Simulated Extravehicular Activity Research Articles

Augmented Reality in Extra-Vehicular Activities: Optimizing Alert Detection and Cognitive Workload

This study investigates the impact of augmented reality (AR) system complexity (active vs. passive) and lighting conditions (normal, dark, glare) on task performance, cognitive workload, and alert detection during simulated extravehicular activities (EVAs). Fifty-eight undergraduate students participated, using AR systems to complete tasks. Key metrics included task completion rate, completion time, alert detection, and cognitive workload assessed by the NASA Task Load Index (NASA-TLX). Results showed no significant differences in task performance or cognitive workload across lighting conditions and AR types. Notably, less than 15% of participants noticed any alerts, suggesting AR alerts may lack sufficient salience. The study’s lack of significant findings is attributed to the small sample size and potential insensitivity of measures. Future research should involve larger samples and refined methodologies to better understand AR’s impact on astronaut performance and safety during EVAs.

Walking on ‘‘Mars’’: Gendered Group Processes in Space Analog Missions

Most research on mixed-gender teams in space analog environments focuses on individual-level variation and overlooks structural causes of inequality. Status characteristics theory posits how socially recognized traits, such as gender, contribute to the formation of informal hierarchies by denoting perceived levels of competence to group members. We investigated gender as a status characteristic in groups in space analog environments. We used data from the Mars Desert Research Station (MDRS) and hypothesized that women crew members are less likely than men to be selected to participate in simulated extravehicular activities during a Mars simulation at the MDRS. We used reports and biographies from 30 randomly selected crews (n = 177) posted on the MDRS website to construct our dataset and multilevel generalized regression models to test our hypothesis. Women crew members were 6% less likely to participate in simulated extravehicular activities than men, controlling for crew role, education, and other factors. Our study shows that gender acts as a status characteristic and influences group decisions in crews in space analogs. These results highlight the need for more studies on interactional inequalities in preparation for a long-term human spaceflight.

Effects Of Exercise On Executive Functions During A Simulated Extravehicular Activity (EVA) Underwater

Effects Of Exercise On Executive Functions During A Simulated Extravehicular Activity (EVA) Underwater

Mission enhancing capabilities for science-driven exploration extravehicular activity derived from the NASA BASALT research program

The NASA-funded Biologic Analog Science Associated with Lava Terrains (BASALT) program is investigating candidate Mars extravehicular activity (EVA) concepts of operations and capabilities through science-driven terrestrial fieldwork under Mars-mission operational constraints. The third BASALT field test consisted of ten simulated EVAs in the Kilauea Caldera and Kilauea Iki regions of Hawai’i in November 2017. Each EVA included two extravehicular (EV) crewmembers completing real (non-simulated) geobiochemical science objectives (e.g., collecting detailed imagery and scientific instrument data and extracting samples of basalt for further laboratory investigations), two intravehicular crewmembers who supported the EV crew in real-time, and a Mission Support Center comprised of remote scientists and operators who provided scientific expertise during the EVAs across Mars-relevant communication latencies. Throughout this field test, a series of new capabilities, including high-resolution panoramic imagery, mobile automated light detection and ranging data, immersive mixed-reality terrain models, and augmented-reality field systems for terrain navigation and annotation, were incorporated and evaluated during the mission simulation for their ability to enable and enhance science. The value provided by these capabilities was assessed using a variety of objective and subjective technology impact metrics, including detailed EVA task timing data, space-to-ground interaction data, and heritage subjective assessments of capability assessment (a measure of the potential for mission enhancement), simulation quality (a measure of simulation fidelity), and acceptability (including the specification of desired, warranted, and required improvements). In general, these capabilities were rated at least moderately enhancing for future planetary EVA, meaning that they are likely to moderately enhance one of more aspects of EVA or significantly enhance EVA on rare occasions. A number of specific improvements to these capabilities were identified, which warrant further development and testing, and several complementary future studies are proposed. Together, these data provide critical knowledge to help meet design challenges associated with future science-driven planetary EVA. While this paper specifically focuses on applications for future human spaceflight exploration, many of the results and lessons learned are also applicable to present-day terrestrial scientific fieldwork.

