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Comparison Between an Indoor and an Outdoor 6-Minute Walk Test Among Individuals With Chronic Obstructive Pulmonary Disease Dina Brooks, PhD, Sherra Solway, MSc, Krisztina Weinacht, Dip(PT), David Wang, MSc, Scott Thomas, PhD ABSTRACT. Brooks D, Solway S, Weinacht K, Wang D, Thomas S. 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. Objectives: To investigate the feasibility of an outdoor 6-minute walk test (6MWT) as a measure of functional status among individuals with chronic obstructive pulmonary disease (COPD), and to examine the relationship between performance on an indoor and an outdoor 6MWT. Design: An experimental, repeated-measures crossover design. Subjects were studied on 2 separate days in the same week. Two 6MWTs— one indoors and the other outdoors— were performed on each study day, with a rest in between. The test order was randomly selected on the first day and reversed on the second day. Outdoor tests were performed on days of moderate weather conditions (mean temperature ⫾ standard deviation, 21°⫾3°C; mean wind speed, 15⫾7km/h; no precipitation) and on a flat surface (sidewalk). Setting: Outpatient rehabilitation program in Ontario. Participants: Eighteen subjects with COPD (10 men, 8 women; age, 70⫾8y), 5 using supplemental oxygen at rest (forced expiratory volume in 1s, 1.0⫾0.3L; 42%⫾8% of predicted). Interventions: Not applicable. Main Outcome Measures: Distance walked in 6 minutes (in meters), duration of rest (in seconds), and change in rate of perceived dyspnea. Results: There was no significant effect of setting (indoors vs outdoors) on distance walked (394⫾86m vs 398⫾84m, P⫽0.4), duration of rest (13⫾28s vs 9⫾20s, P⫽0.4), or change in rate of perceived dyspnea (2.3⫾1.7 vs 2.3⫾2.0, P⫽0.8). Testing day had no significant effect on walk test performance (all P⬎0.1). Conclusions: The results indicate that the 6MWT performed outdoors within reasonable climatic parameters may be reflective of 6MWT performance indoors. Key Words: Exercise test; Exercise tolerance; Pulmonary disease, chronic obstructive; Rehabilitation; Walking © 2003 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

From the Department of Physical Therapy (Brooks, Solway, Wang, Thomas), University of Toronto; Institute for Work & Health and Toronto Rehabilitation Institute (Solway); and Toronto East General Hospital (Weinacht), Toronto, ON, Canada. Supported by the Ontario Respiratory Care Society and Ontario Lung Association. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Dina Brooks, PhD, Dept of Physical Therapy, University of Toronto, 500 University Ave, Rm 848, Toronto, ON M5G 1V7, Canada, e-mail: [email protected]. 0003-9993/03/8406-7772$30.00/0 doi:10.1016/S0003-9993(03)00011-X

NDIVIDUALS WITH CHRONIC obstructive pulmonary disease (COPD) show variable exercise capacity and activity Ilevels that do not necessarily correlate with severity of airflow obstruction.1-4 Measures of functional status are commonly used in this population and are essential for optimal clinical management.2,5 In a national survey of pulmonary rehabilitation programs in Canada, 98% of respondents reported inclusion of a measure of functional exercise capacity, usually the 6-minute walk test (6MWT) or the 12-minute walk test.6 The 6MWT is a valid measure of functional status commonly used for evaluating individuals with COPD.7 Typically, this test is administered in an enclosed indoor corridor, free of distractions, and patients are asked to cover as much distance as they can in 6 minutes.2 This environment is somewhat artificial and may not reflect activities that individuals usually do on a daily basis. There is no literature on the amount of time that individuals with advanced pulmonary disease spend indoors versus outdoors. Nevertheless, activities of daily living (ADLs) likely require them to ambulate outdoors regularly (eg, walking from their car to a doctor’s office, shopping mall, or grocery store). Many of these individuals will have experience with pulmonary rehabilitation programs, in which they are encouraged to maintain their exercise program by ambulating. From our clinical experience, individuals with COPD value the opportunity to exercise outdoors. Furthermore, health care professionals in pulmonary rehabilitation programs use the results of the indoor 6MWT to advise individuals with COPD about activities outdoors. Such recommendations are based on the health care professional’s clinical experience and not on any scientific evidence. Finally, clinicians making home visits or those in community clinics may not have access to the indoor space required for the 6MWT, making an outdoor 6MWT a desirable alternative for evaluating functional status in these environments. Thus, the purpose of this study was to investigate the feasibility of an outdoor 6MWT as a measure of functional status among individuals with COPD and to determine the relationship between performance on an indoor and an outdoor 6MWT. METHODS Participants Subjects were recruited from the outpatient pulmonary rehabilitation program at Toronto East General Hospital, in Toronto, ON, Canada. Subjects were considered eligible for the study if they were clinically stable with a diagnosis of COPD,8 were between 55 and 85 years of age, had a forced expiratory volume in 1 second (FEV1) of less than 60% of predicted, and reported dyspnea or fatigue with ADLs. Exclusion criteria included the presence of associated medical conditions that limited exercise tolerance (eg, symptomatic cardiovascular or musculoskeletal conditions, recent surgery), inability to communicate in English, or the use of a mobility aid. The rationale Arch Phys Med Rehabil Vol 84, June 2003

