The circulation of the brain has been the object of more speculation and study than any other part of the body. All present methods of estimating cerebral blood flow, however, leave much to be desired. Since, for many practical purposes we are not in-terested in the blood flow per se but in whether or not the oxygenation of the brain is adequate, we have attempted to approach the problem of quantitative brain oxygen tension directly.The prime consideration was "what" to measure that might justifiably be called "mean tissue oxygen tension." As pointed out by Bronk et al.1 and Bloor et al.2 the spatial resolution of the oxygen electrode is such that the absolute value obtained from the cortex varies greatly with its position relative to blood vessels. Other obvious problems arise in using the exposed cortex, such as pulsation, movement, variations in electrode pressure, etc. However, fluid in equilibrium with the tissue obviates most of these problems, and the brain is fortuitously bathed and filled with readily accessible fluid. Inasmuch as the work of Bering3 in¬ dicated a rapid water exchange in the cere¬ brospinal fluid, it did not seem unreasonable to expect equally rapid changes in the oxygen tension of the fluid. To explore this thesis and to determine the relationship be¬ tween brain, blood, and cerebrospinal fluid oxygen tension, the following study^v as undertaken. MethodsAll oxygen tension studies were carried out with an oxygen microelectrode based on the polarographic principle. The electrode is 2\/i in. in length with an O.D. of 0.038 in., which allows it to be introduced through an 18-gauge thin-walled needle. The assembly can be cold sterilized for aseptic use. The flow artifact is less than 1'% ; temperature coefficient is 3.2% per degree centigrade; readings with known oxygen tensions are reproducible to within 3% or less ; and the electrode is linear from 0 to over 700 mm. Hg oxygen tension.* Observations were made on 51 mongrel dogs and 4 monkeys (African Green). Light thiamylal sodium (Surital) anesthesia was used for the period of surgical preparation. During the experi¬ mental period the animals were intubated, im¬ mobilized with intramuscular succinylcholine, and mechanically respi rated so that the general anes-

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BACKGROUND: NASA has been making efforts to assess the carbon dioxide (CO2) washout capability of spacesuits using a standard CO2 sampling protocol. This study established the methodology for determining the partial pressure of inspired CO2 (PIco2) in a pressurized spacesuit. We applied the methodology to characterize PIco2 for the extravehicular mobility unit (EMU).METHODS: We suggested an automated and mathematical algorithm to find the end-tidal CO2 and the end of inspiration. We provided objective and standardized guidelines to identify acceptable breath traces, which are essential to accurate and reproducible calculation of the in-suit inhaled and exhaled partial pressure of CO2 (Pco2). The mouth guard-based method for measurement of inhaled and exhaled dry-gas Pco2 was described. We calculated all individual concentrations of PIco2 inhaled by 19 healthy subjects classified into 3 fitness groups. The transcutaneous Pco2 was monitored as a secondary measure to validate washout performance.RESULTS: Mean and standard deviation values for the data collection performance and the CO2 metrics were presented (e.g., minimum time weighted average Pco2 at suited workloads of resting, 1000, 2000, and 3000 (BTU h1) were 4.75 1.03, 8.09 1.39, 11.39 1.26, and 14.36 1.29 (mmHg s1). All CO2 metrics had a statistically significant association and all positive slopes with increasing metabolic rate. No significant differences in CO2 metrics were found between the three fitness groups.DISCUSSION: A standardized and automated methodology to calculate PIco2 exposure level is presented and applied to characterize CO2 washout in the EMU. The EMU has been operated successfully in over 400 extravehicular activities (EVAs) and is considered to provide acceptable CO2 washout performance. Results provide a basis for establishing verifiable Pco2 requirements for current and future EVA spacesuits.Kim KJ, Bekdash OS, Norcross JR, Conkin J, Garbino A, Fricker J, Young M, Abercromby AFJ. The partial pressure of inspired carbon dioxide exposure levels in the extravehicular mobility unit. Aerosp Med Hum Perform. 2020; 91(12):923931.

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