Abstract
BACKGROUND:
It is important to include large sample sizes and different factors that influence the six-minute walking distance (6MWD) in order to propose reference equations for the six-minute walking test (6MWT).
OBJECTIVE:
To evaluate the influence of anthropometric, demographic, and physiologic variables on the 6MWD of healthy subjects from different regions of Brazil to establish a reference equation for the Brazilian population.
METHOD:
In a multicenter study, 617 healthy subjects performed two 6MWTs and had their weight, height, and body mass index (BMI) measured, as well as their physiologic responses to the test. Delta heart rate (∆HR), perceived effort, and peripheral oxygen saturation were calculated by the difference between the respective values at the end of the test minus the baseline value.
RESULTS:
Walking distance averaged 586±106m, 54m greater in male compared to female subjects (p<0.001). No differences were observed among the 6MWD from different regions. The quadratic regression analysis considering only anthropometric and demographic data explained 46% of the variability in the 6MWT (p<0.001) and derived the equation: 6MWDpred=890.46-(6.11×age)+(0.0345×age2)+(48.87×gender)-(4.87×BMI). A second model of stepwise multiple regression including ∆HR explained 62% of the variability (p<0.0001) and derived the equation: 6MWDpred=356.658-(2.303×age)+(36.648×gender)+(1.704×height)+(1.365×∆HR).
CONCLUSION:
The equations proposed in this study, especially the second one, seem adequate to accurately predict the 6MWD for Brazilians.
walking; exercise test; reference values; regression analysis; rehabilitation
Introduction
The ability to walk a distance is an easy and inexpensive way to assess physical
capacity in health and illness. Among the field tests proposed in the literature, the
6-minute walk test (6MWT) has been widely used to assess the functional exercise
capacity of patients with limiting conditions such as cardiopulmonary diseases11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
. The test is simple, safe and provides a global and integrated response of all
systems involved during exercise based on the distance walked in a level corridor during
6 minutes (6MWD)11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
,
22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
. The 6MWT has a submaximal design22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
and, because most activities of daily living are performed at submaximal levels,
it is a good reflection of the functional exercise level for daily physical
activities11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
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,
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. The distance covered during the test has been used to assess response to
therapeutic interventions (pharmacological and non-pharmacological)44. Puhan MA, Mador MJ, Held U, Goldstein R, Guyatt GH, Schünemann HJ.
Interpretation of treatment changes in 6-minute walk distance in patients with COPD.
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.
According to the guidelines of the American Thoracic Society (ATS)11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
, the interpretation of the 6MWT should ideally be done by considering age,
height, weight, and gender, variables which independently affect the 6MWT in healthy
adults. Thus, equations for predicting reference values of the 6MWT are necessary. In
fact, some reference equations have been proposed previously for people from different
countries, including Brazilian samples22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
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,
99. Enright PL, Sherrill DL. Reference equations for the six-minute walk
in healthy adults. Am J Respir Crit Care Med. 1998;158(5):1384-7. PMid:9817683.
http://dx.doi.org/10.1164/ajrccm.158.5.9710086
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10. Gibbons WJ, Fruchter N, Sloan S, Levy RD. Reference values for a
multiple repetition 6-minute walk test in healthy adults older than 20 years. J
Cardiopulm Rehabil. 2001;21(2):87-93. PMid:11314289.
http://dx.doi.org/10.1097/00008483-200103000-00005
http://dx.doi.org/10.1097/00008483-20010...
11. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
12. Poh H, Eastwood PR, Cecins NM, Ho KT, Jenkins SC. Six-minute walk
distance in healthy Singaporean adults cannot be predicted using reference equations
derived from Caucasian populations. Respirology. 2006;11(2):211-6. PMid:16548908.
http://dx.doi.org/10.1111/j.1440-1843.2006.00820.x
http://dx.doi.org/10.1111/j.1440-1843.20...