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for the last criteria was that the use of a mobility aid could alter performance outdoors because of changes in the terrain and that these alterations may be difficult to differentiate from differences unrelated to aids in a small sample. Protocol The Research Ethics Boards at the University of Toronto and Toronto East General Hospital approved the study. All subjects gave informed written consent. A randomized crossover design was used. Each subject was studied at the same time of the day on 2 separate days in the same week. Subjects were asked to abstain from caffeine for 4 hours before each session and to administer their inhaled bronchodilator 30 minutes before each session. Subjects wore the same footwear on both study days. On each study day, two 6MWTs were performed with a minimum of a 30-minute rest between tests (or when perceived dyspnea, heart rate, and oxygen saturation returned to baseline levels). One walk test was performed indoors and the other outdoors. The test order was randomized for the first day and reversed on the second day. The indoor 6MWT was administered in a corridor, 30m in length. Pylons were placed at either end of the course. The tests were performed under quiet conditions, with a minimum of distractions and corridor traffic. The outdoor 6MWT was performed by using the same length of flat sidewalk, in a quiet neighborhood. Outdoor tests were performed on days in which the weather was “reasonable,” which was defined as an “apparent” temperature (a composite of ambient temperature and humidity) of 10° to 25°C, no precipitation, wind speed of less than 20km/h, and an air quality index of less than 32. The air quality index ranges from 0 to more than 100, with 0 to 15 representing very good air quality; 16 to 31, good; 32 to 49, moderate; 50 to 99, poor; and 100 or more, very poor.9 A value below 32 has no known health effects for the majority of the population.9 Weather variables were not directly measured; this information was taken from a weather report website corresponding to the location and time each outdoor walk test was administered. All subjects performed at least 2 practice walks before data collection, to control for learning and practice effects.7 Standardized instructions were provided to subjects and no encouragement was offered during the tests.10 Subjects were requested to cover as much ground as possible during the test period, stopping only if they felt too tired or too breathless to continue, and to resume walking as soon as they were able to do so. The tester accompanied the subjects for each walk test and walked behind the subject to avoid pacing. A folding chair was used for sitting if the subject required a rest. Subjects using supplemental oxygen carried or pulled their tank during each 6MWT. Measures For each 6MWT, we recorded distance walked and the number and duration of rests, and we monitored oxyhemoglobin saturation and heart rate continuously by using a pulse oximeter.a The oximeter provided a printout of values reflecting maximum and minimum recorded and the mean of the complete duration (value used in analysis). At the start and at the end of each test, subjects rated their perceived rate of dyspnea (“difficulty of breathing”) by using a modified Borg Scale.11 By using the same scale, subjects were also asked to rate their perceived leg effort. Furthermore, a self-report of how the climate affected performance and preference between the indoor and the outdoor 6MWT was recorded by means of a simple standardized questionnaire. This brief questionnaire was piloted for suitability and clarity. Subjects were asked to identify factors that influenced their ability to walk outdoors Arch Phys Med Rehabil Vol 84, June 2003

