Mechanisms of Exertional Dyspnea in Patients with Mild COPD and a Low Resting DL.

Patients with mild chronic obstructive pulmonary disease (COPD) and lower resting diffusing capacity for carbon monoxide (DL) often report troublesome dyspnea during exercise although the mechanisms are not clear. We postulated that in such individuals, exertional dyspnea is linked to relatively high inspiratory neural drive (IND) due, in part, to the effects of reduced ventilatory efficiency. This cross-sectional study included 28 patients with GOLD I COPD stratified into two groups with ( = 15) and without ( = 13) DL less than the lower limit of normal (<LLN; Global Lung Function Initiative criteria) and 16 healthy controls. We compared dyspnea (Borg scale), IND (by diaphragm electromyography), ventilatory equivalent for CO (/CO), and respiratory mechanics during incremental cycle exercise in the three groups. Spirometry and resting lung volumes were similar between COPD groups. During exercise, dyspnea, IND and /CO were higher at equivalent work rates (WR) in the DL<LLN group compared with the other two groups (all  < 0.05). In patients with DL<LLN, severe respiratory mechanical constraints, indicated by end-inspiratory lung volume of approximately 90% of total lung capacity, occurred at a lower WR than the other two groups ( < 0.05). The dyspnea/IND relationship was similar across groups; therefore, the increased dyspnea at a standardized WR in the low DL<LLN group reflected the higher corresponding IND. Higher dyspnea ratings in patients with mild COPD and DL<LLN were associated with higher IND and /CO at a given work rate. Higher ventilatory requirements in the DL<LLN group accelerated dynamic mechanical abnormalities earlier in exercise, further increasing IND and dyspnea.