13. Chetta A, Zanini A, Pisi G, Aiello M, Tzani P, Neri M, et al.
Reference values for the 6-min walk test in healthy subjects 20-50 years old. Respir
Med. 2006;100(9):1573-8. PMid:16466676. http://dx.doi.org/10.1016/j.rmed.2006.01.001
http://dx.doi.org/10.1016/j.rmed.2006.01...
14. Alameri H, Al-Majed S, Al-Howaikan A. Six-min walk test in a healthy
adult Arab population. Respir Med. 2009;103(7):1041-6. PMid:19230640.
http://dx.doi.org/10.1016/j.rmed.2009.01.012
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15. Soares MR, Pereira CA. Six-minute walk test: reference values for
healthy adults in Brazil. J Bras Pneumol. 2001;37(5):576-83.
16. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
-
1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
. However, aspects that needed to be better considered in future studies were
identified1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
,
1212. Poh H, Eastwood PR, Cecins NM, Ho KT, Jenkins SC. Six-minute walk
distance in healthy Singaporean adults cannot be predicted using reference equations
derived from Caucasian populations. Respirology. 2006;11(2):211-6. PMid:16548908.
http://dx.doi.org/10.1111/j.1440-1843.2006.00820.x
http://dx.doi.org/10.1111/j.1440-1843.20...
,
1616. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
,
1818. Dourado VZ. Reference equations for the 6-minute walk test in
healthy individuals. Arq Bras Cardiol. 2011;96:e128-38.
http://dx.doi.org/10.1590/S0066-782X2011005000024
http://dx.doi.org/10.1590/S0066-782X2011...
. Firstly, it is essential that new studies be performed with larger sample
sizes1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
,
1616. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
,
1818. Dourado VZ. Reference equations for the 6-minute walk test in
healthy individuals. Arq Bras Cardiol. 2011;96:e128-38.
http://dx.doi.org/10.1590/S0066-782X2011005000024
http://dx.doi.org/10.1590/S0066-782X2011...
. Secondly, since the age range is limited in some of the studies22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
,
1313. Chetta A, Zanini A, Pisi G, Aiello M, Tzani P, Neri M, et al.
Reference values for the 6-min walk test in healthy subjects 20-50 years old. Respir
Med. 2006;100(9):1573-8. PMid:16466676. http://dx.doi.org/10.1016/j.rmed.2006.01.001
http://dx.doi.org/10.1016/j.rmed.2006.01...
,
1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
, not all equations are applicable to young people. Finally, although the
interference of physiologic variables (e.g., heart rate) in the 6MWD has been suggested
and considered important1212. Poh H, Eastwood PR, Cecins NM, Ho KT, Jenkins SC. Six-minute walk
distance in healthy Singaporean adults cannot be predicted using reference equations
derived from Caucasian populations. Respirology. 2006;11(2):211-6. PMid:16548908.
http://dx.doi.org/10.1111/j.1440-1843.2006.00820.x
http://dx.doi.org/10.1111/j.1440-1843.20...
,
1313. Chetta A, Zanini A, Pisi G, Aiello M, Tzani P, Neri M, et al.
Reference values for the 6-min walk test in healthy subjects 20-50 years old. Respir
Med. 2006;100(9):1573-8. PMid:16466676. http://dx.doi.org/10.1016/j.rmed.2006.01.001
http://dx.doi.org/10.1016/j.rmed.2006.01...
,
1818. Dourado VZ. Reference equations for the 6-minute walk test in
healthy individuals. Arq Bras Cardiol. 2011;96:e128-38.
http://dx.doi.org/10.1590/S0066-782X2011005000024
http://dx.doi.org/10.1590/S0066-782X2011...
, few studies have considered these variables in reference equations.