from a list of possible factors (temperature, wind, pollution, humidity, other). Functional status was also evaluated by using the modified version of the Pulmonary Functional Status and Dyspnea Questionnaire (PSFDQ-M) on the first study day before the start of testing. The PSFDQ is a 40-item, self-administered functional status and symptoms (dyspnea and fatigue) questionnaire that requires less than 10 minutes to complete.12,13 Psychometric properties of this questionnaire (eg, internal consistency, testretest reliability, construct validity) have been established in adult patients with pulmonary disease.12-14 The questionnaire includes 3 domains: dyspnea, fatigue, and activity level. For the activity domain, a list of 10 ADLs (eg, brushing/combing hair, walking) are included, and subjects were asked to indicate “involvement with the activity now as compared to before you developed lung problems.” A scale of 0 (as active as I have ever been) to 10 (have omitted entirely) was used. The same activities were listed again, and the subjects were asked to rate each according to the degree of shortness of breath (dyspnea domain) and tiredness (fatigue domain) experienced. A scale of 0 (none) to 10 (very severe) was used. Within each domain, the values assigned for each activity were summed and divided by the number of activities that applied. Thus, for each domain, the score could range from 0 (minimal limitation) to 100 (maximal limitation).11 Statistical Analysis A sample size estimation using a 2-tailed paired t test with a type I error of .05 and power of 90% determined that a clinically significant difference in 6MWT distance5 (ie, 54m) would be detected with a minimum of 16 subjects (standard deviation [SD], 86m). Means and SDs were calculated for all outcomes. Descriptive statistics (including frequencies) were also used for subject characteristics (eg, age, FEV1) and subject preference. A total of four 6MWTs were performed; two 6MWTs were performed on each day, one indoors and the other outdoors. To determine the effect of environment, analyses were performed by using 2-way repeated-measures analysis of variance. The 2 factors were day (1, 2) and environment (indoors, outdoors). Linear regression was calculated to examine the relationship between the distance walked indoors and outdoors. Intraclass correlation coefficients (ICCs) were calculated to represent test-retest reliability for day and setting. To determine if there was an association between self-report level of functional status and outdoor walk test performance, univariate regression was used to explore the relationship between distance walked outdoors and PFSDQ-M scores. To determine whether more functionally disabled individuals had a greater deterioration in the outdoor walk test as compared with the indoor walk test, we also explored the relationship between the change in the distance walked indoors minus outdoors and PFSDQ-M scores. SigmaStat, version 2.03,b and SigmaPlot, version 5.0,b statistical softwares were used for all analyses; a P value of .05 or less was considered significant. RESULTS Eighteen individuals with a medical diagnosis of COPD participated in the study. The characteristics of these subjects are presented in table 1. The outdoor 6MWTs were performed on comparable days with respect to climatic variables—that is, temperature, humidex, and wind speed (all P⬎0.2). The combined data for all days are presented in table 2.

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Table 1: Characteristics of the Subjects (Nⴝ18) Mean ⫾ SD (Range)

Age (y) FEV1 (L) FEV1 (% predicted) Sex Oxygen use (L) (n⫽5)

70⫾8 (52–81) 1.0⫾0.3 (0.5–1.5) 42⫾8 (28–42) 10 men, 8 women 2.9⫾0.7 (2–4)

Figure 1 shows distance walked indoors and outdoors for each subject. There was no effect of setting on distance walked (indoors: 394⫾86m vs outdoors: 398⫾84m, P⫽0.4). Duration of rest was 13⫾28 seconds for the indoor tests and 9⫾20 seconds for the outdoor tests (P⫽0.4). Furthermore, setting had no effect on the change in rate of perceived dyspnea (indoors: 2.3⫾1.7 vs outdoors: 2.3⫾2.0, P⫽0.8) or perceived leg effort (indoors: 1.1⫾1.0 vs outdoors: 0.9⫾0.8, P⫽0.6) (fig 2). Actual values for rate of perceived exertion are presented in table 3. Similarly, setting did not influence mean heart rate or oxygen saturation (SpO2) during the test (P⬍0.3) (fig 3). Day of testing had no significant effect on any of the variables examined (all P⬎0.1), which indicates that the test was reliable on 2 separate days (ICC⫽.94). Linear regression between distance walked indoors from the 2 trials and the distance walked outdoors revealed a significant correlation (r⫽.97, P⬍.001, slope⫽.99, intercept⫽⫺.64), and an ICC of .95. Forty-two percent of the subjects had no preference for the environment of testing, whereas 36% preferred the indoor setting and 22% preferred the outdoor. When asked which climatic variable influenced their performance, 30% of respondents identified temperature, 14% wind, 19% pollution, and 11% humidity. The PFSDQ-M reflected minimal impairment for dyspnea (1.9⫾1.6; range, 0.1–5.0), fatigue (1.7⫾1.5; range, 0.1–5.6), and activity (2.1⫾1.6; range, 0.1–5.0). There was no significant correlation between distance walked outdoors and each of the 3 dimensions (r⬍0.2, P⬎0.6) or between the distance walked indoors and the 3 dimensions (r⬍0.1, P⬎0.7). Similarly, there were no significant correlation between the difference in distance walked indoors and outdoors and the PFSDQ-M subscales scores (r⬍0.2, P⬎0.5). DISCUSSION The findings indicate that when performed within reasonable climatic conditions, the outdoor 6MWT is a feasible measure of functional status among individuals with COPD. Furthermore, the outdoor 6MWT test was reproducible on 2 separate days, and subjects’ performance did not differ significantly from that indoors. We were unable to locate any studies that included an outdoor 6MWT. However, considerable attention has been given to the indoor 6MWT, especially in recent years. The use of the 6MWT has been studied in different populations, including individuals with COPD, individuals with heart failure, individuals with pacemakers, individuals with peripheral arte-

