NASA Exploration Atmosphere Test 6: Back to Back EVAs

INTRODUCTION:
Space suits operate at low pressures to optimize astronauts’ performance. The drop in pressure from the habitat environment to the suit carries a risk of decompression sickness (DCS), which is mitigated by a 95% O2 prebreathe protocol. Prebreathe protocols for the lunar surface have a significantly higher risk compared to International Space Station microgravity Extravehicular Activities (EVA). In previous studies, we validated a prebreathe protocols for single (independent) EVA exposures. However, Artemis mission plans include daily (‘back to back’) EVAs. NASA has minimal data exploring the DCS risk impacts of ~16hrs ‘surface intervals’, ie, breaks between EVAs.

METHODS:
Sixteen volunteers completed 7-day hypobaric chamber studies simulating daily EVAs. Subjects acclimated for 48 hrs at the proposed Exploration Atmosphere (66.2kPa, 28.5% O2) and then participated in five 6-hour simulated EVAs at 29.6 kPa, 95% O2. Prior to EVA, subjects underwent a 120 or 150-minute prebreathe. The EVA simulation was designed to be representative of future planetary EVAs, proportionate to the subject’s VO2max. Decompression stress was monitored by real-time doppler and echocardiograph alternating every 15 min, as well as clinical monitoring for DCS signs/symptoms. Venous gas emboli (VGE) and DCS outcomes were verified against NASA-STD-3001 acceptance criteria.

RESULTS AND DISCUSSION:
Two neurological DCS cases occurred in the 120-min/29.6kPa prebreathe, terminating further testing of this protocol. One Type I DCS case was identified in the 150 min/29.6kPa protocol. All DCS symptoms resolved with treatment, with no residual sequelae post HBOT treatments of DCS. Venous gas emboli (VGE) were identified during all EVAs. Within the statistical power of these limited exposures, there was no apparent increase or decrease in VGE as a result of back-to-back EVAs (within +/- 1 VGE grade). This test highlighted the importance of characterizing DCS risk factors, and the difficulty in assessing and managing DCS during sustained EVA operations in remote planetary surfaces.