The 6-min walk distance in healthy subjects: reference standards from seven countries

The 6-min walk distance (6MWD) predicted values have been derived from small cohorts mostly from single countries. The aim of the present study was to investigate differences between countries and identify new reference values to improve 6MWD interpretation. We studied 444 subjects (238 males) from seven countries (10 centres) ranging 40-80 yrs of age. We measured 6MWD, height, weight, spirometry, heart rate (HR), maximum HR (HRmax) during the 6-min walk test/the predicted maximum HR (HRmax % pred), Borg dyspnoea score and oxygen saturation. The mean +/- SD 6MWD was 571 +/- 90 m (range 380-782 m). Males walked 30 m more than females (p<0.001). A multiple regression model for the 6MWD included age, sex, height, weight and HRmax % pred (adjusted r(2)=0.38; p<0.001), but there was variability across centres (adjusted r(2)=0.09-0.73) and its routine use is not recommended. Age had a great impact in 6MWD independent of the centres, declining significantly in the older population (p<0.001). Age-specific reference standards of 6MWD were constructed for male and female adults. In healthy subjects, there were geographic variations in 6MWD and caution must be taken when using existing predictive equations. The present study provides new 6MWD standard curves that could be useful in the care of adult patients with chronic diseases.