Exposure to 16 h of normobaric hypoxia induces ionic edema in the healthy brain

Following prolonged exposure to hypoxic conditions, for example, due to ascent to high altitude, stroke, or traumatic brain injury, cerebral edema can develop. The exact nature and genesis of hypoxia-induced edema in healthy individuals remain unresolved. We examined the effects of prolonged, normobaric hypoxia, induced by 16 h of exposure to simulated high altitude, on healthy brains using proton, dynamic contrast enhanced, and sodium MRI. This dual approach allowed us to directly measure key factors in the development of hypoxia-induced brain edema: (1) Sodium signals as a surrogate of the distribution of electrolytes within the cerebral tissue and (2) Ktrans as a marker of blood–brain–barrier integrity. The measurements point toward an accumulation of sodium ions in extra- but not in intracellular space in combination with an intact endothelium. Both findings in combination are indicative of ionic extracellular edema, a subtype of cerebral edema that was only recently specified as an intermittent, yet distinct stage between cytotoxic and vasogenic edemas. In sum, here a combination of imaging techniques demonstrates the development of ionic edemas following prolonged normobaric hypoxia in agreement with cascadic models of edema formation.

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Sample size
Sample size was determined based on power analyses assuming a medium effect size (GPower; Faul F, Erdfelder E, Buchner A, Lang A-G. Statistical power analyses using G*Power 3.1: Tests for correlation and regression analyses. Behavior Research Methods 41, 1149Methods 41, -1160Methods 41, , 2009, and a beta of 0.8.
Data exclusions Data exclusion was done separately for each statistical test, to-be-excluded outliers were defined as values smaller or larger than 3 standard deviations than the mean for t-tests and 2 standard deviations for correlation analyses, as the latter are more sensitive to outliers.

Replication
The experimental manipulation was replicated for each participant.
Randomization Each participant was tested under the same experimental conditions, there were no groups in this study. The design relies on a withinparticipant comparison of values obtained before and after the experimental manipulation.

Blinding
There were no groups in this study, the design relies on a within-participant comparison of values obtained before and after the experimental manipulation. Therefore, blinding is not applicable, there is simply no information that could be subject to blinding.

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Recruitment
Posters at the University and in sports facilities in the community. This resulted in a very homogeneous sample of mostly young men. As outlined in our Discussion, we refrain from analyzing any predictors of altitude mountain sickness due to the homogeneity of the sample.

Ethics oversight Ethics Committee of the Medical Department of the University of Heidelberg
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Magnetic resonance imaging Experimental design
Design type pre-and post MR scans with artificially induced normobaric hypoxia in between Design specifications all participants were exposed to 16 hours under hypoxic conditions, MR scans were obtained before and after the exposure; there were no trials or blocks in this experiment.

Normalization
Sodium data of brain parenchyma was normalized to CSF signals. Spatial transformation: Physiologic brain data was linearly registered to anatomical data with 12 degrees of freedom (flirt, Part of FSL

Effect(s) tested
The difference between pre-and post-exposure measurements normalized by pre-exposure values was tested against zero using t-tests and correlated across ROIs and measures using Pearson's correlation coefficient.