Fluid Redistribution in Spaceflight Using Venoconstrictive Thigh Cuffs

The cephalad fluid shift that occurs in weightlessness is thought to contribute to multiple health risks including venous flow stasis and thrombosis, making the development of effective countermeasures a critical research priority. Venoconstrictive thigh cuffs (VTC) have been used during spaceflight to sequester fluid in the legs and reverse the headward fluid shift caused by removal of a hydrostatic pressure gradient due to lack of gravity. The purpose of this study was to quantify the magnitude of fluid shift reversal and characterize venous flow patterns with use of VTC by measuring anthropometric and hemodynamic responses before and during 6 hours of VTC use during spaceflight. We hypothesized that use of VTC donned at 50 mmHg for up to 6 hours will lead to increased thigh circumference, decreased stroke volume (SV) and internal jugular vein (IJV) area and changes in flow patterns in the IJV. Astronauts (n=7) flying ~6-month missions to the International Space Station donned VTC for up to 6 hours on flight day (FD) 45. Left and right high circumference was recorded both with and without VTC application, and SV and bilateral IJV area were measured via ultrasound while supine during preflight testing and inflight. Bilateral IJV flow was assessed using Doppler ultrasound. Inflight measurements were obtained before and at 30 min, 3 hours, and 6 hours of VTC use. Left and right thigh circumference and bilateral IJV area were averaged, and mixed-effects analyses were conducted to assess for differences thigh circumference, SV, and IJV area between supine preflight, inflight without VTC, and inflight with VTC at each timepoint. P-values were adjusted using Dunnett’s or Šídák's multiple comparisons test. Thigh circumference decreased from preflight in the supine posture to FD45 before VTC use (mean: -4.9 cm, 95% CI: -6.6 to -3.2 cm, p< 0.0001), and increased during use of VTC at 30 min (2.2 cm, 1.2 to 3.2 cm, p< 0.0001), 3h (3.4 cm, 2.0 to 4.9 cm, p< 0.0001), and at 6h (4.3 cm, 3.2 to 5.3 cm, p< 0.0001), which did not differ from preflight supine (52.5 cm, 50.9 to 54.1 cm, p=0.7392). After donning VTC, SV was lower from pre-VTC use at 30 min (-17.9 ml, -29.5 to -6.4, p=0.0072), 3h (-17.0 mL, -24.7 to –9.3 ml, p=0.0012), and 6h (-19.1 mL, -30.5 to -7.6, p=0.0065). After donning VTC, IJV area was not significantly different after 30 min (-0.13 mm2, -0.28 to -0.02 mm2, p=0.094), 3h (-0.28 mm2, -0.64 to 0.07 mm2, p=0.1342) or 6h (-0.30 mm2, -0.69 to -0.08 mm2, p=0.1345). In 43% of participants, VTC application inflight returned flow patterns to those seen in upright posture on Earth. These findings indicate that the application of a VTC induces a redistribution of fluid volume, characterized by reduced venous return and increased lower limb fluid accumulation within 30 min of application. The application of VTCs can be safely worn for up to 6 hours during spaceflight and appear to counteract the headward fluid shift induced by microgravity, indicating potential efficacy for mitigating blood flow stasis, which is one contributing risk factor for thrombus. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.