INTRODUCTION

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The assessment of functional capacity in patients with chronic pulmonary disease is a complex issue. Objective measurements are needed that accurately reflect a patient's exercise performance. However, these measures are affected by a multitude of components such as cardiovascular, pulmonary, neurologic, and musculoskeletal function. Psychological factors, including motivation, tolerance of dyspnea or muscle discomfort, and learning effects, also influence exercise performance. Objective exercise measurements should be reproducible over time yet be sensitive enough to reflect changes in performance caused by progression of disease, pulmonary rehabilitation, and medical or surgical intervention. In addition, any measurement of exercise capacity should be safe, inexpensive, simple to perform, and require minimal medical personnel. A self-paced walk test performed in a corridor has probably been the most widely used test for the purpose of repeated measures of exercise capacity in patients with advanced lung disease. Initially, patients were walked in a hallway for 12 min and the distance recorded (1, 2). Subsequently, it was shown that there was a close correlation between the 6- and 12-min walk test (3). Since then the 6-min walk test (SMWT) has become the measure of exercise capacity used most commonly.

The SMWT is often utilized before and after interventions such as pulmonary rehabilitation, lung volume reduction surgery, and lung transplantation (4). However, the SMWT has become much more elaborate since the addition of supplemental oxygen, if required, along with heart rate and oxygen saturation monitoring. A physical space that allows performance of the SMWT is also required. To ensure standardization this corridor should be at least 100 feet in length, not be carpeted, and allow uninterrupted performance of the test by other hallway traffic. Because of the cumbersome nature of this test we sought to develop a modified measure of exercise capacity for patients with chronic pulmonary disease utilizing a self-paced treadmill, monitors, and supplemental oxygen.

	METHODS

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Subjects were recruited from two supervised pulmonary rehabilitation programs. One program was at an urban tertiary care academic medical center and the other at a suburban community medical center. Newly enrolled and long-term rehabilitation patients were candidates for the study. Participants were enrolled over a 12-mo time span. Subjects were approached for enrollment in the study after identification by the rehabilitation supervisor and chart review. Exclusion criteria included unstable pulmonary status, orthopedic or cardiovascular complications affecting exercise performance, or inability to utilize a treadmill. In addition, patients were excluded if there was a change in inhaled medication during the prior 2 wk or a change in oral corticosteroid use in the prior 4 wk. After enrollment subjects were randomized by sealed envelope into two groups. One group performed the treadmill (TM) SMWT on the first day and the hallway (HW) SMWT on the subsequent day and the other group performed the HW SMWT as the initial test and the TM SMWT as the second test. There was at least 48 h of rest between each test day and all subjects completed both test days within 7 d.

Spirometry was performed and recorded prior to the SMWT on each test day. A standardized protocol was followed in which each subject performed three SMWTs both in the hallway and on the treadmill. There was at least a 30-min rest between each SMWT.

For the HW SMWT, patients were instructed to walk back and forth along the corridor at their own pace. They were instructed to cover as much ground as possible in the 6 min. They were also told a prompt would be given when 4 and 2 min remained in the test.

The following standardized prompt was used: "You have two (or four) minutes remaining. You are doing very well. You may speed up at any time, slow down at any time, continue at the same pace or rest if you need to. Just try to cover as much distance as you can in the time remaining. You are doing very well." Patients were allowed to rest and the total rest time was recorded. At the conclusion of each walk the total distance covered by the patient was recorded along with a Borg dyspnea scale rating. Heart rate and oxygen saturation were monitored using a portable oxygen saturation monitor, which was pushed or carried by the tester. Oxygen saturation and heart rate were recorded pretest and at 2-, 4-, and 6-min intervals. Supplemental oxygen was utilized if oxygen saturation fell below 88%. The portable oxygen tank was pushed or carried by the tester.

For the TM SMWT the total distance covered was recorded by a rolling measure device attached directly to the treadmill. The treadmill's data panel showing speed and distance traveled was not visible to the subject. Only the start, stop, and speed up or slow down buttons were accessible to the subject. The initial speed of the TM was set at 1 mile per hour and the minimal speed was 0.6 miles per hour. The participant was given the same instructions regarding the goal of the test as in the hallway SMWT. Prior to the TM SMWT, subjects were instructed on how to increase and decrease the speed of the treadmill along with how to stop and restart the treadmill if they needed to rest. The same prompt was given at 2- and 4-min intervals. Heart rate and oxygen saturation were monitored by a stationary oxygen saturation monitor and supplemental oxygen was utilized if oxygen saturation fell below 88%. Heart rate and oxygen saturation were recorded similarly as in the HW SMWT. In addition, total rest time and the Borg dyspnea scale were recorded. Patients were not told any results until both hallway and TM SMWT were completed.

