| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Guest Access | Sign In via User Name/Password
|
|
|
|||||||
Guest Access | Sign In via User Name/Password |
|||||||||
* From the Department of Physiotherapy (Ms. Leung), and the Pulmonary and Palliative Care Unit (Drs. K.K. Chan and K.S. Chan), Haven of Hope Hospital, Hong Kong; and the Centre for Exercise and Nutrition Science (Dr. Sykes), University of Chester, Chester, UK.
Correspondence to: K.S. Chan, MB BS, FCCP, Haven of Hope Hospital, Pulmonary and Palliative Care Unit, Tseung Kwan O, Hong Kong 852; e-mail: chanks{at}ha.org.hk
Abstract
Purpose: To investigate the reliability, validity, and responsiveness of a 2-min walk test (2MWT) in Chinese COPD patients with moderate-to-severe disease.
Methods: This study consisted of two parts. Forty-seven stable COPD patients with moderate-to-severe disease participated in the first part of study for the investigation of reliability and validity. The demographic and anthropomorphic data collected included age, gender, body weight, height, and body mass index. Each subject performed a pulmonary function test, a cardiopulmonary exercise test, three trials of a 2MWT, and two trials of 6-min-walk test (6MWT) in random order within a 5-day period. Fifteen of these subjects participated in the second part of study for the evaluation of the responsiveness of the 2MWT following pulmonary rehabilitation. The 2MWT and 6MWT were conducted before and after the program for comparison.
Results: Forty-five subjects (mean [± SD] age, 71.8 ± 8.3 years; mean FEV1, 0.88 ± 0.27 L) completed the first part of study, and 9 subjects (mean age, 74 ± 8.7 years; mean FEV1, 0.94 ± 0.28 L) completed the second part of study. The intraclass correlation coefficient of the repeated 2MWTs was high (R = 0.9994; p < 0.05), mean differences across trials ranged from 0.3 to 0.8 m (95% confidence interval, –3.1 to 4.6 m) demonstrated its high test-retest reliability. Significant correlations were found between the 2MWT and the 6MWT, the maximum oxygen uptake (
O2max) in milliliters per minute, and the O2max in milliliters per kilogram per minute (r = 0.937, 0.454, and 0.555 respectively; p < 0.0125). Following rehabilitation, there were significant improvements in the mean 2MWT and 6MWT walking distance of 17.2 ± 13.8 m and 60.6 ± 35.5 m, respectively, with moderate effect sizes (0.61 and 0.53, respectively) and large standardized mean responses (1.25 and 1.70, respectively). High correlation was found between changes in the 2MWT and the 6MWT (r = 0.70; p < 0.05).
Conclusion: The 2MWT was shown to be a reliable and valid test for the assessment of exercise capacity and responsive following rehabilitation in patients with moderate-to-severe COPD. It is practical, simple, and well-tolerated by patients with severe COPD symptoms.
Key Words: COPD • 2-min walk test • reliability • responsiveness • validity
Patients with COPD experience a wide variation in their levels of disability. Rehabilitation is recommended for COPD patients with moderate-to-severe disease who are aiming at reducing symptoms, improving quality of life, and increasing physical and emotional participation in everyday activities.1 Baseline and outcome assessments should be made in order to (1) quantify the patient’s functional ability, (2) prescribe an appropriate exercise regime, and (3) evaluate the effectiveness of the rehabilitation program.1 Progressive, incremental tests using treadmill or cycle ergometry with online monitoring of cardiac and ventilatory functions are the "gold standard" for the measurement of exercise capacity (maximum oxygen uptake [O2max]), which is a measure of disability as well as an outcome measurement for treatment and rehabilitation.2
However, these technically intense and relatively expensive measures are of questionable benefit when predicting physical functioning for daily living.3 In particular, frail, elderly patients with severe cardiac or pulmonary disease become exhausted after only a few minutes of conventional maximal exercise testing, and exercise capacity may be underestimated.
