Reduced exercise tolerance in mild chronic obstructive pulmonary disease: The contribution of combined abnormalities of diffusing capacity for carbon monoxide and ventilatory efficiency

Background and objective The combination of both reduced resting diffusing capacity of the lung for carbon monoxide (DL CO ) and ventilatory efficiency (increased ventilatory requirement for CO 2 clearance [V˙ E / V˙ CO 2 ]) has been linked to exertional dyspnoea and exercise intolerance in chronic obstructive pulmonary disease (COPD) but the underlying mechanisms are poorly understood. The current study examined if low resting DL CO and higher exercise ventilatory requirements were associated with earlier critical dynamic mechanical constraints, dyspnoea and exercise limitation in patients with mild COPD. Methods In this retrospective analysis, we compared V˙ E / V˙ CO 2 , dynamic inspiratory reserve volume (IRV), dyspnoea and exercise capacity in groups of patients with Global Initiative for Chronic Obstructive Lung Disease stage 1 COPD with (1) a resting DL CO at or greater than the lower limit of normal (≥LLN; Global Lung Function Initiative reference equations [ n = 44]) or (2) below the <LLN ( n = 33), and age‐ and sex‐matched healthy controls ( n = 81). Results Spirometry and resting lung volumes were similar in the two COPD groups. During exercise, V˙ E / V˙ CO 2 (nadir and slope) was consistently higher in the DL CO < LLN compared with the other groups (all p < 0.05). The DL CO < LLN group had lower IRV and greater dyspnoea intensity at standardized submaximal work rates and lower peak work rate and oxygen uptake than the other two groups (all p < 0.05). Conclusion Reduced exercise capacity in patients with DL CO < LLN was related to higher ventilatory requirements, a faster rate of decline in dynamic IRV and greater dyspnoea during exercise. These simple measurements should be considered for the clinical evaluation of unexplained exercise intolerance in individuals with ostensibly mild COPD.