Comparison Between an Indoor and an Outdoor 6-MinuteWalk Test Among Individuals With Chronic ObstructivePulmonary DiseaseDina 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 outdoor6-minute walk test among individuals with chronic obstructivepulmonary disease. Arch Phys Med Rehabil 2003;84:873-6.

Objectives: To investigate the feasibility of an outdoor6-minute walk test (6MWT) as a measure of functional statusamong individuals with chronic obstructive pulmonary disease(COPD), and to examine the relationship between performanceon an indoor and an outdoor 6MWT.

Design: An experimental, repeated-measures crossover de-sign. Subjects were studied on 2 separate days in the sameweek. Two 6MWTs—one indoors and the other outdoors—were performed on each study day, with a rest in between. Thetest order was randomly selected on the first day and reversedon the second day. Outdoor tests were performed on days ofmoderate weather conditions (mean temperature� standarddeviation, 21°�3°C; mean wind speed, 15�7km/h; no precip-itation) 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 pre-dicted).

Interventions: Not applicable.Main Outcome Measures: Distance walked in 6 minutes

(in meters), duration of rest (in seconds), and change in rate ofperceived dyspnea.

Results: There was no significant effect of setting (indoorsvs outdoors) on distance walked (394�86m vs 398�84m,P�0.4), duration of rest (13�28s vs 9�20s,P�0.4), or changein 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 performedoutdoors within reasonable climatic parameters may be reflec-tive of 6MWT performance indoors.

Key Words: Exercise test; Exercise tolerance; Pulmonarydisease, chronic obstructive; Rehabilitation; Walking

© 2003 by the American Congress of Rehabilitation Medi-cine and the American Academy of Physical Medicine andRehabilitation

INDIVIDUALS WITH CHRONIC obstructive pulmonarydisease (COPD) show variable exercise capacity and activity

levels that do not necessarily correlate with severity of airflowobstruction.1-4 Measures of functional status are commonlyused in this population and are essential for optimal clinicalmanagement.2,5 In a national survey of pulmonary rehabilita-tion programs in Canada, 98% of respondents reported inclu-sion of a measure of functional exercise capacity, usually the6-minute walk test (6MWT) or the 12-minute walk test.6

The 6MWT is a valid measure of functional status com-monly used for evaluating individuals with COPD.7 Typically,this test is administered in an enclosed indoor corridor, free ofdistractions, and patients are asked to cover as much distanceas they can in 6 minutes.2 This environment is somewhatartificial and may not reflect activities that individuals usuallydo on a daily basis.

There is no literature on the amount of time that individualswith advanced pulmonary disease spend indoors versus out-doors. Nevertheless, activities of daily living (ADLs) likelyrequire them to ambulate outdoors regularly (eg, walking fromtheir car to a doctor’s office, shopping mall, or grocery store).Many of these individuals will have experience with pulmo-nary rehabilitation programs, in which they are encouraged tomaintain their exercise program by ambulating. From our clin-ical experience, individuals with COPD value the opportunityto exercise outdoors. Furthermore, health care professionals inpulmonary rehabilitation programs use the results of the indoor6MWT to advise individuals with COPD about activities out-doors. Such recommendations are based on the health careprofessional’s clinical experience and not on any scientificevidence. Finally, clinicians making home visits or those incommunity clinics may not have access to the indoor spacerequired for the 6MWT, making an outdoor 6MWT a desirablealternative for evaluating functional status in these environ-ments.

Thus, the purpose of this study was to investigate the feasi-bility of an outdoor 6MWT as a measure of functional statusamong individuals with COPD and to determine the relation-ship between performance on an indoor and an outdoor6MWT.

