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

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.

Fig. 1.

Cross-sectional magnitude images from a phase-contrast MRI sequence taken between the second and third vertebrae in one subject. The images show the left and right internal jugular veins (triangles), internal carotid arteries (solid arrows), and vertebral arteries (dashed arrows) at 0° baseline (A), −6° HDT (B), −12° HDT (C), and −18° HDT (D).

Fig. 2.

Effects of different head-down tilt angles on total cross-sectional area (CSA; A) and total flow (B) for the arterial (●) and internal jugular venous (■) systems. Measurements were taken at baseline (0°) and after 4.5-h head-down tilt at various angles with phase-contrast MRI. Flow was calculated by multiplying blood flow velocity by the CSA for each vessel. Blood flow through the bilateral internal carotid and vertebral arteries were summed to give total arterial inflow, and blood flow through the bilateral internal jugular veins were summed to give jugular venous outflow. Values are means ± SE. *P <0.05. **P <0.01. ***P <0.001. ⁺P <0.05.

Fig. 3.

Blood flow velocity for the internal carotid arteries (●), vertebral arteries (▲), and internal jugular veins (■) at various head-down tilt angles. Measurements were taken at baseline (0°) and after 4.5-h head-down tilt with phase-contrast MRI. Values are means ± SE. *P < 0.05. **P < 0.01. ***P < 0.001.

Fig. 4.

Effects of −12° head-down tilt with and without a 1% CO₂ atmosphere on phase-contrast MRI-derived total arterial inflow (●; A) and internal jugular venous outflow (■; A) and total arterial CSA (●) and internal jugular venous CSA (■; B). Measurements were taken at baseline (0°) and after 4.5-h head-down tilt. Flow was calculated by multiplying blood flow velocity by the CSA for each vessel. Flow through the left and right internal carotid arteries and vertebral arteries were summed to give total arterial inflow. Flow through the left and right jugular veins were summed to give venous outflow. Values are means ± SE. *P < 0.05. **P < 0.01. ⁺P < 0.05. ⁺⁺P < 0.01.