Abstract
BACKGROUND: Six-minute step test (6MST) has been used to assess functional capacity in chronic conditions; however, its reproducibility in the COPD population has not been evaluated. Our study objective was to evaluate 6MST reproducibility.
METHODS: The test was performed in a single 20-cm height step, and subjects were instructed to step up and down (cadence-free) for 6 min. Subjects underwent three tests. The first and second were controlled by the same assessor with a 30-min interval. The third test was controlled by a different assessor one week later. For intra-rater comparison, the first and second performances of the test were used, and for inter-assessor comparison, the better performance of the first two tests was compared with the third test.
RESULTS: Excellent intra-rater and inter-rater relative reproducibility was observed (intraclass correlation coefficient > 0.8), and there was no statistical difference (repeated measures of analysis of variance) among the performances of the three tests. Intra-rater error values were acceptable (mean error of 5.7 steps and limits of agreement between −7 and 18 steps). Inter-rater error values were not acceptable (mean error of 4.4 steps and limits of agreement between −20 and 29 steps.
CONCLUSIONS: 6MST proved to be reproducible in the COPD population when performed by the same assessor.
Introduction
COPD is characterized by chronic air-flow limitation, pulmonary function deterioration, significant weight loss, and decreased strength and endurance of respiratory muscles and lower and upper limbs,1 in addition to a decreased functional capacity and intolerance of physical efforts.2
Exercise intolerance in COPD subjects has important implications regarding quality of life, number of hospitalizations, and survival.3 Thus, functional physical tests have been considered to be essential components in the routine clinical assessment of exercise capacity in these subjects.3 The most accurate way of assessing physical state and determining the cause of exercise intolerance is maximal cardiopulmonary exercise testing3,4 with measurements of exhaled gases. However, the complexity of the equipment, the high operating costs, and the need for trained technicians make its use limited in clinical practice.5
Alternatives to the maximal tests are the 6-min walk6 and step7 tests because they are practical and easy to perform in a clinical routine.2,3,7 The walk test has good reproducibility, reliability, and low cost;4,6,8,9 is a predictor of morbidity and mortality;10,11 and reflects the exercise capacity of COPD subjects to perform the physical activities of daily living.12 However, the physical space required for its execution often limits its use.6,7 Moreover, one step test protocol, the 6-min step test (6MST), is considered to be a good alternative for assessing exercise capacity in chronic subjects.7,13
Recently, 6MST has been validated in subjects with interstitial lung disease.7 It was reproducible, safe, and sensitive to oxygen desaturation induced by exercise, proving it to be easy to use, economical, and portable.7 Although there are studies showing the physiological responses to 6MST,14,15 we did not find reproducibility studies of this test in subjects with COPD, justifying this study. Therefore, the objective of this study was to evaluate the reproducibility of 6MST in subjects with COPD. We hypothesized that 6MST would be reproducible in either intra-rater or inter-rater analysis, which would make it the perfect choice for use in large-sample analysis of exercise capacity.
QUICK LOOK
Current knowledge
The 6-min walk test is the standard test for evaluating exercise intolerance in patients with COPD. The 6-min step test has been used to assess functional capacity in chronic conditions; however, its reproducibility in COPD has not been evaluated.
What this paper contributes to our knowledge
The cadence-free 6-min step test is a reproducible test for the population of COPD subjects when performed by the same examiner.
Methods
Study Design and Participants
This study is part of a larger prospective, cross-sectional, observational study being conducted by the Spirometry and Respiratory Physiotherapy Laboratory at the Federal University of São Carlos in Brazil. The goal of this larger study was to verify the clinimetrics of 6MST and the 6-min walk test in three different populations (COPD patients, young adults, and elderly adults); the study was registered at clinicaltrials.gov (Identifier NCT01298661). Here, we have concentrated on the reproducibility of 6MST in patients with COPD.
