The supine position improves but does not normalize the blunted pulmonary capillary blood volume response to exercise in mild COPD.

Patients with mild chronic obstructive pulmonary disease (COPD) demonstrate resting pulmonary vascular dysfunction as well as a blunted pulmonary diffusing capacity (DLCO) and pulmonary capillary blood volume (V) response to exercise. The transition from the upright to supine position increases central blood volume and perfusion pressure, which may overcome microvascular dysfunction in an otherwise intact alveolar-capillary interface. The present study examined whether the supine position normalized DLCO and V responses to exercise in mild COPD. Sixteen mild COPD participants and 13 age-, gender-, and height-matched controls completed DLCO maneuvers at rest and during exercise in the upright and supine position. The multiple -DLCO method was used to determine DLCO, V, and membrane diffusion capacity (D). All three variables were adjusted for alveolar volume (DLCOAdj, VAdj, and DAdj). The supine position reduced alveolar volume similarly in both groups, but oxygen consumption and cardiac output were unaffected. DLCOAdj, DAdj, and VAdj were all lower in COPD. These same variables all increased with upright and supine exercise in both groups. DLCOAdj was unaffected by the supine position. VAdj increased in the supine position similarly in both groups. DAdj was reduced in the supine position in both groups. While the supine position increased exercise VAdj in COPD, the increase was of similar magnitude to healthy controls; therefore, exercise V remained blunted in COPD. The persistent reduction in exercise DLCO and V when supine suggests that pulmonary vascular destruction is a contributing factor to the blunted DLCO and V response to exercise in mild COPD. Patients with mild chronic obstructive pulmonary disease demonstrate a combination of reversible pulmonary microvascular dysfunction and irreversible pulmonary microvascular destruction.