http://informahealthcare.com/dre ISSN 0963-8288 print/ISSN 1464-5165 online Disabil Rehabil, 2015; 37(12): 1102–1106 ! 2014 Informa UK Ltd. DOI: 10.3109/09638288.2014.956815
ASSESSMENT PROCEDURES
The 6-minute walk distance cannot be accurately assessed at home in people with COPD Anne E. Holland1,2,3, Tshepo Rasekaba1, Julio F. Fiore Jr2,3, Angela T. Burge1, and Annemarie L. Lee1,2,3
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1
Department of Physiotherapy, Alfred Health, Melbourne, Australia, 2Department of Physiotherapy, La Trobe University, Melbourne, Australia, and Institute for Breathing and Sleep, Melbourne, Australia
3
Abstract
Keywords
Purpose: The 6-minute walk test (6MWT) is commonly used to measure exercise capacity in COPD, but it is unclear if this test is accurate when performed at home. This study aimed to determine whether exercise capacity can be accurately assessed at home using the 6MWT in COPD. Methods: A total of 19 participants with stable COPD (mean [SD] FEV1/FVC 52[13]) undertook the 6MWT at home and at the hospital, in random order, with two tests performed on each occasion. Hospital tests were conducted on a 30-metre walking track whilst home tests (indoor or outdoor) were conducted using the longest available track. Agreement for 6-minute walk distance (6MWD) was examined using the Bland and Altman method. Results: The track length at home was mean [SD] of 17[9] m. The home 6MWD was shorter than the hospital 6MWD (mean 30 m shorter, limits of agreement 167 to 102 m). For the home tests, a shorter track length was associated with a greater reduction in 6MWD (rs ¼ 0.59, p ¼ 0.01), but not an increased number of turns (rs ¼ 0.41, p ¼ 0.08). Conclusions: The 6MWD underestimates exercise capacity when conducted at home in COPD. Alternative tests suitable for the home environment should be considered if a comprehensive assessment is to be performed at home.
6MWT, COPD, rehabilitation History Received 10 December 2013 Revised 11 July 2014 Accepted 18 August 2014 Published online 1 September 2014
ä Implications for Rehabilitation
The 6-minute walk test is commonly used to assess change in exercise capacity following pulmonary rehabilitation in patients with chronic obstructive pulmonary disease, and may be conducted on varying track lengths, indoors or outdoors. When conducted at home, the 6-minute walk test underestimates exercise capacity in chronic obstructive pulmonary disease, due to a shorter track length available in the home environment. This suggests that results from 6-minute walk tests performed at home environment cannot be directly compared to results from centre-based tests
Introduction The 6-minute walk test (6MWT) is a self-paced test of exercise capacity which is commonly used in patients with cardiorespiratory disease [1]. Its main outcome, the 6-minute walk distance (6MWD), is a valid measure of exercise capacity in chronic obstructive pulmonary disease (COPD) [2]. The 6MWT is commonly used to measure improvement in exercise capacity following pulmonary rehabilitation [3], and has been conducted under a variety of conditions, including using a straight or circular track of varying lengths, indoors or outdoors [4–6]. A comparison of indoor and outdoor testing found that an indoor 6MWD was comparable with outdoor testing, but both environments were tightly controlled, conducted in quiet conditions with minimal distraction or corridor traffic on a flat
Address for correspondence: Anne Holland, Department of Physiotherapy, La Trobe University, Melbourne, Victoria, Australia. Tel: 00613 94796744. Fax: 00613 9533 2104. E-mail: A.Holland@ alfred.org.au
track [5]. Outdoor testing was conducted on a track of equal length, in conditions of an optimal balance of ambient temperature and humidity, with no precipitation, wind speeds less than 20 km/hr and at an air quality index associated with no health effects [5]. While the current guidelines for the 6MWT recommend the use of a standardized 30 meter straight track with strict environmental control [4], it is not known whether this can be achieved in the home setting. The reliability of home-based testing and factors that may influence the accuracy of exercise capacity assessment in this environment is not known. Two studies have examined the impact of track length on the 6MWD [6,7]. In patients with low levels of exercise capacity, track length had no effect on the 6MWD [6]. In contrast, a comparison of 10 meter versus 30 meter tracks demonstrated a shorter 6MWD achieved in patients with mild to very severe COPD [7]. Within the home setting testing must be conducted on tracks of varying lengths, which influences the number of turns during a test, and tests may be completed inside or outside. There has been no comparison of the distance achieved during a home versus hospital-based 6MWT in COPD.
6MWT at home in COPD
DOI: 10.3109/09638288.2014.956815
Research into home testing using the 6MWT is important because home-based pulmonary rehabilitation is proving to be a safe and effective alternative to hospital-based programs and a way of improving access to evidence-based care in this disabled population [8]. If exercise testing is found to be accurate at home, it will reduce the need for patients to attend the hospital for testing and further reduce the barriers to this important intervention. The primary aim of this study was to determine if the 6MWD can be accurately assessed in the home setting compared to the hospital setting in patients with COPD. A secondary aim was to identify the effect of track length, subsequent frequency of turning and environmental conditions on the 6MWD.
Methods
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This study was approved by the Alfred Health Human Research Ethics Committee. This was a within subjects crossover design. Participants Patients with a physician diagnosis of COPD, confirmed on spirometry [9] who were attending pulmonary rehabilitation at the Alfred Hospital and were clinically stable (absence of acute exacerbation of COPD over the past 4 weeks) were eligible to participate. Patients were excluded if they exhibited signs of an abnormal exercise response (e.g. failure of heart rate or blood pressure to increase with exercise, chest pain or intolerable dyspnoea) documented during their 6MWT on admission to pulmonary rehabilitation or if they were unable to comprehend and follow instructions in English. All participants provided written, informed consent prior to participation. Study design and methodology Participants undertook a 6MWT in two settings – hospital and at home. Participants were randomly allocated to setting order, the sequence determined by a computer-generated randomization schedule. Two tests were conducted at home and two tests in the hospital, with the best test used for comparison. To minimize differences being attributed to changes in disease state, fitness levels or the time period between tests, all tests were performed at the same time of day and within one week, as previous studies have shown similar reliability of 6MWD with test-retest periods ranging from one day (intraclass correlation coefficient (ICC) 0.93) [10] to seven days (ICC 0.92) [11]. All tests were completed at the conclusion of a pulmonary rehabilitation program. All participants were already familiar with the 6MWT protocol, having completed at least two tests prior to commencing the pulmonary rehabilitation program, to minimize the impact of learning effects [12]. The 6MWT performed in the hospital was completed on a level, temperature controlled corridor in accordance with standardized guidelines [4] using a straight track of 30 m in length. Home-based tests were performed outdoors when the environmental conditions were reasonable (temperature between 10 and 25 C, no rain, wind speed less than 20 km/h, air quality index of less than 32), a hard level surface was available and any current noise and distractions were unlikely to affect walking performance [5]. The suitability of weather conditions was based on the Australian Bureau of Meterology weather report [13] while the appropriateness of surface, noise level and distractions was determined by the assessor. When conditions were not reasonable, the tests were completed indoors, using the greatest track length that could be practically achieved in the home environment, allowing for turns. As changes in track shape from straight to continuous may also significantly affect distance walked [6,14], where possible, a straight track was used for home testing.
