Methods. Internal jugular vein cross-sectional area (IJVA) and inferior vena cava (IVC) diameter (VividQ, GE Health Care, Chicago, IL), intraocular pressure (Triggerfish, Sensimed, Switzerland), heart rate, and beat-to-beat finger blood pressure (Finapres Medical Systems, Amsterdam-Zuidoost, Netherlands) were measured in 9 subjects (5F, 4M) while supine before flight and while seated when exposed to 1-G, 0.75-G, 0.50-G, and 0.25-G during parabolic flights flown by Novespace, Inc. (Bordeaux-Mérignac, France) as part of the first International Space Life Sciences Working Group Campaign in June 2018.

Results. There was a main effect of condition on IJVA (p=0.0001). IJVA was smallest in the 1-G seated posture, progressively increased in the seated subjects as G-levels decreased, and was largest during 1-G supine, our surrogate for 0-G. IJVA during 1-G seated was smaller than when subjects were exposed to 0.25- and when supine in 1-G. In contrast, there was no difference (p=0.71) in IVC diameter at any level of acceleration or while supine in 1-G. There was a main effect of condition on mean arterial pressure (p=0.004) such that MAP during 1-G seated rest was significantly greater than during any of the other conditions. Conversely, there was no effect of condition (p=0.09) on heart rate.

Conclusions. Preliminary analysis of these data, particularly IJVA, suggest (a) that a G-level greater than 0.25-G may be required in the z-axis to reverse weightlessness-induced venous fluid shift to the extent that artificial gravity would be a viable countermeasure to SANS development during long-duration spaceflight and (b) that G-levels experienced on the moon and Mars may not be sufficient to prevent the development of SANS. Future work should include validation of these findings with similar measures during the application of countermeasures in spaceflight and with exposure to partial gravity environments.