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Physiol Rep. 2017 Jun;5(11). pii: e13302. doi: 10.14814/phy2.13302.

Effects of short-term mild hypercapnia during head-down tilt on intracranial pressure and ocular structures in healthy human subjects.

Author information

1
KBRwyle, Houston, Texas steven.laurie@nasa.gov.
2
Department of Ophthalmology and Visual Sciences, The University of Texas Medical Branch, Galveston, Texas.
3
MEI Technologies, Houston, Texas.
4
Department of Physiologic Nursing, University of California, San Francisco, San Francisco, California.
5
KBRwyle, Houston, Texas.
6
University of Michigan School of Nursing Department of Applied Biostatistics Laboratory, Ann Arbor, Michigan.
7
NASA Lyndon B. Johnson Space Center, Houston, Texas.
8
Department of Preventive Medicine & Community Health, The University of Texas Medical Branch, Galveston, Texas.

Abstract

Many astronauts experience ocular structural and functional changes during long-duration spaceflight, including choroidal folds, optic disc edema, globe flattening, optic nerve sheath diameter (ONSD) distension, retinal nerve fiber layer thickening, and decreased visual acuity. The leading hypothesis suggests that weightlessness-induced cephalad fluid shifts increase intracranial pressure (ICP), which contributes to the ocular structural changes, but elevated ambient CO2 levels on the International Space Station may also be a factor. We used the spaceflight analog of 6° head-down tilt (HDT) to investigate possible mechanisms for ocular changes in eight male subjects during three 1-h conditions: Seated, HDT, and HDT with 1% inspired CO2 (HDT + CO2). Noninvasive ICP, intraocular pressure (IOP), translaminar pressure difference (TLPD = IOP-ICP), cerebral and ocular ultrasound, and optical coherence tomography (OCT) scans of the macula and the optic disc were obtained. Analysis of one-carbon pathway genetics previously associated with spaceflight-induced ocular changes was conducted. Relative to Seated, IOP and ICP increased and TLPD decreased during HDT During HDT + CO2 IOP increased relative to HDT, but there was no significant difference in TLPD between the HDT conditions. ONSD and subfoveal choroidal thickness increased during HDT relative to Seated, but there was no difference between HDT and HDT + CO2 Visual acuity and ocular structures assessed with OCT imaging did not change across conditions. Genetic polymorphisms were associated with differences in IOP, ICP, and end-tidal PCO2 In conclusion, acute exposure to mild hypercapnia during HDT did not augment cardiovascular outcomes, ICP, or TLPD relative to the HDT condition.

KEYWORDS:

NASA ; Head‐down tilt; hypercapnia; one‐carbon metabolism; translaminar pressure difference

PMID:
28611153
PMCID:
PMC5471441
DOI:
10.14814/phy2.13302
[Indexed for MEDLINE]
Free PMC Article

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