The effect of mild decrement in plasma volume simulating short-duration spaceflight on intracranial pressure.

Short-duration spaceflight induces an approximately 10% reduction in plasma volume, which leads to mild volume depletion. In a previous study, we found that mild volume depletion improved dynamic cerebral autoregulation. However, the effect of mild volume depletion on intracranial pressure (ICP) remains unknown. Therefore, we estimated ICP noninvasively (nICP), and calculated two indices relating to ICP, the cerebral artery compliance and the cerebral blood flow pulsatility index (PI), to examine whether ICP would decrease due to a mild decrement in plasma volume. In our previous experiment, fourteen subjects were administered 0.2 mg/kg of furosemide in a supine position to simulate an approximately 10% reduction in plasma volume induced by short-duration spaceflight. We re-analyzed the cerebral blood flow velocity waveform from the middle cerebral artery obtained by transcranial Doppler and the arterial blood pressure waveform at the radial artery obtained by tonometry to estimate nICP and to calculate cerebral artery compliance and PI using mathematical analysis based on an intracranial hydraulic model. All indices were compared between before and after furosemide administration. There were no significant changes in nICP and cerebral artery compliance. However, PI decreased significantly from before to after furosemide administration (0.78 ± 0.10 to 0.74 ± 0.09, p  = 0.009). Decreases in ICP were not observed during the 10% reduction in plasma volume. Although cerebral artery compliance did not change, PI decreased significantly. These findings suggest that the impedance of distal cerebral arteries would be reduced in response to mild decreases in plasma volume induced by short-duration spaceflight.