METHODS

Participants

Subjects were recruited from the outpatient pulmonary re-habilitation program at Toronto East General Hospital, in To-ronto, ON, Canada. Subjects were considered eligible for thestudy if they were clinically stable with a diagnosis of COPD,8

were between 55 and 85 years of age, had a forced expiratoryvolume in 1 second (FEV1) of less than 60% of predicted, andreported dyspnea or fatigue with ADLs. Exclusion criteriaincluded the presence of associated medical conditions thatlimited exercise tolerance (eg, symptomatic cardiovascular ormusculoskeletal conditions, recent surgery), inability to com-municate in English, or the use of a mobility aid. The rationale

From the Department of Physical Therapy (Brooks, Solway, Wang, Thomas),University of Toronto; Institute for Work & Health and Toronto RehabilitationInstitute (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 anyorganization with which the author(s) is/are associated.

Reprint requests to Dina Brooks, PhD, Dept of Physical Therapy, University ofToronto, 500 University Ave, Rm 848, Toronto, ON M5G 1V7, Canada, e-mail:dina.brooks@utoronto.ca.

0003-9993/03/8406-7772$30.00/0doi:10.1016/S0003-9993(03)00011-X

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for the last criteria was that the use of a mobility aid could alterperformance outdoors because of changes in the terrain andthat these alterations may be difficult to differentiate fromdifferences unrelated to aids in a small sample.

ProtocolThe Research Ethics Boards at the University of Toronto and

Toronto East General Hospital approved the study. All subjectsgave informed written consent. A randomized crossover designwas used. Each subject was studied at the same time of the dayon 2 separate days in the same week. Subjects were asked toabstain from caffeine for 4 hours before each session and toadminister their inhaled bronchodilator 30 minutes before eachsession. Subjects wore the same footwear on both study days.

On each study day, two 6MWTs were performed with aminimum of a 30-minute rest between tests (or when perceiveddyspnea, heart rate, and oxygen saturation returned to baselinelevels). One walk test was performed indoors and the otheroutdoors. The test order was randomized for the first day andreversed on the second day. The indoor 6MWT was adminis-tered in a corridor, 30m in length. Pylons were placed at eitherend of the course. The tests were performed under quiet con-ditions, with a minimum of distractions and corridor traffic.The outdoor 6MWT was performed by using the same lengthof flat sidewalk, in a quiet neighborhood. Outdoor tests wereperformed on days in which the weather was “ reasonable,”which was defined as an “apparent” temperature (a compositeof ambient temperature and humidity) of 10° to 25°C, noprecipitation, wind speed of less than 20km/h, and an airquality index of less than 32. The air quality index ranges from0 to more than 100, with 0 to 15 representing very good airquality; 16 to 31, good; 32 to 49, moderate; 50 to 99, poor; and100 or more, very poor.9 A value below 32 has no knownhealth effects for the majority of the population.9 Weathervariables were not directly measured; this information wastaken from a weather report website corresponding to thelocation and time each outdoor walk test was administered.

All subjects performed at least 2 practice walks before datacollection, to control for learning and practice effects.7 Stan-dardized instructions were provided to subjects and no encour-agement was offered during the tests.10 Subjects were re-quested to cover as much ground as possible during the testperiod, stopping only if they felt too tired or too breathless tocontinue, and to resume walking as soon as they were able todo so. The tester accompanied the subjects for each walk testand walked behind the subject to avoid pacing. A folding chairwas used for sitting if the subject required a rest. Subjects usingsupplemental oxygen carried or pulled their tank during each6MWT.

MeasuresFor each 6MWT, we recorded distance walked and the

number and duration of rests, and we monitored oxyhemoglo-bin saturation and heart rate continuously by using a pulseoximeter.a The oximeter provided a printout of values reflect-ing maximum and minimum recorded and the mean of thecomplete duration (value used in analysis). At the start and atthe end of each test, subjects rated their perceived rate ofdyspnea (“difficulty of breathing” ) by using a modified BorgScale.11 By using the same scale, subjects were also asked torate their perceived leg effort. Furthermore, a self-report ofhow the climate affected performance and preference betweenthe indoor and the outdoor 6MWT was recorded by means ofa simple standardized questionnaire. This brief questionnairewas piloted for suitability and clarity. Subjects were asked toidentify factors that influenced their ability to walk outdoors

from a list of possible factors (temperature, wind, pollution,humidity, other).