Toward Better Understanding of Function Allocation Requirements for Planetary Eva and Habitat Tasks

The authors participated in a Mars planetary science research analog that included simulated extravehicular activity (EVA) and intravehicular (IV) crewmembers with delayed communications with Earth. Within the analog context, the authors considered which information streams and sources were monitored by IV crewmembers throughout the simulated EVAs. The capabilities of IV crewmember monitoring were compared to information considered critical for the successful completion of EVA on Mars, suggesting gaps for improved human-automation function allocation. The authors’ findings support available literature that indicate that new mission architectures for planetary exploration mandate increased autonomy from Earth-based controllers. This autonomy will result in too many mission-critical data streams for the attentional capabilities of relatively small crew. Results of this work can inform further investigation into evolving concepts of mission operations that incorporate effective human-automation teaming to support dynamic function allocation for spaceflight exploration tasks.

Prediction of Lunar- and Martian-Based Intra- and Site-to-Site Task Performance.

This study aimed to investigate the feasibility of determining the physiological parameters associated with the ability to complete simulated exploration type tasks at metabolic rates which might be expected for lunar and Martian ambulation. Running V̇O2max and gas exchange threshold (GET) were measured in 21 volunteers. Two simulated extravehicular activity field tests were completed in 1 G in regular athletic apparel at two intensities designed to elicit metabolic rates of ∼20.0 and ∼30.0 ml · kg(-1) · min(-1), which are similar to those previously reported for ambulation in simulated lunar- and Martian-based environments, respectively. All subjects were able to complete the field test at the lunar intensity, but 28% were unable to complete the field test at the Martian intensity (non-Finishers). During the Martian field test there were no differences in V̇O2 between Finishers and non-Finishers, but the non-Finishers achieved a greater %V̇O2max compared to Finishers (78.4 ± 4.6% vs. 64.9 ± 9.6%). Logistic regression analysis revealed fitness thresholds for a predicted probability of 0.5, at which Finishing and non-Finishing are equally likely, and 0.75, at which an individual has a 75% chance of Finishing, to be a V̇O2max of 38.4 ml · kg(-1) · min(-1) and 40.0 ml · kg(-1) · min(-1) or a GET of 20.1 ml · kg(-1) · min(-1) and 25.1 ml · kg(-1) · min(-1), respectively (χ(2) = 10.2). Logistic regression analysis also revealed that the expected %V̇O2max required to complete a field test could be used to successfully predict performance (χ(2) = 19.3). The results of the present investigation highlight the potential utility of V̇O2max, particularly as it relates to the metabolic demands of a surface ambulation, in defining successful completion of planetary-based exploration field tests.

Relationship between simulated extravehicular activity tasks and measurements of physical performance

The purpose was to evaluate the relationships between tests of fitness and two activities that simulate components of Lunar- and Martian-based extravehicular activities (EVA). Seventy-one subjects completed two field tests: a physical abilities test and a 10km Walkback test. The relationships between test times and the following parameters were determined: running V˙O2max, gas exchange threshold (GET), speed at V˙O2max (s-V˙O2max), highest sustainable rate of aerobic metabolism [critical speed (CS)], and the finite distance that could be covered above CS (D′): arm cranking V˙O2peak, GET, critical power (CP), and the finite work that can be performed above CP (W′). CS, running V˙O2max, s-V˙O2max, and arm cranking V˙O2peak had the highest correlations with the physical abilities field test (r=0.66–0.82, P<0.001). For the 10km Walkback, CS, s-V˙O2max, and running V˙O2max were significant predictors (r=0.64–0.85, P<0.001). CS and to a lesser extent V˙O2max are most strongly associated with tasks that simulate aspects of EVA performance, highlighting CS as a method for evaluating astronaut physical capacity.

Lunar and Mars simulated extravehicular activity evoked physiological responses (882.7)

