Application of a new parameter in the 6-minute walk test for manifold analysis of exercise capacity in patients with COPD

BACKGROUND: New parameters in the 6-minute walk test (6MWT) are required for comprehensive analysis of exercise capacity in patients with chronic obstructive pulmonary disease (COPD). The aim of the present study was to apply a novel index, the desaturation distance ratio (DDR), to clinical research on COPD as an estimate of exercise capacity and to examine whether DDR is a potential parameter for manifold analysis of exercise capacity in patients with COPD. METHODS: A total of 41 patients with COPD (median age [interquartile range] =75 [68–79] years; and body mass index [BMI] =22.3 [19.4–23.8] kg/m(2)) participated in the study. The 6MWT was performed along with anthropometric measurements and a pulmonary function test. The “desaturation area” was measured as the total area above the curve created using peripheral oxygen saturation (SpO(2)) values observed at each minute during the 6MWT. Then the DDR was calculated as the ratio of the desaturation area to the 6-minute walk distance (6MWD). RESULTS: The 6MWD was 370 (328–445) m, and the decline in SpO(2) values (ΔSpO(2)) was −5.0% (−8.0% to −1.5%). The DDR correlated modestly with baseline pulmonary function in patients with COPD (forced expiratory volume in 1 second [% of predicted value]: r=−0.658, P<0.001; and diffusing capacity of the lung for carbon monoxide [DL(CO)]: r=−0.470, P=0.002), comparable with the findings of the 6MWD. The DDR correlated well with ΔSpO(2) (r=−0.656, P<0.001) and with the increase in subjective sense of dyspnea during the 6MWT, as assessed by Borg scale scores (ΔBorg) (r=0.486, P=0.001), in contrast with the 6MWD, which was not significantly correlated with ΔSpO(2) and ΔBorg scale scores. CONCLUSION: The DDR is more informative for manifold analysis of exercise capacity associated with oxygen desaturation and subsequent sense of dyspnea by exercise in patients with COPD.