Factors Affecting CNS Oxygen Toxicity in Humans

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Factors Affecting CNS Oxygen Toxicity in Humans

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Title: Factors Affecting CNS Oxygen Toxicity in Humans
Author: Natoli, M; Vann, RD
Abstract: The onset time of central nervous system (CNS) oxygen toxicity symptoms is reduced by elevated oxygen partial pressure, inspired CO2, immersion, and work. This project investigated the effects of these factors in humans on ventilatory response to CO2, brain cortical oxygenation, and the occurrence of symptoms of CNS oxygen toxicity. Relative changes in cerebral oxygenation from a baseline were measured by near-infrared (NIR) spectroscopy. Ventilatory response was measured during CO2 rebreathing in a computer- controlled, closed-circuit breathing apparatus. The results of this study suggest that inspired C02 is a potent instigator of CNS oxygen toxicity, more potent than oxygen alone or in combination with immersion and work. Thirty-three of the 35 CNS oxygen toxicity symptoms reported, including a convulsion, involved breathing inspired C02 at a Pio2 of 2.80 ATA. Most (31) symptoms were reported while the inspired CO2 was 7.0% -7.5%. Hyperoxia, Pio2= 1.75 or 2.80 ATA, leads to increased cerebrovascular oxygenation despite vasoconstriction. Inspired CO2 reversed vasoconstriction and increasing cerebral oxygenation further. Subjects with symptoms consistent with oxygen toxicity did not show a CO2-induced vasoconstrictive reversal despite increased cerebral oxygenation. This result is contrary to previous findings (Lambertsen 1955) where inspired C02 caused symptoms of CNS oxygen toxicity while increasing cerebral blood flow and increasing cerebral oxygenation. This disparity is most likely due to the difference in the cerebral oxygenation measurements made. NIRS measures regional physiology while cerebral blood flow and blood sampling measurements are global physiological measurements. Ventilatory response to CO2 did not correlate well with symptom incidence but the subject who convulsed had the lowest ventilatory response to CO2 with a Pio2 of 0.21 ATA. The subject who convulsed also had the largest increase in cytochrome a,a3 oxidation with inspired Cog. The combination of the low ventilatory response to CO2 with increased cerebral oxygenation resulting in an oxygen seizure supports the hypothesis that susceptibility to CNS oxygen toxicity may be related to CO2 retention as a result of depressed ventilatory response to CO2 (Lanphier 1975). Facial immersion lead to increased cerebrovascular blood volume and cytochrome a,a3 oxidation at both 0.21 and 2.80 ATA Pio2 and with both positive and negative static lung loads.
Description: MS thesis submitted as Technical Report to the Office of Naval Research
URI: http://archive.rubicon-foundation.org/21
Date: 1996

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