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Keywords:

  • chronic obstructive pulmonary disease;
  • exercise test;
  • oxygen uptake;
  • repeatability;
  • 6-min walk test

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Background and objective:  Field and laboratory-based tests are used to measure exercise capacity in people with COPD. A comparison of the cardiorespiratory responses to field tests, referenced to a laboratory test, is needed to appreciate the relative physiological demands. We sought to compare peak and submaximal cardiorespiratory responses to the 6-min walk test, incremental shuttle walk test and endurance shuttle walk test with a ramp cycle ergometer test (CET) in patients with COPD.

Methods:  Twenty-four participants (FEV1 50 ± 14%; 66.5 ± 7.7 years; 15 men) completed four sessions, separated by ≥24 h. During an individual session, participants completed either two 6-min walk tests, incremental shuttle walk tests, endurance shuttle walk tests using standardized protocols, or a single CET, wearing a portable gas analysis unit (Cosmed K4b2) which included measures of heart rate and arterial oxygen saturation (SpO2).

Results:  Between tests, no difference was observed in the peak rate of oxygen uptake (F3,69 = 1.2; P = 0.31), end-test heart rate (F2,50 = 0.6; P = 0.58) or tidal volume (F3,69 = 1.5; P = 0.21). Compared with all walking tests, the CET elicited a higher peak rate of carbon dioxide output (1173 ± 350 mL/min; F3,62 = 4.8; P = 0.006), minute ventilation (48 ± 17 L/min; F3,69 = 10.2; P < 0.001) and a higher end-test SpO2 (95 ± 4%; F3,63 = 24.9; P < 0.001).

Conclusions:  In patients with moderate COPD, field walking tests elicited a similar peak rate of oxygen uptake and heart rate as a CET, demonstrating that both self- and externally paced walking tests progress to high intensities.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

In people with COPD, impaired lung1 and peripheral muscle function compromise exercise capacity.2 Accurate assessment of exercise capacity yields prognostic information3 and has important implications for an individual's clinical management such as evaluating the response to an intervention and prescribing exercise training intensities capable of inducing adaptation.4–6 Laboratory-based incremental cycle ergometer tests (CETs) are generally accepted as the gold standard for quantifying exercise capacity.7 Such tests are, however, costly and require sophisticated equipment and resources that are not available in all facilities or to all clinicians. Therefore, field walking tests are often used.8,9 Such tests are reproducible after one practice walk10,11 and responsive to exercise training.12–14 However, it is unclear how the peak cardiorespiratory responses during walking tests compare with a CET. That is, although the incremental shuttle walk test (ISWT) consistently elicits a similar peak rate of oxygen uptake (inline image) and heart rate as a CET,15–18 the 6-min walk test (6MWT) elicits similar peak responses in some16,18,19 but not all studies.17,20–22 No study has compared response during the endurance shuttle walk test (ESWT)10 with the ISWT, 6MWT and CET. Such information is needed to appreciate the physiologic demands of each test, relative to one another. Further, the pattern of submaximal cardiorespiratory responses is needed to explain previous findings of higher dyspnoea at time points prior to test completion during the 6MWT relative to the ISWT and CET.16

The primary aim of this study was to compare peak and submaximal cardiorespiratory responses during the 6MWT, ISWT and ESWT with a CET in people with COPD. A secondary objective was to report the coefficient of repeatability of gas exchange and breathing pattern variables.

METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Design

This cross-sectional study was approved by the relevant Human Research Ethics Committees. Written informed consent was obtained from every person before participation. Each participant attended four 2-h assessment sessions, each separated by at least 24 h. During a session, participants completed either 6MWTs, ISWTs, ESWTs or a CET. To account for potential improvements resulting from familiarization,10,11 each walking test was performed twice, separated by a 20- to 30-min rest. To eliminate day-to-day variability,23 the same test protocol was performed twice on a given day. The order of sessions differed among participants.

Study criteria

Participants were eligible for the study if they had a diagnosis of COPD,24 a smoking history > 10 pack-years and were clinically stable (i.e. had not used oral corticosteroids or antibiotics within the preceding 2-week period). Exclusion criteria comprised evidence of a comorbid condition that might adversely affect exercise performance, a history of lung surgery, the use of a rollator or long-term oxygen therapy or evidence of marked desaturation during exercise (i.e. SpO2 < 80%) that would have necessitated premature cessation of the test.7 Most participants were familiar with the 6MWT prior to participating in this study.