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The goal was for the model for the home track to be as similar as possible to the hospital track for both length and shape (straight versus continuous), with the priority of track shape (straight) over track length. All instructions and monitoring were consistent for the home and hospital based testing [4]. Before and after each 6MWT in both the hospital and at home, participants rated their dyspnoea and level of perceived exertion on a Borg scale [15] and had their blood pressure measured. Heart rate and SpO2 were monitored continuously during the test using a portable pulse oximeter (Nellcor N550 Pulse Oximeter; Nellcor; Pleasanton, CA). The 6MWT was performed twice at home and twice at the hospital, to control for the known learning effect on this test [10], with a minimum of 30 min of rest between tests. The distance walked, the number of turns and the length of the track were recorded for each test. To minimize the influence of other factors known to affect 6MWD, the use of walking aids and supplementary oxygen were controlled for all tests. For those participants requiring a walking aid, the same walking aid was used for both the home and hospital testing. For those who required supplemental oxygen therapy for their 6MWT, the same dosage, mode of delivery and method of oxygen transport was applied for all tests. Statistical analysis Prospective power analyses determined that a sample size of 19 participants were needed to detect a clinically important difference in walking distance of 25 m, assuming a standard deviation of 36 m (taken from data collected in our own program) between the hospital and home based tests with a power of 80% at an alpha of 0.05. A difference between tests of 25 m was chosen as this is the minimal important change in 6MWD in our population of patients with COPD [16]. Primary aim Differences between home and hospital 6MWDs were examined using paired t-tests for normally distributed data or a Mann–Whitney U test for data which was not normally distributed. Agreement between the 6MWD measured at home and in hospital was calculated using the methods described by Bland and Altman [17]. Secondary aim The extent to which differences in 6MWD between home and hospital-based tests was related to differences in number of turns and track length was analyzed using Spearman’s rho. To assess whether the reproducibility of the home based tests contributed to any differences in 6MWD between testing locations, an intra-class correlation coefficient (ICC)2,1 using a two-way random effect model was calculated to compare 6MWD on test 1 and test 2 at home. A similar approach was used for tests in the hospital. The ICC scores were interpreted according to the following guidelines: values above 0.75 are indicative of good reliability, values between 0.5 to 0.74 indicate moderate reliability and less than 0.50 indicate poor reliability [18]. Data were analyzed using SPSS version 21.0 (IBM Corp., Armonk, NY) and an alpha level of 0.05 used.
Results A total of 24 potential participants were screened for this study, with three excluded on the basis of declining to undertake a 6MWT at home and two excluded on the basis of limited understanding of English. A total of 19 participants were included in this study. The majority were male (n ¼ 11), with an average FEV1 of 54% predicted. The severity of COPD according to the
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Table 1. Subject demographics. Mean (SD)
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Gender (male:female) Age (yrs) BMI (kg/m2) FEV1 (L) FEV1 % predicted FVC (L) FVC % predicted FEV1/FVC 6MWD in Hospital (m) Use of O2 therapy (yes:no) O2 flow rate 2 L/min 3 L/min 4 L/min 6 L/min Use of gait aid (yes:no) 4WF
11:8 71 (6.8) 29.9 (6.0) 1.39 (0.51) 53.6 (18.8) 2.74 (0.92) 79.2 (21.2) 51.6 (13.0) 421 (116) 5:14 1 (5%) 1 (5%) 2 (11%) 1 (5%) 4:15 4 (21%)
Range 56–86 19.8–42.5 0.58–2.44 25–91 1.37–4.63 49–122 27.0–70.0 200–655
Data are mean (SD) or n (%) unless otherwise specified. 6MWD – 6-minute walk distance; BMI – Body mass index; FEV1 – Forced expiratory volume in one second; FVC – Forced vital capacity; O2 – Oxygen; 4WF – 4-wheeled framt.
GOLD classification [9] was mild (n ¼ 2), moderate (n ¼ 9), severe (n ¼ 6) and very severe disease (n ¼ 2). Five participants used oxygen therapy, all delivered by portable cylinder, and four participants used gait aids during the tests. The demographics of participants are outlined in Table 1. For the home-based tests, the mean (SD) track length was 17(9) m (range of 7 to 45 m). A total of 11 (58%) participants completed their test outdoors and 18 (95%) were on a straight track. There was a trend towards a significantly longer mean (SD) track length for outdoor tests at 20(10) m compared to indoor tests at 13(7)m, p ¼ 0.06. The mean number of turns for indoor versus outdoor tests was 30(SD 18) versus 20(10 m), p ¼ 0.20, and the mean number of stops for indoor versus outdoor tests was 0.1 (0.4) versus 0.5 (1.0), p ¼ 0.23. For those who completed the home-based 6MWT outdoors, the mean (SD) temperature was 16.0(0.05) degrees Celsius, wind speed of 23.5 (13.4) km/hr and air quality was 24.5(12), all of which were classified as very good conditions [13]. There was a significant increase in distance between the two home-based tests of 10 m (95% CI 2.8 to 17.0), p ¼ 0.009, with 14 (74%) of participants walking further on the second test. The percentage change was 4(6)%. Similarly for the hospitalbased test, there was an increase of 16 m (95% CI 5 m to 28 m), p ¼ 0.008, with 15 (79%) participants walking further on the second test, with percentage change being 3(4)%. Sixteen participants (84%) achieved a shorter 6MWD in the home setting compared to the hospital setting. There was a significant difference in the distance walked between hospital versus home-based 6MWT, with the 6MWD being 30 m shorter in home-based test (95% CI 0.4 to 63.2), with limits of agreement of 167 m and 102 m (Figure 1). There was a moderate relationship between the difference in the 6MWD between settings and the track length at home (rs ¼ 0.59, p ¼ 0.01; Figure 2), showing a greater reduction in 6MWD between settings when the track length was shorter. There was a weak relationship between the number of turns required for the home-based test and the difference in 6MWD between settings (rs ¼ 0.41, p ¼ 0.08). The maximum HR achieved was similar across settings, with a mean difference of 1.2 beats per minute. While a lower nadir SpO2 was measured during the home-based test (mean difference of 3%), this was not significant. There was no significant difference in symptoms or number of stops during the tests (Table 2). While correlation between the number of turns and the
Figure 1. Bland Altman plot of the mean 6MWD against the difference between hospital and home based tests. Mean difference is 30.4 m.
