Lethal physiological effects of carbon dioxide exposure at high concentration in rats.

The acute toxicity of high concentrations of carbon dioxide (CO) was investigated in anesthetized rats using physiological parameters. At an oxygen concentration of 21%, the survival time decreased in a concentration-dependent manner from ≥7.3 h at 20% CO to 1.0 h at 50% CO. The animals were divided into groups that were exposed to 40% CO and 21% O balanced with nitrogen (CO group), 40% CO and 12.6% O (CO-Hypoxia group), 0% CO and 12.6% O (Hypoxia group), and 0% CO and 21% O (Control group) for 3 h. In the CO group, mean blood pressure (MBP) increased temporarily in the first 60 min followed by a gradual decrease, while breathing rate (BR) decreased immediately up to 3 h and the concentration of serum indicators reflecting organ damage increased. Most of these effects progressed in the CO-Hypoxia group. The Hypoxia group showed a contrasting response to the CO groups in MBP and BR, and a slight partial increase in the serum indicators. Histological changes were not observed in any primary organs of any group, except for eosinophilic or necrosis of pyramidal cells in the hippocampal CA1 region of the CO group. These results indicate that high concentrations of CO inhalation are toxic, likely due to BR suppression, and that hypoxia produced under a high CO environment, while showing little effect on its own, enhances the toxic effects of CO.