Results are reported as means ± standard deviations. The longest distance of the three walks was used for statistical analysis. The distance of the HW and TM SMWT among each subject was compared by paired samples t tests. The differences in distances between the two tests were evaluated by multiple linear regression. The intratest variability of the HW and TM SMWTs was also compared. The three HW SMWT distances were evaluated by tests of repeated measures along with the 3 TM SMWT distances.

	RESULTS

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Twenty-one subjects were recruited and completed the study protocol. There were 12 women and nine men. Mean age was 65 ± 10.9 yr (Table 1). The majority of patients had chronic obstructive pulmonary disease (COPD), but other pulmonary diseases were represented. There was a wide range in the length of time each patient had spent in pulmonary rehabilitation. Four patients were newly referred to rehabilitation. In seven subjects the time from initial referral to rehabilitation was greater than one year, and one patient had been referred 9 yr previously. However, most of these patients had not been in rehabilitation continuously. They had attended rehabilitation only intermittently over this time span. Ten of the subjects were receiving supplemental oxygen. The FEV₁ did not differ significantly between the two test days.

The mean HW SMWT distance (1,228 ± 255 ft; range, 612 to 1,679 ft) was significantly greater than the mean TM SMWT distance (1,060 ± 389 ft; range, 475 to 1,819 ft) (p < 0.05). The comparison of TM and HW SMWT distances with the line of best fit plotted is illustrated in Figure 1. In seven subjects the difference between the HW and TM SMWT was less than 100 ft, but there was a wide variability in difference, with a mean of 168 ft and a range of 326 to 743 ft. Using absolute values the range was 27 to 743 ft. Fourteen of 21 patients walked a greater distance in the HW than on the TM.

View larger version (15K): [in this window] [in a new window]   Figure 1.   Hallway (HW) versus treadmill (TM) 6-min walk test distance (n = 21).

Subjects were also analyzed by whether they performed the TM or HW SMWT first to test if SMWT order affected the results, but the differences were not explained by test order. Multiple linear regression was performed using age, sex, time in rehabilitation, and oxygen use as independent variables, but none was significantly associated with the difference between the two tests.

The mean SMWT distance for all patients on the first, second, and third HW and TM SMWTs is shown in Table 2. For both the HW and TM there was a statistically significant increase in the distance comparing the first and second walks, as shown in Figures 2 and 3. However, there was no significant difference between the second and third HW and TM SMWT. Tests of repeated measures of the distance walked in the HW and TM SMWTs were similar, and the coefficients of variation for the HW and TM SMWTs were not significantly different.

View larger version (9K): [in this window] [in a new window]   Figure 2.   Repeated hallway (HW) 6-min walk distance.

View larger version (10K): [in this window] [in a new window]   Figure 3.   Repeated treadmill (TM) 6-min walk distance.

	DISCUSSION

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A treadmill SMWT protocol was developed as an alternative to the cumbersome standard hallway SMWT. We assessed the feasibility of a TM SMWT protocol and compared the results with those obtained from a standard HW SMWT. The TM SMWT was technically simple to perform and was tolerated by all subjects. However, we found a statistically significant difference between the two tests, with the majority of patients walking a greater distance in the HW. Therefore, the two tests are not interchangeable. It is possible that different skills (i.e., adjustment of lower extremity speed with an upper extremity motion) may have influenced the results.

Although the TM and HW SMWT distances were statistically different, it is debatable what difference in distance is clinically significant. Guyatt and colleagues (7) studied this and concluded that 54 meters or 177 feet is the smallest difference that is noticeable to patients when comparing their walking abilities with those of other subjects. Using this cutoff, 11 of the 21 subjects had a clinically significant difference between the HW and TM SMWT distances. The clinical relevance of smaller differences may be argued, although most clinicians would agree that the larger differences we found in some subjects were significant. We attempted to find clinical characteristics that would predict the difference between the two tests but were unable to find any factors. Age, sex, FEV₁ and oxygen use did not correlate with the differences in the two tests by multiple linear regression analysis. Time from initial referral to rehabilitation as a marker of familiarity with the treadmill was also included as an independent variable. Once again it was not a significant predictor of the difference in the two tests by regression analysis. Although more severely impaired subjects may have a greater difference between the HW and TM SMWTs in comparison with less impaired subjects we were unable to find data to support this possibility. Analysis of the data with different markers of impairment, including need for oxygen and whether subjects traveled more or less than 1,000 ft in the HW were not significant predictors of the difference between the HW and TM SMWTs. However, more subjects may be required to adequately prove or disprove this hypothesis.

