The Influence of the Haldane Effect on Alveolar CO₂ Tension Equilibrated with Mixed Venous Blood in Man

抄録

When blood in the pulmonary capillary is oxygenated in hypercapnic air, P_CO2 in the red cell has been thought to exceed alveolar P_CO2 due to the Haldane effect, inducing outward CO₂ diffusion.As long as the inward CO₂ diffusion and, consequently, HCO₃-formation are prevented in the red cell, the CO₂ gain in plasma is reduced down to the level predicted from a CO₂ dissociation curve of separated plasma. Therefore, if the direction of the CO₂ diffusion is not reversed during the contact time, the virtual venous P_CO2 (P_Eq), where the CO₂ loss due to the Haldane effect is balanced with the gain due to the venoalveolar P_CO2 gradient, becomes higher than the oxygenated venous P_CO2 in proportion to the CO₂ difference between the true and separated plasma. In order to verify the validity of the above assumption, the P_Eq value was measured in normo- and hypercapnia by using the Defares'extrapolation method in six normal subjects.The results obtained revealed that the P_Eq estimated in hypercapnia was obviously higher than that in normo-capnia. The above difference was significantly greater in normoxia than in hypoxia.Furthermore, it agreed fairly well with the theoretical difference presumed by taking the difference in CO₂ content between separated and true plasma and the R.Q.effect on the alveolar gas volume into account, suggesting that the inward CO₂ diffusion following the oxygenation reaction could be disregarded in normoxic hypercapnia.