Field Tests of Exercise in COPD: The Six-Minute Walk Test and the Shuttle Walk Test

Abstract

Exercise testing is useful to assess the degree of disability, prognosis for survival, presence of exercise-induced hypoxemia, and response to treatment in individuals with chronic obstructive pulmonary disease. Simple walking tests have been developed and are increasingly used in assessment of chronic obstructive pulmonary disease patients for clinical and research purposes. This article reviews how these tests are performed and to what degree they are reliable, and how these tests are used in assessment of individuals with chronic obstructive pulmonary disease.

Keywords::

• COPD
• Exercise
• 6-minute walk test

INTRODUCTION

Exercise testing in individuals with chronic obstructive pulmonary disease (COPD) is useful to assess degree of disability, prognosis for survival, presence of exercise-induced hypoxemia, and response to treatment. Because of its ability to provide measures of physiologic data in addition to diagnostic information, maximum cardiopulmonary exercise testing using a cycle ergometer with measurement of gas exchange has long been the standard method for measuring exercise capacity in COPD. However, maximum exercise testing with cycle ergometry has some limitations that have hindered its use in routine clinical practice. Cycle ergometry employs an activity that is less familiar than walking, and measurement of gas exchange requires expensive and complex technology.

In addition, exercise capacity by cycle ergometry, expressed in units of oxygen consumption or power output, can be difficult to interpret, particularly in relationship to an individual's ability to perform daily activities. In recent years, simple walking tests of exercise have been developed and are increasingly used in assessment of COPD patients for clinical and research purposes. The most widely used walking test in the United States is the 6-minute walk test (6MWT), which simply measures the distance walked over 6 minutes. The walking test more commonly used in the United Kingdom is the shuttle walk test (SWT). This test employs incrementally faster walking speeds until the individual is no longer able to maintain the walking pace. Recently, a variant of the SWT, the endurance shuttle walk test (ESWT), has also been introduced into clinical practice. The purpose of this manuscript is to review how these tests are performed and to review the validity, reliability, and interpretation of these tests in assessment of individuals with COPD.

The 6-minute walk test

The 6MWT was introduced into clinical practice in the early 1980s after being adapted from a 12-minute running test of physical fitness in healthy individuals ([1]). Since then, the 6MWT has become the most widely used field exercise test in COPD. One of the main reasons for the widespread use of the 6MWT is its ease of administration. This test is performed on an indoor course measuring from 30 to 100 meters, and the primary measurement is the distance walked in six minutes (6MWD) ([2]). Individuals are allowed to walk at their own pace, to stop if necessary, and to use oxygen. In 2002, the American Thoracic Society published guidelines to standardize performance of the 6MWT (Table 1), including standardized wording for instructing patients prior to the 6MWT and encouraging patients at each minute of the test ([2]). The use of standardized instructions and encouragements is essential to limit the effects of coaching on performance during the 6MWT ([3]). Secondary measurements that are commonly assessed during a 6MWT include oxygen saturation, heart rate, dyspnea, and leg fatigue by the Borg Category Ratio Scale or Visual Analog Scale.

Table 1 Key elements of ATS recommendations for 6-minute walk test

• The straight walking course should be 30–100 m long with a flat, comfortable surface.

• The patient should wear loose comfortable clothes and walking shoes.

• Perform serial tests at same time of day.

• Oxygen may be used if necessary.

• No warm-up or practice period is permitted.

• Precede test with at least 10–minute rest period.

• Pulse oximetry measurement during and after test is optional.

• Borg dyspnea and fatigue measurements should be performed at start and end of test.

• A standardized scripted explanation of the test is provided the patient.

• The tester should not walk with the patient and should not pace the patient or use unnecessary encouragement by speech or body language.

• Standard scripted phrases of encouragement at 1-minute intervals are given, e.g., “Keep up the good work, you have 4 minutes to go.”

• Congratulate the patient and offer a drink of water at the end of the test.

Several technical issues may affect performance on the 6MWT. The technician should be cautioned to use only scripted encouragement at one-minute intervals to prevent coaching effects. As the 6MWT is self-paced, many individuals improve their performance with repeated testing as they learn pacing strategies and become familiar with the testing protocol. Previous studies have shown that there is an increase in 6MWD from the first to second tests, but improvements in 6MWD plateau after the second test ([3], [4], [5]). The amount of improvement in 6MWD from the first to second test is variable and ranges from 0 to 17% ([2], [3], [4], [5]). In order to maximize performance due to learning effects, clinicians may utilize a practice 6MWT. However, the ATS Guidelines recommend only one test session on a given day.

