Figure 4.

Inspiratory capacity (IC), inspiratory reserve volume (IRV), tidal volume (V _T), and breathing frequency (F _b) responses versus minute ventilation during constant work rate exercise across the continuum of health and COPD severity. The IC at rest and throughout exercise progressively decreases with advancing disease. Note the clear inflection (plateau) in the V _T-ventilation relationship, which coincides with a simultaneous inflection in the IRV-ventilation relationship. After this point, further increases in ventilation are accomplished by accelerating F _b. Data from Normal subjects and GOLD stage I (i.e., mild COPD) are from Ofir et al. [61]. Quartiles (Q) of COPD severity are based on forced expiratory volume in 1 second (FEV₁) expressed as percent predicted (ranges: Q1 = 54.5–85.1; Q2 = 43.8–54.1; Q3 = 34.9–43.6; Q4 = 16.5–34.9) from O'Donnell et al. [12]. VC, vital capacity; TLC, total lung capacity; GOLD, Global Initiative for Obstructive Lung Disease.