The discussion is probably as old as scuba diving itself: Does oxygen have a narcotic effect under pressure?
It is well known that breathing compressed air can trigger nitrogen narcosis from a diving depth of approx. 30 m. Strategies to avoid this include the use of nitrox (breathing gas enriched with oxygen to reduce the nitrogen content) for recreational diving and the use of helium, which has only a minimal narcotic effect, for deeper dives.
Estimating the narcotic effect
To estimate the narcotic effect of the breathing gas mixture, the END (equivalent narcotic depth) can be calculated from the fractions of the individual gases in the mixture and their specific narcotic potency. The method is used to calculate, for a given breathing gas mixture and depth of dive, the equivalent depth that would produce approximately the same narcotic effect when breathing air. Whether the oxygen fraction should also be included for this calculation is a matter of ongoing debate. The extreme diver and (former) world record holder Nuno Gomes, for example, categorically advocates the inclusion of the oxygen fraction, calculating the narcotic potency of the breathing gas mixture using methods that go beyond the END.
The limitations of the Meyer-Overton-rule
The rational basis is provided by the Meyer-Overton correlation, which classifies the narcotic potency of a substance by its lipid solubility. Accordingly, oxygen would have to cause considerable narcosis - it has twice the lipid solubility of nitrogen and thirty-eight times that of helium - and would lead to more severe symptoms than those caused by nitrogen. However, experience shows that this is not the case, and it is also known that many exceptions to the Meyer-Overton correlation exist. For diving, therefore, the question arises as to how high the narcotic effect of oxygen actually is in practice and how this should be measured. Thus, the mentioned calculation of Gomes is also based on his subjective impression of the effect of the breathing gas mixture.
The study
A recent study by Vrijdag et al. explores this question with a clever approach. Twelve subjects breathed air and 100% oxygen at normobaric conditions and 100% oxygen at 1.4 bar and 2.8 bar, while psychometric performance and perception of "task load" were recorded and an electroencephalogram (EEG, brain waveform) was derived. Hyperbaric values were based on the maximum allowed oxygen partial pressure values for diving (1.4 bar) and for pressure chamber treatments (2.8 bar). Hyperbaric oxygen breathing did not cause any relevant change in EEG or psychometric test in contrast to hyperbaric air breathing (6 bar) in a previous experiment of the same group, which was interpreted as an expression of an onset of nitrogen narcosis. In contrast, the EEG under hyperbaric oxygen showed evidence of hyperexcitability of the cerebral cortex as an expression of CNS toxicity.
Oxygen does not have a narcotic effect if it is breathed within the partial pressure limits for diving.
These results are strong evidence that hyperbaric oxygen is not narcotic within its generally accepted partial pressure limits for diving (1.4 bar) and for pressure chamber treatments (2.8 bar).
High relevance to diving practice
The practical relevance of this study should be emphasized, as the recommended limits for oxygen partial pressure were deliberately tested using a combination of objective (EEG and psychomotor tests) and subjective markers (perception of task load). The results give divers a lot of confidence in choosing appropriate breathing gas mixtures. For recreational and technical diving in "non-extreme" areas, this means that the oxygen fraction need not be considered for estimating the narcotic effect of the breathing gas mixture, provided that the recommended upper limit of a maximum oxygen partial pressure of 1.4 bar is not exceeded.
CNS oxygen toxicity is measureable at an early stage of a dive.
As a side effect, the experiment demonstrates hyperexcitability of the cerebral cortex as an expression of CNS oxygen toxicity, which can already be detected in the EEG at the level of a "safe" oxygen partial pressure. This underlines that the recommended upper limit (1.4 bar for diving) for hyperbaric oxygen must not be exceeded.
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