Tidal volume expandability affected by flow, dynamic hyperinflation, and quasi-fixed inspiratory time in patients with COPD and healthy individuals

Exertional dyspnea (ED) and impaired exercise performance (EP) are mainly caused by dynamic hyperinflation (DH) in chronic obstructive pulmonary disease (COPD) patients by constraining tidal volume expansion at peak exercise (V(Tpeak)). As V(Tpeak) is the product of inspiratory time (T(Ipeak)) and flow (V(T)/T(Ipeak)), it was hypothesized that V(Tpeak) and V(Tpeak)/total lung capacity (V(Tpeak)/TLC) may be affected by T(Ipeak) and V(T)/T(Ipeak). Hence, the study investigated the (1) effect of T(Ipeak) and V(T)/T(Ipeak) on V(Tpeak) expansion, (2) factors associated with T(Ipeak), expiratory time (T(Epeak)), V(T)/T(Ipeak), and V(Tpeak)/TLC, and (3) relationships between V(T)/T(Ipeak) and V(Tpeak)/TLC with ED and EP in COPD patients and controls. The study enrolled 126 male stable COPD patients and 33 sex-matched controls. At peak exercise, T(Ipeak) was similar in all subjects (COPD versus controls, mean ± SD: 0.78 ± 0.17 s versus 0.81 ± 0.20 s, p = NS), whereas the COPD group had lower V(T)/T(Ipeak) (1.71 ± 0.49 L/s versus 2.58 ± 0.69 L/s, p < .0001) and thus the COPD group had smaller V(Tpeak) (1.31 ± 0.34 L versus 2.01 ± 0.45 L,p < .0001) and V(Tpeak)/TLC (0.22 ± 0.06 vs 0.33 ± 0.05, p < .0001). T(Ipeak), T(Epeak,) and V(T)/T(Ipeak) were mainly affected by exercise effort, whereas V(Tpeak)/TLC was not. T(Epeak,) V(T)/T(Ipeak), and V(Tpeak)/TLC were inversely changed by impaired lung function. T(Ipeak) was not affected by lung function. Dynamic hyperinflation did not occur in the controls, however, V(Tpeak)/TLC was strongly inversely related to DH (r = −0.79) and moderately to strongly related to lung function, ED, and EP in the COPD group. There was a slightly stronger correlation between V(Tpeak)/TLC with ED and EP than V(T)/T(Ipeak) in the COPD group (|r| = 0.55–0.56 vs 0.38–0.43). In summary, T(Ipeak) was similar in both groups and the key to understanding how flow affects lung expansion. However, the DH volume effect was more important than the flow effect on ED and EP in the COPD group.