Figure 2. Relationship between track length at home and difference in 6MWD between the home and hospital settings.
Table 2. Comparison of conditions, physiological and symptom responses to the 6MWT in differing environments. Variable 6MWD (m) Track length (m) Maximum HR (b/min) Nadir SpO2 End Borg Dyspnoea score End Borg RPE score Number of stops
Hospital 421 30 103 86 4 12 0.2
(116) (0) (21) (8) (2) (3) (0.5)
Home 391 17 102 85 4 11 0.4
(99) (9) (27) (13) (1) (3) (0.8)
p Value 0.04 50.001 0.88 0.20 0.55 0.07 0.10
Data are mean (SD). 6MWD – 6-minute walk distance; HR – heart rate; b/min – beats per minute; SpO2 – Percutaneous oxygen saturation; RPE – Rate of perceived exertion.
distance walked for the hospital-based test was high (rs ¼ 0.83, p50.001), there was no relationship for the home-based test (rs ¼ 0.08, p ¼ 0.721). Correlation between home 6MWD and track length was rs ¼ 0.44 (p ¼ 0.06). The test-retest reliability for the hospital test was high, with an ICC2,1 of 0.98 (95% CI 0.95 to 0.99). Reliability for the
6MWT at home in COPD
DOI: 10.3109/09638288.2014.956815
home-based test was high, regardless of the order of completion, with an ICC2,1 of 0.990 (95% CI 0.970 to 0.990) in participants who completed the home-based test on the second testing day, and an ICC2,1 of 0.996 (95% CI 0.978 to 0.999) in those completing the home-based test first. This high reliability was maintained regardless of whether the test was indoors or outdoors (ICC2,1 of 0.970 [95% CI 0.836 to 0.995] for an indoor test; ICC2,1 of 0.998 [95% CI 0.993 to 0.999] for an outdoor test).
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Discussion This study has shown that, in patients with moderate to severe COPD, the 6MWD is likely to be shorter when the 6MWT is conducted at home using a standardized protocol. The primary reason appears to be the shorter tracks available at home, regardless of whether the test is performed indoors or outdoors. The difference in distance walked between hospital and homebased tests is large enough to be clinically important if the 6MWD is being used to evaluate change over time. The influence of a shorter track length resulting in a shorter 6MWD due to the increased number of turns was initially proposed by Enright [19]. However, recent studies in COPD found that patients with a high exercise capacity who would undertake a greater number of turns on a straight track did not demonstrate greater gains in 6MWD when performing the test on a circular track [6,14]. This suggests there may be other possible contributing factors unrelated to the number of turns influencing the 6MWD. In healthy elderly subjects, a higher gait speed with greater stride velocity was used over longer tracks (greater than 20 m) when walking at habitual speed, with a slower speed demonstrated on a shorter track (less than 10 m) [20]. In patients with mild to severe COPD, walking patterns are influenced by the need to minimize energy and maintain stability [21]. In comparison to healthy controls, patients with COPD have a reduced cadence and a greater degree of medio-lateral variability (due to lateral foot placement) which is an active control strategy to compensate for balance disturbances and to ensure stability in the forward direction (direction of propulsion) [21]. If this variation in walking pattern is evident during a 6MWT conducted on a 30 m straight track, it is possible that further changes in gait strategy may be adopted on a shorter track length, resulting in a reduced 6MWD. Tracks of longer length permit a greater amount of acceleration and higher top speed, with a distance of up to 3.2 m required to accelerate to steady walking pace and a distance of up to 1.9 m required to decelerate in healthy adults [22]. With a shorter track, the ability to achieve steady pace walking and allow for changes in acceleration and deceleration may be limited. Evidence of slower walking speed during a 6MWT conducted over a 10 m course in a group of eight patients with COPD [23] supports this suggestion and should be confirmed in studies with larger samples. The difference in distance between the home and hospitalbased tests appears to be unrelated to test reproducibility, as the ICCs for both test conditions were greater than 0.90, and the randomized testing order acted to minimize the impact of any learning effect across testing occasions. Although a learning effect was evident between tests 1 and 2 regardless of the location where they were performed, the magnitude did not differ from previous reports [10,12,24]. This small effect is likely to be attributable to the familiarity of all participants with the 6MWT prior to study commencement. The home-based test induced equivalent cardiovascular and ventilatory responses to exercise [25] as the increase in HR, decline in SpO2 and symptoms of dyspnoea and fatigue did not differ between settings. These factors are therefore unlikely to be responsible for the differences in 6MWD between settings.
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The difference in 6MWD between hospital and home tests is smaller (30.4 m) than that recently described in a comparison of distances achieved on a 30 m and 10 m track length (49.5 m) [7]. The greater range of options for track lengths at home (7–45 m) is the most likely explanation. Irrespective of this, this difference between home-based and hospital-based tests is beyond to the minimal important difference described for patients with moderate to severe COPD [16] and is within the 95% CI of the described by Puhan et al [26], both of which were completed on a 30 m course. The average difference in walk distance exceeded the minimal important difference in 58% of participants and the limits of agreement were wide. This suggests that a home-based 6MWD is not an ideal method to assess change over time compared to hospital-based measures, and that existing minimal important differences may not be relevant for home-based tests. There may be similar limitations when the 6MWD is used to estimate prognosis. Previous studies establishing threshold values of the 6MWD associated with hospitalization or survival were conducted on tracks of 30 meters or more [27–29]. Given the broad clinical importance of having accurate measures of 6MWD and the relationship of this measure with track length demonstrated in this and other studies [7], a home-based 6MWT should not be used to compare a patient’s performance to existing threshold values of 6MWD, unless adequate track length (425 m)[4] is assured. The limitations of the 6MWT at home suggest that alternative measures of exercise capacity which require less space to complete and reflect a similar level of function should be considered as outcome measures in this environment. Recent reports of the five-repetition sit-to-stand test in COPD [30] and the 4-metre gait speed [31], which have been validated in this patient population may be suitable options. Barriers to attending pulmonary rehabilitation include difficulty in attending assessments conducted in the outpatient environment due to transport [32]. With home-based rehabilitation emerging as an effective alternative for undertaking this intervention [8], it is important to identify outcome measures which may also be reliably completed within the home environment. This study is limited by the small number of participants, which influences the ability to perform multivariate analysis to determine the independent effect of factors such as track length, number of turns, outdoor and indoor conditions and disease severity on the 6MWT performed at home. Further analysis involving a larger number of participants is necessary to determine whether there is any interaction between these factors. In conclusion, the 6MWD will underestimate exercise capacity when conducted at home in people with COPD. Alternative exercise tests suitable for the home environment should be developed if a comprehensive assessment is to be performed at home.
