Mechanical countermeasures to headward fluid shifts

Karina Marshall-Goebel, Brandon R. Macias, Steven S. Laurie, Stuart M.C. Lee, Douglas J. Ebert, David T. Kemp, Annelise Miller, Scott H. Greenwald, David S. Martin, Millennia Young, Alan R. Hargens, Benjamin D. Levine, Michael B. Stenger

Research output: Contribution to journalArticlepeer-review

Abstract

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 three mechanical countermeasures [lower body negative pressure (LBNP), venoconstrictive 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 with the supine posture, IJV area decreased 48% with application of LBNP [mean ratio: 0.52, 95% confidence interval (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 (D -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, represents promising headward fluid shift countermeasures but will require future testing in analog and spaceflight environments. NEW & NOTEWORTHY As a weightlessness-induced headward fluid shift is hypothesized to be a primary factor underlying several physiological consequences of spaceflight, countermeasures aimed at reversing the fluid shift will likely be crucial during exploration-class spaceflight missions. Here, we tested three mechanical countermeasures individually and in various combinations to reduce a posture-induced headward fluid shift as a ground-based spaceflight analog.

Original languageEnglish (US)
Pages (from-to)1766-1777
Number of pages12
JournalJournal of applied physiology
Volume130
Issue number6
DOIs
StatePublished - Jun 2021

Keywords

  • Impedance threshold device
  • Lower body negative pressure
  • SANS
  • Spaceflight
  • Thigh cuffs

ASJC Scopus subject areas

  • Physiology
  • Physiology (medical)

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