Measurements

Exercise capacity

All walking tests were overseen by two experienced physiotherapists on a quiet, level, enclosed, temperature-controlled corridor. Before and after each walk test, and every minute during the CET, measurements were obtained of dyspnoea and leg fatigue using the modified Borg scale.25 During all tests, breath-by-breath measurements of gas exchange and breathing pattern were collected using a calibrated portable gas analysis system (K4b2, Cosmed, Rome, Italy). Arterial oxygen saturation (forehead sensor, Nellcor Max Fast, Pleasanton, CA, USA) and heart rate (Polar a1 heart rate monitor, Polar Electro Oy, Kempele, Finland) were monitored continuously.

Walk tests.  The ISWTs and ESWTs were performed according to standardized protocols,10,20 modified to include one standardized warning to increase walking speed the first time each participant lagged behind the pace dictated by the audio-signal.16 The speed selected for the ESWTs was equivalent to 85% of the peak inline imageestimated using the distance achieved during the best ISWT.10 This test was preceded by a 90-s warm-up period during which the participant walked slowly. The ESWT was terminated by the tester at 20 min.10 For patients who achieved 20 min during their first ESWT, the second test was conducted at a faster walking pace. Similarly, to reduce the likelihood of walking for 20 min during the second ESWT,13,26 those who walked for more than 10 min with minimal progression in heart rate or change in arterial oxygen saturation (SpO2) for three consecutive minutes during the first test, also completed their second test at a faster pace. The 6MWTs were performed according to the American Thoracic Society guidelines.27

Cycle-ergometry test.  A symptom-limited ramp CET was performed on an electronically braked bicycle ergometer (Lode Excalibur 926851V3.00, Lode, Groningen, the Netherlands). Peak exercise responses during CET are reproducible in people with COPD28,29 and therefore the CET was performed once. Participants pedalled without a load for 3 min and thereafter the load increased by 5, 10 or 15 W/min based on each participant's history of daily physical activity and symptoms, to induce symptom limitation within approximately 10 min.

Anthropometric and resting lung function

During the first assessment session, age, sex and Modified Medical Research Council dyspnoea grade30 were recorded and measurements were made of height and weight. The most recent measurements of resting lung function were extracted from the medical notes.

Analyses

Breath-by-breath data were exported to Sigmaplot (version 11.0) for analyses. To plot the submaximal responses, for every test, measures of inline image, rate of carbon dioxide output (inline image), minute ventilation (inline image), tidal volume, heart rate and SpO2 were grouped into epochs equivalent to 10% increments of the total test duration (i.e. deciles) using a two-dimensional data transformation. Data collected during the 90-s warm-up and 3 min of unloaded pedalling that preceded the ESWT and CET, respectively, were excluded from analyses. Data collected across both 6MWTs, ISWTs and ESWTs were averaged for analysis, with the exception of those participants who completed the ESWTs at different speeds. In this instance, only data performed at the faster walking speed were used in the analysis.

Before undertaking statistical analysis, a natural logarithmic transformation was applied to all cardiorespiratory variables to improve the extent to which they approached a normal distribution. Peak (end-test) responses were compared among tests using one-way repeated measures analysis of variance with paired t-tests for post-hoc comparisons (SPSS version 17.0, SPSS Inc., Chicago, IL, USA). For heart rate, inline imageand leg fatigue, Mauchly's test indicated that the assumption of sphericity was violated, and therefore the degrees of freedom were corrected using Huynh–Feldt estimates. P-values ≤ 0.05 were considered significant.

Repeatability of variables collected on completion of the walk tests was assessed using methods of Bland and Altman.31 We calculated the bias, defined as the mean difference between two 6MWTs, ISWTs and ESWTs as well as the coefficient of repeatability, defined as 1.96 times the standard deviation of the difference. Patients who performed ESWTs at different speeds were excluded from this analysis.

Sample size calculations

A prospective sample size calculation was based on detecting a 10% difference in the primary outcome, peak inline image, between any walking test and CET. Using data available in the literature that reported a peak inline imageduring a CET of 1.41 L/min,19 we estimated that 24 participants would yield 80% power (α = 0.05) to detect a difference of 0.14 L/min in the peakinline image (i.e. 10% difference and the coefficient of repeatability for this measure)29 with a standard deviation of 0.24 L/min (average standard deviation of the two tests reported by Troosters et al.)19 using a paired t-test.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Of the 33 individuals who consented to participate in this study, five (15%) were unable to tolerate the mask for portable gas analysis equipment, one (3%) was withdrawn due to hypertension and one (3%) withdrew. Data from two individuals (6%) was ineligible due to equipment failure. Characteristics of the 24 participants who completed the study are summarized in Table 1.

