Astronauts aboard the International Space Station (ISS) have exhibited hyperopic shifts, posterior eye globe flattening, dilated optic nerve sheaths, and even optic disc swelling from spaceflight. Elevated intracranial pressure (ICP) consequent to cephalad fluid shifts is commonly hypothesized as contributing to these ocular changes. Head-down tilt (HDT) is frequently utilized as an Earth-based analog to study similar fluid shifts. Sealed environments like the ISS also exhibit elevated carbon dioxide (CO2), a potent arteriolar vasodilator that could further affect cerebral blood volume and flow, intracranial compliance, and ICP. A collaborative pilot study between the National Space Biomedical Research Institute and the German Aerospace Center tested the hypotheses that: (1) HDT and elevated CO2 physiologically interact, and (2) cerebrovascular pulsatility is related to HDT and/or elevated CO2. In a double-blind crossover study (n=6), we measured cerebral blood volume (CBV) pulsatility via near-infrared spectroscopy, alongside non-invasive ICP and intraocular pressure (IOP) during 28-hr -12° HDT at both nominal (0.04%) and elevated (0.5%) ambient CO2. In our cohort, CBV pulsatility increased significantly over time at cardiac frequencies (0.031±0.009 μM/hr increase in [HbT] pulsatility amplitude) and Mayer wave frequencies (0.019±0.005 μM/hr increase). The HDT-CO2 interaction on pulsatility was not robust, but rather driven by an individual. Significant differences between atmospheres were not detected in ICP or IOP. Further work is needed to reproduce these findings in a larger cohort, to determine whether a "water hammer" effect in cerebral pulsatility is also present during spaceflight, and whether it is associated with ocular changes in astronauts.
- visual impairment intracranial pressure
- vision alterations
- spaceflight analogs
- spaceflight induced intracranial hypertension
- tissue perfusion
- Copyright © 2016, Journal of Applied Physiology