It is currently unclear what physiological measurements from astronauts performing EVAs best indicate impending task failure. To date 7 subjects have completed a treadmill test for determination of maximal O2 uptake (V̇O2max) and two simulated EVAs at governed paces to evoke Lunar (V̇O2 ~20 ml/kg/min) and Mars (V̇O2 ~30 ml/kg/min) intensities, which have previously been reported for these environments. Subjects were separated into two groups based upon completion (COM) or failure (FAIL) of the Mars EVA. There were no significant (p &gt; 0.05) differences between V̇O2 (Lunar COM 19.6 ± 1.3 ml/kg/min vs. FAIL 20.2 ± 0.9 ml/kg/min; Mars COM 29.9 ± 2.4 ml/kg/min vs. FAIL 30.0 ± 1.8 ml/kg/min), while FAIL %V̇O2max was significantly (p &lt; 0.05) higher (Lunar COM 42.1 ± 3.9 % vs. FAIL 55.9 ± 1.3%; Mars COM 64.5 ± 8.5% vs. FAIL 83.1 ± 7.1%). FAIL V̇O2 rose throughout the Mars EVA until failure, while steady‐states were achieved in both groups for all other EVAs. Vastus lateralis muscle oxygenation and electromyographic data were not significantly (p &gt; 0.05) different. These results indicate that global systemic measurements of EVA intensity are more informative of impending EVA failure than peripheral. The V̇O2 responses are characteristic of activities performed above &amp; below critical power/speed. Thus critical speed may be vital to know for an astronaut prior to and during a mission, as EVA intensities above these will lead to task failure.Grant Funding Source: Supported by NASA Grant NNX10AK60G

Mission control team structure and operational lessons learned from the 2009 and 2010 NASA desert RATS simulated lunar exploration field tests

The NASA Desert Research and Technology Studies (Desert RATS) is an annual field test of advanced concepts, prototype hardware, and potential modes of operation to be used on human planetary surface space exploration missions. For the 2009 and 2010 NASA Desert RATS field tests, various engineering concepts and operational exercises were incorporated into mission timelines with the focus of the majority of daily operations being on simulated lunar geological field operations and executed in a manner similar to current Space Shuttle and International Space Station missions. The field test for 2009 involved a two week lunar exploration simulation utilizing a two-man rover. The 2010 Desert RATS field test took this two week simulation further by incorporating a second two-man rover working in tandem with the 2009 rover, as well as including docked operations with a Pressurized Excursion Module (PEM). Personnel for the field test included the crew, a mission management team, engineering teams, a science team, and the mission operations team. The mission operations team served as the core of the Desert RATS mission control team and included certified NASA Mission Operations Directorate (MOD) flight controllers, former flight controllers, and astronaut personnel. The backgrounds of the flight controllers were in the areas of Extravehicular Activity (EVA), onboard mechanical systems and maintenance, robotics, timeline planning (OpsPlan), and spacecraft communicator (Capcom). With the simulated EVA operations, mechanized operations (the rover), and expectations of replanning, these flight control disciplines were especially well suited for the execution of the 2009 and 2010 Desert RATS field tests. The inclusion of an operations team has provided the added benefit of giving NASA mission operations flight control personnel the opportunity to begin examining operational mission control techniques, team compositions, and mission scenarios. This also gave the mission operations team the opportunity to gain insight into functional hardware requirements via lessons learned from executing the Desert RATS field test missions. This paper will detail the mission control team structure that was used during the 2009 and 2010 Desert RATS Lunar analog missions. It will also present a number of the lessons learned by the operations team during these field tests. Major lessons learned involved Mission Control Center (MCC) operations, pre-mission planning and training processes, procedure requirements, communication requirements, and logistic support for analogs. This knowledge will be applied to future Desert RATS field tests, and other Earth based analog testing for space exploration, to continue the evolution of manned space operations in preparation for human planetary exploration. It is important that operational knowledge for human space exploration missions be obtained during Earth-bound field tests to the greatest extent possible. This allows operations personnel the ability to examine various flight control and crew operations scenarios in preparation for actual space missions.

Evaluation of dual multi-mission space exploration vehicle operations during simulated planetary surface exploration

