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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.
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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 () 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.
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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)
|Variable||Mean ± SD||Range|
|Age (years)||66.5 ± 7.7||48–83|
|Height (m)||1.64 ± 0.10||1.42–1.86|
|Weight (kg)||71.5 ± 14.1||46.7–91.6|
|BMI (kg/m2)||26.8 ± 5.5||19.1–39.7|
|MRC dyspnoea grade||2 ± 1†||0–2|
|FEV1 (L)||1.18 ± 0.44||0.57–2.51|
|FEV1 (% predicted)||50 ± 14||20–85|
|FEV1/FVC (%)||40 ± 9||25–69|
|FRC (L)||5.19 ± 1.48||1.46–8.04|
|FRC (% predicted)||169 ± 37||98–213|
|DLCO (mL/min/mm Hg)||12.6 ± 3.3||6.3–17.9|
|DLCO (% predicted)||58 ± 18||26–95|
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 ± SD||Range|
|6-min walk test|| || |
| Learning effect (m)†||3 ± 11||−24 to 24|
| Distance (m)‡||459 ± 66||355–631|
| End-test dyspnoea‡||3.6 ± 1.7||0.5–7.5|
| End-test leg fatigue‡||2.3 ± 2.1||0–6|
|Incremental shuttle walk test|| || |
| Learning effect (m)†||25 ± 35||−20 to 100|
| Distance (m)‡||338 ± 102||180–540|
| End-test dyspnoea‡||4.0 ± 1.1||1–6|
| End-test leg fatigue‡||2.2 ± 2.2||0–6.5|
|Endurance shuttle walk test|| || |
| Learning effect (s)§||50 ± 83||−90 to 256|
| Time (s)‡||313 ± 160||123–765|
| Distance (m)‡||384 ± 193||136–873|
| End-test dyspnoea‡||4.4 ± 1.7||1–8|
| End-test leg fatigue‡||3.0 ± 2.4||0–8.5|
|Cycle ergometry test|| || |
| Peak power (W)||72 ± 28||27–129|
| End-test dyspnoea||6.2 ± 2.0||0.5–9|
| End-test leg fatigue||5.7 ± 2.7||0.5–10|
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.
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The similar peak and 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 and 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 and than the 6MWT, exercise intensity is usually expressed in terms of or heart rate (as a surrogate for ) 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 , 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 and 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 , respiratory exchange ratio and during the CET compared with the walking tests.15,18 These differences have been demonstrated to persist when walking and cycling modalities were matched for 36 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 .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, was 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 .
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.
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 peak 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.