Effects of acute and chronic hypercapnia on oxygen tolerance in rats

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Abstract
Groups of 16-52 normal or CO2-adapted rats were exposed to 100% O2 or to O2 with 60 Torr PI[inspired partial pressure]CO2 (O2-CO2) at pressures of 1.0, 1.5, 2.0, 3.0 and 4.0 ATA [atmospheres of absolute pressure]. Exposure durations for 50% mortality (LD50) in normal rats at 4.0, 3.0, 2.0, 1.5 and 1.0 ATA O2 were 6.3, 9.3, 17.2, 27.4 and 76.1 h, respectively. Corresponding LD50 values for normal rats exposed to O2-CO2 were 2.0, 2.0, 16.3, 24.8 and 74.8 h. Survival times of CO2-adapted rats exposed to O2 were nearly identical to those of normal rats. LD50 values for CO2-adapted rats exposed to O2-CO2 were 4.1, 7.5, 17.9, 23.6 and 65.4 h, respectively. These data confirm acceleration of O2 intoxication by acute hypercapnia at 4.0 and 3.0 ATA, but they show less prominent effects at 2.0, 1.5 and 1.0 ATA. Hypercapnia adaptation clearly has a protective effect in rats exposed to O2-CO2 at 4.0 and 3.0 ATA. At 2.0, 1.5 and 1.0 ATA, where acute hypercapnia has less effect, the effects of CO2 adaptation are less prominent. The observed changes in O2 tolerance can be explained by cerebral vasodilation with increased brain oxygenation in acute hypercapnia and by significant amelioration of this response during chronic hypercapnia.