Table 1.  Participant characteristics (n = 24; 15 men)
VariableMean ± SDRange
  •  

    Data are median ± interquartile range.

  • MRC, modified Medical Research Council (scores range from 0 to 4).

Age (years)66.5 ± 7.748–83
Height (m)1.64 ± 0.101.42–1.86
Weight (kg)71.5 ± 14.146.7–91.6
BMI (kg/m2)26.8 ± 5.519.1–39.7
MRC dyspnoea grade2 ± 10–2
FEV1 (L)1.18 ± 0.440.57–2.51
FEV1 (% predicted)50 ± 1420–85
FEV1/FVC (%)40 ± 925–69
FRC (L)5.19 ± 1.481.46–8.04
FRC (% predicted)169 ± 3798–213
DLCO (mL/min/mm Hg)12.6 ± 3.36.3–17.9
DLCO (% predicted)58 ± 1826–95

ISWTs were performed before 6MWTs by 13 (54%) participants. Three (12%) participants rested during the 6MWT. The ESWT and ISWT were each performed only once by one participant (4%) due to an abnormally high end-test heart rate. A higher walking speed was selected for the second ESWT in five (21%) participants.

Performance during each test is summarized in Table 2. Average speed during the 6MWT was 76.5 m/min (95% confidence interval (CI): 71.8–81.1 m/min). Peak walking speed achieved during the ISWT was 85.9 m/min (95% CI: 80.3–91.5 m/min). Average speed during the ESWT was 73.4 m/min (95% CI: 68.3–78.4 m/min). Compared with each walking test, the CET elicited greater peak dyspnoea (F3,69 = 27.9; P < 0.001) and leg fatigue (F3,54 = 27.5; P < 0.001). Patterns of response for cardiorespiratory variables are illustrated in Figure 1a to 1f. The submaximal pattern of change during the ESWT and 6MWT was curvilinear for all variables and the submaximal pattern of change during the ISWT and CET was linear for all variables.

Table 2.  Results of exercise tests
 Mean ± SDRange
  •  

    Difference between second and first test.

  •  

    Average of two tests (for ESWT, only tests performed the same walk speed were averaged, otherwise the test performed at the faster speed was included in the analysis).

  • § 

    Includes data from n = 18.

6-min walk test  
 Learning effect (m)3 ± 11−24 to 24
 Distance (m)459 ± 66355–631
 End-test dyspnoea3.6 ± 1.70.5–7.5
 End-test leg fatigue2.3 ± 2.10–6
Incremental shuttle walk test  
 Learning effect (m)25 ± 35−20 to 100
 Distance (m)338 ± 102180–540
 End-test dyspnoea4.0 ± 1.11–6
 End-test leg fatigue2.2 ± 2.20–6.5
Endurance shuttle walk test  
 Learning effect (s)§50 ± 83−90 to 256
 Time (s)313 ± 160123–765
 Distance (m)384 ± 193136–873
 End-test dyspnoea4.4 ± 1.71–8
 End-test leg fatigue3.0 ± 2.40–8.5
Cycle ergometry test  
 Peak power (W)72 ± 2827–129
 End-test dyspnoea6.2 ± 2.00.5–9
 End-test leg fatigue5.7 ± 2.70.5–10
image

Figure 1. Data are mean and standard error. All participants contribute to each data point. Figures are patterns of response for; (a) rate of oxygen uptake (inline image), (b) rate of carbon dioxide output (inline image), (c) minute ventilation (inline image), (d) tidal volume (VT), (e) heart rate (HR) and (f) arterial oxygen saturation measured via pulse oximetry (SpO2) for each test. inline image, cycle ergometry test; inline image, 6-min walk test; inline image, incremental shuttle walk test; inline image, endurance shuttle walk test; *P < 0.05 for difference between cycle ergometry with all other tests.

Download figure to PowerPoint

Peak cardiorespiratory responses for each test are presented in Table 3. Compared with each walking test, the CET elicited a greater inline image, respiratory exchange ratio and inline imageas well as a smaller decrease in SpO2. There was no difference in peak inline image, heart rate or tidal volume among the tests. The small variability between tests observed in resting measures in Figure 1a to 1f, reflects the metabolic load associated with the warm-up period and unloaded cycling that preceded the ESWT and CET, respectively.

Table 3.  Peak (end-test) cardiorespiratory responses
 6MWTISWTESWTCETanova results
  1. Data are mean ± SD.

