Respiratory timing and dynamic hyperinflation during incremental exercise in patients with COPD

Abstract

Background: The ventilatory limitation in patients with COPD is related to expiratory flow limitation (EFL) and static and dynamic lung hyperinflation (DH). Respiratory timing (duty cycle) is the ratio of inspiratory time (T i ) to total time of a breath (T tot ). A shorter duty cycle could compensate for EFL and DH by giving more time for expiration. Aim: The aim of this study was to examine the relationship between T i /T tot and EFL and DH in patients with COPD during incremental exercise. We hypothesised that T i /T tot was negatively related to DH and EFL. Methods: Sixty three patients (35 men), mean (SD) age 66 (6)yrs and mean (SD) FEV 1 48 (15)% of predicted, performed an incremental treadmill exercise test. DH was measured as the change in inspiratory capacity (ΔIC) from rest to peak exercise, and EFL as the fraction of the tidal volume where flow was equal to or higher than the maximal expiratory flow rate. The relationship between T i /T tot and tidal volume was analyzed for each subject and fitted to a linear regression line. The relationship between T i /T tot and the explanatory variables age, sex, height, weight, ΔIC, EFL and FEV 1 were analyzed by multivariate linear regression analysis. Results: T i /T tot was constant throughout the exercise test with a coefficient of variation less than 5% for each subject. Mean T i /T tot was 0.40 (SD=0.04). In multivariate regression analysis, T i /T tot was related to ΔIC (Standardized Beta (SB) -0.35, p=0.009), FEV 1 (SB 0.54, p=0.004) and EFL at rest (SB -0.29, p=0.04), but not to EFL at peak exercise. Conclusion: T i /T tot was lower with increasing airway obstruction, DH and resting EFL, which could be an adaptive phenomenon allowing more time for expiration.