Effect of body position on measurements of diffusion capacity after exercise

Background-Pulmonary diffusing capacity for carbon monorride (Dlco), alveol ar capillary membrane diffusing capacity (Dm), and pulmonary capillary bloo d volume (Vc) are all significantly reduced after exercise. Objective-To investigate whether measurement position affects this impaired gas transfer. Methods-Before and one, two, and four hours after incrementa l cycle ergometer exercise to fatigue, single breath Dlco, Dm, and Vc measu rements were obtained in 10 healthy men in a randomly assigned supine and u pright seated position. Results-After exercise, Dice, Dm, and Vc were signi ficantly depressed compared with baseline in both positions. The supine pos ition produced significantly higher values over time for DIco (5.22 (0.13) v 4.66 (0.15) maximum HgCl, p = 0.022) and Dm (6.78 (0.19) v 6.03 (0.19) ml /min/mm Hg/l, p = 0.016), but there was no significant position effect for Vc. There was a similar pattern of change over time for Dice, Dm, and Vc in the two positions. Conclusions The change in Dice after exercise appears to be primarily due t o a decrease in Vc. Although the mechanism for the reduction in Vc cannot b e determined from these data, passive relocation of blood to the periphery as the result of gravity can be discounted, suggesting that active vasocons triction of the pulmonary vasculature and/or peripheral vasodilatation is o ccurring after exercise.