Acknowledgements The authors would like to thank the patients for taking part in this study and to Sarah Foss who also assisted with data collection.
Declaration of interest The authors report no declarations of interest.
References 1. Rasekaba T, Lee A, Naughton M, et al. The six-minute walk test: a useful metric for the cardiopulmonary patient. Intern Med J 2009; 39:495–501. 2. Jenkins S. 6-minute walk test in patients with COPD – clinical applications in pulmonary rehabilitation. Physiotherapy 2007;93: 175–82.
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3. Lacasse Y, Goldstein R, Lasserson TJ, Martin S. Pulmonary rehabilitation for chronic obstructive pulmonary disease. Cochrane Database Syst Rev 2006;(4):CD003793. 4. American Thoracic Society. ATS Statement: guidelines for the 6-minute walk test. American Am J Respir Crit Care Med 2002;166: 111–17. 5. Brooks D, Solway S, Weinacht K, et al. Comparison between an indoor and an outdoor 6- minute walk test among individuals with chronic obstructive pulmonary disease. Arch Phys Med Rehabil 2003;84:873–6. 6. Sciurba F, Criner G, Lee S, et al.; National Emphysema Treatment Trial Research Group. Six-minute walk distance in chronic obstructive pulmonary disease. Reproducibility and effect of walking course layout and length. Am J Respir Crit Care Med 2003;167:1522–7. 7. Beekman E, Mesters I, Hendriks E, et al. Course length of 30 metres versus 10 metres has a significant influence on six-minute walk distance in patients with COPD: an experimental crossover study. J Physiother 2013;59:169–76. 8. Maltais F, Bourbeau J, Shapiro S, et al. Effects of home-based pulmonary rehabilitation in patients with chronic obstructive pulmonary disease: a randomized trial. Ann Intern Med 2008;149: 869–78. 9. Global strategy for the diagnosis, management and prevention of COPD, Global Initiative for Chronic Obstructive Lung Disease (GOLD) [Internet]. 2013. Available from: http://www.goldcopd.org/ [last accessed 9 Sept 2013]. 10. Hernandes N, Wouters E, Meijer K, et al. Reproducibility of 6-minute walking test in patients with COPD. Eur Respir J 2011;38: 261–7. 11. Eiser N, Willsher D, Dore CJ. Reliability, repeatability and sensitivity to change of externally and self-paced walking tests in COPD patients. Respir Med 2003;97:407–14. 12. Spencer L, Alison J, McKeough Z. Six-minute walk test as an outcome measure. Are two six-minute walk tests necessary immediately after pulmonary rehabilitation and at three-months follow-up? Am J Phys Med Rehabil 2007;87:224–8. 13. Australian Government: Bureau of Meterology. 2013. Available from: http://www.bom.gov.au/[last accessed 18 Sept 2013]. 14. Bansal V, Hill K, Dolmage T, et al. Modifying track layout from straight to circular has a modest effect on the 6-min walk distance. Chest 2008;133:1155–60. 15. Borg G. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982;14:377–81. 16. Holland AE, Hill C, Rasekaba T, et al. Updating the minimal important difference for six-minute walk distance in patients with chronic obstructive pulmonary disease. Arch Phys Med Rehabil 2010;91:221–5.
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17. Bland J, Altman D. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1: 307–10. 18. Portney L, Watkins M. Foundations of clinical research–applications to practice. 3rd ed. New Jersey: Pearson Education; 2009. 19. Enright P. The six-minute walk test. Respir Care 2003;48:783–5. 20. Najafi R, Helbostad J, Moe-Nilssen R, et al. Does walking strategy in older people change as a function of walking distance? Gait Posture 2009;29:261–6. 21. Annegarn J, Spruit MA, Savelberg HH, et al. Differences in walking pattern during the 6-minute walk test between patients with COPD and healthy subjects. PLoS One 2012;7:e37329. 22. Macfarlane P, Looney M. Walking length determination for steady state walking in young and older adults. Res Q Exerc Sport 2008;79: 261–7. 23. Casas A, Vilaro J, Rabinovich R, et al. Encouraged 6-min walking test indicates maximum sustainable exercise in COPD patients. Chest 2005;125:55–61. 24. Troosters T, Gosselink R, Decramer M. Six minute walking distance in health elderly subjects. Eur Respir J 1999;14:270–4. 25. Troosters T, Vilaro J, Rabinovich R. Physiological responses to the 6-min walk test in patients with chronic obstructive pulmonary disease. Eur Respir J 2002;20:564–9. 26. Puhan M, Mador M, Held U, et al. Interpretation of treatment changes in 6-minute walk distance in patients with COPD. Eur Respir J 2008;32:637–43. 27. Pinto-Plata VM, Cote C, Cabral H, et al. The 6-minute walk distance: change over time and value as a predictor of survival in severe COPD. Eur Respir J 2004;23:28–33. 28. Spruit MA, Polkey MI, Celli B, et al.; Evaluation of COPD Longitudinally to identify predictive surrogate endpoints (ECLIPSE) study investigators. Predicting outcomes from 6-minute walk distance in chronic obstructive pulmonary disease. J Am Med Dir Assoc 2012;13:291–7. 29. Cote CG, Pinto-Plata V, Kasprzyk K, et al. The 6-min walk distance, peak oxygen uptake, and mortality in COPD. Chest 2007;132: 1778–85. 30. Jones S, Kon SS, Canavan JL, et al. The five-repetition sit-to-stand test as a functional outcome measure in COPD. Thorax 2013;68: 1015–20. 31. Kon S, Patel MS, Canavan JL, et al. Reliability and validity of 4-metre gait speed in COPD. Eur Respir J 2013;42:33–40. 32. Keating A, Lee AL, Holland AE. Lack of perceived benefit and inadequate transport influence uptake and completion of pulmonary rehabilitation in people with chronic obstructive pulmonary disease: a qualitative study. J Physiother 2011;57: 183–90.