In the past few years, walk tests have gained prominence in both clinical practice and research. These tests are used for measuring functional capacity, monitoring treatment effectiveness, and establishing prognosis.4 In the pulmonary rehabilitation setting, the self-paced timed walk tests are conducted for these purposes. The most commonly used timed walk tests are the 6-min walk test (6MWT) and the 12-min walk test (12MWT). Subjects are instructed to walk as much as possible in the allotted time period. Both tests have been quite extensively evaluated, and have been shown to be reliable and valid in reflecting the exercise capacity in COPD patients.345 They have been proved to be responsive to improvements in physical functioning following pulmonary rehabilitation.67 However, these tests may be too exhausting for patients with severe COPD to perform and possibly too time-consuming in the busy health-care setting.
A 2-min walk test (2MWT) was first proposed by Butland et al8 in 1982 and was reported to be a valid test in COPD patients. However, the statistical method was in doubt as no p value had been mentioned in the whole study and only 13 subjects were recruited. A small-scale study (n = 9) performed by Bernstein et al9 in 1994 also reported on the validity and responsiveness of the 2MWT. However, the results should be interpreted with caution because the authors compared the distance walked in a 2-min interval of the 12MWT. It was argued that the distance covered in a 2-min interval of a 12MWT might be different from the distance walked in a 2MWT, as the pacing by the subjects might be different for the two different intervals. It was reported10 that a patient who knew that a walk test would only last for 2 min would walk further than a patient would in the first 2 min of a longer walk test. A more recent study by Eiser et al11 reported that the 2MWT was a reliable test and was sensitive to change after bronchodilator therapy. However, the patients’ performance may have been greatly influenced by the investigators as the investigators walked in front of the patients during the tests. A few studies of 2MWT in other populations including children with cystic fibrosis, frail elderly persons, and those patients having undergone amputation have been conducted.121314 Since both the reliability and validity of a test are population-specific,15 and the responsiveness of the 2MWT to rehabilitation is lacking, this study was designed to investigate the reliability and validity of the 2MWT as well as its responsiveness to change following a pulmonary rehabilitation program.
Materials and Methods
Subjects
This study consisted of the following two parts: first, to evaluate the reliability and validity of the 2MWT; and second, to evaluate the responsiveness of the 2MWT to pulmonary rehabilitation. Forty-seven COPD patients were recruited from the outpatient pulmonary clinic and waiting list for the pulmonary rehabilitation program in Haven of Hope Hospital. The inclusion criteria included the following: (1) moderate-to-severe COPD; (2) stable condition without experiencing an acute exacerbation in the month prior to or during the study; (3) no change in medications during the study; and (4) no requirement for long-term oxygen therapy. The exclusion criteria included dementia, concomitant heart failure, diseases with mobility impairment, and medical conditions that were contraindicated for exercise testing according to American Thoracic Society (ATS) guidelines.1617 After completing the data collection for the first part of the study, 15 of these patients joined a 5-week intensive pulmonary rehabilitation program and were recruited into the second part of study. The program consisted of educational and aerobic exercise components as well as components of relaxation and coping skills in activities of daily living in each session. The patients attended the hospital 3 days a week (from 9:00 AM to 3:00 PM). Training strategy and intensity were individually tailored according to patients’ goals and abilities. All patients were required to give written consent before the study. The study was approved by the research ethics committees of the Haven of Hope Hospital and the University of Chester.
Study Design
A prospective correlational study design was selected. Demographic and anthropomorphic data were collected (ie, age, gender, body weight, height, body mass index [BMI], and dominant hand grip strength). Two walking tests (the 2MWT and the 6MWT), a pulmonary function test, and a maximal cardiopulmonary exercise test (CPET) on a cycle ergometer were conducted in random order within a 5-day period. The 2MWT was repeated three times in order to examine its reliability. The results from the 2MWT were correlated with those of the 6MWT and the CPET (ie, O2max, O2max per kilogram, and maximum work capacity [Wmax]) for validation evaluation. For the evaluation of responsiveness, the 2MWT and 6MWT were conducted at the start and at the end of the pulmonary rehabilitation program for comparison.
Methods
Demographics and Anthropomorphic Assessment:
Body weight (in kilograms) and height (in meters) were recorded, and the BMI (in kilograms per square meter) was calculated.
