The 6-min walk (6MW) test is commonly used to assess exercise capacity in patients with COPD and to track functional change resulting from disease progression or therapeutic intervention. Not surprisingly, distance covered has been the preferred outcome for this test. However, distance walked does not account for differences in body weight that are known to influence exercise capacity.
Objective: The aim of this study was to evaluate the 6-min distance × body weight product (6MWORK) as an improved outcome measure with a solid physiologic foundation.
Patients and methods: One hundred twenty-four men and women with moderate-to-severe COPD volunteered and completed the testing sequence, which included pulmonary function, a peak effort ramp cardiopulmonary exercise study with gas exchange, and the 6MW. Means and SD were generated for the variables of interest. Differences were analyzed using analysis of variance techniques. Correlation coefficients and receiver operating characteristic (ROC) curves were calculated for the 6-min walk distance (6MWD) and 6MWORK with indexes of pulmonary function, work performance, and Borg scores for dyspnea and effort.
Results: Men and women presented with a significant smoking history that also differed by gender (48 vs 66 pack-years, respectively; p < 0.01). The mean (± SD) FEV1 values were 45 ± 12.6% and 48 ± 12.1%, respectively (not significant), while the diffusing capacity of the lung for carbon monoxide (Dlco) was 14.7 ± 6.1 vs 10.3 ± 3.9 mL/min/mm Hg, respectively (p < 0.001), for men and women. The 6MWD averaged 416.8 ± 79.0 m for men and 367.8 ± 78.6 m for women, and these differences were significant (p < 0.002). When 6MWD was compared as the percent predicted of normal values, each gender presented with a similar reduction of 78.6 ± 14.5% vs 79.9 ± 17.5% (p > 0.05), respectively. 6MWORK averaged 35,370 ± 9,482 kg/m and 25,643 ± 9,080 kg/m (p < 0.0001) for men and women, respectively. 6MWORK yielded higher correlation coefficients than did 6MWD when correlated with Dlco, lung diffusion for alveolar ventilation, FEV1, FEV1/FVC ratio, watts, peak oxygen uptake, peak minute ventilation, and peak tidal volume. The ROC curve demonstrated that 6MWORK had a significantly larger calculated area under the curve (p < 0.05) [plot of 100-sensitivity to specificity for each variable of interest for all subjects] than 6MWD when differentiating an objectively selected definition of low work capacity vs high work capacity (bike ergometry work, < 55 vs > 55 W, respectively).
Conclusions: We conclude that work calculated as the product of distance × body weight is an improved outcome measure for the 6MW. 6MWORK can be used whenever the 6MW is required to estimate a patient’s functional capacity. This measure is also a common measure, which can be converted to indexes of caloric expenditure for direct cross-modality comparisons.