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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.
Highlights
This is the first study to examine cerebral hemodynamics using phase-contrast MRI during various angles of head-down tilt
Total jugular venous cross-sectional area (CSA) increased during Ϫ6° (0.9 Ϯ 0.4 cm2, P ϭ 0.015), Ϫ12° (1.1 Ϯ 0.5 cm2, P ϭ 0.0001), and Ϫ18° Head-down tilt (HDT) (1.3 Ϯ 0.5 cm2, P Ͻ 0.0001), with the largest increase seen at Ϫ18°, compared with 0.6 Ϯ 0.2 cm2 at supine baseline
The addition of a 1% CO2 atmosphere during HDT led to an increase in blood flow on both the arterial and venous side compared with HDT with ambient atmosphere, bringing blood flow closer to supine baseline values
Summary
This is the first study to examine cerebral hemodynamics using phase-contrast MRI during various angles of head-down tilt. The study investigated the additional effects of increased ambient carbon dioxide during head-down tilt as an analog to the environment onboard the International Space Station. Given that cerebral venous outflow is directly related to cerebrospinal fluid outflow, this may lead to some of the underlying structural and functional ophthalmic changes seen in astronauts with the Visual Impairment and Intracranial Pressure (VIIP) syndrome, including optic disk edema, globe flattening, and hyperopic shifts [19, 21]. Long-duration HDT bed-rest studies at Ϫ6° have failed to reproduce ophthalmic findings similar to those seen in the VIIP syndrome [33]. This raises the question as to whether the Ϫ6° HDT angle creates a sufficient hydrostatic pressure gradient to induce cephalad fluid shifting as in microgravity. 2) exposure to increased ambient CO2 in combination with an intermediate HDT angle (Ϫ12°) as an additional analog to the ISS environment
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