Pulmonary Functions Assessment: Pulmonary functions were assessed (Pulmonary Diagnostic Systems model PF/DX; Medical Graphics Corp; St. Paul, MN). FVC and FEV1 were determined from three trials, and the values for FVC and FEV1 were selected that fulfilled the reproducibility and acceptability criteria in accordance with the ATS criteria.18 Lung volumes, including residual volume and total lung capacity, were measured with a body plethysmograph, and diffusion capacity of the lung for carbon monoxide was measured by the single-breath technique.19 Absolute values and percentages of predicted values2021 of the lung function parameters were used for analysis.
2MWT: This test was administered according to the protocol as described by Guyatt et al.10 Subjects were asked to walk as far as they could in 2 min, back and forth along an indoor premeasured corridor of 30 m. Subjects were allowed to rest during the 2-min time period, and no encouragement was given during the test. Immediately before and after the walk test, arterial oxygen saturation (SaO2) and heart rate (HR) were measured by pulse oximetry (Oxypleth; Novametrix; Wallingford, CT), the rating of perceived dyspnea (RPD) and rating of perceived exertion (RPE) were measured by modified Borg scales, and the respiratory rate (RR) was also monitored. No talking was permitted during the tests. The distance walked (in meters) was recorded.
To control for learning and practice effects, the subjects were allowed to become familiar with the test by undertaking a practice walk 1 day prior to the actual testing day. In conducting the actual testing, each subject was asked to perform three 2MWTs with adequate rest of about 20 min between tests. All parameters (ie, SaO2, HR, RPD, RPE, and RR) had to return to baseline levels before the patient commenced another trial. The longest distance walked was used for the analysis of validity and responsiveness.
6MWT: The test was conducted according to the ATS guidelines.16 Subjects were asked to walk back and forth at their own pace along a corridor, attempting to cover as much ground as possible in 6 min. Subjects were permitted to slow down, to stop, and to rest as necessary but were instructed to resume walking as soon as possible. Standard phrases of encouragement were given every minute during the test. The distance walked was recorded in meters. Two trials were performed with adequate recovery between them, and the longest distance walked was used for analysis. Parameters including SaO2, HR, RPD, RPE, and RR were monitored before and immediately after the test.
CPET:
A symptom-limited maximal exercise test was performed by the patient under the supervision of a pulmonologist and a qualified nurse. A standardized ramp exercise protocol was conducted using a calibrated, electronically braked cycle ergometer (model CPE/DCE; Medical Graphics Corp) according to the ATS/American College of Chest Physicians guidelines.17 Functional and metabolic data were measured at rest and during the test. Mixed-expired gas data, minute ventilation, breathing pattern, oxygen uptake, carbon dioxide production, and respiratory exchange ratio were generated at 30-s intervals. SaO2 and HR were monitored continuously with a pulse oximeter, and were recorded every minute. ECG activity was monitored continuously, and systemic arterial BP was registered every 2 min. Subjects were asked to quantify their perceived dyspnea and exertion by pointing at a 10-point modified Borg scale before and at the termination of the test. Wmax (measured in Watts) was defined as the highest work level reached, and O2max (measured in milliliters per minute and milliliters per kilogram per minute) was defined as the highest oxygen uptake achieved during maximal effort for an incremental exercise test.22
Statistical Analysis
All statistical analyses were performed using a statistical software package (SPSS, version 11.5; SPSS; Chicago, IL), except for the Bland-Altman plot (MedCalc; Mariakerke, Belgium). Data were checked for normal distribution, and the means ± SDs were calculated. The results of the Borg ratings were analyzed by appropriate nonparametric tests. A p value of 
0.05 was considered to be statistically significant. A Bonferroni adjustment was performed as indicated for the analysis. The reliability of the 2MWT was determined by the intraclass correlation method (a two-way random-effects model) and the methods described by Bland and Altman.23 Pearson correlation tests were used to analyze the relationships between the 2MWT and the 6MWT, O2max, O2max per unit of body weight (O2max/kg), and Wmax, respectively. The interpretation of the strength of the correlations was based on a grading scheme used by Cohen and Holiday.24 Paired t tests were used to assess the effect of pulmonary rehabilitation on exercise capacity. Effect size (mean change/SD of baseline scores) and standardized response mean (mean change/SD of change scores) were calculated to evaluate the internal responsiveness (characterizing the ability of a measure to change over a prespecified time frame) of the walk tests. Values of 0.20, 0.50, and 
0.80, respectively, have been proposed to represent small, moderate, and large responsiveness.25 A correlation analysis between the change in the 2MWT and the change in the 6MWT distances walked was conducted to evaluate the external responsiveness (reflecting the extent to which change in a measure relates to a corresponding change in a reference measure of clinical or health status) of the 2MWT.
