Pulmonary capillary blood volume response to exercise is diminished in mild chronic obstructive pulmonary disease

Previous work suggests that mild chronic obstructive pulmonary disease (COPD) patients have greater lung dysfunction than previously appreciated from spirometry alone. There is evidence of pulmonary microvascular dysfunction in mild COPD, which may reduce diffusing capacity (DLCO) and increase ventilatory inefficiency during exercise. The purpose of this study was to determine if DLCO, pulmonary capillary blood volume (Vc), and membrane diffusing capacity (Dm) are diminished during exercise in mild COPD, and whether this is related to ventilatory inefficiency and dyspnea.

Seventeen mild COPD patients (FEV₁/FVC: 64 ± 4%, FEV₁ = 94 ± 11%_pred) and 17 age- and sex-matched controls were recruited. Ten moderate COPD patients were also tested for comparison (FEV₁ = 66 ± 7%_pred). DLCO, Vc, and Dm were determined using the multiple-fraction of inspired oxygen (F_IO₂) DLCO method at baseline and during steady-state cycle exercise at 40W, 50%, and 80% of V˙O_2peak. Using expired gas data, ventilatory inefficiency was assessed by V˙_E/V˙CO₂.

Compared to controls, mild COPD had lower DLCO at baseline and during exercise secondary to diminished Vc (P < 0.05). No difference in Dm was observed between controls and mild COPD at rest or during exercise. Patients with high V˙_E/V˙CO₂ (i.e. ≥34) had lower Vc and greater dyspnea ratings compared to control at 40W. Moderate COPD patients were unable to increase Vc with increasing exercise intensity, suggesting further pulmonary vascular impairment with increased obstruction severity.

Despite relatively minor airflow obstruction, mild COPD patients exhibit a diminished DLCO and capillary blood volume response to exercise, which appears to contribute to ventilatory inefficiency and greater dyspnea.