Prognostic ability of the distance-saturation product in the 6-minute walk test in patients with chronic obstructive pulmonary disease

Introduction: The product (DSP) of the distance walked (meters) and minimum oxygen saturation obtained during the 6-minute walk test (6MWT) has been proposed as a predictor of mortality in idiopathic pulmonary fibrosis and in bronchiectasis not related to cystic fibrosis.

Objective: The aim of this study was to determine the DSP's ability to predict mortality in patients with chronic obstructive pulmonary disease (COPD) at the outpatient level and compare it to the BODE index and meters walked in the 6MWT.

Material and methods: Descriptive observational study in a cohort of patients with COPD being treated at outpatient pulmonology clinics. Each of the patients completed the 6MWT following ATS/ERS protocols and their BODE index and DSP were calculated.

Results: About 103 patients were included. The average length of follow-up was 36 months. Patients who died showed a lower number of meters walked in the 6MWT (P < 0.001), as well as a lower DSP (P < 0.001). A 6MWT < 334 m, a DSP < 290 and a BODE ≥ 4 showed good prognostic ability at 3 years (AUC 71%, 69% and 70.4%, respectively). The 6MWT was superior to the BODE index in predicting mortality during the first year of follow-up (P = 0.023). We did not find any differences between DSP and meters walked in the 6MWT.

Conclusions: The DSP is a good predictor of mortality, although it does not offer a better prognostic ability than that of meters walked in the 6MWT.