The enrollment period for participants was January 2011 to June 2012, and the participants were invited using posters distributed in the university and its neighborhood, local radio and television, and newspapers. In addition, patients referred for treatment to the Special Unit of Respiratory Physiotherapy at the Federal University of São Carlos were invited to participate. All subjects willing to participate were asked via telephone if they matched the inclusion criteria for participation in the study. The inclusion criteria adopted were: subjects with clinical and spirometric COPD diagnosis16 who did not show associated diseases that would prevent them from performing the proposed tests. The exclusion criteria adopted were: subjects who had already performed 6MST before this study and those with COPD exacerbation. Moreover, subjects who could not complete the first assessment day were not included in any analysis, and those who missed the second assessment day were not included in the inter-rater reproducibility analysis. All accepted patients signed a consent form for the larger study, which was approved by the Human Ethics Committee of the university (decision 009/2011). The consent and the ethics committee-approved project included all of the objectives and measurements used here.
Experimental Procedure
Subject assessment was conducted in 2 days with a one-week interval. On the first day, the subjects' history and their anthropometric and spirometric characteristics were collected. On that same day, they performed two 6MSTs with a 30-min interval between them. On the second day, a body composition analysis and a third 6MST by a different assessor were performed.
6MST
Two assessors conducted each of the tests, one to command the test and the other to count the number of steps the subject climbed. A 20-cm height step was used as an ergometer.7 The same standardized verbal incentives for each 6-min walk test6 were used. Prior to the beginning of the test, the subjects received standard instructions, which are described in Figure 1.
Instructions given before six-minute step test.
Throughout the test, heart rate (Vantage NV, model 1901001, Polar, Kempele, Oulu, Finland) and pulse oximetry (Nonin, model 2500, Minneapolis, Minnesota) were monitored, and if the individual heart rate presented higher than the submaximal heart rate or if pulse oximetry was < 85%, the assessor asked the individual to rest until the heart rate was down by 10 beats/min under the submaximal heart rate or until pulse oximetry increased to 88% or higher. Submaximal heart rate (SubHR) was calculated using the following equations: SubHR (beats/min) = [220 − age (y)] × 0.85 for men and SubHR (beats/min) = [210 − age (y)] × 0.85 for women. The participant could also choose to stop the test to rest, but in either case, the chronometer was not stopped during the interruption.
Statistical Analysis
Initially, the normality of the data was verified using the Shapiro-Wilk test, and parametric data were expressed using mean ± SD. P values < .05 was considered as significant. To verify the intra-rater reproducibility, first and second 6MSTs were compared, and the inter-rater reproducibility was verified by comparison of the third test with the others and the choice of the better test.
Relative reproducibility was analyzed by the intraclass correlation coefficient (ICC), and reproducibility was considered to be excellent when the ICC values were higher than 0.75.17 Absolute reproducibility was verified by comparison of the tests using repeated measures analysis of variance, standard error of measurement (SEM = SD × √(1 − ICC)) and its 95% CI (SEM − 95% CI = SEM × 1.96), and minimal detectable difference (MDD = 1.64 × √(2) × SEM).18 Furthermore, the mean error and limits of agreement were calculated to construct Bland-Altman plots.
The sample size required to perform the proposed reliability analysis was at least 19 subjects,19 considering α < 0.05 and β < 0.2, two test repetitions, the null hypothesis of ICC < 0.7, and the expected hypothesis of ICC > 0.9. The expected ICC value used was consistent with the mean value of ICC found in 6 min walk test reproducibility studies: 0.88–0.94.20–22
Results
This study included 34 subjects, two of whom were excluded for not completing the first assessment day. Of the 32 subjects included in the analysis, 2 presented FEV1 > 80% of the predicted value, 9 presented FEV1 < 80% and > 50% of the predicted value, 15 presented FEV1 < 50% and > 30% of the predicted value, and 6 presented FEV1 < 30% of the predicted value.16 Sample characteristics are shown in Table 1. Three subjects did not appear on the second day of evaluation, so they were included only in the intra-rater analysis.