  2. P < 0.05 vs CET;  P < 0.01 vs CET. The subscripted numbers that accompany the F-statistics refer to the degrees of freedom available for the between and within-group comparisons.

  3. anova, analysis of variance; CET, cycle ergometry test; ESWT, endurance shuttle walk test; ISWT, incremental shuttle walk test; 6MWT, 6-min walk test; SpO2, arterial oxygen saturation measured via pulse oximetry; inline image, rate of oxygen uptake; inline image, rate of carbon dioxide output; inline image, minute ventilation.

inline image(mL/min)1168 ± 3441227 ± 3101232 ± 3681186 ± 314F3,69 = 1.2; P = 0.31
inline image(mL/min)1009 ± 2701036 ± 3271060 ± 342*1173 ± 350F3,62 = 4.8; P = 0.006
Respiratory exchange ratio0.87 ± 0.110.84 ± 0.100.86 ± 0.120.99 ± 0.17F3,69 = 18.7; P < 0.001
inline image(L/min)41 ± 1743 ± 1544 ± 16*48 ± 17F3,69 = 10.2; P < 0.001
Tidal volume (L)1.39 ± 0.461.35 ± 0.421.36 ± 0.461.45 ± 0.46F3,69 = 1.5; P = 0.21
Heart rate (beats/min)128 ± 17127 ± 14130 ± 15128 ± 19F2,50 = 0.6; P = 0.58
SpO2 (%)88 ± 588 ± 588 ± 595 ± 4F3,63 = 24.9; P < 0.001

The bias and coefficient of repeatability for gas exchange and breathing pattern variables collected during each walking test are presented in Table 4. Compared with data collected during the ISWT, the coefficients of repeatability tended to be narrower for data collected during the 6MWT and ESWT.

Table 4.  Mean difference (bias) and coefficient of repeatability for peak (end-test) cardiorespiratory responses
 6MWTISWTESWT
BiasCoefficientBiasCoefficientBiasCoefficient
  •  

    Only tests performed the same walk speed were included in these analyses. Differences between tests were not systematic and therefore coefficients of repeatability could be calculated for all variables.

  • ESWT, endurance shuttle walk test; ISWT, incremental shuttle walk test; 6MWT, 6-min walk test; SpO2, arterial oxygen saturation measured via pulse oximetry; inline image, rate of oxygen uptake; inline image, rate of carbon dioxide output; inline image, minute ventilation.

inline image(mL/min)−33322−56414−138287
inline image(mL/min)−3221856329−76218
Respiratory exchange ratio−0.010.190.090.240.0270.12
inline image(L/min)−0.619.623.2112.60−1.3010.84
Tidal volume (L)−0.030.380.030.45−0.050.27
Heart rate (beats/min)21041329
SpO2 (%)−160418

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

This is the first study to compare the peak and submaximal cardiorespiratory responses across three field walking tests with a laboratory-based CET in patients with COPD. The important findings of this study are: (i) all tests elicited similar peak inline image, heart rate and tidal volume; (ii) the CET elicited a greater peak inline image, respiratory exchange ratio, inline imageand smaller decrease in SpO2 compared with all walking tests; (iii) the submaximal pattern of change during the ISWT and CET were similar, being linear for all variables; and (iv) the submaximal pattern of change during the 6MWT and ESWT were similar, being curvilinear for all variables. Further, this is the first study to report the bias and coefficient of repeatability for measures of gas exchange and breathing pattern variables measured during walking tests. Generally, the bias for each measure was small with variables measured on completion of the 6MWT and ESWT demonstrating narrower coefficients of repeatability compared to the ISWT.

The similar peak inline imageand heart rate during the ISWT and CET corroborates earlier reports.15–18 It has been suggested that the 6MWT is most likely to elicit peak cardiorespiratory responses in people with COPD following an acute exacerbation,32 or in those with profound functional limitation33 or severe airflow obstruction.16,19 However, our data reveal that among stable participants with, on average, moderate disease (FEV1 = 50 ± 14 % predicted) and modest functional limitation (6-min walk distance = 459 ± 66 m), the 6MWT elicited similar peak inline imageand cardiac demands as both the CET and ISWT. This relates, at least in part, to the utilization of a 6MWT protocol that included standardized instructions, encouragement and consistent performance of two 6MWTs under identical conditions on the same day. Many of the participants had experience with the 6MWT; a factor likely to account for the modest learning effect and may have contributed to the magnitude of response elicited during the 6MWT. Although the CET elicited greater inline imageand inline imagethan the 6MWT, exercise intensity is usually expressed in terms of inline imageor heart rate (as a surrogate for inline image) and therefore our data reveals that a standardized, encouraged 6MWT elicited a maximum exercise response in people with moderate COPD, with modest functional limitation, who were clinically stable.

