Absence of dynamic hyperinflation during exhaustive exercise in severe COPD reflects submaximal IC maneuvers rather...

Approximately 20% of chronic obstructive pulmonary disease (COPD) patients have been considered to have a "nonhyperinflator phenotype." However, this judgment depends on patients making a fully maximal inspiratory capacity (IC) maneuver at rest, since the IC during exercise is compared with this baseline measurement. We hypothesized that IC maneuvers at rest are sometimes submaximal and tested this hypothesis by measuring IC and associated neural respiratory drive at rest and during inhalation of CO₂ and exercise in patients with COPD. Twenty-six COPD patients [age 66 ± 6 yr, mean forced expiratory volume in 1 s (FEV₁) 40 ± 11% predicted] and 39 healthy subjects (age 39 ± 14 yr, FEV₁ 98 ± 12% predicted) were studied. IC and the diaphragm electromyogram (EMG_di) associated with it (EMG_di-IC) and forced inspiratory vital capacity (FIVC) and its corresponding EMG_di (EMG_di-FIVC) were measured during inhalation of 8% CO₂ (8% CO₂-92% O₂) and room air. Incremental exhaustive cycle ergometer exercise was also performed in both patients with COPD and healthy subjects. IC, EMG_di-IC, FIVC, and EMG_di-FIVC during breathing 8% CO₂ were significantly greater than those during breathing room air in both patients with COPD and healthy subjects (all P < 0.001). EMG_di-IC in patients with COPD constantly increased during exercise from 145 ± 40 µV at rest to 185 ± 52 µV at the end of exercise but change in IC was variable. Neural respiratory drive and its relevant IC increased during hypercapnia. Exercise-related hypercapnia in patients with COPD raises neural respiratory drives, which compensate for IC reduction, leading to underestimation of dynamic hyperinflation measured by IC at rest breathing room air.NEW & NOTEWORTHY Inspiratory capacity measured during hypercapnia is higher than that during eucapnia. Thus total lung capacity is not always be achieved by a standard inspiratory capacity maneuver, leading to risk of underestimation of dynamic hyperinflation in patients with severe chronic obstructive pulmonary disease after exhaustive exercise.