Multivariable assessment of the 6-min walking test in patients with chronic obstructive pulmonary disease

Functional exercise tolerance in patients with chronic obstructive pulmonar y disease (COPD) is often assessed by the 6-min walking test (6MWT). To ass ess if the use of multiple factors adds to walking distance in describing p erformance in the 6MWT, an exploratory factor analysis was performed on phy siological measurements and dyspnea ratings recorded during testing. Eighty -three patients with mild to severe COPD performed repeated 6MWTs before in patient pulmonary rehabilitation. Factor analysis on 15 variables yielded a stable four-factor structure explaining 78.4% of the total variance. Recor ded heart rate variables contributed to factor 1 (heart rate pattern), walk ing distance, heart rate increase, and decrease contributed to factor 2 (en durance capacity), oxygen desaturation variables contributed to factor 3 (i mpairment of oxygen transport), and dyspnea and effort variables contribute d to factor 4 (perceived symptoms). Walking distance decreased in half of t he 53 patients measured posttreatment, but self-perceived change in exercis e tolerance improved in 84% and was explained by change in walking distance , by less desaturation, and by less dyspnea (R-2 = 0.55, p = 0.005). Qualit ative analysis showed that 29 of 53 patients improved in three or four fact ors. Performance in the 6MWT can be described with four statistically indep endent and clinically interpretable factors. Because clinically relevant ch anges consist of more than only walking distance, assessment of functional exercise tolerance in patients with COPD improves by reporting multiple var iables.