During weightlessness astronauts experience a headward fluid shift which results in alterations of the cardiovascular system (CV), including a decreased plasma volume (PV) and CV deconditioning. Without the use of countermeasures crew members are at risk of post spaceflight orthostatic intolerance (OI) upon return to a gravitational environment when in the upright posture. Orthostatic intolerance can be defined as the inability to maintain an upright position due to a significant drop in blood pressure and is known to affect over 60% of long-duration crewmembers with a higher incidence in females2 . The Lunar human landing system will be of higher mass as compared to the Apollo lander; therefore, Artemis crew may experience higher Gz acceleration profiles during descent to and ascent from the Lunar surface. Determining the CV response to orthostatic challenges in a deconditioned state at simulated gravitational stresses less than 1G can provide valuable information on the efficacy of the novel Orthostatic Intolerance Garment (OIG) to protect crewmembers against OI. In assessing the efficacy of the OIG, it is imperative formal guidelines are established to properly assess pressure within these custom garments.
Methods and Materials Terrestrially Furosemide, a diuretic, was utilized to induce similar PV losses as seen during weightlessness to mimic spaceflight induced PV reductions. Seventeen subjects consented to two sessions on separate days. The first session included an eighty degree head up tilt for a twentyminute duration under normovolemic conditions. The second session included three randomized twenty-minute tilts at 30°, 50°, and 80° under hypovolemic conditions. The CV responses and tilt tolerance were measured throughout the duration of the study to determine the need for countermeasures. A protocol was then established to adequately assess pressure in the custom made OIGs.
Results Preliminary heart rate (HR) data of five subjects suggest HR response was greater during the hypovolemic 80° head-up tilt than the normovolemic 80° head-up tilt, as average HR during the hypovolemic and normovolemic head-up tilts were 59 bpm and 54 bpm, respectively. Differences in HR responses were also seen in differing tilt degrees. Seeing as though average HR during the hypovolemic 30° and 50° head-up tilts were 55 bpm and 54 bpm, respectively, it is evident average HR remained lower during hypovolemic 30° and 50° head-up tilts than the hypovolemic 80° head-up tilt. In addition to average HR, mean arterial pressure remained lower during hypovolemic 30° and 50° head-up tilts. Mean arterial pressure during the hypovolemic 30° and 50° head-up tilts were 86 mmHg and 85 mmHg, respectively, whereas mean arterial pressure during the hypovolemic 80° head-up tilt was 88mmHg. Additional blood pressure (BP) analysis demonstrated the onset of presycopal events as early as six minutes after the tilts were conducted. A sudden 60 mmHg drop in systolic arterial pressure from 140 mmHg to 80 mmHg was seen in one presyncopal subject. The incidence of presyncopal events in subjects during hypovolemic 30°, 50°, and 80° tilts were 1,4, and 6, respectively.
Discussion, and Conclusions Limited data on CV impacts of sustained weightlessness in addition to the ongoing debate of effective countermeasures, indicate a need to investigate the protective capabilities of the OIG. Through trends in heart rate data, blood pressure analysis, and the prevalence of subjects experiencing presyncopal events, it is evident a need for countermeasures is imperative in mitigating the risks of developing OI. This data will be supplemented to the Human Landing System (HLS) in determining burn rates for future Artemis missions.
References 1. Hargens AR, et al. Eur J Appl Phtysiol. (2013). 2. Arzeno NM, et al. Am J Physiol Heart Circ Physiol. (2013). 3. Lee SMC, et al. Front Physiol. (2020).
Acknowledgements Thank you to the NASA Cardiovascular and Vision Laboratory, subject participants, NASA Human Health and Performance Directorate, and the Space Life Sciences Summer Institute.
