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

Abstract

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.

Keywords: Inspiratory time; breathing pattern; dynamic hyperinflation; exercise capacity; exertional dyspnea; tidal inspiratory flow; tidal volume and total lung capacity ratio.

Figure 1.

Flow chart. A total of 209 subjects were assessed for eligibility. 126 subjects with chronic obstructive pulmonary disease (COPD) and 33 healthy subjects were enrolled. All of the participants with COPD had respiratory symptoms, risk factors and a post-bronchodilator forced expired volume in one second (FEV₁)/forced vital capacity (FVC) of < 0.7 without a significant post-bronchodilator effect. Healthy subjects were recruited among the hospital staff and the local community through personal contacts. They were free of known significant diseases. A total of 50 subjects were excluded due to the reasons shown. For details about the inclusion and exclusion criteria of the participants, please refer to the text.

Figure 2.

Mean values of expiratory time (T_E), inspiratory time (T_I), tidal volume (V_T), tidal inspiratory flow, V_T and total lung capacity ratio (V_T/TLC), and tidal inspiratory flow (V_T/T_I) in the healthy individuals (open circles) and subjects with chronic obstructive pulmonary disease (solid circles) in response to incremental exercise at rest, unloaded, anaerobic threshold, and peak exercise. The AT was indeterminate in 29 (23%) COPD subjects. Bars indicate standard deviations. No symbol, p = NS,^* ≤0.05, ⁺ ≤0.01, ^# ≤0.001, ^$ ≤0.0001 for group comparison.