Latency to CNS oxygen toxicity in rats as a function of PCO2 and PO2

Central nervous system (CNS) oxygen toxicity can occur as convulsions and loss of consciousness. without any premonitory symptoms. We have made a quantitative study of the effect of inspired carbon dioxide on sensitivity to oxygen toxicity in the rat. Rats were exposed to four oxygen pressures (PO2; 456, 507, 608 and 709 kPa) and an inspired partial pressure of carbon dioxide (PCO2) in the range 0-12 kPa until the appearance of the electroencephalograph first electrical discharge (FED) that preceeds the clinical convulsions. Exposures were conducted at a thermoneutral temperature of 27 degrees C. Latency to the FED decreased linearly with the increase in PCO2 at all four PO2 values studied. This decrease, which is probably related to the cerebral vasodilatory effect of carbon dioxide, reached a minimal value that remained constant on further elevation of PCO2. The slopes (absolute value) and intercepts of latency to the FED as a function of carbon dioxide decreased with the increase in PO2. This log-linear relationship made possible the derivation of equations that describe latency to the FED as a function of both PO2 and PCO2 in the PCO2 - dependent range: Latency (min) = e((5.19-0.0040PO2)) - e((2.77-0.0034PO2)) X PCO2 (kPa), and in the PCO2-independent range: Latency(min) = e((2.44-0.0009PO2)). A PCO2 as low as 1 kPa significantly reduced the latency to the FED. It is suggested that in closed-circuit oxygen diving, any accumulation of carbon dioxide should be avoided in order to minimize the risk of CNS oxygen toxicity.