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

• Published in: Physiological reports Vol. 5; no. 11
• Main Authors: Laurie, Steven S., Vizzeri, Gianmarco, Taibbi, Giovanni, Ferguson, Connor R., Hu, Xiao, Lee, Stuart M. C., Ploutz‐Snyder, Robert, Smith, Scott M., Zwart, Sara R., Stenger, Michael B.
• Format: Journal Article
• Language: English
• Published: United States John Wiley & Sons, Inc 01.06.2017; John Wiley and Sons Inc
• Subjects: Acuity; Adult; Carbon dioxide; Distension; Edema; Environmental Physiology; Fluid Shifts - physiology; Functional anatomy; Head-Down Tilt - physiology; Head‐down tilt; Humans; Hypercapnia; Hypercapnia - physiopathology; Intracranial Pressure - physiology; Intraocular Pressure - physiology; Male; Metabolism and Regulation; Middle Aged; NASA; one‐carbon metabolism; Optic nerve; Original Research; Physiology; Pressure; Respiratory Conditions Disorder and Diseases; Retina; Sensory Neuroscience; Space Flight; Structure-function relationships; Tomography; Tomography, Optical Coherence; translaminar pressure difference; Ultrasound; Weightlessness; Weightlessness Simulation; translaminar pressure difference; one‐carbon metabolism; NASA; hypercapnia; Head‐down tilt
• DOI: 10.14814/phy2.13302
• ISSN: 2051-817X

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. Many astronauts experience ocular structural and functional changes during long‐duration spaceflight, including choroidal folds, optic disc edema, globe flattening, optic nerve sheath diameter distension, retinal nerve fiber layer thickening, and decreased visual acuity. This study investigated whether acute exposure to mild hypercapnia combined with a cephalad fluid shift induced by head‐down tilt (HDT) would increase cerebral or ocular blood flow, result in an increase in intracranial pressure, a reduction in translaminar pressure difference, and a mild accumulation of fluid at the optic nerve. Breathing 1% CO2 for 60 min did not change our cardiovascular, ocular, or intracranial pressure measures, but genetic polymorphisms within the one‐carbon pathway may lead to differential responses between subjects.