Six-minute-walk distance and accelerometry predict outcomes in chronic obstructive pulmonary disease independent of Global Initiative for Chronic Obstructive Lung Disease 2011 Group

Abstract

Rationale: The 2011 combined Global Initiative for Chronic Obstructive Lung Disease (GOLD) assessment incorporates symptoms, exacerbation history, and spirometry in discriminating risk of exacerbations in patients with chronic obstructive pulmonary disease (COPD). Six-minute-walk distance (6MWD) and accelerometry also have been used to assess disease severity in COPD. The association between these measures and the risks of hospitalization and mortality in the context of GOLD 2011 is unknown.

Objectives: To describe changes in exercise tolerance and physical activity over time in patients with COPD and to test the hypothesis that lower baseline 6MWD or accelerometry step count is associated with increased risk of COPD-related hospitalization or all-cause mortality, independent of GOLD 2011 group.

Methods: Physical function and medical outcomes were prospectively assessed in 326 patients with moderate to severe COPD in INSPIRE-II, a randomized controlled trial of a coping skills training intervention. Cox models were used to determine if GOLD 2011 group, 6MWD, or accelerometry steps were associated with risk of COPD-related hospitalization or all-cause mortality.

Measurements and main results: Physical function declined over time in GOLD group D but remained stable in groups A, B, and C. GOLD classification was associated with time to death or first COPD-related hospitalization. Baseline 6MWD was more strongly associated with time to death or first COPD-related hospitalization (hazard ratio, 0.50 [95% confidence interval, 0.34, 0.73] per 150 m, P=0.0003) than GOLD 2011 classification. A similar relationship was observed for accelerometry steps (hazard ratio, 0.80 [95% confidence interval, 0.70, 0.92] per 1,000 steps, P=0.002).

Conclusions: Exercise tolerance and daily physical activity are important predictors of hospitalization and mortality in COPD, independent of GOLD 2011 classification. Physical function may represent a modifiable risk factor that warrants increased attention as a target for interventions to improve clinically meaningful outcomes in COPD.

Keywords: accelerometry; chronic obstructive pulmonary disease; exercise; hospitalization; mortality.

Figure 1.

Changes in exercise tolerance and daily physical activity. Bars represent mean changes in (A) 6-minute-walk distance, and (B) total daily accelerometry steps over 16 weeks, by Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2011 group. Vertical brackets represent SEM changes. *P < 0.05 for pairwise comparison of change in group D versus B. **P < 0.05 for pairwise comparisons of change in group D versus all other groups.

Figure 2.

Risk of chronic obstructive pulmonary disease (COPD)-related hospitalization or all-cause mortality associated with Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2011 classification. Curves represent GOLD 2011 group A (A, n = 27), group B (B, n = 91), group C (C, n = 17), and group D (D, n = 191). P = 0.0007 for Kaplan-Meier log-rank test. Subjects at risk are below the x axis.

Figure 3.

Impact of baseline 6-minute-walk distance on risk of chronic obstructive pulmonary disease (COPD)-related hospitalization or all-cause mortality. Stratified as less than 300 m (n = 96), 300 to 449 m (n = 164), and 450 m or more (n = 66). P < 0.0001 for Kaplan-Meier log-rank test. Subjects at risk are below the x axis.

Figure 4.

Impact of baseline daily accelerometry steps on risk of chronic obstructive pulmonary disease (COPD)-related hospitalization or all-cause mortality. Stratified as less than 2,000 steps (n = 99), 2,000 to 3,999 steps (n = 104), and 4,000 or more steps (n = 121). P < 0.0001 for Kaplan-Meier log-rank test. Subjects at risk are below the x axis.