Demographic, Anthropometric, and Spirometric Characteristics of the 32 Evaluated Subjects with COPD
Intra-rater and inter-rater relative reproducibilities were excellent in COPD subjects (Table 2). There were no statistically significant differences among the tests (T1/T2/T1 or T2 and T3) (Table 3), which indicates that 6MST has absolute reproducibility as well. Error analysis, verified by a Bland-Altman plot (Fig. 2), the standard error of measurement, and the minimal detectable difference (Table 4), showed acceptable error in the intra-rater comparison. However, they were unacceptably high in the inter-rater comparison.
Relative Reproducibility of 6-min Step Test in COPD Subjects Evaluated
Performance on 6-min Step Test Assessed in Subjects with COPD
Reproducibility in COPD patients. Left: Intra-rater six-minute step tests. Mean error = 5.75 steps. Right: Inter-rater six-minute step test (best of first 2 tests). Mean error = 9.25 steps.
Absolute Reproducibility of 6-min Step Test assessed in subjects with COPD
Discussion
6MST is a reproducible test in COPD subjects when performed by the same assessor. Furthermore, it presents an excellent inter-rater relative reproducibility,17 but with high error values that are too high, which does not encourage the comparison of two 6MSTs performed by different assessors.
Relative reproducibility values (ICC) were > 0.8 in the intra-rater comparison, and there were no statistical differences in the first two tests (means of T1 = 76 and T2 = 82 steps). This indicates that, for this population, the test is reproducible with reproducibility values similar to those obtained using other clinical exercise tests, such as 6-min walk test, which presents an ICC of 0.88–0.94,20–22 and shuttle-walk test, which presents an ICC of 0.87.23
Moreover, the error value for this comparison was acceptable, presenting a mean error of 5.7 steps and limits of agreement ranging from −7.1 to 18.6 steps. Dal Corso and colleagues7 also concluded that this test is reproducible in subjects with interstitial pulmonary fibrosis with even lower error values (mean error of one step and limits of agreement ranging from −0.5 to 2.5 steps). Furthermore, standard error of measurement and the minimal detectable difference were 4.1 and 11 steps, respectively, which confirms previous results from a study24 in which 6MST was used to determine an 11-step difference between two groups of COPD subjects, one group of which was included in an aerobic physical training protocol and the other was not. Standard error of measurement as a percentage of the mean was 5%, which was similar to the same error analysis using the 6-min walk test in other studies that also represented ∼5% of the mean performance of the test.21,25
These results show that the performance in this test was not influenced by the learning effect in the COPD population, which leads to the affirmation that this test can be done without a familiarization test, spending only 15 min of assessment time. This characteristic could be an advantage over the 6-min walk test, which requires at least one previous test and a recovery time between them, totaling a time expenditure of at least 50 min.20 However, since the 6-min walk test has been largely used, and it has already been validated and related to morbidity/mortality in COPD subjects, we suggest that 6MST should substitute for the walk test only under certain circumstances. The 6MST should be used when there is not an adequate area in which to perform it or when the time is too short to allow a familiarization test or an adequate recovery interval because of large sample sizes.
Inter-rater relative reproducibility was also excellent, and there were no significant differences among the comparisons, but it presented higher limits of agreement and standard error of measurement and the minimal detectable difference, which were considered above acceptable. Thus, we believe that performances in two tests controlled by different assessors should not be compared. Taking this into consideration, questions should be raised whether other functional tests, such as the 6-min walk test, should be performed by different assessors.
The extrapolation of this study could be limited because of the use of a convenience sample. Moreover, subjects could already being in physical training, but since one week is not enough to change exercise capacity, one can affirm that it did not influence the differences between the first two tests and the third test.
Conclusions
We conclude that cadence-free 6MST is a reproducible test in the population of COPD subjects when performed by the same examiner.
Footnotes
- Correspondence: Joyce NF da Costa PT, Spirometry and Respiratory Physiotherapy Laboratory, Universidade Federal de São Carlos (UFSCar), Rod, Washington Luis, Km 235, Caixa Postal 696, CEP 13565-905, São Carlos, SP, Brazil. E-mail: joy.nog{at}bol.com.br.
This work was supported by scholarships from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). The authors have disclosed no conflicts of interests.
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