Jugular venous flow dynamics during acute weightlessness and partial gravity in parabolic flight

Weightlessness (0.00-G) and partial gravity exposures may contribute to internal jugular vein (IJV) distension and altered cerebral hemodynamics, which may increase the risk of venous thrombosis. Nine participants were studied while in supine and seated position in normal gravity and during parabolic flight while seated. Participants were exposed to 10 parabolas at each G-level: 0.00-G, 0.25-G, 0.50-G, and 0.75-G. Bilateral IJV cross-sectional area (CSA), pressure, and flow were assessed using two-dimensional (2-D) and Doppler ultrasound. Compared with seated preflight, left IJV CSA increased during 0.00-G, 0.25G-, and 0.50-G, and right IJV CSA increased during 0.00-G and 0.25-G exposures (<i>P</i> < 0.05). IJV CSA was not significantly different between preflight supine and seated position 0.00-G. Left IJV pressure during all reduced G-levels and right IJV pressure during 0.00- and 0.50-G were significantly greater than preflight seated. Normal forward flow was observed in the right and left IJV in all participants preflight and in the right IJV during all G-levels. In seven of nine participants, the left IJV presented with normal flow across partial-G levels and weightlessness. Stagnant flow was observed in the left IJV during 0.00-G in two participants and during 0.25- and 0.50-G in one participant. Together these data reveal a graded effect in the left and right IJV CSA and pressure across increasing G-levels and normal forward flow in most individuals. Venous stasis developed in the left IJV during acute reduced gravity exposures in two participants, suggesting that astronauts should be monitored for flow abnormalities early in their mission and while on the Moon and Mars.<b>NEW & NOTEWORTHY</b> Weightlessness-induced blood flow stasis in internal jugular veins (IJV) may contribute to an increased risk of thrombosis during spaceflight. Here we report that reduced gravity leads to IJV distension in healthy participants and that 0.00-G, 0.25-G, and 0.50-G exposures resulted in stagnant blood flow in the left IJV in one participant. Reduced gravity on the Moon and Mars may be insufficient to protect against venous flow abnormalities.