Biomedical Support of U.S. Extravehicular Activity M. L. Gernhardt, PhD, J. P. Dervay MD, D. Gillis MD, H. J. McMann, K. S. Thomas Medical Operations and Crew Systems NASA Johnson Space Center (JSC) NASA JSC EVA Project Office (Retired) Hamilton Sundstrand Human Space Systems Introduction The world’s first extravehicular activity (EVA) was performed by A. A. Leonov on March 18, 1965 during the Russian Voskhod-2 mission. The first US EVA was executed by Gemini IV astronaut Ed White on June 3, 1965, with an umbilical tether that included communications and an oxygen supply. A hand-held maneuvering unit (HHMU) also was used to test maneuverability during the brief EVA; however the somewhat stiff umbilical limited controlled movement. That constraint, plus difficulty returning through the vehicle hatch, highlighted the need for increased thermal control and improved EVA ergonomics. Clearly, requirements for a useful EVA were interrelated with the vehicle design. The early Gemini EVAs generated requirements for suits providing micro-meteor protection, adequate visual field and eye protection from solar visual and infrared radiation, gloves optimized for dexterity while pressurized, and thermal systems capable of protecting the astronaut while rejecting metabolic heat during high workloads. Subsequent Gemini EVAs built upon this early experience and included development of a portable environmental control and life support systems (ECLSS) and an astronaut maneuvering unit. The ECLSS provided a pressure vessel and controller with functional control over suit pressure, oxygen flow, carbon dioxide removal, humidity, and temperature control. Gemini EVA experience also identified the usefulness of underwater neutral buoyancy and altitude chamber task training, and the importance of developing reliable task timelines. Improved thermal management and carbon dioxide control also were required for high workload tasks. With the Apollo project, EVA activity was primarily on the lunar surface; and suit durability, integrated liquid cooling garments, and low suit operating pressures (3.75 pounds per square inch absolute [psia] or 25.8 kilopascal [kPa],) were required to facilitate longer EVAs with ambulation and significant physical workloads with average metabolic rates of 1000 BTU/hr and peaks of up to 2200 BTU/hr [6] . Mobility was further augmented with the Lunar Roving Vehicle. The Apollo extravehicular mobility unit (EMU) was made up of over 15 components, ranging from a biomedical belt for capturing and transmitting biomedical data, urine and fecal containment systems, a liquid cooling garment, communications cap, a modular portable life support system (PLSS), a boot system, thermal overgloves, and a bubble helmet with eye protection. Apollo lunar astronauts performed successful EVAs on the lunar surface from a 5 psia (34.4 kPa) 100% oxygen environment in the Lunar Lander. A maximum of three EVAs were performed on any mission. For Skylab a modified A7LB suit, used for Apollo 15, was selected. The Skylab astronaut life support assembly (ALSA) provided umbilical support through the life support umbilical (LSU) and used open loop oxygen flow, rather than closed-loop as in Apollo missions. Thermal control was provided by liquid water circulated by spacecraft pumps and electrical power also was provided from the spacecraft via the umbilical. The cabin atmosphere of 5 psia (34.4 kPa), 70% oxygen, provided a normoxic atmosphere and because of the very low nitrogen partial pressures, no special protocols were required to protect against decompression sickness (DCS) as was the case with the Apollo spacecraft with a 5 psi, 100% oxygen environment. Space Shuttle EVA The Shuttle was designed to provide crew and cargo transfer from Earth to low earth orbit for deployment and capture of satellites and scientific spacecraft as well as for future space stations. Shuttle EVAs were initially anticipated to be for contingency use associated with various malfunctions of the payload bay doors. The Shuttle EMU was derived from the advanced Apollo configuration and later enhanced. The enhanced EMU is the currently used US EVA integrated space-suit system. 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This combination of cabin suit. space This combination ofcabin pressure decompression protocol, t protocol, decompression [5], suggesting very low decompression stresses after stresses very lowdecompression suggesting [5], 2 109 e rateof23.7%[10],withthe majorityofthose 82 EV3/EV4 preferredusing planners 1.65. The R-value is defined as the ratio between ratio between asthe 1.65.TheR-valueisdefined ment of special protocols to protect against DCS. against toprotect protocols ment ofspecial 7 psia (101.2 kPa) with 21% oxygen.Thishigher kPa)with21% psia(101.2 7 86 (h) protocols These pressure. space-suit tment andthe shows the predictions of theoretical bubble growth growth theoretical bubble of predictions the shows 87 (b) 35.5 26.5%oxygen.The with (70.2kPa) to 10.2psia 88 (c ) 128 psia(70.2 a10.2 kPa), and psia(101.2 it at14.7 96 38.5 rts ofDCSinover140EVAs using these 103 EV1/EV2 108 103 EV3/EV4 67.5 101 14.5 during EVAfixedtasks. uttle used this protocol.Additionallyfor uttle usedthis kPa) would be expected tohavea expected kPa) wouldbe 106 16.5 9.6 kPa) was selected to maintain the was selectedtomaintain 9.6 kPa) 92 EV1/EV2 54 92 EV3/EV4 78 12- ofthetested lly farinexcess ebreathe timesareafunctionof 97( b ) 123.5 mission, kPa)byShuttle ia (70.2 98 (b) 74 102 EV1/EV2 15.5 102 EV3/EV4 (d) 14 1 perform he crewmembers the 10.2 psia (70.2 kPa) (70.2 10.2psia the 100 (d) 47 104 (e) 28.5 105 (d) 27 108 15 109 EV1/EV2 127 109 EV3/EV4 106.5 25 12 hr* 20 15 16 hr* 10 20 hr* 5 480 min Tissue 480 min 24 hr* Bubble Growth Index Index Growth Bubble 0 0 100 200 300 400 EVA Time (min) Figure 2. Theoretical bubble growth during a 6-hour EVA after spending 12, 16, 20, and 24 hours at 10.2 psia (70.2 kPa) with 26.5% oxygen. In addition to the increased exposure time at 10.2 psia (70.2 kPa), the suit itself provides some increased decompression protection in the form of additional operational oxygen prebreathe time and higher metabolic rates during prebreathe compared with the resting test subjects of the laboratory trials. Once the suit is donned a series of configuration and leak checks are performed, which are followed by an 8-12 minute purge cycle before the prebreathe clock is started. Then, during depressurization to vacuum, the suit pressure is set by the positive pressure relief valves keeping the suit 5 psia (34.4 kPa) over the ambient pressure; this results in more oxygen prebreathe time before the tissues becoming supersaturated creating decompression stresses. The combined effect of the suit operational overhead is to result in between 20 to 30 minutes additional prebreathe at elevated oxygen concentration levels. Also, the metabolic rates of crewmembers “resting in the suit” have been measured at 6.8 mL/kg-min compared to typical resting metabolic rates of approximately 3.8 mL/kg-min. Recent research has shown that even small increases in metabolic rate can decrease DCS incidence, presumably through increased nitrogen washout [1, 3, 7]. The combination of all suit-related operational effects reduce decompression stress compared to shirt-sleeve laboratory subjects and offer one explanation as to why the incidence of DCS in suited ground-based vacuum chamber tests and spaceflight EVAs are much lower than the initial laboratory trials used to develop these decompression protocols.