Activity monitoring and energy expenditure in COPD patients: a validation study

Abstract

There is increasing interest in the objective measurement of physical activity in chronic obstructive pulmonary disease (COPD) patients due to the close relationship between physical activity level, health, disability and mortality. We aimed to (a) determine the validity and reproducibility of an activity monitor that integrates accelerometry with multiple physiologic sensors in the determination of energy expenditure in COPD subjects and (b) to document the independent contribution of the additional physiologic sensors to accelerometry measures in improving true energy expenditure determination. Eight subjects (4 male, FEV(1) 56.4 +/- 14.1%, RV 145.0 +/- 75.7%) performed 2 separate 6-minute walk and 2 incremental shuttle walk exercise tests. Energy expenditure was calculated during each exercise test using the physiologic activity monitor and compared to a validated exhaled breath metabolic system. Test-retest reproducibility of physiologic activity monitor during the walking tests was comparable to an exhaled breath metabolic system. Physiologic sensor data significantly improved the explained variance in energy expenditure determination (r(2)=0.88) compared to accelerometry data alone (r(2)=0.68). This physiologic activity monitor provides a valid and reproducible estimate of energy expenditure during slow to moderate paced walking in a laboratory setting and represents an objective method to assess activity in COPD subjects.

Figures

Figure 1

(Left) Cumulative EE (kcal/min) during ISW (●) and 6 MW (□) tests measured by a physiologic activity monitor (EE-PAM) compared to an exhaled breath metabolic system (EE-BXB) (r2 = 0.86, p < 0.001). (Right) Relationship of the accelerometry only data without physiologic sensors (VMU: vector magnitude units) from the physiologic activity monitor to exhaled breath metabolic system (r2 = 0.68, p < 0.001).

Figure 2

EE (kcal/min) measured by a physiologic activity monitor (—■—) vs. an exhaled breath metabolic system (—◆—) during ISW (left panel) and 6 MW tests (right panel). Bars represent standard errors. Walk speed (feet/second) (——) and accelerometer “only” output (vector magnitude units) (—■—).

Figure 3

Kilocalorie difference between physiologic activity monitor (EE-PAM) compared to an exhaled breath metabolic system (EE-BXB). Data points are minute-by-minute values for both the 1st and 2nd ISW (●) and 1st and 2nd 6 MW (□) tests for all subjects.