Although the variability of results with repeated testing can be minimized with use of either the mean or maximum measurement, this practice can place an unnecessary burden on patients if it requires multiple testing days or prolonged resting periods at the test facility. The course layout also can affect performance on the 6MWT. When course length is increased from 10 meters to 90 meters, 6MWD increases by 54 meters, ([6]) and an oval track increases 6MWD by a mean of 28 meters ([5]). The reason that course layout might affect 6MWD presumably relates to the fact that a greater number of turns decreases speed and walking efficiency ([6]).

Performance of a 6MWT on a treadmill is not recommended as treadmill walking does not allow self-pacing. The treadmill 6MWD is significantly lower than a self-paced 6MWT performed in a corridor ([7], [8]). Use of oxygen and bronchodilators also affects performance on the 6MWT. If oxygen desaturation with exercise is present, use of oxygen can improve 6MWD despite the added weight of the oxygen delivery device ([9], [10], [11]). In addition, use of bronchodilators prior to the testing increases 6MWD ([12], [13], [14], [15]). Thus, it is recommended that testing be performed 30 minutes to 2 hours after use of a short-acting bronchodilator in COPD patients ([2]). A final consideration regarding the technical performance of the 6MWT is that significant variability in the performance of the test continues to be a problem despite formal guidelines for standardization of the test. In one survey, only 42% of 71 responding pulmonary rehabilitation programs gave standardized instructions prior to the test, and 76% of programs allowed the walk supervisor to direct an individual to speed up, slow down, or rest ([16]).

While the 6MWT does not provide formal physiologic measurements and is constructed to be an endurance test rather than a maximal exercise test, the 6MWT has been validated with respect to a variety of physiologic and quality of life (QOL) measures. The correlation between the 6MWD and maximum oxygen consumption and work rate on a bicycle ergometer ranges from 0.51 to 0.81 in COPD ([4], [17], [18], [19], [20], [21]). However, correlation with lung function measurements is less strong (r-values 0.17–0.55), ([20], [21], [22], [23]) and 6MWD may be well-preserved despite advanced COPD ([4], [5], [18], [20]). Finally, there is a weak-to-moderate correlation between 6MWD and dyspnea scores with r-values of about –0.30 to –0.40 for the Borg and Visual Analog Scale for dyspnea. Correlation of 6MWD with the Chronic Respiratory Disease Questionnaire, a disease-specific quality of life measure, ranges from −0.03 to −0.25 ([18], [20], [22], [23]). The wide range of correlation between 6MWD and pulmonary function and QOL measures likely reflects the submaximal nature of the test as well as the underlying degree of lung function impairment.

Further evidence validating the 6MWT is its responsivity to treatments that also improve clinical outcomes and QOL in COPD. Short-acting bronchodilators can improve 6MWD if used shortly before testing ([12], [13], [14], [15], [24]). Although long-acting bronchodilators including both anticholinergic and beta-agonist medications have shown little or no improvements in the 6MWD, these medications do reduce perceived dyspnea during the test ([25], [26], [27], [28]). Tiotropium, a once-daily anticholinergic inhaled medication, has been demonstrated to improve endurance time on a cycle ergometer ([29], [30]), but this has not been demonstrated for the 6MWD. The increase in 6MWD after treatment with inhaled corticosteroids has been reported in one study to be 33 m ([31]), but others have failed to show significant improvement in 6MWD after treatment ([32], [33]).

The 6MWT is also particularly responsive to non-pharmacologic therapies in patients with COPD. The 6MWT has been most widely used as a measure of improvement during and after pulmonary rehabilitation. In a meta-analysis of 10 trials including over 450 individuals with a wide range of COPD severity, the mean increase in 6MWD was 49 meters after completion of pulmonary rehabilitation ([34]). The National Emphysema Treatment Trial recorded 6-minute walk tests in 1,218 enrolled participants before and after pulmonary rehabilitation and prior to randomization to either lung volume reduction surgery or medical therapy ([35]).

