Background: Distance walked during the 6-min walk test (6MWT) predicts mortality in COPD. The body weight of the patient affects the work required to walk. Calculated work during the 6MWT (6MWT work) may account for differences in walk distance resulting from change in body weight. Thus, 6MWT work might be a better predictor of mortality than distance walked. This study was designed to test this hypothesis and to assess if other variables measured during the 6MWT, like continuous oximetry recording, offered additional prognostic information.
Methods: This was a retrospective analysis of prospectively collected data; 104 COPD patients were studied. 6MWT was performed in all cases. 6MWT work was calculated as body weight (in kg) × distance walked (in m). Receiver operating characteristic curves were used to assess the value of variables to predict mortality. Additional analysis was performed using Kaplan-Meier survival plots and Cox proportional hazards regression models.
Results: Mean follow-up was 590 ± 472 d. Eleven subjects (10.6%) died. 6MWT work was not better than distance walked to predict mortality (area under the curve 0.77 for 6MWT work vs 0.80 for distance; difference 0.03, 95% CI -0.05 to 0.12, P = .45). Subjects who died had more dyspnea (measured using the Borg scale) after the 6MWT (8.5 vs 4.0, P < .001), lower baseline SpO2 (85% vs 93%, P = .001), worse oxygen saturation during the 6MWT (mean SpO2 while walking 74.0% vs 86.6%, P = .02) and walked less distance (255 m vs 480 m, P = .001). On multivariate analysis, only 6MWT distance and dyspnea after the test correlated independently with mortality (P = .005 for both variables).
Conclusions: 6MWT work was not more useful than 6MWT distance to predict mortality. The study confirms that 6MWT distance and dyspnea on exertion are key elements in prognostic evaluation in COPD, while the value of exercise oxygen desaturation is less clear.
Keywords: 6-min walk test; chronic obstructive; mortality; oximetry; prognosis; pulmonary diseases; work capacity evaluation.