Given that Brazil is a large country with different climates and diverse socioeconomic
and cultural conditions, a multicenter study that can provide samples from different
regions of the country is vital to establishing a more representative reference equation
for the 6MWT1818. Dourado VZ. Reference equations for the 6-minute walk test in
healthy individuals. Arq Bras Cardiol. 2011;96:e128-38.
http://dx.doi.org/10.1590/S0066-782X2011005000024
http://dx.doi.org/10.1590/S0066-782X2011...
. Therefore, taking into consideration the importance of the 6MWT in clinical and
research settings, by circumventing some of the current limitations, we aimed to study
the influence of anthropometric, demographic, and physiologic variables on the 6MWT of a
large, multicenter sample of healthy Brazilian subjects and to establish an equation for
predicting reference values of the 6MWT for the Brazilian population.
Method
Subjects
In the present multicenter study, although a sample size of 328 subjects was calculated to consider four variables in the regression model, 629 healthy subjects from four centers located in the Northeastern (1), Southeastern (2), and Southern (1) regions of Brazil were included. They were recruited from the local community and among students and employees of four universities, as well as their relatives. All of them had their anthropometric and demographic characteristics assessed and performed two 6MWTs. Data were collected from July 2008 to July 2011. The study was approved by the Research Ethics Committee of Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG, Brazil (ETIC approval number 390-04). All participants gave written informed consent.
The inclusion criteria were: subjects of both genders aged 18 years or more; ability to understand and perform all procedures proposed; absence of any severe and/or unstable disease which could limit exercise tolerance, such as chronic obstructive pulmonary disease, asthma, cystic fibrosis, interstitial lung disease, angina, myocardial infarction, congestive heart failure, stroke, transient ischemic attack, peripheral vascular disease, and arthritis1919. Enright PL. The six-minute walk test. Respir Care. 2003;48(8):783-5. PMid:12890299.. Subjects were excluded if they had a body mass index (BMI) under 18kg.m-2 and above 40kg.m-2 or if they could not perform two 6MWTs for any reason.
6-minute walk test
In all centers, subjects performed two 6MWTs according to the ATS standardized
protocol11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
with, at least, 30 min of rest between them. The test was performed in a 30-m
corridor, and the subjects were instructed to keep walking for 6 minutes. The best
walked distance was considered for analysis. The tests were applied by a physical
therapist or previously trained physical therapy student, and both 6MWTs were applied
by the same assessor. Blood pressure, heart rate (HR), peripheral oxygen saturation
(SpO2), and perceived dyspnea and leg fatigue (modified Borg
scale)2020. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports
Exerc. 1982;14(5):377-81. PMid:7154893.
http://dx.doi.org/10.1249/00005768-198205000-00012
http://dx.doi.org/10.1249/00005768-19820...
were assessed before and immediately after the tests. During the tests, HR
and SpO2 were continuously verified and standardized phrases of
encouragement were spoken to the participants every minute11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
(ATS, 2002). The predicted maximal HR (HRmax) was derived from the formula
HRmax=220-age2121. Karvonen MJ, Kentala E, Mustala O. The effects of training on heart
rate: a longitudinal study. Ann Méd Exper Fenn. 1957;35(3):307-15.. The delta of HR (∆HR), perceived effort (∆Borg), and SpO2
(∆SpO2) were calculated by the difference between the respective value
at the end of the test minus the baseline value.
Anthropometric and demographic data and health status
A questionnaire was applied in order to investigate the health status of the participants. Height (cm) and body weight (Kg) were measured, and the BMI was calculated.
