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

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.

Retrospective Evaluation of Clinical Symptoms Due to Mild Hypobaric Hypoxia Exposure in Microgravity.

INTRODUCTION: A habitat atmosphere of 34% oxygen (O2) and 66% nitrogen (N2) at 8.2 psia (56.5 kPa) is proposed to minimize the risk of decompression sickness during extravehicular activity. The resulting inspired O2 partial pressure (PIo2) of 128 mmHg is similar to that experienced during portions of 41 Space Shuttle missions that used a "staged" denitrogenation (prebreathe) protocol with an atmosphere of 26.5% O2 and 73.5% N2 at 10.2 psia (70.3 kPa). We evaluated symptoms possibly linked to mild hypoxia in astronauts breathing a PIo2 of 127 mmHg. METHODS: Environmental data were used to determine time in the shuttle at 10.2 psia and time at 14.7 psia (101.3 kPa). A total of 14 possible hypoxia symptoms were compared with symptoms collected during normoxic shuttle operations at 14.7 psia using logistic regression. RESULTS: There were 134.1 d (788.8 person days) under the 10.2 psia staged condition with a mean of 3.17 ± 2.2 SD d/mission. There were 258.81 d at 14.7 psia (2192.95 person days). An average of 4.31 potentially hypoxia-related symptoms per mission day was documented under the staged condition compared with 4.08 per mission day during the normoxic condition. Logistic regression showed no symptoms were significantly associated with just the 10.2 psia condition. DISCUSSION: Chronic exposure to a PIo2 of 127 mmHg is well-tolerated by healthy humans on Earth. A similar short-duration exposure on the shuttle resulted in no increased reporting of possible hypoxia-related symptoms. However, chronic mild hypoxia interactions with physiological changes due to microgravity adaptations remain unclear.Wessel JH III, Schaefer CM, Thompson MS, Norcross JR, Bekdash OS. Retrospective evaluation of clinical symptoms due to mild hypobaric hypoxia exposure in microgravity. Aerosp Med Hum Perform. 2018; 89(9):792-797.

Hemoglobin Oxygen Saturation with Mild Hypoxia and Microgravity.

INTRODUCTION: Microgravity (µG) exposure and even early recovery from µG in combination with mild hypoxia may increase the alveolar-arterial oxygen (O2) partial pressure gradient. METHODS: Four male astronauts on STS-69 (1995) and four on STS-72 (1996) were exposed on Earth to an acute sequential hypoxic challenge by breathing for 4 min 18.0%, 14.9%, 13.5%, 12.9%, and 12.2% oxygen-balance nitrogen. The 18.0% O2 mixture at sea level resulted in an inspired O2 partial pressure (PIo2) of 127 mmHg. The equivalent PIO2 was also achieved by breathing 26.5% O2 at 527 mmHg that occurred for several days in µG on the Space Shuttle. A Novametrix CO2SMO Model 7100 recorded hemoglobin (Hb) oxygen saturation through finger pulse oximetry (Spo2, %). There were 12 in-flight measurements collected. Measurements were also taken the day of (R+0) and 2 d after (R+2) return to Earth. Linear mixed effects models assessed changes in Spo2 during and after exposure to µG. RESULTS: Astronaut Spo2 levels at baseline, R+0, and R+2 were not significantly different from in flight, about 97% given a PIo2 of 127 mmHg. There was also no difference in astronaut Spo2 levels between baseline and R+0 or R+2 over the hypoxic challenge. CONCLUSIONS: The multitude of physiological changes associated with µG and during recovery from µG did not affect astronaut Spo2 under hypoxic challenge.Conkin J, Wessel JH III, Norcross JR, Bekdash OS, Abercromby AFJ, Koslovsky MD, Gernhardt ML. Hemoglobin oxygen saturation with mild hypoxia and microgravity. Aerosp Med Hum Perform. 2017; 88(6):527-534.