Patients with chronic obstructive pulmonary disease (COPD) often show impaired ability to perform activities of daily living (ADL),1 which results in, exacerbations, hospitalizations, and potentially death.2 Therefore, functional evaluation is essential in routine clinical practice,3 especially in pulmonary rehabilitation (PR).4 The six-minute walk test (6MWT)2 is widely used to measure functional capacity, as it is a simple and inexpensive tool and objectively reflects ADL of individuals with COPD.2 The American Thoracic Society and the European Respiratory Society (ATS/ERS)2 provide standardized guidelines for the performance of the 6MWT on a 30-m track (6MWT30). However, a 30-meter track is not always available in clinical settings, with often used shorter tracks requiring a greater number of deceleration/acceleration phases when changing direction, potentially affecting performance.5

Conducting the 6MWT on a 10-meter track, showed high test-retest reliability (ICC = 0.98) and performance was strongly correlated with those on the 6MWT30 (r = 0.96; p < 0.01).5 However, the use of the shorter track caused a clinically relevant reduction in 6MWT performance. In children,6 healthy older adults,7 and individuals with hepatic cirrhosis8 and stroke,9 the performance on the 6MWT conducted on a 20-meter track (6MWT20) resulted in a difference of less than 10%. Therefore, the 6MWT20 may be an alternative, easier to conduct clinically, for patients with COPD. Because performance on the 6MWT30 is related to several important outcomes, such as hospitalizations and mortality, and is a component of the BODE index,10 which reflects the COPD prognosis, alternative convenient testing distances are important to determine. However, the impact of performing the 6MWT on a 20 versus a 30 m track, in individuals with COPD of various severity and disability level, is unknown.

The objectives of this study, conducted on individuals with COPD, were: 1) to compare performance between the 6MWT30 and the 6MWT20; 2) to determine validity and reliability of the 6MWT30 and the 6MWT20; and 3) to determine if the impact of a shorter track is consistent across all levels of performance on the 6MWT30.

Patients referred to the Nucleus for Assistance, Education and Research on Pulmonary Rehabilitation (NuReab) of Universidade de Santa Catarina (UDESC) participated in this study. The inclusion criteria were: diagnosis of COPD (GOLD 2–4)11; clinically stable status over the previous four weeks; age between 40 and 80 years; and optimized disease-targeted medical therapy (the use of medication prescribed by the pulmonologist for at least one month). The exclusion criteria were: other respiratory diseases or disabling health conditions; use of home oxygen therapy; hospitalization in the previous 12 weeks; participation in PR in the previous six months; inability to perform any of the study evaluations; grade III obesity,12 smoking cessation in the previous six months; and suspicion of COPD exacerbation during the protocol. The study protocol was approved by the Human Research Ethics Committee of the Universidade do Estado de Santa Catarina UDESC, Florianópolis, Santa Catarina, Brazil CAAE: 62638716.0.0000.0118. All participants signed an Informed Consent Document.

Of 33 potentially eligible patients, 30 completed the protocol (Table 1). One patient was excluded due to grade III obesity,12 one patient due to smoking during the protocol; and one patient dropped out. Data related to IM, number of steps, and EE during the 6MWT were not obtained for four patients.

Validity and reliability of 6MWT30 and 6MWT20

The distance walked, IM, number of steps, and EE showed strong correlation between the best 6MWT30 and 6MWT20 (r = 0.86, p < 0.001; r = 0.92, p < 0.001; r = 0.92, p < 0.001; r = 0.94, p < 0.001, respectively). The distances walked in the 6MWT30 and the 6MWT20 negatively correlated with CAT (r = −0.48, p = 0.01; r = −0.53, p = 0.003; respectively) and with mMRC (r = −0.53, p = 0.002; r = −0.62; p < 0.001; respectively) results. The LCADL%total showed a tendency to correlate with the performance on the 6MWT20 (r = −0.33; p = 0.07) and did not show any correlations with the 6MWT30 (p = 0.29). The distances walked in the 6MWT30 and 6MWT20 did not correlate with the pulmonary function variables (p > 0.05).