In this group of severely impaired individuals with a mean FEV₁ of 26.9% predicted, there was a mean increase in 6MWD of 23.0 ± 53.7 meters following pulmonary rehabilitation ([36]). In one study, the regular use of a flutter valve as a mucus clearance device also improved 6MWD in a group of COPD patients ([37]). The 6MWT is also responsive to surgical therapies. In the National Emphysema Treatment trial, 48% of individuals randomized to lung volume reduction surgery (LVRS) plus optimal therapy had an improvement in 6MWD compared to only 21% of individuals who received medical therapy alone. In most individuals, however, the improvement in 6MWD was less than 30 meters, and it is notable that over 50% of individuals did not have any improvement in 6MWD after LVRS ([38]). Other uncontrolled studies have shown much larger effects of LVRS on 6MWD. In one center that performed LVRS on 250 individuals with severe COPD, the mean improvement in 6MWD was 61.9 meters at 6 months and was sustained at 39.3 meters at 3 years ([39]). The best response in 6MWD follows lung transplantation with approximately 100% improvement in 6MWD at 3 months following both single and bilateral lung transplant ([40], [41], [42]). In those who survive, this improvement is sustained for up to 7 years ([40]).

At present, there are no widely accepted normal reference values for the 6MWD. However, two studies have published suggested reference equations for men and women ([43], [44]). The largest study, performed by Enright et al. ([43]), included 117 healthy men and 173 healthy women, ages 40 to 80. There was an equal distribution of age among the individuals who participated, and those with obesity (BMI > 35 kg/m²), prior stroke, evidence of peripheral vascular disease, or FEV₁ < 70% predicted were excluded ([43]). In this population, men walked a median distance of 576 m, and women walked a median of 494 m ([43]). Troosters and colleagues, measured 6MWD in 51 healthy adults, ages 50 to 85 and found higher values. Compared to the study of Enright et al., this study was performed in a longer corridor (50 m vs 30 m), encouragement was provided every 30 seconds, and the test was replicated ([44]).

The mean 6MWD for both men and women was 631 meters with men walking 84 meters farther than women on average ([44]). In addition, 2 other studies have also evaluated the normal 6MWD in healthy adults. Gibbons performed four 6MWTs in a 20 m corridor in 79 healthy adults aged 20–80, finding an average value of 698 m. The substantially larger values found in this study compared to prior studies was attributed to the repetition of the test and the inclusion of a younger population ([45]). Likewise, Camarri et al. conducted triplicate 6MWTs in 70 individuals. They found an average distance of 659 m, with women walking 59 m less than men. In all studies, the factors that best predicted 6MWD were gender, age, height, weight, and absolute value of FEV₁ ([43], [44], [45], [46]).

Thus, it can be reasonably surmised that the choice of reference values depends largely upon the methodology of the test procedure. If multiple training walks are incorporated into the testing procedure, then one of the reference standards using this technique is appropriate. Otherwise the Enright reference standards seem most appropriate for situations where a single walk is performed.

Because of the wide variability in reference values, another approach to interpretation of the 6MWD has been to utilize the absolute value of the 6MWT. Normal or mildly impaired individuals may exhibit a ceiling effect because jogging is not permitted in the testing procedure, and most individuals cannot walk at a pace that exceeds 4 to 5 mph. Table 2 shows the approximate metabolic equivalents and walking speed for a range of 6MWD from normal to severe impairment.

Table 2 Degree of impairment in 6-minute walk distance

Degree of Impairment	Distance Walked (ft)	Distance Walked (m)	Miles per Hour (mph)	Approximate Metabolic Equivalents (METS)
Normal	≥ 2000	≥ 609.6	≥ 3.79	4.5
Mild	1500–1999	457.2–609.5	2.84–3.78	3.3
Moderate	1000–1499	304.8–457.1	1.89–2.83	2.5
Severe	500–999	152.4–304.7	0.95–1.88	2.0–2.3
Very Severe	< 500	< 152.4	< 0.95	< 2.0

*Approximate metabolic equivalents derived from Fletcher GF, Circulation 2001; 104:1694–1740 ([66]),

Mildly impaired individuals with COPD, therefore, may retain normal or near normal 6MWD. As disease progresses, the maximum exercise capacity declines to a greater degree than the 6-minute walk distance. This is thought due to the fact that, in most individuals, the 6MWT reflects the maximum sustainable exercise—about 80% of the maximum exercise capacity (Figure 1) ([6]). Some patients, however, may adopt a strategy whereby they transiently reach maximum oxygen capacity during a 6-minute walk test ([47]). A low value of 6MWD provides helpful prognostic information in COPD patients. Following rehabilitation, a 6MWD less than 155 meters is associated with approximately 45% mortality at 4 years ([48]). Furthermore, a 6MWD less than 200 meters predicts an 84% surgical mortality with LVRS in COPD ([49]), and a 6MWD less than 300 meters is associated with a higher mortality in individuals listed for lung transplantation with a variety of lung disease ([50]). Finally, the 6MWD is a component of the BODE index (body mass index, airflow obstruction, dyspnea, and exercise capacity index) that predicts mortality in COPD ([51]).

