PCO2 threshold for CNS oxygen toxicity in rats in the low range of hyperbaric PO2

Central nervous system (CNS) oxygen toxicity, as manifested by the first electrical discharge (FED) in the electroencephalogram, can occur as convulsions and loss of consciousness. CO2 potentiates this risk by vasodilation and pH reduction. We suggest that CO2 can produce CNS oxygen toxicity at a Po-2 that does not on its own ultimately cause FED. We searched for the CO2 threshold that will result in the appearance of FED at a Po-2 between 507 and 253 kPa. Rats were exposed to a Po-2 and an inspired PCO2 in 1-kPa steps to define the threshold for FED. The results confirmed our assumption that each rat has its own PCO2 threshold, any PCO2 above which will cause FED but below which no FED will occur. As Po-2 decreased from 507 to 456, 405, and 355 kPa, the percentage of rats that exhibited FED without the addition Of CO2 (F-o) dropped from 91 to 62, to 8 and 0%, respectively. The percentage of rats (F) having FED as a function Of PCO2 was sigmoid in shape and displaced toward high PCO2 with the reduction in Po-2. The following formula is suggested to express risk as a function Of PCO2 and Po-2 F = F-o + (100 - Fo)/[1 + (P-50/P-CO2)(N)] [GRAPHICS] where P-50 is the PCO2 for the half response and N is power. A small increase in PCO2 at a Po-2 that does not cause CNS oxygen toxicity may shift an entire population into the risk zone. Closed-circuit divers who are CO2 retainers or divers who have elevated inspired CO2 are at increased risk of CNS oxygen toxicity.