The effect of carbon dioxide on the changes in blood pH during hypoxia

The change in blood pH during hypoxia was examined using rabbits. Rabbits were divided into 4 groups according to the test gases used (O2 5.1%: CO2 5.4%: N2 89.5%, O2 4.9%: N2 95.1%, O2 2.0%: CO2 5.6%: N2 92.4% and O2 1.9%: N2 98.1%). After an intravenous injection of urethane, the rabbit was fixed on its back on an operating table and the trachea was cannulated. Animals inhaled test gases through a cylindrical unidirectional valve box connected to the cannula. Blood samples were drawn from the catheterized femoral artery. The duration of exposure was 90 min. The animals in the CO2-added 5% O2 group survived the exposure, while only 2 animals survived in the CO2-free 5% O2 group. There was a difference of about 5 mmHg in the blood PO2 between both 5% O2 groups. There was also a marked difference in the time course of changes in the hydrogen ion [( H+]) concentrations between the groups. The [H+] in the CO2-free group began to increase after an initial fall and exceeded the value of the other group, reaching acidotic levels. The acidosis (metabolic) was considered to have been caused by the accumulation of lactic acid. Under the present experimental conditions the contribution of hypoxia to this acidosis was considered to be greater than that of hypocapnia. The animals in the 2% O2 groups died by the 11 min after the start of exposure. In the CO2-free group neither hypercapnia nor acidosis was observed during the exposure. The PO2 values at apnea were about 10 mmHg in both groups.