There was high reproducibility of the distance covered between test-retest for the 6MWT20 (ICC = 0.99; 95% CI 0.98, 0.99) and 6MWT30 (ICC = 0.98; 95% CI 0.95, 0.99). There was high reproducibility of the results on the heart rate between test-retest for the 6MWT20 ICC of 0.77 (95% CI 0.49, 0.89; p < 0.05) and 6MWT30 ICC of 0.92 (95% CI 0.83, 0.96; p < 0.05). There was no difference between the test-retest (6MWT30 and 6MWT20) for any of the cardiovascular variables (p > 0.05) (Table 2). The SEM of 6MWT30 and 6MWT20 were 16.3 vs. 19.4 m and the MDC was 45.2 vs. 53.8 m, respectively. In both 6MWT20 and 6MWT30, the patients increased performance in the retest (Table 2). The learning effect in the 6MWT20 and 6MWT30 did not differ (1.63% vs. 2.70%, respectively; p = 0.29). Bland-Altman plots confirmed that most patients showed better performance in the retest in both 6MWT (Fig. 1). It could be observed that patients with better mean distance walked in the 6MWT30 and 6MWT20 also presented greater difference between both tests (Fig. 1A).

Table 2.

Physiological and performance variables for the six minute walk test.

6MWT30					6MWT20
	Test	Retest	ICC(95% CI)	Best test	Test	Retest	ICC(95% CI)	Best test
Δ HR, bpm	28.5 ± 14.8	29.4 ± 14.3	0.92 (0.83, 0.96)§	30.7 ± 15.2	23.4 ± 16.2	29.4 ± 14.3	0.77 (0.49, 0.89) §	27.3 ± 11.25
Δ SpO2	−5.53 ± 4.61	−6.17 ± 5.38	0.90 (0.78, 0.95) §	−6.20 ± 5.03	−5.13 ± 4.84	−6.17 ± 5.38	0.76 (0.51, 0.89) §	−6.40 ± 5.84
Δ dyspnea*	2 [0–8]	2 [0–6]	0.95 (0.90, 0.98) §	2 [0–8]	2 [0–6.5]	2 [0–6]	0.87 (0.72, 0.94) §	1 [0–6.5]
Δ fatigue*	0 [0–5]	0 [0–4]	0.88 (0.74, 0.94) §	0 [0–4]	0 [0–3]	0 [0–4]	0.80 (0.58, 0.91) §	0 [0–3]
Δ SBP, mmHg	23.7 ± 22.0	23.3 ± 20.9	0.72 (0.50, 0.89) §	23.1 ± 30.8	23.7 ± 18.8	23.3 ± 20.9	0.81 (0.60, 0.91) §	22.3 ± 16.3
Δ DBP, mmHg	1.00 ± 19.0	2.33 ± 7.28	0.22 (-0.68, 0.63)	2.00 ± 7.61	2.67 ± 8.27	2.33 ± 7.28	0.22 (-0.68, 0.63)	3.67 ± 7.18
Distance, m	444 ± 115	456 ± 107†	0.98 (0.95, 0.99) §	462 ± 112 ‡	429 ± 97	437 ± 103†	0.99 (0.98, 0.99) §	439 ± 101
MI, m/s2	3.36 ± 1.52	3.50 ± 1.68†	0.99 (0.97, 0.99) §	3.59 ± 1.71‡	3.32 ± 1.50	3.46 ± 1.66†	0.99 (0.97, 0.99) §	3.44 ± 1.55
Steps number	649 ± 110	641 ± 156	0.81 (0.58, 0.92) §	673 ± 104	648 ± 108	641 ± 153	0.81 (0.58, 0.91) §	660 ± 90.2
EE, Kcal	35.8 ± 11.4	36.5 ± 11.6	0.99 (0.97, 0.99) §	36.9 ± 12.1	35.3 ± 11.5	35.9 ± 11.8	0.99 (0.98, 0.99) §	36.4 ± 11.1

Data presented as mean ± standard deviation; 6MWT30: six minute walk test in track of 30 m; 6MWT20: six minute walk test in track of 20 m; ICC: intraclass correlation coefficient; 95% CI: 95% confidence interval; Δ: difference; HR: heart rate; SpO2: pulse oxygen saturation; SBP: systolic blood pressure; DBP: diastolic blood pressure; MI: movement intensity; EE: energy expenditure.*Data presented as median [minimum – maximum]; †p < 0.05 to test vs. retest; ‡p < 0.05 to best 6MWT30 vs. best 6MWT20; §p < 0.05 to ICC.