There are several possibilities that could explain the differences in the two tests. The two tests may require different skills. Familiarity with the TM may be a factor, although time in rehabilitation was not a predictor of difference between the HW and TM SMWT distance when we performed the multiple linear regression analysis. Perhaps some patients are more comfortable with TM use and speed manipulation than are others regardless of time spent using a TM. Walking in a corridor utilizes a skill that is used daily and therefore more familiar to patients than walking on a TM. Also, ambulating in a HW requires little conscious effort to speed up, whereas speed manipulation on a TM requires an additional voluntary movement.

Motivation is also a factor, but it is difficult to measure. Motivation should be similar for the two tests, but perhaps motivation may be superior in a task that is familiar to the subject (i.e., walking without a treadmill). It has previously been shown that encouragement during the SMWT can effect the results, but patients received a similar prompt during both the HW and TM tests (8).

There are well known learning effects in the HW SMWT. Prior studies have shown a marked increase in SMWT distance measured over several days. The majority of the difference occurred during the first three walks (9). We assessed the reproducibility of the TM SMWT and compared the results with the HW SMWT reproducibility. The overall variability of the three walks was similar for the HW and the TM SMWT. In addition, a significant increase was found comparing the first and second HW and TM SMWTs. There was no significant difference between the second and third HW or TM SMWTs.

Comparison of the TM and HW SMWTs was first reported in 1985. Guz and colleagues (10) performed a 12-min TM walk test and compared the results with those of a 12-min corridor walk performed on the same day. The mean 12-min TM distance was 742 meters, whereas the mean distance covered in the HW was 791 meters. They found no statistically significant difference in the distance covered, and the TM test was repeatable after the first walk. Therefore, they concluded that the 12-min TM walk test is a simple repeatable test with advantages over the corridor walk. However, only 10 patients were included in the study, so perhaps a significant difference would have been found if more patients were studied. More recently, Swerts and colleagues (11) compared results in 11 patients with severe COPD using a 12-min corridor and TM walk test. They found that significantly higher distances were covered during the corridor walk than during the TM walk. However, patients performed only one practice TM and HW walk prior to enrollment. Then each subject performed one TM and one HW in random order on two consecutive test days. Performing only one SMWT on a single test day may not reflect a patient's best performance since it has been shown that significant variation may occur if repeated walks are performed. These subjects were not encouraged on either test, which may also have yielded a less than maximal effort. It was not reported whether the control panel of the treadmill was covered during the SMWT.

The ideal test for measuring functional capacity in patients with chronic respiratory disease should provide an objective measure of disability but be simple, inexpensive, reproducible, and responsive to declines or improvement in lung function because of interventions. Although desirable, but not necessary, such a test might also correlate with quality of life measurements and survival. Initially a 12-min walk test was performed in which a patient simply ambulated in a hall and the distance traveled was recorded (1). It was then shown that a 6-min walk test correlated well with the longer 12-min walk test (3). Since that time supplemental oxygen for hypoxemia and monitoring equipment have become standard features of the 6-min walk test. A portable step test has been utilized in which the subject stepped up and down a 12-inch step every 4 s (12). A shuttle walking test has also been described in which the subject walks up and down a 10-meter course with the walking speed dictated by an audio signal (13). There are no great advantages to any of the above tests. The TM SMWT on the other hand is closely related to the HW SMWT in that it reflects walking ability and is more practicable. No transport of monitoring equipment, oxygen, or subjects is required. A second technician is unnecessary since all monitors and oxygen source are stationary. Less physical space is needed for a TM than for the 100-foot corridor needed for the HW SMWT. Therefore, there are distinct advantages to a TM SMWT.

In conclusion, the TM SMWT is feasible and simple to perform. Patient monitoring is easier because of less transport of equipment prior to and during the SMWT. There is a significant difference in the HW and TM SMWT distance. Therefore the two studies are not interchangeable. We were unable to find any factors that predict this difference. In addition, the TM SMWT is reproducible from test to test within a single session, similar to the HW SMWT. Further studies should be directed towards examining the reproducibility of the TM SMWT over time and its potential role in assessing and following patients with chronic lung disease.