Comparing neurophysiological oscillations between normobaric and hypobaric hypoxia

Hypoxia has a detrimental impact on the brain due to its large oxygen demand. Hypoxia induced deficits are of concern for pilots, who undertake training to recognize their own symptoms of hypoxia. This training can be performed inside an altitude chamber (hypobaric hypoxia) or by breathing a hypoxic gas mixture (normobaric hypoxia). A gap exists in the literature as to whether hypobaric and normbaric hypoxia are equivalent, and this thesis aims to bridge this gap by comparing both forms of hypoxia with an EEG alongside middle cerebral artery blood velocity, blood pressure, end tidal gases, oxygen saturation and breathing rate. The participants took part in a randomized within-subjects design study inside a hypobaric chamber with a hypobaric hypoxia condition consisting of depressurization to a simulated altitude of 13,000 ft (~0.62 bar, partial pressure of inspired oxygen = 87.2 mmHg), normbaric hypoxia condition with a hypoxic gas mixture of 12.8% oxygen (partial pressure of inspired oxygen = 87.2 mmHg), and a normobaric normoxic condition (partial pressure of inspired oxygen = 143 mmHg). During each of the randomized conditions, participants took part in a five-minute eyes-open resting state task followed immediately by a 25-minute visuospatial covert attention task. Spectral power in canonical frequency bands and multi-scale entropy were correlated with oxygen saturation data and then compared between groups for the resting state analysis. Participants were found to be desaturating during the first half of the resting state recording in the normobaric hypoxia condition and were steadily desaturated in the hypobaric hypoxia condition. A negative correlation between alpha power and oxygen saturation, and a positive correlation between multi-scale entropy and oxygen saturation were found only during the desaturation phase of normobaric hypoxia. Once participants had steadily desaturated there were no significant conditional differences in correlations. During the attention task, oxygen saturation and end tidal oxygen were found to be significantly higher in normobaric normoxia and similar between both hypoxic conditions. Induced alpha power was found to be significantly higher during normobaric normoxia, and induced lower-beta and evoked higher-beta power were significantly increased in the hypobaric hypoxia condition. This thesis highlights that an EEG can differentiate between normobaric and hypobaric hypoxia.