Revisiting the 6-minute walk test: maximum gait speed–adjusted 6-minute walk distance and long-term outcomes in patients with cardiovascular disease

Background

The 6-minute walk distance (6MWD) is widely used as an indicator of exercise capacity in patients with cardiovascular disease (CVD). Although 6MWD is generally interpreted as reflecting cardiopulmonary function, it has been reported to correlate strongly with maximal gait speed (MGS), measured over short distances, particularly in older patients with CVD, suggesting that 6MWD may not purely reflect endurance alone. In other words, 6MWD may be influenced by baseline walking ability, and its interpretation warrants further consideration. Therefore, we developed an MGS-adjusted 6MWD index (maximum gait speed–modified ratio : MGSMR) to assess its prognostic value in patients with CVD.

Methods

We studied 5,128 patients with CVD who were able to undergo the 6-minute walk test (6MWT) and MGS assessment at discharge.

MGSMR was calculated as mean gait speed during the 6MWT (6MWD/360 s) divided by MGS [MGSMR = (6MWD/360 s)/MGS], representing the percentage of an individual’s maximal walking capacity utilized during the 6MWT.

Patients were classified into MGSMR tertiles (High >79.4%, Intermediate 69.6–79.4%, Low <69.6%). The primary endpoint was 5-year all-cause mortality. Kaplan–Meier analysis (log-rank test) and multivariable Cox proportional hazards models adjusted for clinical covariates were used to examine the association between MGSMR and mortality. Patients were further stratified by 6MWD (<300 m vs ≥300 m), and MGSMR was dichotomized at the median (74.7%) within each stratum.

Results

The median age was 71 years, and 67.4% were male. All-cause mortality decreased stepwise across MGSMR tertiles (Low 15.3%, Intermediate 10.8%, High 8.8%; log-rank P<0.001). Using Low as the reference, higher MGSMR was associated with lower adjusted mortality risk [adjusted HR (95% confidence interval [CI] ): Intermediate vs Low 0.73 (0.61–0.89), P=0.002; High vs Low 0.62 (0.50–0.76), P<0.001]. In stratified analyses, among patients with 6MWD <300 m, MGSMR ≥median was associated with significantly lower adjusted mortality risk compared with MGSMR <median [adjusted HR (95% CI): 0.68 (0.50–0.92); P=0.012], whereas no significant difference was observed among those with 6MWD ≥ 300 m.

Conclusions

MGSMR, a 6MWD index adjusted for MGS, was independently associated with long-term prognosis in patients with CVD. This association was particularly evident among patients with low 6MWD, suggesting the importance of evaluating 6MWD not only as an absolute value but also as a relative performance index against maximal walking capacity.