TISSUE GAS STORES OF THE BODY AND HEAD-OUT IMMERSION IN HUMANS

Breath-by-breath gas exchange was studied in 10 subjects during and af ter transitions between dry conditions and head-out immersion in therm oneutral conditions. Cardiac index (CI) was estimated by means of impe dance cardiography. Previous largely qualitative models of changes in tissue gas stores after blood volume shifts could be confirmed and ext ended to include a quantitative analysis of O2 and CO2 tissue stores. An increase in CI by 47.0% during immersion was associated with an inc rease in the tissue O2 stores by 122 ml/m2 and a decrease in the tissu e CO2 stores by 148 ml/m2. The time constants for the recovery of O2 u ptake (tau(O2)) and CO2 elimination after initial increases after the dry-to-immersion transition were 32.4 and 79.3 s, respectively. The de crease in CI on return to the dry conditions was associated with a dro p in tissue O2 stores and a tau(O2), of 144 s. The increase in tissue O2 stores during immersion as well as the difference in tau(O2) betwee n the two transitions were larger than could be explained by the chang e in CI only. This was attributed to changes in the distributions of p eripheral blood flow and venous blood volume. Compared with the O2 sto res, the decrease in CO2 stores was better predicted by the change in CI. The present results emphasize that the changes in pulmonary and ti ssue gas exchange imposed by head-out immersion transients mainly refl ect movement of gas in and out of body stores.