Statistical analysis
The statistical analysis was performed using the statistical packages SPSS 17.0 (SPSS Inc., USA) and STATA 12.0 (STATA Corp, USA). The normality of data distribution was evaluated using the Kolmogorov-Smirnov test. The Mann-Whitney test or the independent Student t test was used to compare male and female characteristics. The comparisons between regions were assessed using the Kruskall-Wallis test followed by post-hoc Mann-Whitney, when appropriate. Spearman's correlation coefficient was used to verify the simple correlation between the walked distance (dependent variable) and age, gender, weight, height, BMI, SpO2, HRmax % predicted, and ∆HR. Two regression models were applied to derive reference equations both considering the association between the variables and the distance walked in the 6MWT. The best models were constructed considering the variables with the best independent coefficient of determination (R2) and statistical significance as p<0.05. The first model included only demographic and anthropometric variables as independent variables (age, gender, weight, height, and BMI). In a stepwise multiple linear regression model, the assumptions of normality of the residuals score were not met, so a quadratic regression model (Equation 1) was used. The second model, a stepwise multiple linear regression model, included the same independent variables used in the first model in addition to physiologic variables (SpO2, HRmax % predicted, ∆HR) and the normality of the residuals score was met (Equation 2). Since equation 2 showed a much higher coefficient of determination (see results), further analysis focused on this specific equation. In order to verify the reliability of Equation 2, it was applied a posteriori analysis in a different group of healthy subjects, composed of 58 individuals selected according to the same inclusion criteria in the different centers. In addition, Equation 2 was cross-validated against prediction equations existing in the literature in this group, considering only the subjects with the same age range of each study. The level of statistical significance was considered as p<0.05.
6MWDpred=890.46–(6.11xage)+(0.0345xage2)+(48.87xgender)–(4.87xBMI) (where male gender = 1 and female gender=0).
6MWDpred=356.658–(2.303xage)+(36.648xgender)+(1.704xheight)+(1.365xΔHR) (where male gender=1 and female gender=0).
Results
From the 629 healthy subjects selected, 12 (2%) were excluded and 617 were included in the final analysis. The subjects were aged 19 to 79 years and 52% were women. Table 1 shows descriptive data on characteristics of the studied subjects and outcome parameters of the best 6MWT. The participants were distributed by age range as follows: 19-29 years (n=155); 30-39 years (n=66); 40-49 years (n=77); 50-59 years (n=83); 60-69 years (n=110), and 70-79 years (n=126). According to the BMI classification of body composition (kg/m2), 47.1% were normal (18.5 to 24.9), 37.9% were overweight (25.0 to 29.9), and 14.9% were obese (30.0 to 39.9).
Comparison between regions
Table 2 shows that, despite the differences in anthropometric parameters observed among the 3 different regions, the 6MWD was similar.
6MWT determinants and reference equations
There were significant correlations between the walked distance and age (r=0.55, p<0.0001), height (r=0.48, p<0.0001), BMI (r=0.40, p<0.0001), and ∆HR (r=0.43, p<0.0001). A model of quadratic regression analysis showed that age, gender, and BMI explained 46% of the variability in the 6MWT (R2=0.46, p<0.0001; Table 3). The derived equation (Equation 1) for the 6MWT based on this analysis considering only anthropometric and demographic data was:
6MWDpred=890.46-(6.11Xage)+(0.0345Xage2)+(48.87xgender)-(4.87xBMI)
(where male gender = 1 and female gender=0).
The second model of stepwise multiple regression showed that age, gender, height, and ∆HR explained 62% (R2=0.616, p<0.0001) of the variability in the 6MWT (Table 4). The derived equation (Equation 2) for the distance walked in the 6MWT was:
6MWDpred=356.658-(2.303xage)+(36.648xgender)+(1.704xheight)+(1.365xΔHR)
(where male gender=1 and female gender=0).
Reliability of equation 2 and comparison with other reference equations
The characteristics of the other group composed of 58 healthy subjects (23 male and 35 female) included in the a posteriori analysis were: age 52±15 years and BMI 26±4 kg/m2. When equation 2 was applied in this group, there was no difference between the actual and the predicted 6MWD (565 m vs. 582 m, p=0.11, respectively) with a significant correlation between them (r=0.76, p<0.0001).