Fig. 1.

Bland-Altman plot of walking distances in meters (m): (A) of the best 6MWT on tracks of 30 m (6MWT30) and of 20 m (6MWT20); (B) test-retest of 6MWT20; (C) test-retest of 6MWT30. The central continuous line represents the mean difference between two 6MWT, and the upper and lower dotted lines represent the upper limits (UL) and lower limits (LL) of agreement. The dashed lines represent the minimum important difference (MID) of 30 m for the 6MWT. (A) LL: -24.8; UL: 72.7; mean: 22.1; (B) LL: −26.7; UL: 40.9; mean: 7.10; (C) LL: −48.1; UL: 72.9; mean: 12.4.

(0.23MB).

The patients presented difference between test-retest only for the first day of testing, regardless of the randomization. When the 6MWT30 was tested first, the mean difference was 20 m (95% CI 8.69, 31.3; p = 0.004) for the 6MWT30 and −0.08 m (95% CI −14.5, 14.4; p = 0.16) for the 6MWT20. When the 6MWT20 was tested first, the mean differences were 7.33 m (95% CI −11.0, 25.6; p = 0.18) for the 6MWT30 and 11.9 m (95% CI 6.61, 17.2; p = 0.001) for the 6MWT20.

Discriminating patients with clinical impact on performance

A ROC curve (Fig. 2) indicated a cutoff point of 430 m to discriminate patients who showed a difference of 30 m or greater in the distance walked in the 6MWT30 compared to the 6MWT20. The mean difference between the distance walked in the 6MWT30 and 6MWT20 was greater in patients who walked ≥430 m than in patients who walked <430 m (34.5 ± 23.3 m vs. 12.6 ± 24.1 m; respectively; mean difference = 22 m; 95% CI: 3.93, 39.8; p = 0.01).

Fig. 2.

Receiver operating characteristic (ROC) curve for the 6MWT on tracks of 30 m cut-off point to discriminate the patients where the difference in walking distance between the tracks of 30 m and 20 m was clinically relevant, considering the minimum important difference of 30 m for the 6MWT. Cut-off = 430 m; area under the ROC curve (AUC) = 0.75 [95% CI: 0.57, 0.93]; p = 0.02; Sensitivity = 67% and Specificity = 72%.

(0.06MB).

The main results of this study were that the 6MWT30 performance was better than the 6MWT20 performance, with the difference increasing as performance improved. Additionally, it was observed that for patients with walking distance less than 430 m on the 6MWT30, using the 20-meter track, may not lead to a clinically important impact on the results.

Although the 6MWT30 is widely used and has well-established criteria, its clinical applicability is limited to settings that are able to guarantee the measurement properties of the test, as recommended by the ATS/ERS.5,6,21,24 As expected, the 6MWT20 performance was lower when compared to the 6MWT30 in the present study. In healthy older adults, the minimum track length required for acceleration varies from 2.17 to 3.23 m, and 1.80–1.85 m25 for deceleration. Therefore, about 20% of a 20-meter track is used for acceleration and deceleration, while only 13% in a 30-meter track. This may explain the difference found in performance between the 6MWT30 and the 6MWT20.

Another point to be highlighted is that the cardiovascular overload was similar between the tests. This may have occurred as the 6MWT usually has a submaximal character, with stable oxygen uptake beyond the third minute.26 Therefore, the difference between the tests may not have been sufficient to cause a greater cardiovascular overload in the 6MWT.