Figure 1 Mean oxygen consumption (VO₂) in 8 patients with COPD performing 4 exercise tests. The rate of increase and maximum value of VO₂ with a progressive cycle ergometry test and the incremental shuttle walk test is similar. Stair climbing was accomplished by having patients walk up stairs without the use of handrails at a brisk pace for as many steps as they could tolerate. During the 6-minute walk test, the patients reached a plateau of oxygen consumption after 2 minutes, which was sustained for the remainder of the test at about 80% of the maximum value. Error bars indicate ±SEM. Figure modified from Casas A, Vilaro J, Rabinovich R, Mayer A, Barbera JA, Rodriguez-Roisin R, Roca J. Encouraged 6-min walking test indicates maximum sustainable exercise in COPD patients. Chest 2005; 128:55–61. ([6]) (Adapted with permission).

Of 12 factors screened for inclusion in the BODE index, the 4 variables included in the BODE index were the only ones found to have an independent effect on mortality in COPD. This index was then validated in an additional 625 individuals with a wide range of COPD severity ([51]). In the BODE index, a 6MWD less than 350 meters worsens the prognostic index with values less or equal to 150 meters marking those at highest risk ([51]). In addition, a lower 6MWD (< 367 m) predicts a greater number of hospitalizations for acute exacerbations of COPD ([52]) as does a higher BODE score ([53]). Longitudinal change in 6MWD also predicts mortality in COPD with an 18% increased risk of death for every 50 m decrease in 6MWD over the course of at least 1 year ([54]).

Another consideration when interpreting the 6MWT is how much change over time is the minimal difference that is required to be associated with clinical improvement (or deterioration) in COPD. The several ways to assess this minimal clinically important difference (MCID) in the 6MWT have recently been reviewed ([55]). One method that has been used asks individuals to rate their perceived change over time. However, when this method is used, there is a poor correlation between perceived improvement or deterioration and actual change in performance on 6MWT (r = 0.20) ([56]). Furthermore, more than half of the individuals who perceived an improvement in their walking distance actually showed deterioration in their 6MWD ([56]).

This result can be attributed to poor recall of prior health status and optimistic assessment of current health status. These same individuals were also asked to compare their performance to others whom they observed during a 5-day pulmonary rehabilitation program. The subjects were allowed to interact with one another, asking questions about health status ([56]). Following this, the differences in 6MWD were calculated for each category of perceived level of difference in walking ability compared to others in the rehabilitation program: much better, somewhat better, a little bit better, about the same, a little bit worse, somewhat worse, and much worse. The MCID was estimated as the mean difference that was necessary for an individual to change their status from about the same to either a little bit better or a little bit worse.

There was an optimistic bias in this population with those rating themselves about the same walking approximately 10 m less than their peers ([56]). The average change necessary to rate oneself as better than his or her peers was about 40 m and as worse than his or her peers was about 70 m. Based on this analysis, the average distance necessary to cause an individual to change his or her functional status was 54 m (95% confidence interval = 37 to 71 m) ([56]). It is notable that the MCID estimated in this manner did not vary across ranges of baseline functional status, gender, age or pulmonary function. This range of values is the current standard for interpreting the MCID in 6MWT. A similar value can be estimated statistically by using population distributions to estimate the least change that can be confidently assumed to be outside random variation. Using this method, the estimated MCID is 47 m, similar to the value based upon self-assessment ([55]).

The incremental and endurance shuttle walk tests

The incremental shuttle walk test (SWT) was developed in England in the early 1990s as an alternative to the 6MWT. The objective behind the design of the SWT was to use walking as a familiar activity that is relevant to daily functioning, but to limit the influence of strategy, motivation and coaching on the performance of the test ([57]). In addition, the SWT was designed as a test of maximum exercise capacity in that it emulated an incremental treadmill test more closely than the 6MWT. The SWT is performed on a straight and flat indoor course with two traffic cones placed 10 meters apart. The test is externally paced via a recorded metronome that gives a signal for each stride. The walking pace is increased by 0.17 m/s every minute, and the test is concluded when an individual is no longer able to keep up the pace or stops because of symptoms ([57]). There are a maximum of 12 stages, and the primary outcome is distance walked, calculated from the number of laps (“shuttles”) completed ([57]). There are no published normal reference values for the SWT, and standardized guidelines for test performance have not been endorsed by any professional organization.

