Abstract
Microgravity induces a cephalad fluid shift that is responsible for cephalic venous stasis that may increase intracranial pressure (ICP) in astronauts. However, the effects of microgravity on regional cerebral blood flow (rCBF) are not known. We therefore investigated changes in rCBF in a 5-day dry immersion (DI) model. Moreover, we tested thigh cuffs as a countermeasure to prevent potential microgravity-induced modifications in rCBF. Around 18 healthy male participants underwent 5-day DI with or without a thigh cuffs countermeasure. They were randomly allocated to a control (n=9) or cuffs (n=9) group. rCBF was measured 4days before DI and at the end of the fifth day of DI (DI5), using single-photon emission computed tomography (SPECT) with radiopharmaceutical 99mTc-hexamethyl propylene amine oxime (99mTc-HMPAO). SPECT images were processed using statistical parametric mapping (SPM12) software. At DI5, we observed a significant decrease in rCBF in 32 cortical and subcortical regions, with greater hypoperfusion in basal ganglia (right putamen peak level: z=4.71, puncorr<0.001), bilateral occipital regions (left superior occipital peak level: z=4.51, puncorr<0.001), bilateral insula (right insula peak level: 4.10, puncorr<0.001), and bilateral inferior temporal (right inferior temporal peak level: 4.07, puncorr<0.001). No significant difference was found between the control and cuffs groups on change in rCBF after 5days of DI. After a 5-day DI, we found a decrease in rCBF in cortical and subcortical regions. However, thigh cuffs countermeasure failed to prevent hypoperfusion. To date, this is the first study measuring rCBF in DI. Further investigations are needed in order to better understand the underlying mechanisms in cerebral blood flow (CBF) changes after exposure to microgravity.
Highlights
Exposure to microgravity has detrimental effects on human physiology, such as muscle atrophy, bone demineralization, sensorimotor and cardiovascular deconditioning, and immune, hormonal, and metabolic changes (Michel et al, 1976; West, 2000)
Around 32 cortical and subcortical regions that were significantly less perfused at R0 than at baseline data collection (BDC)-4 were highlighted, the decrease in regional cerebral blood flow (rCBF) being greater in basal ganglia, bilateral occipital regions, bilateral insula, and bilateral inferior temporal
There was no significant difference in the change in rCBF at R0 compared with BDC-4 between the cuffs and control groups
Summary
Exposure to microgravity has detrimental effects on human physiology, such as muscle atrophy, bone demineralization, sensorimotor and cardiovascular deconditioning, and immune, hormonal, and metabolic changes (Michel et al, 1976; West, 2000). Body fluid redistribution begins in the first hours of space flight This so-called cephalad fluid shift is responsible for cephalic venous stasis, characterized by dilation of the jugular vein and facial oedema. During long-duration spaceflights, the cephalad fluid shift observed in astronauts may increase intracranial pressure (ICP), as suggested by the assessment of optic nerve sheath diameter (ONSD) by ultrasound and MRI (Kramer et al, 2012; Sirek et al, 2014) and cerebral hemodynamics seem to be modified.
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