Mechanical Countermeasures to Headward Fluid Shifts.

Head-to-foot gravitationally-induced hydrostatic pressure gradients in the upright posture on Earth are absent in weightlessness. This results in a relative headward fluid shift in the vascular and cerebrospinal fluid compartments and may underlie multiple physiological consequences of spaceflight, including the Spaceflight Associated Neuro-ocular Syndrome. Here, we tested 3 mechanical countermeasures (lower body negative pressure [LBNP], veno-constrictive thigh cuffs [VTC] and impedance threshold device [ITD] resistive inspiratory breathing) individually and in combination to reduce a posture-induced headward fluid shift as a ground-based spaceflight analog. Ten healthy subjects (5 male) underwent baseline measures (seated and supine postures) followed by countermeasure exposure in the supine posture. Noninvasive measurements included ultrasound (internal jugular veins [IJV] cross-sectional area, cardiac stroke volume, optic nerve sheath diameter, noninvasive IJV pressure), transient evoked otoacoustic emissions (OAE; intracranial pressure index), intraocular pressure, choroidal thickness from optical coherence tomography imaging, and brachial blood pressure. Compared to the supine posture, IJV area decreased 48% with application of LBNP (mean ratio: 0.52, 95% CI: 0.44-0.60, P<0.001), 31% with VTC (mean ratio: 0.69, 95% CI: 0.55-0.87, P<0.001), and 56% with ITD (mean ratio: 0.44, 95% CI: 0.12-1.70, P=0.46), measured at end-inspiration. LBNP was the only individual countermeasure to decrease the OAE phase angle (Δ -12.9 degrees, 95% CI: -25 to -0.9, P=0.027), and use of combined countermeasures did not result in greater effects. Thus, LBNP, and to a lesser extent VTC and ITD, represent promising headward fluid shift countermeasures, but will require future testing in analog and spaceflight environments.