Because the SWT requires increasing walking pace at pre-determined intervals and is designed to be a maximal exercise test, the SWT is not necessarily indicative of an individual's endurance and ability to perform activities that require pacing and strategy. The endurance shuttle walk test (ESWT), a variant of the SWT, was designed as an alternative to the SWT and 6MWT that would better reflect the submaximal exertion that individuals use in performance of their daily activities, but with external pacing to decrease the inherent variability between subjects ([58]). In order to perform the ESWT, an individual must first undergo a maximal SWT as described here. The walking speed for the ESWT is then calculated as 85% of the maximum sustainable walking speed from the SWT ([58]).

During the ESWT, there is a 2-minute warm-up period with a slower walking speed to allow the individual to become accustomed to the course. Following this, a triple signal indicates when to increase the speed to the pre-determined walking speed from the SWT. The individual is then instructed to continue walking until he or she becomes too breathless or tired to continue or 20 minutes have elapsed ([58]). The primary measurement during the ESWT is the walking time in seconds. Other measurements include heart rate, oxygen saturation, breathlessness, and perceived exertion.

The technical issues that affect performance on the SWT and ESWT have not been assessed as extensively as with the 6MWT. However, because these tests are externally paced, no verbal encouragement is used except to instruct the subject in the correct performance of the test ([57], [58]). A practice walk is recommended because of the learning effect between the first and second walks ([24]). Performance of the ESWT on a treadmill produces longer walking times that the ESWT performed in a corridor, but not to a significant degree with a wide variability in results within individuals ([58]). Currently, there is no consensus statement recommending specific performance standards for the ESWT, and the ESWT should be performed according to the original description ([58]).

The SWT has been correlated with performance on both maximal and submaximal exercise tests as well as with pulmonary function. The distance walked on the SWT correlates strongly with maximum oxygen consumption on cycle ergometry (r-values = 0.73–0.88) ([17], [47], [59]) and distance walked on the 6MWT (r-values = 0.70–0.91) ([17], [47], [59], [60], [61]). Correlation between the SWT and pulmonary function is less strong with r-values of about 0.30 ([59], [62], [63]), similar to the correlation seen with the 6MWT. The ESWT has not been similarly correlated maximum cardiopulmonary exercise tests or 6MWT, but correlates with treadmill endurance time when the ESWT was performed at 75% of maximum speed on the SWT ([58]).

The SWT and ESWT have also demonstrated responsiveness to therapeutic interventions, but again these are less well studied than the 6MWT. Acute administration of short-acting bronchodilators causes a significant improvement in distance walked of about 30 m on the SWT ([24], [62]) Administration of tiotropium also causes an increase in the distance walked on the SWT by a mean of 33 m after 6 weeks of treatment, and this improvement is sustained at 12 weeks ([64]). Tiotropium in combination with pulmonary rehabilitation also improved endurance time after 13 weeks on a treadmill endurance test set at 80% of maximal walking speed, which was determined by treadmill testing ([65]). In this study, endurance time increased by over 10 minutes following treatment and was 5.35 minutes greater in those receiving tiotropium plus pulmonary rehabilitation compared with those receiving placebo and pulmonary rehabilitation ([65]).

Pulmonary rehabilitation alone also improves both SWT and ESWT. The distance walked on SWT improved by an average of 40 m while the distance walked on the ESWT increased by an average of 235 m ([58]). In addition, endurance time in seconds, the primary outcome measurement of the ESWT, also increased by 400 seconds, an improvement of 160% ([58]). Thus, the ESWT is more responsive to the effects of pulmonary rehabilitation than the SWT and likely a better indicator of functional status.

There are no data available about performance on the SWT and ESWT in normal individuals. Thus, it is not possible to interpret these tests with respect to reference values, and they have their greatest utility in assessing longitudinal changes in individual patients or comparing groups of patients. Because the minimal clinically important difference has not been investigated, it is not known how much of a change will be important for an individual patient.

SUMMARY

Field tests of exercise capacity can provide reliable, responsive, and informative data about exercise capability in patients with COPD. The selection of the appropriate test for clinical or research purposes depends on whether one is interested in peak exercise capacity or endurance exercise capacity. Translation of these tests to functional status and ability to conduct activities of daily living has not been established, but it seems reasonable to speculate that the 6-minute walk test correlates better with ability to perform sustained exercise such as walking, whereas the shuttle walking test is a better predictor of activities that require bursts of heavier exercise such as stair climbing. Although these tests require little in the way of equipment, attention to detail is important for making the tests reproducible. The 6-minute walk test has formal guidelines for conduct, minimal clinically important difference, and normal reference values, whereas these are not currently available for the shuttle walk test. In most cases, however, the usefulness of these tests is to follow patients longitudinally and to compare groups of patients.