Effects of short-term exposure to head-down tilt on cerebral hemodynamics: a prospective evaluation of a spaceflight analog using phase-contrast MRI

J Appl Physiol (1985). 2016 Jun 15;120(12):1466-73. doi: 10.1152/japplphysiol.00841.2015. Epub 2016 Mar 24.

Abstract

Alterations in cerebral hemodynamics in microgravity are hypothesized to occur during spaceflight and could be linked to the Visual Impairment and Intracranial Pressure syndrome. Head-down tilt (HDT) is frequently used as a ground-based analog to simulate cephalad fluid shifts in microgravity; however, its effects on cerebral hemodynamics have not been well studied with MRI techniques. Here, we evaluate the effects of 1) various HDT angles on cerebral arterial and venous hemodynamics; and 2) exposure to 1% CO2 during an intermediate HDT angle (-12°) as an additional space-related environmental factor. Blood flow, cross-sectional area (CSA), and blood flow velocity were measured with phase-contrast MRI in the internal jugular veins, as well as the vertebral and internal carotid arteries. Nine healthy male subjects were measured at baseline (supine, 0°) and after 4.5 h of HDT at -6°, -12° (with and without 1% CO2), and -18°. We found a decrease in total arterial blood flow from baseline during all angles of HDT. On the venous side, CSA increased with HDT, and outflow decreased during -12° HDT (P = 0.039). Moreover, the addition of 1% CO2 to -12° HDT caused an increase in total arterial blood flow (P = 0.016) and jugular venous outflow (P < 0.001) compared with -12° HDT with ambient atmosphere. Overall, the results indicate decreased cerebral blood flow during HDT, which may have implications for microgravity-induced cerebral hemodynamic changes.

Keywords: MRI; cerebral blood flow; head-down tilt; microgravity; visual impairment and intracranial pressure.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Cerebellum / blood supply
  • Cerebellum / physiology*
  • Cerebral Arteries / physiology
  • Cerebral Veins / physiology
  • Cerebrovascular Circulation / physiology
  • Head-Down Tilt / physiology*
  • Hemodynamics / physiology*
  • Humans
  • Intracranial Pressure / physiology
  • Jugular Veins / physiology
  • Magnetic Resonance Imaging / methods
  • Male
  • Prospective Studies
  • Space Flight / methods
  • Weightlessness
  • Weightlessness Simulation / methods