Results
Of the 47 subjects who were initially recruited to the study, two subjects dropped out because they had exacerbations during the assessment period. Demographic and anthropomorphic data, and pulmonary function test and exercise test results are shown in Tables 1, 2 . Forty-five subjects (37 men and 8 women; mean age, 71.8 ± 8.3 years; mean FEV1, 41.9 ± 13.0% predicted) completed the study. There were significant increases in RPDs and RPEs after pulmonary rehabilitation for both the 2MWT and 6MWT. The magnitudes of RPDs and RPEs were comparable to those of other studies.11 Small but significant differences in the end of test RPEs between the 2MWT and the 6MWT indicated that the 6MWT was more exhausting to perform than the 2MWT (mean RPE in 2MWT, 3.0 ± 2.0; mean RPE in 6MWT, 3.3 ± 0.3; p < 0.05).
|
|
|
. The mean increases were 0.3 to 0.8 m with limits of agreement in the 95% confidence interval ranging from –3.1 to 4.6 m. These were very small variations compared with the mean 2MWT distances. Therefore, we can conclude that the reliability of the 2MWT in terms of limits of agreement was high.
|
|
O2max, O2max/kg, and Wmax) are shown in Table 4
. Since four statistical tests were conducted, a Bonferroni adjustment was performed to minimize the type 1 error. Therefore, the significance level was set at 0.0125 (0.05/4).
|
O2max, and the O2max/kg respectively (all p < 0.0125). A very high correlation was found between the 2MWT and the 6MWT (r = 0.937), whereas modest correlations were found between the 2MWT and the O2max (r = 0.454), and the 2MWT and the O2max/kg (r = 0.555).
Responsiveness
Part 2 of this study investigated the responsiveness of the 2MWT as a measure of change following a 5-week pulmonary rehabilitation program. Of the 15 subjects recruited to this program, 3 subjects dropped out due to poor motivation and 3 subjects failed the postassessment review due to acute exacerbations. Therefore, data from nine subjects (mean age, 74 ± 8.7 years; mean FEV1, 0.94 ± 0.29 L and 42.4 ± 12.1% predicted) were used in the analysis (Table 5
). There were significant improvements in both the 2MWT and 6MWT after pulmonary rehabilitation (2MWT, 17.2 ± 13.8 m; 6MWT, 60.6 ± 35.5 m; p < 0.01). Moderate effect sizes were found for both the 2MWT and 6MWT. The effect size of the 2MWT was larger than that of the 6MWT. Large responsiveness to pulmonary rehabilitation was found for both the 2MWT and the 6MWT, as indicated by the standardized mean responses (2MWT, 1.25; 6MWT, 1.70). A correlation analysis between the mean changes of the 2MWT and 6MWT showed a high correlation coefficient (r = 0.70; p < 0.05), which indicated that the external responsiveness of the 2MWT was good.
|
Reliability is a fundamental measurement property that is relatively easy to determine, and is quantified in terms of the degree of consistency and repeatability when properly administered under similar circumstances for a specific population.15 This study demonstrated that with this group of COPD patients, the distance walked in the three trials in 2MWTs was highly reproducible. This was different from the results of other studies2627 in which there were significant improvements in subsequent trials. In the study of Larson et al,26 the improvements were probably due to learning effects as subjects remembered their performance and were encouraged through external cues such as "try harder" on subsequence tests. It is known that motor learning occurred as a result of practice and experience with a task, resulting in a relatively permanent increase in proficiency at the task.28 However, in this study, the learning effect was minimized as a practice walk was given before the actual tests were conducted and the testing procedures were standardized.
In the study by O’Reilly et al,27 there was a significant mean variation of 9.1% in walking distance when tests were performed 2 weeks apart. In the study by Eiser et al,11 significant differences in mean walking distance were found among trials conducted on 3 days of consecutive weeks. The difference may have been caused by the changes in airway condition in these COPD patients, as day-to-day variability in spirometry results was present even in stable COPD patients.2 In this study, there was only very small variation across the trials as the effect caused by lung function variability was minimized since the different trials were conducted on the same day during a 1.5-h test period.
