title: Increased cerebral blood volume pulsatility during head-down tilt with elevated carbon dioxide: the SPACECOT Study.
abstract: Astronauts aboard the International Space Station (ISS) have exhibited hyperopic shifts, posterior eye globe flattening, dilated optic nerve sheaths, and even optic disk 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 CO2, a potent arteriolar vasodilator that could further affect cerebral blood volume (CBV) and cerebral blood flow, intracranial compliance, and ICP. A collaborative pilot study between the National Space Biomedical Research Institute and the German Aerospace Center tested the hypotheses that1) HDT and elevated CO2physiologically interact and2) cerebrovascular pulsatility is related to HDT and/or elevated CO2In a double-blind crossover study (n= 6), we measured CBV pulsatility via near-infrared spectroscopy, alongside noninvasive ICP and intraocular pressure (IOP) during 28-h -12° HDT at both nominal (0.04%) and elevated (0.5%) ambient CO2In our cohort, CBV pulsatility increased significantly over time at cardiac frequencies (0.031 ± 0.009 μM/h increase in total hemoglobin concentration pulsatility amplitude) and Mayer wave frequencies (0.019 ± 0.005 μM/h increase). The HDT-CO2interaction on pulsatility was not robust but rather driven by one individual. Significant differences between atmospheres were not detected in ICP or IOP. Further work is needed to determine whether individual differences in pulsatility responses to CO2relate to visual changes in space.NEW & NOTEWORTHYCerebral blood volume (CBV) pulsatility-as measured by near-infrared spectroscopy-increases over time during -12° head-down tilt at both cardiac and Mayer wave frequencies. CBV pulsatility appeared to increase more under elevated (0.5%) CO2at Mayer wave frequencies in some individuals. If similar dynamic pulsatility increases occur in astronauts, there is the potential to initiate vascular and possibly other remodeling processes that lead to symptoms associated with sustained increases in intracranial pressure.
authors.affiliation: Neural Systems Group, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts; strang@nmr.mgh.harvard.edu.|Center for Space Medicine, Baylor College of Medicine, Houston, Texas.|National Space Biomedical Research Institute, Houston, Texas; and.
authors.forename: Gary E
authors.initials: GE
authors.lastname: Strangman
authors.affiliation: Neural Systems Group, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts.|Center for Space Medicine, Baylor College of Medicine, Houston, Texas.
authors.forename: Quan
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Neural Systems Group, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts.|Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany.
authors.forename: Karina
authors.identifier: http://orcid.org/0000-0002-5240-7625
authors.initials: K
authors.lastname: Marshall-Goebel
authors.affiliation: Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany.
authors.forename: Edwin
authors.initials: E
authors.lastname: Mulder
authors.affiliation: Department of Neurology, Baylor College of Medicine, Houston, Texas.
authors.forename: Brian
authors.initials: B
authors.lastname: Stevens
authors.affiliation: Department of Neurology, Baylor College of Medicine, Houston, Texas.|Center for Space Medicine, Baylor College of Medicine, Houston, Texas.|National Space Biomedical Research Institute, Houston, Texas; and.
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Clark
authors.affiliation: Department of Neurology, Baylor College of Medicine, Houston, Texas.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Bershad
mesh_terms: D061725:Accelerometry / Q000379:methods; D000328:Adult; D018480:Astronauts; D002245:Carbon Dioxide / Q000008:administration & dosage* / Q000009:adverse effects; D000071937:Cerebral Blood Volume / Q000187:drug effects / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000187:drug effects / Q000502:physiology*; D005858:Germany; D018475:Head-Down Tilt / Q000009:adverse effects / Q000502:physiology*; D006801:Humans; D038622:Internationality; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D011897:Random Allocation; D013026:Space Flight / Q000379:methods; D019265:Spectroscopy, Near-Infrared / Q000379:methods; D014481:United States; D018474:Weightlessness Simulation / Q000009:adverse effects / Q000379:methods*
chemical_list: D002245:Carbon Dioxide
keywords: near-infrared spectroscopy; spaceflight analogs; spaceflight-induced intracranial hypertension; tissue perfusion; vision alterations; visual impairment and intracranial pressure