Functional status was also evaluated by using the modifiedversion of the Pulmonary Functional Status and Dyspnea Ques-tionnaire (PSFDQ-M) on the first study day before the start oftesting. The PSFDQ is a 40-item, self-administered functionalstatus and symptoms (dyspnea and fatigue) questionnaire thatrequires less than 10 minutes to complete.12,13 Psychometricproperties of this questionnaire (eg, internal consistency, test-retest reliability, construct validity) have been established inadult patients with pulmonary disease.12-14

The questionnaire includes 3 domains: dyspnea, fatigue, andactivity level. For the activity domain, a list of 10 ADLs (eg,brushing/combing hair, walking) are included, and subjectswere asked to indicate “ involvement with the activity now ascompared to before you developed lung problems.” A scale of0 (as active as I have ever been) to 10 (have omitted entirely)was used. The same activities were listed again, and the sub-jects were asked to rate each according to the degree of short-ness 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 weresummed and divided by the number of activities that applied.Thus, for each domain, the score could range from 0 (minimallimitation) to 100 (maximal limitation).11

Statistical Analysis

A sample size estimation using a 2-tailed paired t test with atype I error of .05 and power of 90% determined that aclinically significant difference in 6MWT distance5 (ie, 54m)would be detected with a minimum of 16 subjects (standarddeviation [SD], 86m).

Means and SDs were calculated for all outcomes. Descrip-tive statistics (including frequencies) were also used for subjectcharacteristics (eg, age, FEV1) and subject preference. A totalof four 6MWTs were performed; two 6MWTs were performedon each day, one indoors and the other outdoors. To determinethe effect of environment, analyses were performed by using2-way repeated-measures analysis of variance. The 2 factorswere day (1, 2) and environment (indoors, outdoors). Linearregression was calculated to examine the relationship betweenthe distance walked indoors and outdoors. Intraclass correla-tion coefficients (ICCs) were calculated to represent test-retestreliability for day and setting. To determine if there was anassociation between self-report level of functional status andoutdoor walk test performance, univariate regression was usedto explore the relationship between distance walked outdoorsand PFSDQ-M scores. To determine whether more function-ally disabled individuals had a greater deterioration in theoutdoor walk test as compared with the indoor walk test, wealso explored the relationship between the change in the dis-tance walked indoors minus outdoors and PFSDQ-M scores.SigmaStat, version 2.03,b and SigmaPlot, version 5.0,b statis-tical softwares were used for all analyses; a P value of .05 orless was considered significant.

RESULTS

Eighteen individuals with a medical diagnosis of COPDparticipated in the study. The characteristics of these subjectsare presented in table 1.

The outdoor 6MWTs were performed on comparable dayswith respect to climatic variables—that is, temperature, humi-dex, and wind speed (all P�0.2). The combined data for alldays are presented in table 2.

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Figure 1 shows distance walked indoors and outdoors foreach subject. There was no effect of setting on distance walked(indoors: 394�86m vs outdoors: 398�84m, P�0.4). Durationof rest was 13�28 seconds for the indoor tests and 9�20seconds for the outdoor tests (P�0.4). Furthermore, setting hadno 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). Actualvalues for rate of perceived exertion are presented in table 3.Similarly, setting did not influence mean heart rate or oxygensaturation (SpO2) during the test (P�0.3) (fig 3). Day of testinghad no significant effect on any of the variables examined (allP�0.1), which indicates that the test was reliable on 2 separatedays (ICC�.94). Linear regression between distance walkedindoors from the 2 trials and the distance walked outdoorsrevealed 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 theenvironment of testing, whereas 36% preferred the indoorsetting and 22% preferred the outdoor. When asked whichclimatic variable influenced their performance, 30% of respon-dents identified temperature, 14% wind, 19% pollution, and11% 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 significantcorrelation between distance walked outdoors and each of the3 dimensions (r�0.2, P�0.6) or between the distance walkedindoors and the 3 dimensions (r�0.1, P�0.7). Similarly, therewere no significant correlation between the difference in dis-tance walked indoors and outdoors and the PFSDQ-M sub-scales scores (r�0.2, P�0.5).