ASSESSMENT PROCEDURES
The 6-minute walk distance cannot be accurately assessed at home in people with COPD Anne E. Holland1,2,3, Tshepo Rasekaba1, Julio F. Fiore Jr2,3, Angela T. Burge1, and Annemarie L. Lee1,2,3
Disabil Rehabil 2015.37:1102-1106. Downloaded from informahealthcare.com by Kainan University on 04/30/15. For personal use only.
1
Department of Physiotherapy, Alfred Health, Melbourne, Australia, 2Department of Physiotherapy, La Trobe University, Melbourne, Australia, and Institute for Breathing and Sleep, Melbourne, Australia
3
Abstract
Keywords
Purpose: The 6-minute walk test (6MWT) is commonly used to measure exercise capacity in COPD, but it is unclear if this test is accurate when performed at home. This study aimed to determine whether exercise capacity can be accurately assessed at home using the 6MWT in COPD. Methods: A total of 19 participants with stable COPD (mean [SD] FEV1/FVC 52[13]) undertook the 6MWT at home and at the hospital, in random order, with two tests performed on each occasion. Hospital tests were conducted on a 30-metre walking track whilst home tests (indoor or outdoor) were conducted using the longest available track. Agreement for 6-minute walk distance (6MWD) was examined using the Bland and Altman method. Results: The track length at home was mean [SD] of 17[9] m. The home 6MWD was shorter than the hospital 6MWD (mean 30 m shorter, limits of agreement 167 to 102 m). For the home tests, a shorter track length was associated with a greater reduction in 6MWD (rs ¼ 0.59, p ¼ 0.01), but not an increased number of turns (rs ¼ 0.41, p ¼ 0.08). Conclusions: The 6MWD underestimates exercise capacity when conducted at home in COPD. Alternative tests suitable for the home environment should be considered if a comprehensive assessment is to be performed at home.
6MWT, COPD, rehabilitation History Received 10 December 2013 Revised 11 July 2014 Accepted 18 August 2014 Published online 1 September 2014
ä Implications for Rehabilitation
The 6-minute walk test is commonly used to assess change in exercise capacity following pulmonary rehabilitation in patients with chronic obstructive pulmonary disease, and may be conducted on varying track lengths, indoors or outdoors. When conducted at home, the 6-minute walk test underestimates exercise capacity in chronic obstructive pulmonary disease, due to a shorter track length available in the home environment. This suggests that results from 6-minute walk tests performed at home environment cannot be directly compared to results from centre-based tests
Introduction The 6-minute walk test (6MWT) is a self-paced test of exercise capacity which is commonly used in patients with cardiorespiratory disease [1]. Its main outcome, the 6-minute walk distance (6MWD), is a valid measure of exercise capacity in chronic obstructive pulmonary disease (COPD) [2]. The 6MWT is commonly used to measure improvement in exercise capacity following pulmonary rehabilitation [3], and has been conducted under a variety of conditions, including using a straight or circular track of varying lengths, indoors or outdoors [4–6]. A comparison of indoor and outdoor testing found that an indoor 6MWD was comparable with outdoor testing, but both environments were tightly controlled, conducted in quiet conditions with minimal distraction or corridor traffic on a flat
Address for correspondence: Anne Holland, Department of Physiotherapy, La Trobe University, Melbourne, Victoria, Australia. Tel: 00613 94796744. Fax: 00613 9533 2104. E-mail: A.Holland@ alfred.org.au
track [5]. Outdoor testing was conducted on a track of equal length, in conditions of an optimal balance of ambient temperature and humidity, with no precipitation, wind speeds less than 20 km/hr and at an air quality index associated with no health effects [5]. While the current guidelines for the 6MWT recommend the use of a standardized 30 meter straight track with strict environmental control [4], it is not known whether this can be achieved in the home setting. The reliability of home-based testing and factors that may influence the accuracy of exercise capacity assessment in this environment is not known. Two studies have examined the impact of track length on the 6MWD [6,7]. In patients with low levels of exercise capacity, track length had no effect on the 6MWD [6]. In contrast, a comparison of 10 meter versus 30 meter tracks demonstrated a shorter 6MWD achieved in patients with mild to very severe COPD [7]. Within the home setting testing must be conducted on tracks of varying lengths, which influences the number of turns during a test, and tests may be completed inside or outside. There has been no comparison of the distance achieved during a home versus hospital-based 6MWT in COPD.
6MWT at home in COPD
DOI: 10.3109/09638288.2014.956815
Research into home testing using the 6MWT is important because home-based pulmonary rehabilitation is proving to be a safe and effective alternative to hospital-based programs and a way of improving access to evidence-based care in this disabled population [8]. If exercise testing is found to be accurate at home, it will reduce the need for patients to attend the hospital for testing and further reduce the barriers to this important intervention. The primary aim of this study was to determine if the 6MWD can be accurately assessed in the home setting compared to the hospital setting in patients with COPD. A secondary aim was to identify the effect of track length, subsequent frequency of turning and environmental conditions on the 6MWD.
Methods
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This study was approved by the Alfred Health Human Research Ethics Committee. This was a within subjects crossover design. Participants Patients with a physician diagnosis of COPD, confirmed on spirometry [9] who were attending pulmonary rehabilitation at the Alfred Hospital and were clinically stable (absence of acute exacerbation of COPD over the past 4 weeks) were eligible to participate. Patients were excluded if they exhibited signs of an abnormal exercise response (e.g. failure of heart rate or blood pressure to increase with exercise, chest pain or intolerable dyspnoea) documented during their 6MWT on admission to pulmonary rehabilitation or if they were unable to comprehend and follow instructions in English. All participants provided written, informed consent prior to participation. Study design and methodology Participants undertook a 6MWT in two settings – hospital and at home. Participants were randomly allocated to setting order, the sequence determined by a computer-generated randomization schedule. Two tests were conducted at home and two tests in the hospital, with the best test used for comparison. To minimize differences being attributed to changes in disease state, fitness levels or the time period between tests, all tests were performed at the same time of day and within one week, as previous studies have shown similar reliability of 6MWD with test-retest periods ranging from one day (intraclass correlation coefficient (ICC) 0.93) [10] to seven days (ICC 0.92) [11]. All tests were completed at the conclusion of a pulmonary rehabilitation program. All participants were already familiar with the 6MWT protocol, having completed at least two tests prior to commencing the pulmonary rehabilitation program, to minimize the impact of learning effects [12]. The 6MWT performed in the hospital was completed on a level, temperature controlled corridor in accordance with standardized guidelines [4] using a straight track of 30 m in length. Home-based tests were performed outdoors when the environmental conditions were reasonable (temperature between 10 and 25 C, no rain, wind speed less than 20 km/h, air quality index of less than 32), a hard level surface was available and any current noise and distractions were unlikely to affect walking performance [5]. The suitability of weather conditions was based on the Australian Bureau of Meterology weather report [13] while the appropriateness of surface, noise level and distractions was determined by the assessor. When conditions were not reasonable, the tests were completed indoors, using the greatest track length that could be practically achieved in the home environment, allowing for turns. As changes in track shape from straight to continuous may also significantly affect distance walked [6,14], where possible, a straight track was used for home testing.
