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 the 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 × 3 h daily, n = 12) or use of venoconstrictive thigh cuffs (VTC) following 1 h of moderate aerobic cycle exercise (Ex + CUFF, 6 h daily for 6 of 7 days, n = 12) across 30 days of HDTBR. The study also included a seated posture group (Upright, 2 × 3 h daily, n = 11), which was conducted in the two campaigns with the LBNP group, and a control group that did not receive a countermeasure (n = 12) that was studied across the two campaigns with the Ex + CUFF group. All LBNP sessions and Ex + CUFF sessions were well tolerated. On HDTBR day 3, 17, and 30, both countermeasures decreased internal jugular vein cross-sectional area (LBNP: -0.97 cm2; Ex + CUFF: -0.64 cm2), stroke volume (SV) (LBNP: -40.2 mL; Ex + CUFF: -33.5 mL), and intraocular pressure (LBNP: -0.5 mmHg; Ex + CUFF: not significant), 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.NEW & NOTEWORTHY We demonstrate that daily use of lower body negative pressure or venoconstrictive thigh cuffs following aerobic exercise is well tolerated throughout 30 days of exposure to the spaceflight analog strict head-down tilt bed rest. Both countermeasures reduced SV and internal jugular vein cross-sectional area, indicators of the venous fluid shift, but to a lesser degree than the upright posture.