Influence of body position on pulmonary diffusing capacity in yound and old men

Steady-state diffusing capacity (DLCOSS2), studied in healthy nonsmokers, was significantly higher in five old men when seated than when supine, but was unchanged in five young men. However, "corrected" for the increased ventilation, it decreased in the young--but not in the old--when erect compared with when supine. The change of DLCOSS2 in the young during the first 20 min when seated was related to a change of the fractional dead space ventilation (VD/VT)mThe old showed less venous admixture when seated than when supine, whereas VD/VT and regional ventilation-perfusion index of the lung bases was the same in both positions. The decrease in venous admixture was probably due to the closing capacity (CC) being less than the functional residual capacity (FRC) when seated. The elimination of closed off regions when seated made more pulmonary capillaries available for diffusion.