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The goal was for the model for the home track to be as similar as possible to the hospital track for both length and shape (straight versus continuous), with the priority of track shape (straight) over track length. All instructions and monitoring were consistent for the home and hospital based testing [4]. Before and after each 6MWT in both the hospital and at home, participants rated their dyspnoea and level of perceived exertion on a Borg scale [15] and had their blood pressure measured. Heart rate and SpO2 were monitored continuously during the test using a portable pulse oximeter (Nellcor N550 Pulse Oximeter; Nellcor; Pleasanton, CA). The 6MWT was performed twice at home and twice at the hospital, to control for the known learning effect on this test [10], with a minimum of 30 min of rest between tests. The distance walked, the number of turns and the length of the track were recorded for each test. To minimize the influence of other factors known to affect 6MWD, the use of walking aids and supplementary oxygen were controlled for all tests. For those participants requiring a walking aid, the same walking aid was used for both the home and hospital testing. For those who required supplemental oxygen therapy for their 6MWT, the same dosage, mode of delivery and method of oxygen transport was applied for all tests. Statistical analysis Prospective power analyses determined that a sample size of 19 participants were needed to detect a clinically important difference in walking distance of 25 m, assuming a standard deviation of 36 m (taken from data collected in our own program) between the hospital and home based tests with a power of 80% at an alpha of 0.05. A difference between tests of 25 m was chosen as this is the minimal important change in 6MWD in our population of patients with COPD [16]. Primary aim Differences between home and hospital 6MWDs were examined using paired t-tests for normally distributed data or a Mann–Whitney U test for data which was not normally distributed. Agreement between the 6MWD measured at home and in hospital was calculated using the methods described by Bland and Altman [17]. Secondary aim The extent to which differences in 6MWD between home and hospital-based tests was related to differences in number of turns and track length was analyzed using Spearman’s rho. To assess whether the reproducibility of the home based tests contributed to any differences in 6MWD between testing locations, an intra-class correlation coefficient (ICC)2,1 using a two-way random effect model was calculated to compare 6MWD on test 1 and test 2 at home. A similar approach was used for tests in the hospital. The ICC scores were interpreted according to the following guidelines: values above 0.75 are indicative of good reliability, values between 0.5 to 0.74 indicate moderate reliability and less than 0.50 indicate poor reliability [18]. Data were analyzed using SPSS version 21.0 (IBM Corp., Armonk, NY) and an alpha level of 0.05 used.
Results A total of 24 potential participants were screened for this study, with three excluded on the basis of declining to undertake a 6MWT at home and two excluded on the basis of limited understanding of English. A total of 19 participants were included in this study. The majority were male (n ¼ 11), with an average FEV1 of 54% predicted. The severity of COPD according to the
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Table 1. Subject demographics. Mean (SD)
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Gender (male:female) Age (yrs) BMI (kg/m2) FEV1 (L) FEV1 % predicted FVC (L) FVC % predicted FEV1/FVC 6MWD in Hospital (m) Use of O2 therapy (yes:no) O2 flow rate 2 L/min 3 L/min 4 L/min 6 L/min Use of gait aid (yes:no) 4WF
11:8 71 (6.8) 29.9 (6.0) 1.39 (0.51) 53.6 (18.8) 2.74 (0.92) 79.2 (21.2) 51.6 (13.0) 421 (116) 5:14 1 (5%) 1 (5%) 2 (11%) 1 (5%) 4:15 4 (21%)
Range 56–86 19.8–42.5 0.58–2.44 25–91 1.37–4.63 49–122 27.0–70.0 200–655
Data are mean (SD) or n (%) unless otherwise specified. 6MWD – 6-minute walk distance; BMI – Body mass index; FEV1 – Forced expiratory volume in one second; FVC – Forced vital capacity; O2 – Oxygen; 4WF – 4-wheeled framt.
GOLD classification [9] was mild (n ¼ 2), moderate (n ¼ 9), severe (n ¼ 6) and very severe disease (n ¼ 2). Five participants used oxygen therapy, all delivered by portable cylinder, and four participants used gait aids during the tests. The demographics of participants are outlined in Table 1. For the home-based tests, the mean (SD) track length was 17(9) m (range of 7 to 45 m). A total of 11 (58%) participants completed their test outdoors and 18 (95%) were on a straight track. There was a trend towards a significantly longer mean (SD) track length for outdoor tests at 20(10) m compared to indoor tests at 13(7)m, p ¼ 0.06. The mean number of turns for indoor versus outdoor tests was 30(SD 18) versus 20(10 m), p ¼ 0.20, and the mean number of stops for indoor versus outdoor tests was 0.1 (0.4) versus 0.5 (1.0), p ¼ 0.23. For those who completed the home-based 6MWT outdoors, the mean (SD) temperature was 16.0(0.05) degrees Celsius, wind speed of 23.5 (13.4) km/hr and air quality was 24.5(12), all of which were classified as very good conditions [13]. There was a significant increase in distance between the two home-based tests of 10 m (95% CI 2.8 to 17.0), p ¼ 0.009, with 14 (74%) of participants walking further on the second test. The percentage change was 4(6)%. Similarly for the hospitalbased test, there was an increase of 16 m (95% CI 5 m to 28 m), p ¼ 0.008, with 15 (79%) participants walking further on the second test, with percentage change being 3(4)%. Sixteen participants (84%) achieved a shorter 6MWD in the home setting compared to the hospital setting. There was a significant difference in the distance walked between hospital versus home-based 6MWT, with the 6MWD being 30 m shorter in home-based test (95% CI 0.4 to 63.2), with limits of agreement of 167 m and 102 m (Figure 1). There was a moderate relationship between the difference in the 6MWD between settings and the track length at home (rs ¼ 0.59, p ¼ 0.01; Figure 2), showing a greater reduction in 6MWD between settings when the track length was shorter. There was a weak relationship between the number of turns required for the home-based test and the difference in 6MWD between settings (rs ¼ 0.41, p ¼ 0.08). The maximum HR achieved was similar across settings, with a mean difference of 1.2 beats per minute. While a lower nadir SpO2 was measured during the home-based test (mean difference of 3%), this was not significant. There was no significant difference in symptoms or number of stops during the tests (Table 2). While correlation between the number of turns and the
Figure 1. Bland Altman plot of the mean 6MWD against the difference between hospital and home based tests. Mean difference is 30.4 m.
