Percent predicted value for the 6-minute walk test: Using norm-referenced equations to characterize severity in persons with CHF

Background: The 6-minute walk test (6MWT) is a widely used measure of functional capacity in patients with chronic heart failure (CHF). Norm-referenced equations that predict the 6-minute walk distance (6MWD) according to age, height, weight, and gender have been proposed for healthy patients. We explored whether these equations apply to CHF patients. Methods and Results: The sample consisted of 213 patients newly admitted to specialized CHF clinics in Montreal, Canada. Percent predicted value (PPV) for 6MWD was calculated using norm-referenced equations. We explored correlations between different measures: PPV, 6MWD, Minnesota Living with Heart Failure Quality of Life score (MLHF-QOL). We compared severity among different age, gender, and BMI (body mass index) subgroups and assessed consistency using different measures of severity. Mean age was 65.5 years and 77.5% were men. Compared with the 6MWD, PPV had a slightly better correlation with MLHF-QOL score (-0.26 versus -0.20), and slightly more predictive power in linear regressions (adjusted r(2) = 6.5% versus 4.2%). When PPV was used to differentiate severity between different age, gender, and BMI subgroups, it consistently led to similar conclusions as the MLHF-QOL score, unlike 6MWD. Conclusion: The 6MWD in meters may give misleading results when used as an indicator of severity of CHF condition to compare groups with different sex, age, and BMI distributions. It may be necessary to standardize it using norm-referenced equations.