Fig 1. Total distance walked indoors and outdoors. Open circles represent individual data points. Solid squares represent mean ⴞ SD.

rial disease, surgical patients, and pediatric patients.7 A recent systematic review of functional walk tests by our group concluded that the psychometric properties of the 6MWT have been well researched and established, and we have recommended the 6MWT as the test of choice for clinical and research use.7 An outdoor 6MWT test is more reflective of real life and is not as artificial as the conventional 6MWT, in which an indoor corridor, free of distractions, is used. One of the objectives of this study was to compare performance on the indoor and on the outdoor walk tests. The data indicate that the 6MWT indoors is strongly correlated to the test performed outdoors. This finding will enable health care professionals to use the results of indoor performance to predict outdoor performance and vice versa. Additionally, the results of this study will allow health care professionals to use the 6MWT in the community (ie, during a home visit), to measure functional status and to monitor treatment effectiveness, when an indoor corridor is not available. The perception of individuals with COPD of the effect of the environment on functional capacity has not been investigated. Despite no difference between performance indoors and outdoors, slightly more subjects preferred the indoor setting (36%)

Table 2: Summary of Climatic Variables for the Outdoor Tests Climatic Variables

Mean ⫾ SD (Range)

Temperature (°C) “Apparent” temperature (°C) Wind speed (km/h)

21⫾3 (16–25.5) 25⫾4 (17–32) 15⫾7 (5–33)

Fig 2. Comparison of change in perceived dyspnea and leg effort ratings when walking indoors and outdoors. Open circles represent individual data points. Solid squares represent mean ⴞ SD. There are several individual points at 0 that are reflected by 1 data point only.

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Table 3: Actual Values for Modified Borg Ratings of Perceived Dyspnea and Leg Effort Before and After 6MWT Indoors and Outdoors Day 1 (mean ⫾ SD) Indoors

Day 2 (mean ⫾ SD) Outdoors

Indoors

Outdoors

Perceived Rating

Before

After

Before

After

Before

After

Before

After

Dyspnea Leg effort

0.5⫾0.8 0.5⫾0.9

2.7⫾1.9 1.6⫾1.3

0.8⫾0.8 0.3⫾0.4

3.0⫾2.3 1.4⫾1.3

0.5⫾0.5 0.4⫾0.5

2.9⫾1.8 1.4⫾1.3

0.5⫾0.7 0.4⫾0.6

3.0⫾2.2 1.1⫾0.9

to the outdoor setting (22%). Temperature was the main factor perceived as influencing ability to walk outside. The finding that the distance walked outdoors did not correlate with PFSDQ-M scores was not surprising. The 2 measures provide different information on functional capacity: the PFSDQ-M examines various ADLs (1 component of which is walking), and the 6MWT assesses the ability to walk. However, our sample included individuals at a high level of function, as shown by the low scores on the PFSDQ-M and indoor 6MWT distances greater than 300m. Studies have indicated that 300m on the 6MWT is a threshold for level of disability among individuals with respiratory disease, including those with COPD.15-17 Thus, it is possible that individuals with greater disability (eg, 6MWT distances ⬍300m) may be influenced by the outdoor environment. Two studies18,19 have reported distances walked in 6 minutes by healthy subjects. In the healthy elderly (aged 60 – 65y), the distances walked in 6 minutes ranged from 494 to 631m. These values are higher than those observed in our sample, in which the distances walked in 6 minutes were between 390 and 400m. Therefore, although our sample was high functioning compared with others with COPD, they had more disability than a healthy population. Limited ranges of climatic parameters were used in this study. Future studies will need to investigate the effect of a broader range of environmental factors (eg, more extreme conditions, ie, colder temperature, higher winds, pollution humidity levels, and different terrains) and walking aids on outdoor performance, to expand our understanding of the feasibility and validity of using the 6MWT as a measure of functional status in an outdoor setting. CONCLUSION The data indicate that the 6MWT performed outdoors within reasonable climatic parameters may be reflective of 6MWT performance indoors. Therefore, an outdoor 6MWT may be an alternative option to the traditional 6MWT for use in the community when an indoor corridor is not available. Future

