Subjective measurement of physical activity using questionnaires has prognostic value in COPD. However, their lack of accuracy and large individual variability limit their use for evaluation on an individual basis. We evaluated the capacity of the objective measurement of daily physical activity in patients with COPD using accelerometers to estimate their prognostic value.
In 173 consecutive subjects with moderate to very severe COPD, daily physical activity was measured using a triaxial accelerometer providing a mean of 1-min movement epochs as vector magnitude units (VMUs). Patients were evaluated by lung function testing and 6-min walk, incremental exercise, and constant work rate tests. Patients were followed for 5 to 8 years, and the end points were all-cause mortality, hospitalization for COPD exacerbation, and annual declining FEV1.
After adjusting for relevant confounders, a high VMU decreased the mortality risk (adjusted hazard ratio [HR], 0.986; 95% CI, 0.981-0.992), and in a multivariate model, comorbidity, endurance time, and VMU were retained as independent predictors of mortality. The time until first admission due to COPD exacerbation was shorter for the patients with lower levels of VMU (adjusted HR, 0.989; 95% CI, 0.983-0.995). Moreover, patients with higher VMU had a lower hospitalization risk than those with a low VMU (adjusted incidence rate ratio, 0.099; 95% CI, 0.033-0.293). In contrast, VMU was not identified as an independent predictor of the annual FEV1 decline.
The objective measurement of the daily physical activity in patients with COPD using an accelerometer constitutes an independent prognostic factor for mortality and hospitalization due to severe exacerbation.