When previous reference equations published in the literature were applied in the
a posteriori analysis, positive and significant correlations with
the walked distance by the 58 subjects were observed (Table 5). The comparison indicated differences in four of
them22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
,
1010. Gibbons WJ, Fruchter N, Sloan S, Levy RD. Reference values for a
multiple repetition 6-minute walk test in healthy adults older than 20 years. J
Cardiopulm Rehabil. 2001;21(2):87-93. PMid:11314289.
http://dx.doi.org/10.1097/00008483-200103000-00005
http://dx.doi.org/10.1097/00008483-20010...
,
1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
,
1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
.
Discussion
This study presented new equations to predict the distance covered during the 6MWT in
Brazilian subjects and showed that approximately 62% of the variance was explained by
gender, age, height, and the change in heart rate (∆HR) during the test. When no
physiologic variables were entered into the logistic analysis, approximately 47% of the
variance was explained by the combination of gender, age, age22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
, and BMI.
The present study has two strong methodological characteristics. Firstly, to the best of our knowledge, this is the study with the largest sample to propose a reference equation for the 6MWT in this population. Secondly, this study had a multicenter design and was therefore the first to have a sample composed of subjects from three regions of the country.
The sample was well distributed according to age, considering the classification of the
American College of Sports Medicine2222. American College of Sports Medicine Position Stand. Position Stand
on The Recommended Quantity and Quality of Exercise for Developing and Maintaining
Cardiorespiratory and Muscular Fitness, and Flexibility in Adults. Med Sci Sports
Exerc. 1998;30(6):975-91. PMid:9624661.
http://dx.doi.org/10.1097/00005768-199806000-00032
http://dx.doi.org/10.1097/00005768-19980...
, with 221 (35.8%) young (<39yr), 200 (32.4%) middle-aged (40-64yr), and 196
(31.8%) elderly (65-79yr) subjects. The number of overweight and obese subjects (levels
I and II) is similar to the one identified in the Brazilian population2323. Instituto Brasileiro de Geografia e Estatística. Sala de Imprensa.
Available from:
http://saladeimprensa.ibge.gov.br/noticias?view=noticia&id=1&busca=1%idnotica=1699.
Available from:
http:/...
. Thus, the sample reflects the characteristics and variations as they exist in
the real population, increasing the external validity. Previous studies have reported
the influence of age1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
,
2020. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports
Exerc. 1982;14(5):377-81. PMid:7154893.
http://dx.doi.org/10.1249/00005768-198205000-00012
http://dx.doi.org/10.1249/00005768-19820...
,
2424. Pires SR, Oliveira AC, Parreira VF, Britto RR. Six-minute walk test
at different ages and body mass index. Rev Bras Fisioter.
2007;11(2):147-51. and BMI on the 6MWD1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
,
1616. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
,
2424. Pires SR, Oliveira AC, Parreira VF, Britto RR. Six-minute walk test
at different ages and body mass index. Rev Bras Fisioter.
2007;11(2):147-51..
The results indicated differences between the anthropometric characteristics of subjects
from different regions. However, it was not sufficient to produce differences in walking
distance when the three regions were compared. Region-to-region difference occurred when
comparing the Northeast to the other two regions, although this difference was probably
due to the younger age of this specific subgroup. Taking into consideration the absence
of differences (Table 4), this indicates the
possibility of generalizing the 6MWD reference equations for all Brazilian regions, at
least in studies where a large age-range is considered1515. Soares MR, Pereira CA. Six-minute walk test: reference values for
healthy adults in Brazil. J Bras Pneumol. 2001;37(5):576-83.
,
1616. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
.
The influence of the relative intensity of the test represented by the ∆HR was
identified as an important variable with 41% of partial correlation in Equation 2. The study by Chetta et al.1313. Chetta A, Zanini A, Pisi G, Aiello M, Tzani P, Neri M, et al.
