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 (IJVs) 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 to -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.
- VIIP syndrome
- cerebral blood flow
- head down tilt
- Copyright © 2015, Journal of Applied Physiology