DISCUSSIONThe findings indicate that when performed within reasonable

climatic conditions, the outdoor 6MWT is a feasible measureof functional status among individuals with COPD. Further-more, the outdoor 6MWT test was reproducible on 2 separatedays, and subjects’ performance did not differ significantlyfrom that indoors.

We were unable to locate any studies that included anoutdoor 6MWT. However, considerable attention has beengiven to the indoor 6MWT, especially in recent years. The useof the 6MWT has been studied in different populations, includ-ing individuals with COPD, individuals with heart failure,individuals with pacemakers, individuals with peripheral arte-

rial disease, surgical patients, and pediatric patients.7 A recentsystematic review of functional walk tests by our group con-cluded that the psychometric properties of the 6MWT havebeen well researched and established, and we have recom-mended the 6MWT as the test of choice for clinical andresearch use.7

An outdoor 6MWT test is more reflective of real life and isnot as artificial as the conventional 6MWT, in which an indoorcorridor, free of distractions, is used. One of the objectives ofthis study was to compare performance on the indoor and onthe outdoor walk tests. The data indicate that the 6MWTindoors is strongly correlated to the test performed outdoors.This finding will enable health care professionals to use theresults of indoor performance to predict outdoor performanceand vice versa. Additionally, the results of this study will allowhealth care professionals to use the 6MWT in the community(ie, during a home visit), to measure functional status and tomonitor treatment effectiveness, when an indoor corridor is notavailable.

The perception of individuals with COPD of the effect of theenvironment on functional capacity has not been investigated.Despite no difference between performance indoors and out-doors, slightly more subjects preferred the indoor setting (36%)

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

Fig 2. Comparison of change in perceived dyspnea and leg effortratings when walking indoors and outdoors. Open circles representindividual data points. Solid squares represent mean � SD. Thereare several individual points at 0 that are reflected by 1 data pointonly.

Table 1: Characteristics of the Subjects (N�18)

Mean � SD (Range)

Age (y) 70�8 (52–81)FEV1 (L) 1.0�0.3 (0.5–1.5)FEV1 (% predicted) 42�8 (28–42)Sex 10 men, 8 womenOxygen use (L) (n�5) 2.9�0.7 (2–4)

Table 2: Summary of Climatic Variables for the Outdoor Tests

Climatic Variables Mean � SD (Range)

Temperature (°C) 21�3 (16–25.5)“Apparent” temperature (°C) 25�4 (17–32)Wind speed (km/h) 15�7 (5–33)

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to the outdoor setting (22%). Temperature was the main factorperceived as influencing ability to walk outside.

The finding that the distance walked outdoors did not cor-relate with PFSDQ-M scores was not surprising. The 2 mea-sures provide different information on functional capacity: thePFSDQ-M examines various ADLs (1 component of which iswalking), and the 6MWT assesses the ability to walk. How-ever, our sample included individuals at a high level of func-tion, as shown by the low scores on the PFSDQ-M and indoor6MWT distances greater than 300m. Studies have indicatedthat 300m on the 6MWT is a threshold for level of disabilityamong individuals with respiratory disease, including thosewith COPD.15-17 Thus, it is possible that individuals withgreater disability (eg, 6MWT distances �300m) may be influ-enced by the outdoor environment.

Two studies18,19 have reported distances walked in 6 minutesby healthy subjects. In the healthy elderly (aged 60–65y), thedistances walked in 6 minutes ranged from 494 to 631m. Thesevalues are higher than those observed in our sample, in whichthe distances walked in 6 minutes were between 390 and 400m.Therefore, although our sample was high functioning com-pared with others with COPD, they had more disability than ahealthy population.