Reference values for the 6-min walk test in healthy subjects 20-50 years old. Respir
Med. 2006;100(9):1573-8. PMid:16466676. http://dx.doi.org/10.1016/j.rmed.2006.01.001
http://dx.doi.org/10.1016/j.rmed.2006.01...
mentioned the importance of considering the heart rate during the 6MWT, however
this variable was not included in the reference equation proposed by those authors. Poh
et al.1212. Poh H, Eastwood PR, Cecins NM, Ho KT, Jenkins SC. Six-minute walk
distance in healthy Singaporean adults cannot be predicted using reference equations
derived from Caucasian populations. Respirology. 2006;11(2):211-6. PMid:16548908.
http://dx.doi.org/10.1111/j.1440-1843.2006.00820.x
http://dx.doi.org/10.1111/j.1440-1843.20...
considered the % of predicted HRmax in the equation as well as age, height, and
weight and explained 78% of the variance of the 6MWD, despite the small sample size
(n=35). The authors considered that the use of the % of predicted HRmax in the equation
may be limited when measuring the 6MWD in subjects with diseases or medications which
have an influence on HRmax. However, the use of the change in HR during the test (∆HR)
could, at least in part, counteract this limitation. This may happen since these
diseases and medications interfere not only in the HR at the end of the test but also in
the resting HR, and therefore their influence on this specific outcome (∆HR) may be
counterbalanced and reduced.
A recent international multicenter study evaluated the geographic variations of the 6MWD
in a cohort of healthy adults and suggested that the resulting distance could be
influenced by factors such as the speed of habitual walking and other aspects related to
lifestyle such as the motivation of the subject and/or the assessor2525. Casanova C, Celli BR, Barria P, Casas A, Cote C, De Torres JP, et
al. The 6-min walk distance in healthy subjects: reference standards from seven
countries. Eur Respir J. 2011;37(1):150-6. PMid:20525717.
http://dx.doi.org/10.1183/09031936.00194909
http://dx.doi.org/10.1183/09031936.00194...
. These aspects were not evaluated in the present study, however they could
influence the ∆HR. As the 6MWT is self-paced, the use of the ∆HR considers the freedom
of the subjects to choose the speed, despite the standardized instructions and
encouragement. Furthermore, the influence of encouragement on the subject's performance
has been discussed since 1984 by Guyatt et al.2626. Guyatt GH, Pugsley SO, Sullivan MJ, Thompson PJ, Berman L, Jones NL,
et al. Effect of encouragement on walking test performance. Thorax.
1984;39(11):818-22. PMid:6505988 PMCid:PMC459930.
http://dx.doi.org/10.1136/thx.39.11.818
http://dx.doi.org/10.1136/thx.39.11.818...
, and the ATS Guidelines (2002)11. ATS Committee on Proficiency Standards for Clinical Pulmonary
Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J
Respir Crit Care Med. 2002;166(1):111-7. PMid:12091180.
http://dx.doi.org/10.1164/ajrccm.166.1.at1102
http://dx.doi.org/10.1164/ajrccm.166.1.a...
consider it an important determinant of the distance covered in the 6MWT.
Nevertheless, motivation involves two aspects: extrinsic and intrinsic motivation2727. Markland D, Tobin V. A modification to the behavioural regulation in
exercise questionnaire to include an assessment of amotivation. J Sport Exerc
Psychol. 2004;26(2):191-6.. Standardizing motivation phrases is a way to control the extrinsic factor,
which produces different responses in subjects depending on their intrinsic motivation.
The ∆HR could indicate how the subjects respond to the extrinsic motivation during the
6MWT. This self-determination theory has been used to evaluate the response of changes
in exercise training behavior2626. Guyatt GH, Pugsley SO, Sullivan MJ, Thompson PJ, Berman L, Jones NL,
et al. Effect of encouragement on walking test performance. Thorax.