Fig 3. Mean oxygen saturation and heart rate during the 6MWT when walking indoors and outdoors. Open circles represent individual data points. Solid squares represent mean ⴞ SD. Note that the y axis does not start at zero.

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studies are needed to increase the generalizability of these findings. References 1. Guyatt GH, Sullivan MJ, Thompson PJ, et al. The 6-minute walk: a new measure of exercise capacity in patients with chronic heart failure. Can Med Assoc J 1985;132:919-23. 2. Guyatt GH, Thompson PJ, Berman LB, et al. How should we measure function in patients with chronic heart and lung disease? J Chronic Dis 1985;38:517-24. 3. Epstein SK, Celli BR. Cardiopulmonary exercise testing in patients with chronic obstructive pulmonary disease. Cleve Clin J Med 1993;60:119-28. 4. Bittner V. Six-minute walk test in patients with cardiac dysfunction. Cardiologia 1997;42:897-902. 5. Redelmeier DA, Bayoumi AM, Goldstein RS, Guyatt GH. Interpreting small differences in functional status: the six-minute walk test in chronic lung disease patients. Am J Respir Crit Care Med 1997;155:1278-82. 6. Brooks D, Lacasse Y, Goldstein RS. Pulmonary rehabilitation programs in Canada: national survey. Can Respir J 1999;6:55-63. 7. Solway S, Brooks D, Lacasse Y, Thomas S. A qualitative systematic overview of the measurement properties of functional walk tests used in the cardiorespiratory domain. Chest 2001;119:256-70. 8. American Thoracic Society. Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease. Am J Respir Crit Care Med 1995;152:S77-121. 9. Ontario Ministry of Environment and Energy. Available at: http:// www.airqualityontario.com. Accessed June 10, 2002. 10. Guyatt GH, Pugsley SO, Sullivan MJ, et al. Effect of encouragement on walking test performance. Thorax 1984;39:818-22. 11. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982;14:377-81. 12. Lareau SC, Carrieri-Kohlman V, Janson-Bjerklie S, Roos PJ. Development and testing of the Pulmonary Functional Status and Dyspnea Questionnaire (PFSDQ). Heart Lung 1994;23:242-50. 13. Lareau SC, Meek PM, Roos PJ. Development and testing of a modified version of the pulmonary status and dyspnea questionnaire. Heart Lung 1998;27:159-68. 14. Lareau SC, Meek PM. Testing of a modified version of the pulmonary functional status and dyspnea questionnaire [abstract]. Am J Respir Crit Care Med 1996;153:A421. 15. Solway S, Brooks D, Lau L, Goldstein R. The short-term effect of a rollator on functional exercise capacity among individuals with severe COPD. Chest 2002;122:56-65. 16. Honeyman P, Barr P, Stubbing DG. Effect of a walking aid on disability, oxygenation, and breathlessness in patients with chronic airflow limitation. J Cardiopulm Rehabil 1996;16:63-7. 17. Goldstein RS, Gort EH, Stubbing D, Avendano MA, Guyatt GH. Randomized control trial of respiratory rehabilitation. Lancet 1994;344:1394-7. 18. Troosters T, Gosselink R, Decramer M. Six minute walking distance in healthy elderly subjects. Eur Respir J 1999;14:270-4. 19. Enright PL, Sherrill DL. Reference equations for the six-minute walk in healthy adults. Am J Respir Crit Care Med 1998;158: 1384-7. Suppliers a. Model 8800; Nonin Medical Inc, 2605 Fernbrook Ln N, Plymouth, MN 55447-4755. b. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.