Divers may be subject to narcotic effects caused by hyperbaric gases (particularly nitrogen). The principal hazard of gas narcosis is the induction of euphoria, overconfidence and loss of judgment. This may cause the diver to become less alert, take extra risks, and start a chain of events culminating in a serious diving accident. No early warning signal is available. Therefore, an objective method for detecting narcosis could improve diver safety and prove useful in related research.

In three experiments, divers were exposed to 1) three low-dose concentrations of nitrous oxide (20, 30 and 40%), 2) air and heliox at 2.8 and 6 ATA (284 and 608 kPa) and 3) oxygen at 1.0, 1.4 and 2.8 ATA (101, 142 and 284 kPa). The cerebral effects were measured with pupillometry, critical flicker fusion frequency (CFFF), psychometric tests and electroencephalogram (EEG). The complexity of the time course of the EEG was estimated using a novel metric – default mode network (DMN) complexity. The degree of spatial functional connectivity – estimated using mutual information – was summarised with the global efficiency network measure. Pupillometry and CFFF showed no differences between conditions. In the nitrous oxide EEG recording, the DMN complexity correlated with the psychometric test results in a mixed-effects model (r2 = 0.67; receiver operating characteristic area [95% CI], 0.72 [0.59 to 0.85], P < 0.001). DMN complexity did not detect a difference with the hyperbaric air exposure. However, air-breathing at 6 ATA (608 kPa) (experienced as mild nitrogen narcosis) caused a significant increase in brain connectivity, compared to surface air-breathing (p=0.001). Air-breathing at 2.8 ATA (284 kPa) trended in a similar direction, showing the early signs of nitrogen narcosis. Hyperbaric oxygen did not increase global efficiency as hyperbaric nitrogen did, which could be interpreted as oxygen not being narcotic. Oxygen caused a decrease in DMN complexity, which might indicate oxygen hyperexcitability at 1 ATA (101 kPa) and up. In conclusion, the results of these studies have provided two novel quantitative EEG analysis algorithms that quantify the distinct cerebral effects of nitrous oxide, nitrogen and oxygen, which might correlate with their different molecular mechanisms of action.

About Mr. Xavier Vrijdag MSc

Research fellow, Department of Anaesthesiology, University of Auckland

Xavier Vrijdag has a master's degree in Technical Medicine. He has worked as a researcher and technical physician at the department of Diving and Hyperbaric Medicine of the Academic Medical Center, the Netherlands. In his Master's thesis he developed a qEEG algorithm to quantify cerebral arterial gas embolism in a swine model in the hyperbaric environment. Between his Master’s and his current employment he continued to work with EEG in the hyperbaric environment, while he was also part of the hyperbaric treatment team in the Academic Medical Center, the Netherlands. After, Xavier worked as a hyperbaric technician and researcher at Deep Dive Dubai. Currently, he works a s a research fellow at the department of Anaestesiology at the University of Auckland investigating the possibility to monitor hyperbaric gas narcosis. Besides work, Xavier is an avid technical diver in caves and wrecks, using open circuit and rebreather. He is a (technical) diving and freediving instructor for SSI and an instructor trainer for the medical courses through DAN.

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