A gravity-induced head-to-foot (Gz) hydrostatic pressure gradient exists in the fluid-filled systems of the body in the upright position on Earth. In the supine position, the gravity vector no longer pulls in the Gz axis; therefore, blood and tissue-fluid pressures and volumes redistribute across the body. By spending approximately two-thirds of the day upright and the remaining one-third of the day supine at night, humans experience fluid shifts daily. However, crew members on the International Space Station (ISS) are weightless and thus experience a sustained redistribution of fluids toward the head that is not subject to daily diurnal posture-induced change in hydrostatic pressure.1 Headward fluid shifts during prolonged weightlessness result in facial puffiness, decreased leg volume, increased stroke volume, and decreased plasma volume.2-4 This fluid shift may also affect cerebral venous outflow as internal jugular vein (IJV) volume has been showed to be increased during 4.0 to 5.5 months of spaceflight exposure.5

The purpose of this study was to quantify the cross-sectional area and pressure of the IJV as well as characterize the Doppler flow velocity profile to describe cerebral venous outflow during spaceflight compared with various postures on Earth. A secondary aim was to evaluate if the use of lower body negative pressure (LBNP) would be associated with negating the effects of the spaceflight-induced headward fluid shift on the IJV.


This cohort study’s findings of abnormal and stagnant cerebral venous outflow in the IJV during spaceflight and subsequent development of jugular vein thrombosis are novel findings that may have significant human health implications for civilian spaceflight as well as future exploration-class missions, such as a mission to Mars. The potential relationship between altered cerebral venous outflow and the spaceflight associated neuro-ocular syndrome31 and neurocognitive performance should be further investigated. Our findings highlight the need for a more comprehensive evaluation of bilateral venous hemodynamics during spaceflight, as well as for investigation of countermeasures, including LBNP, that can restore vascular physiology to a state similar to that seen in the upright and supine positions on Earth.