Carbon Dioxide Tolerance After Adaptation to Hypercarbia

Abstract

Three unanesthetized rhesus monkeys, seated upright, were exposed to graded hypercarbia (30% carbon dioxide per hour) after 72 hours at 4 ± 0.5% CO₂ in an environmental chamber with ambient pressure (4,300-foot elevation), oxygen tension, and temperature. Narcotic and lethal levels of CO₂ were compared to those in three monkeys exposed to 30% CO₂ per hour after 72 hours at ambient air in the same chamber. No significant difference was found between the two groups. When these data were compared to those in three monkeys from a previous study, restrained in the supine position, differences were significant for both narcosis and death, with upright animals more tolerant to CO₂. These findings indicate that CO₂ adaptation fails to extend tolerance to acute exposures to CO₂; however, the upright position affords more protection than does the supine position.