Figure 2. Relationship between track length at home and difference in 6MWD between the home and hospital settings.
Table 2. Comparison of conditions, physiological and symptom responses to the 6MWT in differing environments. Variable 6MWD (m) Track length (m) Maximum HR (b/min) Nadir SpO2 End Borg Dyspnoea score End Borg RPE score Number of stops
Hospital 421 30 103 86 4 12 0.2
(116) (0) (21) (8) (2) (3) (0.5)
Home 391 17 102 85 4 11 0.4
(99) (9) (27) (13) (1) (3) (0.8)
p Value 0.04 50.001 0.88 0.20 0.55 0.07 0.10
Data are mean (SD). 6MWD – 6-minute walk distance; HR – heart rate; b/min – beats per minute; SpO2 – Percutaneous oxygen saturation; RPE – Rate of perceived exertion.
distance walked for the hospital-based test was high (rs ¼ 0.83, p50.001), there was no relationship for the home-based test (rs ¼ 0.08, p ¼ 0.721). Correlation between home 6MWD and track length was rs ¼ 0.44 (p ¼ 0.06). The test-retest reliability for the hospital test was high, with an ICC2,1 of 0.98 (95% CI 0.95 to 0.99). Reliability for the
6MWT at home in COPD
DOI: 10.3109/09638288.2014.956815
home-based test was high, regardless of the order of completion, with an ICC2,1 of 0.990 (95% CI 0.970 to 0.990) in participants who completed the home-based test on the second testing day, and an ICC2,1 of 0.996 (95% CI 0.978 to 0.999) in those completing the home-based test first. This high reliability was maintained regardless of whether the test was indoors or outdoors (ICC2,1 of 0.970 [95% CI 0.836 to 0.995] for an indoor test; ICC2,1 of 0.998 [95% CI 0.993 to 0.999] for an outdoor test).
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Discussion This study has shown that, in patients with moderate to severe COPD, the 6MWD is likely to be shorter when the 6MWT is conducted at home using a standardized protocol. The primary reason appears to be the shorter tracks available at home, regardless of whether the test is performed indoors or outdoors. The difference in distance walked between hospital and homebased tests is large enough to be clinically important if the 6MWD is being used to evaluate change over time. The influence of a shorter track length resulting in a shorter 6MWD due to the increased number of turns was initially proposed by Enright [19]. However, recent studies in COPD found that patients with a high exercise capacity who would undertake a greater number of turns on a straight track did not demonstrate greater gains in 6MWD when performing the test on a circular track [6,14]. This suggests there may be other possible contributing factors unrelated to the number of turns influencing the 6MWD. In healthy elderly subjects, a higher gait speed with greater stride velocity was used over longer tracks (greater than 20 m) when walking at habitual speed, with a slower speed demonstrated on a shorter track (less than 10 m) [20]. In patients with mild to severe COPD, walking patterns are influenced by the need to minimize energy and maintain stability [21]. In comparison to healthy controls, patients with COPD have a reduced cadence and a greater degree of medio-lateral variability (due to lateral foot placement) which is an active control strategy to compensate for balance disturbances and to ensure stability in the forward direction (direction of propulsion) [21]. If this variation in walking pattern is evident during a 6MWT conducted on a 30 m straight track, it is possible that further changes in gait strategy may be adopted on a shorter track length, resulting in a reduced 6MWD. Tracks of longer length permit a greater amount of acceleration and higher top speed, with a distance of up to 3.2 m required to accelerate to steady walking pace and a distance of up to 1.9 m required to decelerate in healthy adults [22]. With a shorter track, the ability to achieve steady pace walking and allow for changes in acceleration and deceleration may be limited. Evidence of slower walking speed during a 6MWT conducted over a 10 m course in a group of eight patients with COPD [23] supports this suggestion and should be confirmed in studies with larger samples. The difference in distance between the home and hospitalbased tests appears to be unrelated to test reproducibility, as the ICCs for both test conditions were greater than 0.90, and the randomized testing order acted to minimize the impact of any learning effect across testing occasions. Although a learning effect was evident between tests 1 and 2 regardless of the location where they were performed, the magnitude did not differ from previous reports [10,12,24]. This small effect is likely to be attributable to the familiarity of all participants with the 6MWT prior to study commencement. The home-based test induced equivalent cardiovascular and ventilatory responses to exercise [25] as the increase in HR, decline in SpO2 and symptoms of dyspnoea and fatigue did not differ between settings. These factors are therefore unlikely to be responsible for the differences in 6MWD between settings.
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The difference in 6MWD between hospital and home tests is smaller (30.4 m) than that recently described in a comparison of distances achieved on a 30 m and 10 m track length (49.5 m) [7]. The greater range of options for track lengths at home (7–45 m) is the most likely explanation. Irrespective of this, this difference between home-based and hospital-based tests is beyond to the minimal important difference described for patients with moderate to severe COPD [16] and is within the 95% CI of the described by Puhan et al [26], both of which were completed on a 30 m course. The average difference in walk distance exceeded the minimal important difference in 58% of participants and the limits of agreement were wide. This suggests that a home-based 6MWD is not an ideal method to assess change over time compared to hospital-based measures, and that existing minimal important differences may not be relevant for home-based tests. There may be similar limitations when the 6MWD is used to estimate prognosis. Previous studies establishing threshold values of the 6MWD associated with hospitalization or survival were conducted on tracks of 30 meters or more [27–29]. Given the broad clinical importance of having accurate measures of 6MWD and the relationship of this measure with track length demonstrated in this and other studies [7], a home-based 6MWT should not be used to compare a patient’s performance to existing threshold values of 6MWD, unless adequate track length (425 m)[4] is assured. The limitations of the 6MWT at home suggest that alternative measures of exercise capacity which require less space to complete and reflect a similar level of function should be considered as outcome measures in this environment. Recent reports of the five-repetition sit-to-stand test in COPD [30] and the 4-metre gait speed [31], which have been validated in this patient population may be suitable options. Barriers to attending pulmonary rehabilitation include difficulty in attending assessments conducted in the outpatient environment due to transport [32]. With home-based rehabilitation emerging as an effective alternative for undertaking this intervention [8], it is important to identify outcome measures which may also be reliably completed within the home environment. This study is limited by the small number of participants, which influences the ability to perform multivariate analysis to determine the independent effect of factors such as track length, number of turns, outdoor and indoor conditions and disease severity on the 6MWT performed at home. Further analysis involving a larger number of participants is necessary to determine whether there is any interaction between these factors. In conclusion, the 6MWD will underestimate exercise capacity when conducted at home in people with COPD. Alternative exercise tests suitable for the home environment should be developed if a comprehensive assessment is to be performed at home.