1984;39(11):818-22. PMid:6505988 PMCid:PMC459930.
http://dx.doi.org/10.1136/thx.39.11.818
http://dx.doi.org/10.1136/thx.39.11.818...
but has not been evaluated in self-regulated exercise tests. All of these
factors help to explain, at least in part, the positive role of including Î"HR in the
prediction of the 6MWD's normal values. Recently, the use of rest HR was considered to
predict maximal oxygen consumption based on the 6MWT2828. Burr JF, Bredin SS, Faktor MD, Warburton DE. The 6-minute walk test
as a predictor of objectively measured aerobic fitness in healthy working-aged
adults. Phys Sportsmed. 2011;39(2):133-9. PMid:21673494.
http://dx.doi.org/10.3810/psm.2011.05.1904
http://dx.doi.org/10.3810/psm.2011.05.19...
.
Equation 1, which considered only anthropometric and demographic variables, explained almost half of the variance in the 6MWT. This showed that age, gender, and BMI are important to predict the 6MWD in agreement with the vast majority of the studies designed to propose reference equations for this test. It is, therefore, an option to be used in case it is not possible to obtain the Î"HR and use Equation 2.
The comparison with other studies allowed us to identify that the equations proposed by
Troosters et al.22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
, Gibbons et al.1010. Gibbons WJ, Fruchter N, Sloan S, Levy RD. Reference values for a
multiple repetition 6-minute walk test in healthy adults older than 20 years. J
Cardiopulm Rehabil. 2001;21(2):87-93. PMid:11314289.
http://dx.doi.org/10.1097/00008483-200103000-00005
http://dx.doi.org/10.1097/00008483-20010...
, Camarri et al.1111. Camarri B, Eastwood PR, Cecins NM, Thompson PJ, Jenkins S. Six
minute walk distance in healthy subjects aged 55-75 years. Respir Med.
2006;100(4):658-65. PMid:16229997. http://dx.doi.org/10.1016/j.rmed.2005.08.003
http://dx.doi.org/10.1016/j.rmed.2005.08...
, and Dourado et al.1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
predicted 6MWD with a statistical difference in comparison to the distance
observed in the present study. These discrepancies could be related to differences in
the test protocol, encouragement, and different aspects of motivation, as previously
discussed. Sample size22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
,
33. Simões LA, Dias JM, Marinho KC, Pinto CL, Britto RR. Relationship
between functional capacity assessed by walking test and respiratory and lower limb
muscle function in community-dwelling elders. Rev Bras Fisioter. 2010;14(1):24-30.
PMid:20414558. http://dx.doi.org/10.1590/S1413-35552010000100005
http://dx.doi.org/10.1590/S1413-35552010...
,
1010. Gibbons WJ, Fruchter N, Sloan S, Levy RD. Reference values for a
multiple repetition 6-minute walk test in healthy adults older than 20 years. J
Cardiopulm Rehabil. 2001;21(2):87-93. PMid:11314289.
http://dx.doi.org/10.1097/00008483-200103000-00005
http://dx.doi.org/10.1097/00008483-20010...
,
1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
, combined with different characteristics of each population22. Troosters T, Gosselink R, Decramer Ml. Six minute walking distance in
healthy elderly subjects. Eur Respir J. 1999;14(2):270-4. PMid:10515400.
http://dx.doi.org/10.1034/j.1399-3003.1999.14b06.x
http://dx.doi.org/10.1034/j.1399-3003.19...
,
33. Simões LA, Dias JM, Marinho KC, Pinto CL, Britto RR. Relationship
between functional capacity assessed by walking test and respiratory and lower limb
muscle function in community-dwelling elders. Rev Bras Fisioter. 2010;14(1):24-30.
PMid:20414558. http://dx.doi.org/10.1590/S1413-35552010000100005
http://dx.doi.org/10.1590/S1413-35552010...