The significant correlations between the 2MWT and the 6MWT, O2max, and O2max/kg support the validity of the 2MWT as a measure of exercise capacity in COPD patients with moderate-to-severe disease. As anticipated, the strongest correlation was observed between the 2MWT and 6MWT, since they are both walking tests and their results were in agreement with those of previous studies.89 The modest but significant correlations observed between the 2MWT, and the O2max and O2max/kg were also similar to those of previous studies.9
When comparing the correlations of the 2MWT, 6MWT, and 12MWT with O2max and O2max/kg, the Pearson correlation coefficients for the 2MWT were only slightly lower than those of the 6MWT and 12MWT (O2max, 0.454 vs 0.49 to 0.51; O2max/kg, 0.555 vs 0.65 to 0.67).59 This demonstrates the fact that a 2MWT can therefore assess exercise capacity as accurately as walking tests of longer duration.
The distances walked in both the 6MWT and 2MWT improve significantly after pulmonary rehabilitation. This finding echoes that of Eiser et al,11 who reported that the 2MWT is sensitive to change after bronchodilator therapy. However, the 6MWT is more exhausting for patients with severe COPD and more time-consuming in a busy health-care setting. In fact, some patients with severe symptoms may not be able to complete a 6MWT, making the data difficult to interpret. In this study, the less tiring 2MWT was as effective as the 6MWT in demonstrating good responsiveness to change after a 5-week program of pulmonary rehabilitation and may be especially useful for this group of patients.
This study has demonstrated that a 2MWT is a reliable, valid, and sensitive tool for the assessment of exercise capacity in COPD patients with moderate-to-severe disease. However, several factors need to be considered when using this test. This test may be valid only for persons experiencing moderate-to-severe exercise limitation. The short duration of the timed walk test may not stress the cardiopulmonary function adequately for patients who have mild cardiopulmonary disease, and the ceiling effect may limit the evaluation of the effectiveness of an intervention. Besides, because of its short duration, the magnitude of improvement following an intervention will be small and perhaps difficult to interpret. Further study to investigate the minimal clinically important difference is recommended.
Since the walk test uses a self-pacing protocol, results may be affected by the effects of learning and motivation. Practice walks and standardized procedures should be considered to produce reproducible and comparable results. A walk test provides only a measure of distance covered. However, when walking ability is impaired and is considered a disability in regard to independent ambulation, both qualitative parameters (eg, dynamic balance sufficient for safety to prevent falls) and quantitative parameters (eg, velocity and distance tolerated) need to be described and compared with the realistic functional demands of daily living in the community.
The 6MWT has been shown429 to be an excellent predictor of morbidity and mortality in patients with COPD. A further study to investigate the predictive capability of 2MWT in this regard may therefore be worthy of investigation.
Conclusion
This study has demonstrated that the 2MWT is a reliable, valid, and sensitive test for the assessment of exercise capacity in patients with moderate-to-severe COPD. It is practical, simple, quick, easy to administer, and well-tolerated by patients with severe COPD symptoms. Further studies are recommended to extend and validate the findings presented.
|
The authors thank Dr. Stephen Fallows for his helpful suggestions in the study, Mr. Raymond Tsang for statistical advice, and the staff of Haven of Hope Hospital for help in obtaining the data.
Footnotes
Abbreviations: ATS = American Thoracic Society; BMI = body mass index; CPET = cardiopulmonary exercise test; HR = heart rate; 2MWT = 2-min walk test; 6MWT = 6-min walk test; 12MWT = 12-min walk test; RPD = rating of perceived dyspnea; RPE = rating of perceived exertion; RR = respiratory rate; SaO2 = arterial oxygen saturation; O2max = maximum oxygen uptake; O2max/kg = maximum oxygen uptake per unit of body weight; Wmax = maximum work capacity
Ms. Leung has no financial or other potential conflicts of interest. Dr. K.K. Chan has no financial or other potential conflicts of interest. Dr. Sykes has no financial or other potential conflicts of interest. Dr. K.S. Chan has no financial or other potential conflicts of interest.
Received for publication August 24, 2005. Accepted for publication January 26, 2006.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
