The present study was aimed to elucidate the relationships between the capacity of oxygen uptake and low atmospheric pressure.
Four anesthetized male dogs were exposed to low pressure of different grades ; 560 mmHg, 460 mmHg, 360 mmHg, and 260 mmHg in a decompression chamber. Blood was drawn from femoral artery and vein through polyethylen tube to outside the chamber. The tube was filled with heparin in order to avoid coagulation when it was not in use. The blood was subjected to determination of oxygen content and oxygen capacity by means of Van Slyke method. Oxygen tension was determined from Hb-Oxygen dissociation curve at pH 7.40.
Arterial and venous oxygen content decreased with lowering the pressure, and the rate of decrease was higher in CaO₂ than in CvO₂. Thus Ca-vO₂ difference decreased from 5.5 vol% (at the control level) to 2.6 vol% (at 260 mmHg.) .
Though the mean oxygen capacity was 20.0 vol% and did not change at 560 mmHg, a significant increase was found at 460 mmHg and 360 mmHg ; they were 20.3 vol% and 20.9 vol% respectively.
PaO₂ fell parallel with lowering of environmental pressure from 92.0 mmHg at 760 mmHg to 29.7 mmHg at 260 mmHg ambient pressure. As venous oxygen tension exhibited a slight decline in contrast to PaO₂, Pa-vO₂ difference became smaller according to lowering the environmental pressure. The difference was 41.4 mmHg at the control level and 4.2 mmHg at 260 mmHg.
In conclusion, we should say that there seems to be two phases in the change of DLO₂ in hypoxic condition. In the first phase, the diffusion gradient decreases lineally with the ambient pressure and VO₂ is unchanged or shows only a slight decrease even in a relatively lower pressure. There may be an increase of DLO₂ accompanied with increase of cardiac output. In much lower ambient pressure, however, there appears the other phase in which QH and also DLO₂ decrease and in turn an apparent decrease of VO₂ can be seen.