IntroductionA pair of small pressurized rovers (multi-mission space exploration vehicles, or MMSEVs) is at the center of the Global Point-of-Departure architecture for future human lunar exploration. Simultaneous operation of multiple crewed surface assets should maximize productive crew time, minimize overhead, and preserve contingency return paths. MethodsA 14-day mission simulation was conducted in the Arizona desert as part of NASA's 2010 Desert Research and Technology Studies (DRATS) field test. The simulation involved two MMSEV earth-gravity prototypes performing geological exploration under varied operational modes affecting both the extent to which the MMSEVs must maintain real-time communications with the mission control center (Continuous [CC] versus Twice-a-Day [2/D]) and their proximity to each other (Lead-and-Follow [L&F] versus Divide-and-Conquer [D&C]). As part of a minimalist lunar architecture, no communication relay satellites were assumed. Two-person crews (an astronaut and a field geologist) operated each MMSEV, day and night, throughout the entire 14-day mission, only leaving via the suit ports to perform simulated extravehicular activities. Metrics and qualitative observations enabled evaluation of the extent to which the operating modes affected productivity and scientific data quality (SDQ). Results and discussionSDQ was greater during CC mode than during 2/D mode; metrics showed a marginal increase while qualitative assessments suggested a practically significant difference. For the communications architecture evaluated, significantly more crew time (14% per day) was required to maintain communications during D&C than during L&F (5%) or 2/D (2%), increasing the time required to complete all traverse objectives. Situational awareness of the other vehicle's location, activities, and contingency return constraints were qualitatively enhanced during L&F and 2/D modes due to line-of-sight and direct MMSEV-to-MMSEV communication. Future testing will evaluate approaches to operating without real-time space-to-earth communications and will include quantitative evaluation and comparison of the efficacy of mission operations, science operations, and public outreach operations.

NASA Desert RATS 2010: Preliminary results for science operations conducted in the San Francisco Volcanic Field, Arizona

The National Aeronautics and Space Administration (NASA) is working with international partners to develop the space architectures and mission plans necessary for human spaceflight beyond Earth orbit. The Apollo missions to the Moon demonstrated conclusively that surface mobility is a key asset that improves the efficiency of human explorers on a planetary surface. NASA's Desert Research and Technology Studies (Desert RATS), a multi-year series of tests of hardware and operations carried out annually in the high desert of Arizona, has tested a crewed pressurized rover concept referred to as the Space Exploration Vehicle (SEV). During NASA's Desert RATS 2010, four 2-person crews driving two SEVs collectively conducted 12 days of field exploration in the San Francisco Volcanic Field in northern Arizona. They collected 461 samples, with a total mass of 161.2kg, on 70 simulated extravehicular activities (EVAs). Each SEV crew traveled over 60km during their field explorations. This paper illustrates where the actual field sites, or ‘science stations’, were located, provides a brief description of the types of samples collected at each station, and highlights some of the more interesting sites. Most of the geologic samples collected at Desert RATS 2010 were well documented at the site of collection, and upon delivery to the Johnson Space Center the samples were given a preliminary examination. The samples are available for further study by interested researchers developing scientific instruments for use on the surfaces of the Moon and Mars, or for geological investigations of the San Francisco Volcanic Field.

Assessment of robotic recon for human exploration of the Moon

Robotic reconnaissance (“recon”) has the potential to significantly improve scientific and technical return from lunar surface exploration. In particular, robotic recon can be used to improve traverse planning, reduce operational risk, and increase crew productivity. To study how robotic recon can benefit human exploration, we recently conducted a field experiment at Black Point Lava Flow (BPLF), Arizona. In our experiment, a simulated ground control team at NASA Ames teleoperated a planetary rover to scout geology traverses at BPLF. The recon data were then used to plan revised traverses. Two-man crews subsequently performed both types of traverses using the NASA “Lunar Electric Rover” (LER) and simulated extra-vehicular activity (EVA) suits. This paper describes the design of our experiment, presents our results, and discusses directions for future research.

Venous gas emboli and exhaled nitric oxide with simulated and actual extravehicular activity

The decompression experienced due to the change in pressure from a space vehicle (1013hPa) to that in a suit for extravehicular activity (EVA) (386hPa) was simulated using a hypobaric chamber. Previous ground-based research has indicated around a 50% occurrence of both venous gas emboli (VGE) and symptoms of decompression illness (DCI) after similar decompressions. In contrast, no DCI symptoms have been reported from past or current space activities. Twenty subjects were studied using Doppler ultrasound to detect any VGE during decompression to 386hPa, where they remained for up to 6h. Subjects were supine to simulate weightlessness. A large number of VGE were found in one subject at rest, who had a recent arm fracture; a small number of VGE were found in another subject during provocation with calf contractions. No changes in exhaled nitric oxide were found that can be related to either simulated EVA or actual EVA (studied in a parallel study on four cosmonauts). We conclude that weightlessness appears to be protective against DCI and that exhaled NO is not likely to be useful to monitor VGE.