Our data show remarkable similarity in the pattern of response and walking speeds used for the ESWT and 6MWT. This contrasts with the study by Pepin et al.34 who reported a greater inline image, respiratory rate and heart rate on completion of the ESWT compared with the 6MWT, despite both tests being conducted at similar walking speeds. This disparity may be related to differences in lung function and exercise capacity between the study samples. Although FEV1 (per cent predicted) was similar between samples, the absolute FEV1 and the forced expiratory ratio were lower in our participants (1.31 vs 1.18 L and 46 vs 40%). Further, the inline imageand distances achieved during the walking tests were considerably greater in the study by Pepin et al.34 Taken together, it appears that the participants in this earlier study were characterized by less severe disease and better exercise capacity34 and the capacity of the 6MWT and ESWT to elicit similar cardiorespiratory responses may be contingent on these factors.

In agreement with earlier reports, there was a smaller decrease in SpO216,18,35 together with a higher inline image, respiratory exchange ratio and inline imageduring the CET compared with the walking tests.15,18 These differences have been demonstrated to persist when walking and cycling modalities were matched for inline image36 and appear to relate to the greater specific load placed on the quadriceps during CET37 eliciting a greater accumulation of lactate15 which is buffered by bicarbonate, thereby increasing inline image.36 Regarding the pattern of response, cardiorespiratory variables during the ISWT and CET increased in a linear pattern reflecting the incremental increase in power17 whereas the cardiorespiratory response to the 6MWT and ESWT was characterized by a similar magnitude of exponential increase over the first 50% of the test, followed by a relative plateau in response.34Figure 1c demonstrates that at time points before test completion, inline imagewas higher during the 6MWT and ESWT, compared with the ISWT and CET; data that support a previous finding of greater dyspnoea during the 6MWT at time points before test completion, compared with incremental test protocols.16 It appears that performance during the 6MWT and ESWT was influenced by the capacity of an individual to tolerate near maximum levels of inline image.

The repeatability of cardiorespiratory measures collected at the end of the walking tests was similar to that previously reported for variables measured on completion of CET.29 The somewhat wider coefficients for measures collected during the ISWT, relative to the 6MWT and ESWT, is likely to reflect that any increase in performance during the ISWTs resulting from familiarization may have necessitated an increase in walking speed and considerable increase in cardiorespiratory demand. In contrast, the coefficients of repeatability calculated for data collected during the ESWT pertain only to tests during which the participants walked at identical speeds and there was a trivial difference in walking speeds during the two 6MWTs. A practical application of these data is that, following an intervention, to be 95% confident that any difference in cardiorespiratory response collected during the 6MWT, ISWT or ESWT was not simply due to normal variability inherent in these tests, the magnitude of change must exceed the coefficient of repeatability.

Limitations

As we excluded those who required long-term oxygen therapy or ambulatory aids, our results may not extend to these individuals. It is possible that the use of a treadmill rather than cycle ergometer would have allowed the participants to achieve greater a inline imagepeak during the laboratory test.36 Nevertheless, the CET was chosen as it is the most common laboratory-based cardiopulmonary exercise test in people referred to pulmonary rehabilitation.12 We did not collect measures of inspiratory capacity and therefore cannot comment on differences in hyperinflation among the tests. Although a similar number of participants completed the 6MWT or ISWT during the first assessment session, practical issues (e.g. laboratory availability) prevented full compliance with the randomization sequence, and it is possible that our results were affected by an order bias.

CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

In clinically stable participants with moderate disease, field walking tests, when conducted according to a standardized protocol, elicited similar peak inline imageand heart rate responses as a CET. These data suggest that both self- and externally paced walking tests progress to very high intensities and therefore appear to provide a basis on which to prescribe training intensities capable of inducing physiologic adaptation.38,39

ACKNOWLEDGEMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

For financial support, we acknowledge Physicians Services Incorporated Foundation (Canada). Dr. Brooks is supported by a Canadian Research Chair and Dr. Goldstein by the NSA Chair in Respiratory Rehabilitation Research. We also acknowledge the assistance of Clarissa Muere, Sachi O'Hoski, Gail Lang and Dr. Diane Flood in recruitment and data collection for this study.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES
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