The 6-minute walk test is highly reproducible during trials of patients with interstitial lung disease secondary to scleroderma, but it may not be a valid outcome measurement because it correlates poorly with other standard physiologic measurements for lung functioning, reported Dr. Maya H. Buch, of the University of Michigan Scleroderma Program in Ann Arbor, and her colleagues.
In a study of 163 patients with interstitial lung disease (ILD) secondary to scleroderma (SSc), researchers had each participant perform a 6-minute walk test (6MWT). In a space without additional oxygen, they were instructed to walk back and forth in a hallway as many times as they could for 6 minutes. Participants were allowed to stop, slow down, and rest as needed. The test was stopped if patients were exhausted or experienced chest pain, leg cramps, or diaphoresis. Those with very mild or very severe lung impairment were excluded (Ann. Rheum. Dis. 2006 [Epub ahead of print]; doi:10.1136/ard.2006.054866).
The data were taken from the results of a 12-month, double-blind, placebo-controlled study that compared the drug bosentan with a placebo for the treatment of SSc-ILD. Participants came from 29 centers in 10 countries. There were 122 women and 41 men; 95 (58%) patients were classified as having diffuse SSc and the remaining 42% had limited SSc. Their mean age was 52.3 years, and the mean disease duration was 6.4 years.
When it came to the 6MWT, 152 participants had less than 15% variability between their first and second tests. Eleven patients required a third test, and there was less than 15% variability between their second and third 6MWT. Mean distance walked was 396.6 m on test one, compared with 399.5 m on test two. Mean overall distance for the two walks was 398 m, and “the mean absolute difference for an individual patient between the two walks was 20.75 m,” said Dr. Buch and her colleagues.
After the 6MWT, a rating of dyspnea was measured using a Borg scale. After walk one, the mean Borg dyspnea index was 2.75, and after walk two it was 2.79. The overall mean Borg score was 2.77, with a mean absolute difference between the assessments of 0.8. According to the researchers, the mean percentage predicted diffusing capacity was 46% and mean percentage predicted forced vital capacity (FVC) was 71%.
There was only a minimal correlation between the 6MWT and the Borg dyspnea scale. There was a weak correlation between the 6MWT and percentage predicted FVC, and the walk test did not correlate at all with mean percentage predicted diffusing capacity. Also, there was a “very weak correlation” between mean percentage predicted diffusing capacity and the Borg dyspnea scale.
“The primary finding of this study is the high intertest reproducibility of the test. We, however, failed to demonstrate correlation of the 6MWT with pulmonary function parameters. This raises the question of what exactly is being measured by the test,” commented Dr. Buch and her colleagues.
They speculated that “multiple confounding variables in the assessment of SSc-ILD” might account for the failure to establish a correlation between the 6MWT and the traditional lung function parameters.
These variables would include subtle cardiopulmonary responses and comorbidity not detected by the lung function tests. Other possibilities not related to SSc that might account for the lack of correlation include variability in FVC and interlaboratory variation in measurement of mean percentage predicted diffusing capacity.
Additionally, participants may have exhibited different psychological responses to exercise, which “could lead to either a training effect or decompensation. Separately, temporal and adaptive changes in breathing patterns could also facilitate exercise performance,” said the researchers. They concluded that further studies and greater “clarification of the relative contributions of nonpulmonary manifestations of SSc on exercise capacity” are needed before the 6MWT could be accepted as a valid assessment criterion, the investigators said.