The Partial Pressure of Inspired Carbon Dioxide Exposure Levels in the Extravehicular Mobility Unit.

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 (Pco₂) in a pressurized spacesuit. We applied the methodology to characterize Pco₂ for the extravehicular mobility unit (EMU). 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 Pco₂ inhaled by 19 healthy subjects classified into 3 fitness groups. The transcutaneous Pco₂ was monitored as a secondary measure to validate washout performance. 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 h) were 4.75 1.03, 8.09 1.39, 11.39 1.26, and 14.36 1.29 (mmHg s). 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. A standardized and automated methodology to calculate Pco₂ 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.