An examination of cerebrovascular responses in hypovolemic men and women following exposure to centrifugation- induced artificial gravity

Research has shown that long-term spaceflight leads to the bone, muscle, and cerebrovascular deconditioning. Artificial gravity (AG) induced by centrifugation has previously been proposed as a countermeasure for spaceflight deconditioning and has been evidenced to improve orthostatic tolerance in males and in exercising females (2007 Stenger abstract in Acta Astronautica). This study aims to further investigate the effects of AG training followed by a period of head-down bed rest (HDBR) on cerebrovascular responses in males and females. Additionally, this report introduces ALEASAT, a 1U CubeSat carrying a scale-model human centrifuge to the low Earth orbit (LEO). This payload will provide information on the behaviour of a human-centrifuge on board a spacecraft. The data from this payload will enable the design and development of a spacecraft-compatible short-arm human centrifuge for future Lunar and Martian missions. Hypothesis: It was hypothesized that females would have different cerebral blood flow responses following AG training and head-down tilt test compared to their male counterparts. Methods: Nine men (Mean ± SD: 37.7 ± 4.0 years, 174.8 ± 3.2 cm, 81.2 ± 4.4 kg) and eight women (29.6 ± 2.2 years, 168 ± 1.9 cm, 70.8 ± 3.5 kg) were cardiovascularly deconditioned by administration of 20 mg furosemide on each day of study. All participants partook in a randomized crossover design where on the first visit, they were exposed to either a 90-minute period of AG training or -6-degree HDBR before orthostatic tolerance limit (OTL) determination. Twenty-one days later, upon a second visit, they were exposed to the procedure not previously performed. After each exposure, participants underwent an OTL test, consisting of a 70-degree head-up-tilt (HUT) followed by ramped lower body negative pressure (LBNP) until presyncopal symptoms developed. Cerebral blood flow measurement was obtained from the middle cerebral arteries (MCA) using bilateral Transcranial Doppler (TCD) ultrasound. A three-way repeated measures ANOVA with order was used to assess statistical differences between males' and females' cerebrovascular responses after AG training and HDBR and during the first five minutes of head-up tilt. Only the data from the left MCA was used for the statistical analysis due to the poor quality of the right MCA data. Results: Data analysis showed no effect of the order of exposure, and an examination of MBFV concerning condition and sex*condition showed a statistically significant difference between males and females in MBFV (p=0.0033 and p=0.0026, respectively). Post hoc analysis revealed that females had a higher mean blood flow velocity (MBFV) (64 ± 6, 52 ± 6 cm∙s-1) in the supine position after exposure to AG and during the HUT period (SUP-HUT) compared to their male (42 ± 5, 40 ± 5 cm∙s-1) counterparts (p= 0.01). No statistically significant difference was observed in cerebrovascular autoregulation between males and females and between AG training and HDBR days (p=0.22 and p=0.32). Conclusions: Consistent with the proposed hypothesis, after AG training, females showed an increase in cerebral blood flow that was not seen in males indicating a basic difference in cerebral artery blood flow in men and women in response to gravity. This observation is consistent with similar studies done previously on astronauts returning to Earth after long-term spaceflight missions. The results from this study emphasize the crucial role of artificial gravity training in future exercise protocol design for astronauts in long-term missions to the Moon and Mars. Future investigations are required to assess cerebrovascular autoregulation using different protocols in males and females after exposure to centrifugation-induced AG.