Modulation of sensitivity to hyperbaric oxygen by CO2 in newborn rats

We examined the modifying effects of CO2 on the CNS and pulmonary manifestations of hyperbaric oxygenation (HBO) in newborn rats. Four- to seven-day-old rats were exposed to HBO with inspired PCO2 (PICO2) of approximately 0, 60, 90, 140, 190, and 380 mmHg at a total pressure of 5 atm abs. The PCO2 values studied corresponded with 0, 8, 12, 18, 25, and 50 kPa, respectively, at a total pressure of 507 kPa. The O2-CO2 exposures lasted for 2-8 h. Hypercapnia at PICO2 of 60 and 90 mmHg with HBO produced extensive pulmonary damage with a high post-decompression mortality, compared to HBO alone. In contrast, PICO2 at the anesthetic levels of 140 and 190 mmHg attenuated the visible pulmonary and neurologic manifestations of O2 toxicity, and significantly reduced post-decompression mortality, compared to moderate hypercapnia of 60-90 mmHg. Supercapnia, at PICO2 of 380 mmHg, with HBO also produced no visible neurologic effect, but it caused an early apnea with severe pulmonary damage. These data indicate unique and dose-dependent cerebral and pulmonary responses to hyperoxic-hypercapnia in newborn rats. It is speculated that anesthetic levels of hypercapnia may be utilized to improve tissue oxygenation during O2 therapy in newborns, without increasing the risk of pulmonary and CNS O2 poisoning.