Static hyperinflation and decline in functional capacity in COPD patients

Introduction: Pulmonary hyperinflation contributes to dyspnea, morbidity and mortality in chronic obstructive pulmonary disease (COPD). The inspiratory-to-total lung capacity (IC/TLC) ratio is a measure of static hyperinflation and is strongly associated with dynamic hyperinflation and exercise tolerance. However, knowledge of its effect on longitudinal change in the 6-minute walk distance (6MWD) in COPD is scarce. Aim: We aimed to study if IC/TLC ratio predicted longitudinal change in 6MWD. Methods: This prospective observational cohort study included 389 patients aged 40-75 yrs with clinically stable COPD in GOLD stages II-IV. 6MWDs were measured at baseline, and after 1 and 3 yrs. Predictor variables at baseline included IC/TLC ratio, spirometry, fat mass index (FMI) and assessment of smoking habits and exacerbations by questionnaires. Generalized estimating equations (GEE) analyses were used to analyze predictors for change in 6MWD. Results: Mean (SE) 6MWD at baseline and after 1 and 3 yrs were 423 (5.7), 431 (6.1) and 400 (7.8) m, respectively. In the multivariable adjusted GEE analysis, there was a predicted annual decline of 59,5 m and the IC/TLC ratio also predicted decline in 6MWD over 3-yrs (interaction IC/TLC*Time, B=87 m, p=0.017). With a 0.1-unit decrease in baseline IC/TLC ratio, the annual decline of 6MWD increased with 8,7 m. Other predictors for reduced 6MWD were reduced FEV 1 (p=0.001), increased age (p=0.002), increased FMI (p=0.001), and >2 exacerbations within 12 months prior to inclusion (p=0.006). Conclusion: The level of static hyperinflation at baseline independently predicted longitudinal decline in functional capacity in COPD patients.