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 ) and ventilatory efficiency (increased ventilatory requirement for CO clearance [V˙ /V˙CO ]) 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 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˙ /V˙CO , 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 at or greater than the lower limit of normal (≥LLN; Global Lung Function Initiative reference equations [n = 44]) or (2) below the
Results: Spirometry and resting lung volumes were similar in the two COPD groups. During exercise, V˙ /V˙CO (nadir and slope) was consistently higher in the DL  < LLN compared with the other groups (all p < 0.05). The DL  < 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  < 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.