Subsurface water detection on Mars by astronauts using a seismic refraction method: Tests during a manned Mars mission simulation

The Mars Society has established a Mars Arctic Research Station (MARS) on Devon Island, North of Canada, on the edge of the Haughton crater. The site was selected for its similarities with the surface of the Mars planet. Since 2001, the MARS Flashline Research Station supports international simulation campaigns of human Mars exploration missions. The second rotation crew conducted in 2001 a geophysics experiment assessing the feasibility of an active seismology method to detect subsurface water on the surface of Mars. A crew of three operators wearing simulated space suits deployed a line of 24 sensors. Reflected and refracted signals produced by a mini-quake generated by a sledge hammer were recorded by a seismograph. The experiment was conducted three times, once in a dry run and twice during simulated extra vehicular activities (EVAs) on the edge of the Haughton crater, allowing a three-dimensional characterization of the subsurface ground to a depth of several hundred meters. Several operational lessons were learned from conducting this experiment under simulated EVA conditions.

Heart Rate Variability during Simulated Space Extravehicular Activity

A major factor in human space missions is extravehicular activity (EVA) which requires numerous training hours in underwater microgravity-simulation environments prior to each mission launch. Heart rate variability (HRV) measurements are a valuable and non-invasive tool that can provide extensive insights on the cyclic variations in heart rate for investigating the modulatory effects of neural mechanisms elicited by the autonomic nervous system on intrinsic heart rate during EVA. PURPOSE: Given the limited literature regarding cardiovascular responses and HRV in simulated EVA (EVA) environments and underwater conditions, our purpose was to evaluate the cardiovascular responses to aSEVA using HRV measures. METHODS: Ten healthy adult volunteers (seven male; three female; BMI 23.7±2.1; age 26.4±2.9) performed a 20-minuteSEVA sortie in a neutral buoyancy water tank environment (27°C) at a depth of 5 meters. TheSEVA scenario was a Hubble Telescope repair mission using a high fidelity task mock-up. Data for HRV (three from the time domain and three from the frequency domain) was obtained for five minutes prior to, during, and for five minutes following theSEVA using non-invasive telemetry sensors (Polar, Kempele, Finland). Participants were instructed to refrain from vigorous exercise, as well as consumption of caffeine and alcohol for at least 12 hours prior to diving. RESULTS: Heart rate duringSEVA was increased when compared to pre-dive (74±10 vs. 108±16 beats min−1, p <0.05). In contrast, the time domain measure pNN50 (37.3±16.9vs. 14.1±10.1 %, p <0.05), and the R-R interval (0.72±0.26 vs. 0.59±0.11, p <0.05) showed a significant decrease duringSEVA when compared with pre-dive conditions, while no changes were detected in the frequency domain measures between conditions. Further, general linear models incorporating covariates for age, gender, and BMI demonstrated associations between pre-dive and dive sympathovagal balance values, as well as between dive R-R interval and pre-dive mean heart rate and sympathetic drive (p <0.05). CONCLUSION: In contrast to previous studies examining HRV during scuba diving, we found a decrease in HRV during an underwaterSEVA sortie. The observed results may be due to the increased psychological stress of performing theSEVA scenario tasks.

Fatty acid composition of plasma lipids and erythrocyte membranes during simulated extravehicular activity

Ten subjects (from 27 to 41 years) have been participated in 32 experiments. They were decompressed from ground level to 40-35 kPa in altitude chamber when breathed 100% oxygen by mask and performed repeated cycles of exercises (3.0 Kcal/min). The intervals between decompressions were 3–5 days. Plasma lipid and erythrocyte membrane fatty acid composition was evaluated in the fasting venous blood before and immediately after hypobaric exposure. There were 7 cases decompression sickness (DCS). Venous gas bubbles (GB) were detected in 27 cases (84.4%). Any significant changes in the fatty acid composition of erythrocyte membranes and plasma didn't practically induce after the first decompression. However, by the beginning of the second decompression the total lipid level in erythrocyte membranes decreased from 54.6 mg% to 40.4 mg% in group with DCS symptoms and from 51.2 mg% to 35.2 mg% (p < 0.05) without DCS symptoms. In group with DCS symptoms a tendency to increased level of saturated fatty acids in erythrocyte membranes (16:0, 18:0), the level of the polyunsaturated linoleic fatty acid (18:2) and arachidonic acid (20:4) tended to be decreased by the beginning of the second decompression. Insignificant changes in blood plasma fatty acid composition was observed in both groups. The obtained biochemical data that indicated the simulated extravehicular activity (EVA) condition is accompanied by the certain changes in the blood lipid metabolism, structural and functional state of erythrocyte membranes, which are reversible. The most pronounced changes are found in subjects with DCS symptoms.