Acknowledgements The authors would like to thank the patients for taking part in this study and to Sarah Foss who also assisted with data collection.
Declaration of interest The authors report no declarations of interest.
References 1. Rasekaba T, Lee A, Naughton M, et al. The six-minute walk test: a useful metric for the cardiopulmonary patient. Intern Med J 2009; 39:495–501. 2. Jenkins S. 6-minute walk test in patients with COPD – clinical applications in pulmonary rehabilitation. Physiotherapy 2007;93: 175–82.
Disabil Rehabil 2015.37:1102-1106. Downloaded from informahealthcare.com by Kainan University on 04/30/15. For personal use only.
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3. Lacasse Y, Goldstein R, Lasserson TJ, Martin S. Pulmonary rehabilitation for chronic obstructive pulmonary disease. Cochrane Database Syst Rev 2006;(4):CD003793. 4. American Thoracic Society. ATS Statement: guidelines for the 6-minute walk test. American Am J Respir Crit Care Med 2002;166: 111–17. 5. Brooks D, Solway S, Weinacht K, et al. Comparison between an indoor and an outdoor 6- minute walk test among individuals with chronic obstructive pulmonary disease. Arch Phys Med Rehabil 2003;84:873–6. 6. Sciurba F, Criner G, Lee S, et al.; National Emphysema Treatment Trial Research Group. Six-minute walk distance in chronic obstructive pulmonary disease. Reproducibility and effect of walking course layout and length. Am J Respir Crit Care Med 2003;167:1522–7. 7. Beekman E, Mesters I, Hendriks E, et al. Course length of 30 metres versus 10 metres has a significant influence on six-minute walk distance in patients with COPD: an experimental crossover study. J Physiother 2013;59:169–76. 8. Maltais F, Bourbeau J, Shapiro S, et al. Effects of home-based pulmonary rehabilitation in patients with chronic obstructive pulmonary disease: a randomized trial. Ann Intern Med 2008;149: 869–78. 9. Global strategy for the diagnosis, management and prevention of COPD, Global Initiative for Chronic Obstructive Lung Disease (GOLD) [Internet]. 2013. Available from: http://www.goldcopd.org/ [last accessed 9 Sept 2013]. 10. Hernandes N, Wouters E, Meijer K, et al. Reproducibility of 6-minute walking test in patients with COPD. Eur Respir J 2011;38: 261–7. 11. Eiser N, Willsher D, Dore CJ. Reliability, repeatability and sensitivity to change of externally and self-paced walking tests in COPD patients. Respir Med 2003;97:407–14. 12. Spencer L, Alison J, McKeough Z. Six-minute walk test as an outcome measure. Are two six-minute walk tests necessary immediately after pulmonary rehabilitation and at three-months follow-up? Am J Phys Med Rehabil 2007;87:224–8. 13. Australian Government: Bureau of Meterology. 2013. Available from: http://www.bom.gov.au/[last accessed 18 Sept 2013]. 14. Bansal V, Hill K, Dolmage T, et al. Modifying track layout from straight to circular has a modest effect on the 6-min walk distance. Chest 2008;133:1155–60. 15. Borg G. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982;14:377–81. 16. Holland AE, Hill C, Rasekaba T, et al. Updating the minimal important difference for six-minute walk distance in patients with chronic obstructive pulmonary disease. Arch Phys Med Rehabil 2010;91:221–5.
Disabil Rehabil, 2015; 37(12): 1102–1106
17. Bland J, Altman D. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1: 307–10. 18. Portney L, Watkins M. Foundations of clinical research–applications to practice. 3rd ed. New Jersey: Pearson Education; 2009. 19. Enright P. The six-minute walk test. Respir Care 2003;48:783–5. 20. Najafi R, Helbostad J, Moe-Nilssen R, et al. Does walking strategy in older people change as a function of walking distance? Gait Posture 2009;29:261–6. 21. Annegarn J, Spruit MA, Savelberg HH, et al. Differences in walking pattern during the 6-minute walk test between patients with COPD and healthy subjects. PLoS One 2012;7:e37329. 22. Macfarlane P, Looney M. Walking length determination for steady state walking in young and older adults. Res Q Exerc Sport 2008;79: 261–7. 23. Casas A, Vilaro J, Rabinovich R, et al. Encouraged 6-min walking test indicates maximum sustainable exercise in COPD patients. Chest 2005;125:55–61. 24. Troosters T, Gosselink R, Decramer M. Six minute walking distance in health elderly subjects. Eur Respir J 1999;14:270–4. 25. Troosters T, Vilaro J, Rabinovich R. Physiological responses to the 6-min walk test in patients with chronic obstructive pulmonary disease. Eur Respir J 2002;20:564–9. 26. Puhan M, Mador M, Held U, et al. Interpretation of treatment changes in 6-minute walk distance in patients with COPD. Eur Respir J 2008;32:637–43. 27. Pinto-Plata VM, Cote C, Cabral H, et al. The 6-minute walk distance: change over time and value as a predictor of survival in severe COPD. Eur Respir J 2004;23:28–33. 28. Spruit MA, Polkey MI, Celli B, et al.; Evaluation of COPD Longitudinally to identify predictive surrogate endpoints (ECLIPSE) study investigators. Predicting outcomes from 6-minute walk distance in chronic obstructive pulmonary disease. J Am Med Dir Assoc 2012;13:291–7. 29. Cote CG, Pinto-Plata V, Kasprzyk K, et al. The 6-min walk distance, peak oxygen uptake, and mortality in COPD. Chest 2007;132: 1778–85. 30. Jones S, Kon SS, Canavan JL, et al. The five-repetition sit-to-stand test as a functional outcome measure in COPD. Thorax 2013;68: 1015–20. 31. Kon S, Patel MS, Canavan JL, et al. Reliability and validity of 4-metre gait speed in COPD. Eur Respir J 2013;42:33–40. 32. Keating A, Lee AL, Holland AE. Lack of perceived benefit and inadequate transport influence uptake and completion of pulmonary rehabilitation in people with chronic obstructive pulmonary disease: a qualitative study. J Physiother 2011;57: 183–90.
