Daily use of countermeasures to prevent headward fluid shifts throughout 30 days of strict head-down tilt bed rest.

Strict 6° head-down tilt bed rest (HDTBR) induces a chronic headward fluid shift similar to weightlessness that may underlie development of spaceflight associated neuro-ocular syndrome. We tested the efficacy of two headward fluid shift countermeasures, using either 25 mmHg lower body negative pressure (LBNP, 2 x 3 hours daily, n=12) or use of venoconstrictive thigh cuffs (VTC) following 1 hour of moderate aerobic cycle exercise (Ex+CUFF, 6 hours daily for 6 of 7 days, n=12) across 30 days of HDTBR. The study also included a seated posture group (Upright, 2 x 3 hours daily, n=11) which was conducted in the 2 campaigns with the LBNP group, and a control group that did not receive a countermeasure (n=12) that was studied across the 2 campaigns with the Ex+CUFF group. All LBNP sessions and Ex+CUFF sessions were well-tolerated. On HDT 3, 17, and 30 both countermeasures decreased internal jugular vein cross-sectional area (LBNP: -0.97 cm²; Ex+CUFF: -0.64 cm²), stroke volume (LBNP: -40.2 mL; Ex+CUFF: -33.5 mL), and IOP (LBNP: -0.5 mmHg; Ex+CUFF: NS), but to lesser degrees than occurred in Upright for all variables. Reversal of the headward fluid shift during use of each countermeasure presented with positive comfort scores throughout the 30-day bed rest study, highlighting that use of LBNP or VTCs following exercise should be tested during spaceflight to address risks associated with the chronic headward fluid shift that occurs in weightlessness.