow for a learning effect,16 subjects unfamiliar with the ISWT completed two tests, which were separated by 30 min of rest.

Measurements
HR was recorded before exercise and at the end of each minute during the walking tests using a lightweight telemetric HR monitor (Polar Electro Oy; Kempele, Finland). Before and at the end of each minute during the CET, HR was recorded from an ECG (Morgan Cardiac Monitor; PK Morgan), in accordance with the usual practice at our institution. For all tests, dyspnea was measured before exercise and at the end of each minute using the Borg dyspnea scale.28 Standard instructions were provided for use of the scale. Subjects had been previously familiarized with the Borg scale to ensure minimal interference when scores were obtained. Pulse oximetric saturation (SpO2) was measured prior to and immediately at the termination of each test using a pulse oximeter (Biox 3700e; Ohmeda; Louisville, CO). The lowest postexercise SpO2 was recorded.

Statistical Analysis
Baseline values (pre-exercise) and peak values of HR and dyspnea, and pre-exercise and postexercise SpO2 were compared among the three exercise tests using one-way repeated measures analysis of variance or, when appropriate, analysis of covariance, with pre-exercise values used as a covariant factor.

To compare the pattern of change in HR and dyspnea during each test regression, models were applied that included linear and quadratic terms. In this analysis, the significance of the quadratic term was examined first, and the linear term was examined only when the quadratic term was not significant. To permit the pooling of data from all subjects, HR data, dyspnea scores, and the time at which measurements were obtained were normalized for each individual for each test. Specifically, the time at which a measurement was taken was expressed as a percentage of the maximum time taken for that test. Measurements of HR and dyspnea were expressed as a percentage of the change from baseline to maximum values.

In those subjects who repeated the 6MWT or ISWT, the data used in the analyses were taken from the test in which the greatest distance had been achieved. The distances walked during the 6MWT and ISWT were compared using a paired t test. The relationship between the distances walked during the 6MWT and ISWT was assessed using Pearson correlation coefficients and linear regression analysis, as were the relationships between distance walked during the field tests with peak O2 and peak workload obtained from the CET. Unpaired t tests were used to compare the data between subjects who rested during the 6MWT with those who performed the test without stopping. For all comparisons the level of statistical significance was set at p < 0.05. All data are presented as the mean ± SD.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Submaximal and Maximal HR and Dyspnea Responses During the Three Tests
The pattern of HR and dyspnea responses during each of the three exercise tests is shown in Figures 1 and 2 . HR increased linearly with increasing workload during the CET (r = 0.98; p < 0.001) and the ISWT (r = 0.96; p < 0.001 [although it was notable that the quadratic term was also significant in this regression equation]) but increased alinearly with a disproportionate increase early during the 6MWT (r = 0.88; p < 0.001) [Fig 1]. In contrast, dyspnea increased linearly during the 6MWT (r = 0.90; p < 0.001) but increased alinearly with a disproportionate increase late during the CET (r = 0.93; p < 0.001) and the ISWT (r = 0.96; p < 0.001) [Fig 2].



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  		Figure 1.. Pooled data from 20 subjects of the changes in HR during the 6MWT, ISWT, and incremental CET. The time at which a measurement was taken was expressed as a percentage of the maximum time taken for that test (%max.). Measurements of HR were expressed as a percentage of the change from baseline to maximum values (% max. change). For each test, a regression line that best describes the relationship is shown. While a quadratic function best described the relationship during the ISWT (solid line), for the purpose of comparison a linear regression line is also shown (dashed line).

 


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  		Figure 2.. Pooled data from 20 subjects of the changes in dyspnea during a 6MWT, ISWT, and incremental CET. See the legend of Figure 1 for abbreviations not used in the text. For each test, a regression line that best describes the relationship is shown.

 
Values for peak HR, peak dyspnea, and end-exercise SpO2 for each exercise test are shown in Table 2 . There were no significant differences in peak HR (p = 0.31) or peak dyspnea (p = 0.44) for the three tests. Mean baseline measures of SpO2 were lower before the 6MWT and ISWT (94.9 ± 1.5% and 95.1 ± 1.8%, respectively) than before the CET (95.8 ± 2.2%; p < 0.05). However, even when adjusting SpO2 for baseline levels (analysis of covariance), SpO2 was significantly lower following the 6MWT and ISWT (86.8 ± 5.1% and 86.5 ± 4.8%, respectively) than following the CET (91.8 ± 3.2%; p < 0.001). Postexercise SpO2 was not significantly different between the two field tests (p < 0.05). In nine subjects, SpO2 was < 85% at the end of both walking tests but remained > 85% in all subjects at the termination of the CET.


