Mechanical countermeasures to headward fluid shifts

Area of Science:

• Space Physiology
• Cardiovascular Research
• Neuro-ophthalmology

Background:

• Weightlessness eliminates hydrostatic pressure gradients, causing fluid to shift towards the head.
• This headward fluid shift is linked to spaceflight-associated neuro-ocular syndrome (SANS) and other physiological changes.
• Effective countermeasures are needed for long-duration space exploration missions.

Purpose of the Study:

• To evaluate the efficacy of three mechanical countermeasures in reducing posture-induced headward fluid shifts.
• To assess lower body negative pressure (LBNP), venoconstrictive thigh cuffs (VTC), and impedance threshold device (ITD) as spaceflight analogs.
• To investigate the combined effects of these countermeasures.

Main Methods:

• Ten healthy subjects participated in a ground-based analog study.
• Noninvasive measurements included ultrasound of internal jugular veins (IJV), cardiac stroke volume, optic nerve sheath diameter, and intracranial pressure index via otoacoustic emissions.
• Subjects underwent baseline measurements in seated and supine postures, followed by countermeasure exposure in the supine posture.

Main Results:

• LBNP significantly reduced IJV cross-sectional area by 48% and was the only countermeasure to decrease the otoacoustic emissions (OAE) phase angle.
• VTC reduced IJV area by 31%, while ITD showed a non-significant reduction.
• Combined countermeasures did not yield greater effects than individual applications.

Conclusions:

• LBNP, VTC, and ITD demonstrate potential as countermeasures against headward fluid shifts.
• LBNP appears most effective among the individual countermeasures tested.
• Further research in analog and actual spaceflight environments is recommended to validate these findings.