Assessment of Jugular Venous Blood Flow Stasis and Thrombosis During Spaceflight

Abstract

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