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  		Table 2.. Peak HR, dyspnea, and oxygen desaturation during the 6MWT, ISWT, and CET in 20 Subjects

 
Relationship Between Distance Walked During Field Tests and Performance on CET
The mean distance walked during the 6MWT was 474.9 ± 87.8 m. Six of 20 subjects required between one and four rests during the walk. The total rest time ranged from 13 to 74 s. The distance walked was significantly less for the group of subjects who rested (mean distance walked, 394 ± 77 m) compared with those who performed the test without stopping (mean distance walked, 510 ± 68 m; p < 0.01). The subjects who rested had greater pulmonary impairment, as indicated by a lower FEV1 (23.0 ± 6.7% predicted vs 31.4 ± 7.1% predicted, respectively; p < 0.05) and a lower peak O2 (11.6 ± 2.7 vs 15.3 ± 2.2 mL/kg/min, respectively; p < 0.05), and they were younger (59 ± 4 vs 66 ± 7 years, respectively; p < 0.05) than the subjects who did not rest.

Thirteen subjects terminated the ISWT because of dyspnea, and 7 subjects stopped because they were unable to maintain the required speed. The average duration and peak walking speed of the ISWT were 6.1 min (range, 4.0 to 7.6 min) and 5.0 km per hour (range, 3.6 to 6.1 km per hour), respectively. The mean distance walked during the ISWT was 339.0 ± 97.2 m. There was a strong correlation between the distances walked on the 6MWT and ISWT (r = 0.91; p < 0.001) [Fig 3 ].



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  		Figure 3.. Regression line with 95% CIs for the relationship between the distances walked in the 6MWT and ISWT (20 subjects). {circ} = two overlapping points.

 
All subjects exercised to volitional exhaustion during the CET. Reasons reported for exercise cessation were dyspnea in 17 subjects, leg fatigue in 1 subject, and both dyspnea and leg fatigue in 2 subjects. The mean exercise time was 5.9 ± 1.8 min (range, 4.0 to 10.0 min). Ventilatory responses and workload at peak exercise during the CET are provided in Table 3 . Peak E was low, and subjects demonstrated a ventilatory limitation to exercise with a mean peak E (expressed as a percentage of maximal voluntary ventilation) of 106.9 ± 16.5% (calculated as the postexercise FEV1 x 35).


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  		Table 3.. Ventilatory Responses and Workload at Peak Exercise During the CET*

 
The distance walked during both field tests was correlated to the peak workload achieved during the CET (6MWT: r = 0.83; p < 0.001; ISWT: r = 0.79; p < 0.001). A significant correlation was observed between the distance walked during the 6MWT and peak O2 (r = 0.73; p < 0.001) [Fig 4 ], and between the distance walked on the ISWT and peak O2 (r = 0.73; p < 0.001) [Fig 5 ].



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  		Figure 4.. Regression line with 95% CIs for the relationship between the distances walked in the 6MWT and the peak O2 from an incremental CET (20 subjects).

 


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  		Figure 5.. Regression line with 95% CIs for the relationship between the distances walked in the ISWT and the peak O2 from an incremental CET (20 subjects). {circ} = two overlapping points.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The major findings of this study were as follows: (1) the pattern of change of HR and dyspnea differed between the incremental tests (ie, the ISWT and the CET) and the self-paced test (ie, the 6MWT), such that at any time prior to test completion both HR and dyspnea were greater during the 6MWT than during the incremental tests; (2) the 6MWT, with the addition of standardized encouragement, and the ISWT elicited similar peak HRs and dyspnea scores to those elicited on the CET, suggesting that both of these field tests can challenge patients with moderate-to-severe COPD to a level of cardiovascular and respiratory stress similar to that experienced during a maximal exercise test such as the CET; and (3) the magnitude of hypoxemia was greater during both the 6MWT and ISWT than during the CET, suggesting that a walking test is more suitable to detect exercise-induced hypoxemia and for assessing ambulatory oxygen therapy needs.

In the present study, we observed that the 6MWT and ISWT evoked equivalent peak HRs and dyspnea responses. While this finding is consistent with those of two other reports,1415 it contrasts with the findings of an earlier study by Singh et al,16 who showed that HR and dyspnea were lower in 9 of their 15 subjects during the 6MWT than during the ISWT. We attribute the difference to our use of a 6MWT protocol that allowed for a learning effect, and included standard instructions and encouragement with the aim of overcoming poor motivation as a potential source of submaximal performance. The use of standardized encouragement in the 6MWT protocol may also explain the stronger relationship between the distances walked on the 6MWT and ISWT in the present study (r = 0.91) when compared to the findings of Singh et al16 (r = 0.68).

We observed no significant differences in peak HRs and dyspnea levels among the three tests. The greatest mean difference in peak HR (3.4 beats/min) occurred between the 6MWT and the ISWT, and in dyspnea (0.58) between the 6MWT and CET. The 95% confidence intervals (CI) for these differences were as follows: HR, –0.18 to 6.98 beats/min; dyspnea, –0.65 to 1.8. It is likely that the 95% CI would decrease with a larger sample size, and the difference may become statistically significant. However, it is unlikely that the mean difference would change significantly. Such small differences in HR and dyspnea are not clinically significant, and are most likely to be within the error of the measurements in the clinical setting.

