Abstract
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.
Highlights
The distribution of body fluid is affected by gravity, which is altered in microgravity.[1]
The major finding of the present re-analysis is that a mild decrement in plasma volume induced by administration of 0.2 mg/kg of furosemide did not change noninvasive ICP (nICP) or cerebral artery compliance, but decreased pulsatility index (PI) significantly
The cerebral artery compliance represents the change in arterial blood volume in response to a change in arterial blood pressure (ABP), and the cerebrospinal fluid (CSF) storage compliance refers to changes in the volume of the CSF storage in response to changes in intracranial pressure (ICP).[7]
Summary
The distribution of body fluid is affected by gravity, which is altered in microgravity.[1] Short-duration spaceflight induces an approximately 10% reduction in plasma volume, which is considered to be mild intravascular fluid deprivation.[2,3] Our previous study demonstrated that administration of furosemide (0.2 mg/kg) produced a 10% reduction in plasma volume.[4] Our previous study showed decreases in the magnitude of the transfer from arterial blood pressure (ABP) oscillations to cerebral blood flow velocity (CBFv) fluctuations, which suggested an improved cerebral autoregulation of the enhanced suppression capability from ABP to cerebral blood flow (CBF).[4] This finding was considered to be partially associated with the enhanced cerebral autoregulation that occurs during and after short-duration spaceflight.[5] mild volume depletion alters cerebrovascular regulation; the effect on intracranial pressure (ICP)
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