Efficacy of doppler utrasound for screening symptoms of decompression sickness during simulated extravehicular activities

Doppler ultrasound is frequently used for monitoring circulating microbubbles during decompression to assess the symptoms of Decompression Sickness (DCS). This analysis was carried out to evaluate its effectiveness for screening symptoms of DCS during simulated extravehicular activities (EVA). The information from various hypobaric chamber studies carried out at the NASA Johnson Space Center, Houston, TX was used in this analysis ( n = 516). The circulating microbubbles were detected in the precordial area in 42% ( 218 516 ), and symptoms were reported in 16% ( 81 516 ) of these exposures. The accuracy of Doppler-detectable bubbles (Spencer grades) on all symptoms of DCS was examined by calculating measures of sensitivity and specificity. The efficacy of Doppler as a screening device was examined by calculating their positive predictive value (PPV) and negative predictive value (NPV). The results of these analyses indicated that the sensitivity of Doppler decreased, and the PPV increased with higher Spencer grades. However, the likelihood of detecting true negative cases (NPV) was consistently higher with all bubble grades. Due to the high false-positive rate and low prior probabilities of the risk of DCS, Doppler was found to be more useful to identify those who did not develop DCS, than to detect positive cases of DCS in the simulated EVA exposures.

Early Stopping of Aerospace Medical Trials: Application of Sequential Principles

A two-period, crossover trial was conducted in the hypobaric chamber on human subjects to compare the influence of inflight exercise (experimental) and restricted activity (control) on altitude decompression sickness (DCS) during simulated extravehicular activities. Out of 39 pairs (total of 78 exposures), 4 cases of DCS occurred under control and 5 occurred under experimental conditions. Analysis of the crossover results showed that the P values for differences in DCS occurrence was 0.56. Under these circumstances, it was necessary to decide whether additional information would be obtained by accruing more subjects. This problem was examined by using a skew sequential design in which the "stopping rule" was based on an alpha of 0.05 (one-sided) and power of 80%. The result of this analysis was in favor of the null hypothesis, and the trial was terminated. The authors recommend the use of similar stopping rules in aerospace trials to optimize sample size without compromising statistical validity.

Blood biochemical and cellular changes during decompression and simulated extravehicular activity

Blood biochemical and cellular parameters were measured in human subjects before and after exposure to a decompression schedule involving 6 h of oxygen prebreathing. The exposure was designed to simulate extravehicular activity for 6 h (subjects performed exercise while exposed to 29.6 kPa). There were no significant differences between blood samples from subjects who were susceptible (n = 11) versus those who were resistant (n = 27) to formation of venous gas emboli. Although several statistically significant (P less than 0.05) changes in blood parameters were observed following the exposure (increases in white blood cell count, prothrombin time, and total bilirubin, and decreases in triglycerides, very-low-density lipoprotein cholesterol, and blood urea nitrogen), the changes were small in magnitude and blood factor levels remained within normal clinical ranges. Thus, the decompression schedule used in this study is not likely to result in blood changes that would pose a threat to astronauts during extravehicular activity.

Hematological changes following repetitive decompressions during simulated extravehicular activity.

A study was performed to evaluate decompression procedures suggested for use prior to Space Shuttle extravehicular activity. Hematological parameters were measured in 12 male human subjects before and after exposure in an altitude chamber to a 3-day staged decompression schedule, with simulated extravehicular activity. Following the exposure, significant increases occurred in white blood cell count and activated partial thromboplastin time, and platelet aggregate ratio was significantly decreased. Pre-exposure samples from subjects who were susceptible to formation of venous gas emboli (VGE) exhibited a significantly lower degree of ADP-induced platelet aggregation and a significantly higher amount of lymphocyte blastogenic transformation in response to the mitogen phytohemagglutinin than samples from VGE-resistant subjects. The results indicate that, following this decompression profile, small but significant changes occur in several hematological parameters, and that levels of certain parameters may be related to susceptibility to VGE formation during decompression.