The finding that peak HR and dyspnea were similar among the three tests suggests that the ISWT and 6MWT are not only strenuous, but can provide a valid measure of exercise capacity in patients with moderate-to-severe COPD. Supporting this assertion is the finding that the distance walked during either field walking test was related to the peak O2 achieved during the CET. While we did not study patients with more mild cases of COPD, it is likely that the correlation between distance walked in the 6MWT and O2 would not be as strong in those subjects. A large variation in peak O2 for the same distance walked on the 6MWT has been previously reported29 in subjects with mild heart failure, and lower peak HR and dyspnea levels have been reported13 following an encouraged 6MWT and compared to the CET in patients with moderate COPD (mean FEV1, 1.2 ± 0.41 L).

The pattern of change of HR and dyspnea may be an important determinant of the levels achieved at end-exercise. In the present study, a steady-state HR was not achieved during the 6MWT. HR increased rapidly in the first few minutes, and thereafter continued to increase at a slower rate toward a similar peak value as achieved during the ISWT and CET. In contrast, a linear increase in HR with time was observed during the ISWT and CET, reflecting the progressive incremental nature of these tests (Fig 1). The alinear nature of the HR-time relationship during the 6MWT most likely reflects neuropsychological influences with a magnitude of load-associated stress being "titrated," or "self-paced" by the individual to an end point that represents their maximum.3031

Regardless of the type of exercise, most subjects terminated exercise at a submaximal dyspnea intensity (rating, severe to very severe), which is a finding that is consistent with other studies in similar subjects,113032 indicating that few individuals with COPD are willing to exercise to maximal symptom intensity, instead choosing to withdraw when the intensity is the greatest they are prepared to tolerate.30 It was notable that for each test the relationships between dyspnea and time, and HR and time were different. That is, while HR increased linearly with increasing workload during the incremental tests (the CET and ISWT) and alinearly during the self-paced test (the 6MWT), dyspnea increased alinearly during incremental tests and linearly during the self-paced test.

The difference in the pattern of change of dyspnea between the 6MWT and the incremental tests can most likely be explained by differences in the ventilatory requirement between the two modes of exercise. Figure 2 shows that, throughout the 6MWT, the magnitude of dyspnea is greater than that in either of the incremental tests at equivalent times. It is probable that during the 6MWT, both the level of ventilation and dynamic hyperinflation were greater than during the incremental tests at submaximal levels of exercise.833 Under these conditions, dyspnea would be expected to be greater during the 6MWT, as previous studies have demonstrated a direct relationship between the level of dyspnea and both the level of central respiratory output34 and the magnitude of dynamic hyperinflation.3234 The finding that dyspnea increased linearly throughout the test, reaching maximum values that were comparable to the incremental tests, raises the likelihood that it was the level of dyspnea that these subjects used to set the pace throughout the test. Such a strategy would not be possible during performance of the other tests, in which increments in work rate were imposed on the subject.

The observation of more profound hypoxemia at the end of the field walking tests than at the end of the CET is consistent with those of previous studies, which also have documented the occurrence of greater desaturation following incremental or constant-load walking tests than following bicycle exercise in some, but not all, subjects with COPD.1118192021 The reasons for this difference remain unclear but may relate to the following conditions: increased ventilation/perfusion mismatching during walking compared with cycling, as a result of differences in body posture, functional residual capacity, and/or pulmonary hemodynamics; the effect of reflex impulses to the respiratory centers arising from the upper limb muscles, which may be more active during walking than cycling; an increased chemical drive to breathe and a higher ventilation during cycling as a consequence of the accumulation of higher levels of lactate142135; or possibly the use of chest wall muscles to perform postural tasks during walking at the expense of ventilation.35

In our patients, hypoxemia during field walking tests was common. In 9 of 20 subjects, SpO2 decreased to < 85% at the end of the walking tests, while it remained at > 85% in all subjects at the termination of the CET. These findings lead us to question a published guideline for the 6MWT,10 which states that the use of pulse oximetry is optional. Indeed, an implication of our findings is that a walking test may be more appropriate than a CET for identifying those who may benefit from oxygen therapy during walking.


   Acknowledgements
 
We would like to thank Dr. Marie Blackmore for her assistance with the statistical analyses.


   Footnotes
 
Abbreviations: CET = cycle ergometer test; CI = confidence interval; HR = heart rate; ISWT = incremental shuttle walking test; 6MWT = 6-min walk test; SpO2 = pulse oximetric saturation; E = minute ventilation; O2 = oxygen uptake

This study was funded, in part, by National Health and Medical Research Council (Australia) grant No. 212016.

Received for publication August 8, 2003. Accepted for publication April 8, 2004.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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