Measuring the effects of inert gas narcosis in divers with electroencephalogram (EEG)

In this prospective intervention study a total of 24 participants are needed. There are two parts in this study each requiring 12 participants. Objective 1 – To refine the novel mutual information qEEG algorithm for nitrogen narcosis by exposing subject to a wider range of nitrogen dose In the first experiment, we will recruit 12 participants to attend one session at the hyperbaric chamber, in which they will be exposed to a pressure of 8 atmospheres absolute (ATA) while breathing air. After checking participant eligibility for testing on the day, psychometric test familiarisation will be conducted. The EEG electrode cap will be placed and checked for electrode impedance. Each measurement will consist of one-minute EEG recording with eyes open and one-minute EEG recording with eyes closed. The psychometric tests will then be administered and EEG recordings subsequently repeated. The EEG will also be recorded during the psychometric test administration. At surface pressure, baseline measurements (as above) will be taken inside the hyperbaric chamber. Directly after, the chamber will be pressurised to 8 ATA at a rate of 1 ATA per minute maximum to minimise heat built-up that could cause sweating, which impairs EEG signal quality. At 8 ATA, the measurements will be repeated after five minutes acclimatisation to air-breathing at that pressure. The decompression will be controlled (including oxygen stops) according to the schedule prescribed by the Canadian Navy decompression tables. In our previous research effort, we exposed participants to a maximum of 6 ATA and found subtle effects of impairment. With this new exposure to 8 ATA, we expect to record a stronger narcotic effect, which will allow us to associate the subtle EEG changes with narcosis more confidently. Objective 2 – To benchmark the real-time mutual information algorithm with nitrogen narcosis in a wider group of subjects and with a more sensitive cognitive function test For the second experiment, we will recruit another 12 participants to attend a session at the hyperbaric chamber, in which each participant will be exposed to a pressure of six ATA while breathing air. In a similar approach to that described above, the session will consist of preparation, followed by baseline and depth EEG measurements. At surface pressure, baseline measurements (as above) will be taken inside the hyperbaric chamber. Directly after, the chamber will be pressurised to 6 ATA at a rate of 1 ATA per minute maximum to minimise heat built-up that could cause sweating, which impairs EEG signal quality. At 6 ATA, the measurements will be repeated after five minutes acclimatisation to air-breathing at that pressure. The decompression will be controlled (including oxygen stops) according to the schedule prescribed by the Canadian Navy decompression tables. This will provide an extension of the current database of EEG recordings we collected from participants exposed to 6 ATA in our previous approved study, which will allow us to assess the metric's performance with new participants and (combined with previous data) better characterise sensitivity, specificity, and positive predictive value for manifestations of cognitive impairment. The psychometric task will be used to identify impairment levels that can be considered to increase danger to the diver, and we will correlate those changes against the mutual information EEG changes. A parallel aim of conducting this work is to minimise the number of electrodes required to apply the mutual information algorithm with a view to eventual real-world use. This will be a post-hoc analysis.