Although the mean difference between the 6MWT30 and the 6MWT20 in the total sample was below MID, this difference was clinically relevant in 40% of the patients. The comparison of patients with more and less difference than the MID between the 6MWT30 and the 6MWT20 showed that these patients did not differ in relation to pulmonary function, dyspnea, health status, and perception of functional limitations. However, patients with 6MWT30–6MWT20 ≥MID achieved higher scores for distance, number of steps, IM, EE, and cardiovascular overload in the 6MWT30. A correlation also showed that the greater the difference in the distance walked between the 6MWT30 and the 6MWT20, the longer was the distance walked on the 6MWT30. In addition, in the present study, a ROC curve analysis identified that, in patients who walked more than 430 m, the shorter length track caused a clinically relevant impact on performance. Therefore, it is important to highlight that, especially in patients with better functional capacity, results from the 30-meter and 20-meter tracks may not be interchangeable. Moreover, it is not possible to interpret the performance on the 6MWT20 based on the cut-off points or reference values established for the 6MWT30.

There is a large body of evidence demonstrating validity and reliability of the 6MWT conducted on the 30-meter track in patients with COPD. In the current study, the 6MWT20 was also valid to assess functional capacity of patients with COPD, showing strong correlations with performance in the 6MWT30 (r = 0.86; p < 0.001). Furthermore, the 6MWT20 proved to be reliable and, similar to the 6MWT30, demonstrated weak and moderate correlation with CAT and mMRC, reflecting health status and dyspnea, which are important outcomes of the disease. This demonstrates that in case of limited physical spaces for the patients with greater functional impairment, the 6MWT20 could be a valid and reliable option. Most patients increased the distance covered in the retest, possibly due to the learning effect already demonstrated in several studies.21,27 Bland-Altman plots confirmed the best retest performance of both 6MWT and evidenced that most patients showed variation within the limits of agreement, reinforcing the reliability of the tests. In the current study, the results of the learning effect of the 6MWT30 differed from those reported in the literature: Troosters et al.28 showed a learning effect of 2.6%, while Spencer et al.29 and Hernandes et al.27 reported a 7% increase in distance in the retest. Interestingly, the present study demonstrated similar learning effect in the 6MWT20 and 6MWT30. The small learning effect found in the 6MWT30 and 6MWT20 may have occurred because the protocol design in which the patients performed four 6MWT. Besides, there was statistically difference between test-retest only on the first day. Since “the effect of learning” is likely to occur due to improved motor coordination, stride length adaptation, decreased anxiety, and recognition of the limits of the test,9 this fact may allow the adoption of strategies to turn at the end of each track more efficiently in the retest and this may have occurred in both tests independently of track length.

This study may present some limitations. Patients with use of long-term oxygen were excluded, and thus the results cannot be extrapolated to this population. The small sample size may have compromised the statistical power to run the ROC curve analysis. However, we found an area under the ROC curve of 0.75 (95% CI: 0.57, 0.93), which is considered acceptable.30 Another factor to be considered was the failure to blind the major outcome assessor. Nevertheless, special care was taken to maintain the same assessor responsible for the verbal encouragement and instructions to avoid different intonations.

Notably, to our knowledge, this was the first study to compare the 20-meter track with the track recommended by the ATS/ERS, as well as to demonstrate validity and reliability of the 6MWT20 for assessing functional capacity of patients with COPD. The results have demonstrated that, despite the lower performance when compared to the 30-m track, the 6MWT20 does not underestimate functional capacity of patients who walk shorter distances. Therefore, the 6MWT20 could be an option to those patients in settings in which the performance of the 6MWT30 is not feasible.

Performance was higher on the 6MWT30, with the difference between tracks increasing as performance improved. The 20-meter track replacing 30-meter track may not cause a clinically important impact on the walking distance of patients with lower performance, but it may cause that impact on those with better performance. Therefore, the 6MWT20 is valid and reliable to evaluate functional capacity but should not be considered interchangeable with the 6MWT30, especially for the less disabled patients with COPD.

This study was financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 and supported by the Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina – FAPESC – Brazil (PAP UDESC) (Chamada Pública N° 01/2016). The funding sources had no involvement in the study design; in the collection, analysis, and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

The authors declare no conflicts of interest.