Limited ranges of climatic parameters were used in thisstudy. Future studies will need to investigate the effect of abroader range of environmental factors (eg, more extremeconditions, ie, colder temperature, higher winds, pollution hu-midity levels, and different terrains) and walking aids on out-door performance, to expand our understanding of the feasi-bility and validity of using the 6MWT as a measure offunctional status in an outdoor setting.

CONCLUSIONThe data indicate that the 6MWT performed outdoors within

reasonable climatic parameters may be reflective of 6MWTperformance indoors. Therefore, an outdoor 6MWT may be analternative option to the traditional 6MWT for use in thecommunity when an indoor corridor is not available. Future

studies are needed to increase the generalizability of thesefindings.

References1. Guyatt GH, Sullivan MJ, Thompson PJ, et al. The 6-minute walk:

a new measure of exercise capacity in patients with chronic heartfailure. Can Med Assoc J 1985;132:919-23.

2. Guyatt GH, Thompson PJ, Berman LB, et al. How should wemeasure 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 pa-tients with chronic obstructive pulmonary disease. Cleve ClinJ Med 1993;60:119-28.

4. Bittner V. Six-minute walk test in patients with cardiac dysfunc-tion. Cardiologia 1997;42:897-902.

5. Redelmeier DA, Bayoumi AM, Goldstein RS, Guyatt GH. Inter-preting small differences in functional status: the six-minute walktest in chronic lung disease patients. Am J Respir Crit Care Med1997;155:1278-82.

6. Brooks D, Lacasse Y, Goldstein RS. Pulmonary rehabilitationprograms in Canada: national survey. Can Respir J 1999;6:55-63.

7. Solway S, Brooks D, Lacasse Y, Thomas S. A qualitative systematicoverview of the measurement properties of functional walk tests usedin the cardiorespiratory domain. Chest 2001;119:256-70.

8. American Thoracic Society. Standards for the diagnosis and careof patients with chronic obstructive pulmonary disease. Am JRespir 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 encourage-ment on walking test performance. Thorax 1984;39:818-22.

11. Borg GA. Psychophysical bases of perceived exertion. Med SciSports Exerc 1982;14:377-81.

12. Lareau SC, Carrieri-Kohlman V, Janson-Bjerklie S, Roos PJ.Development and testing of the Pulmonary Functional Status andDyspnea Questionnaire (PFSDQ). Heart Lung 1994;23:242-50.

13. Lareau SC, Meek PM, Roos PJ. Development and testing of amodified version of the pulmonary status and dyspnea question-naire. Heart Lung 1998;27:159-68.

14. Lareau SC, Meek PM. Testing of a modified version of thepulmonary 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 ofa rollator on functional exercise capacity among individuals withsevere COPD. Chest 2002;122:56-65.

16. Honeyman P, Barr P, Stubbing DG. Effect of a walking aid ondisability, oxygenation, and breathlessness in patients withchronic 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. Lancet1994;344:1394-7.

18. Troosters T, Gosselink R, Decramer M. Six minute walkingdistance in healthy elderly subjects. Eur Respir J 1999;14:270-4.

19. Enright PL, Sherrill DL. Reference equations for the six-minutewalk in healthy adults. Am J Respir Crit Care Med 1998;158:1384-7.

Suppliersa. 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.

Table 3: Actual Values for Modified Borg Ratings of Perceived Dyspnea and Leg Effort Before and After 6MWT Indoors and Outdoors

PerceivedRating

Day 1 (mean � SD) Day 2 (mean � SD)

Indoors Outdoors Indoors Outdoors

Before After Before After Before After Before After

Dyspnea 0.5�0.8 2.7�1.9 0.8�0.8 3.0�2.3 0.5�0.5 2.9�1.8 0.5�0.7 3.0�2.2Leg effort 0.5�0.9 1.6�1.3 0.3�0.4 1.4�1.3 0.4�0.5 1.4�1.3 0.4�0.6 1.1�0.9

Fig 3. Mean oxygen saturation and heart rate during the 6MWTwhen walking indoors and outdoors. Open circles represent indi-vidual data points. Solid squares represent mean � SD. Note thatthe y axis does not start at zero.

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