,
1010. Gibbons WJ, Fruchter N, Sloan S, Levy RD. Reference values for a
multiple repetition 6-minute walk test in healthy adults older than 20 years. J
Cardiopulm Rehabil. 2001;21(2):87-93. PMid:11314289.
http://dx.doi.org/10.1097/00008483-200103000-00005
http://dx.doi.org/10.1097/00008483-20010...
, could also contribute to this discrepancy. The equation proposed by Iwama et
al.1616. Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The
six-minute walk test and body weight-walk distance product in healthy Brazilian
subjects. Braz J Med Biol Res. 2009;42(11):1080-5. PMid:19802464.
http://dx.doi.org/10.1590/S0100-879X2009005000032
http://dx.doi.org/10.1590/S0100-879X2009...
for the Brazilian population, based on the same age range and a sample of 134
subjects, explained only 30% of the variation in the 6MWD. Dourado et al.1717. Dourado VZ, Vidotto MC, Guerra RL. Reference equations for the
performance of healthy adults on field walking tests. J Bras Pneumol.
2011;37(5):607-14. PMid:22042392.
http://dx.doi.org/10.1590/S1806-37132011000500007
http://dx.doi.org/10.1590/S1806-37132011...
also found similar results, explaining 54.3% of the 6MWD variance in a model
based on the age, weight, height, and gender of 90 healthy adults, not including young
subjects. Based on 132 volunteers (also 20-80 years of age) better distributed using the
adjusted quadratic model, Soares and Pereira1515. Soares MR, Pereira CA. Six-minute walk test: reference values for
healthy adults in Brazil. J Bras Pneumol. 2001;37(5):576-83. proposed a model based on height, age, and BMI and explained 55% of the variance
in the 6MWD.
Despite a larger sample, our first equation, based only on anthropometric and demographic data, explained 46% of the variance. This relatively low coefficient of determination can be partially explained by the difference in anthropometric variables between different regions included in the study, without difference between 6MWD values. On the other hand, Equation 2 (including HR) explained 62% of the variance in the 6MWD, well above the previous equations. Despite the fact that this physiological variable is a result of the work performed in the test, it can contribute to the understanding of the expected performance for individuals with different anthropometric characteristics and, particularly, in different age ranges. Thus, at present, Equation 2 seems to stand out as superior (i.e. has fewer limitations) for accurately predicting the 6MWD for Brazilians.
The 6MWT is frequently used in physical therapy routine. The ATS recommends the assessment of HR during the test. In this way, the use of Equation 2 could help determine whether changes in the 6MWD are also related to intrinsic motivation, which is an open field of investigation in regard to self-paced tests.
Given that 38.4% (Equation 2) and 53.5% (Equation 1) of the variance could not be explained by these models, future studies could explore the contribution of other behavioral and/or physiologic factors. The repeatability of the 6MWT with the same subject in a sequence of days could contribute to evaluate the influence of HR and intrinsic motivation behavior on 6MWD variation. It would be also interesting to assess the subjects' level of physical activity. This information could be evaluated as an independent variable and/or used to better identify the subjects' HR response.
Limitation
Although this was a multicenter study, the sample was not randomized. However, we evaluated volunteers from different centers, which can, at least in part, guarantee the diversification of the sample.
Conclusion
In summary, the present study showed that the distance covered during six minutes can be better explained and predicted when considering the HR changes during the test in addition to the anthropometric parameters. Furthermore, we have proposed two new reference equations, one of them including the heart rate changes during the test. These equations, especially the second, seem to be adequate to accurately predict the 6MWD for Brazilians.
Acknowledgments
The authors would like to thank the undergraduate students who helped with data collection and Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq, Brazil (process 77137/2008-3) and Fundação de Amparo a Pesquisa do Estado de Minas Gerais-FAPEMIG, Brazil (PPM00072-09) for their financial support.
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Publication Dates
-
Publication in this collection
Nov 2013 -
Date of issue
Dec 2013
History
-
Received
29 Apr 2013 -
Accepted
07 June 2013