Determinants of functional, peak and endurance exercise capacity in people with chronic obstructive pulmonary disease

Background: Exercise limitation in COPD is multi-factorial. The contribution of cardiac function to exercise capacity is not clear. Methods: Potential determinants of the different constructs of exercise capacity (functional, peak and endurance exercise capacity using the 6-minute walk distance (6MWD), peak oxygen uptake (VO(2)peak) and peak work load, and sub-maximal constant work rate test (CWRT) cycle time, respectively) were analysed. Results: Data were collected in 516 people with COPD (56% male, age (mean +/- SD) 64 +/- 9 years, FEV1% predicted 49 +/- 20%). Participants had reduced exercise capacity (6MWD 424 +/- 124 m, 67 +/- 18% predicted; VO(2)peak 1090 +/- 414 ml/min, 66 +/- 30% predicted; peak work load 70 +/- 34 watts, 56 +/- 27% predicted; CWRT cycle time (median (IQR)) 225 (169-328) seconds). Cardiac function measures, including echocardiography and N-terminal pro-brain natriuretic peptide level, were independently significantly correlated with exercise capacity. In multiple regression analyses adjusted for age and gender, 72% (p < 0.001) of the 6MWD variance could be explained, with the timed up-and-go test accounting for 32% of the variance. For VO(2)peak, 60% (p < 0.001) of the variance could be explained, with FEV1 accounting for 30% of the variance. Quadriceps total work was a significant determinant of all exercise tests. Conclusions: Cardiac function is related to exercise capacity, however is not a primary determinant. Determinants of the different constructs of exercise capacity vary, but there is a strong relationship between quadriceps force and functional and endurance exercise performance, and between lung function and peak exercise capacity.