The following text is an abstract from the chapter "Nitrogen narcosis, oxygen narcosis and the high pressure nervous syndrome" in the Conference Technical diving: Proceedings of the Divers Alert Network. January 18-19 2008. All the material writen below comes from that document and is under the speaker's authorship. My intention with this abstract is to summarize the some of the information contained in such paper and to allow a better comprenhension of narcosis. Source is:
Have fun!
Jorge A. Mahauad
Reading your way through a better understanding of narcosis.
Background:
The depth-dependent effect of nitrogen narcosis on psychometric performance was well illustrated by Shilling and Willgrube (1937). They noted the greatest severity immediately on reaching depth and that rapid compression potentiated the narcosis. Acclimatized subjects showed some improvement.
Case and Haldane found that at 250 fsw manual skills showed little deterioration. At 300 fsw, however, the narcosis was severe with marked impairment of practical ability and judgment. Manual dexterity was affected to a much less degree than mental ability. Orders were recognized but ignored; there was a sense of impending loss of consciousness, euphoria and dizziness, manic or depressive states, catalepsy and a disorganization of the sense of time.There is no question that there is an individual susceptibility to the effects of nitrogen narcosis. This may be due to individual accommodation or “learning to cope” with a reduction in anxiety and recognition by the individual of his own signs and symptoms. Thus highly trained and experienced deep divers may be able to function much more effectively at depths which would incapacitate less experienced divers.
Rate of Compression or Descent
It is widely held that rapid compression potentiates nitrogen narcosis. The effect is believed due to an increase in alveolar and cerebral carbon dioxide. Carbon dioxide increase is reputed to increase susceptibility to nitrogen narcosis, oxygen toxicity and decompression sickness.
In regard to carbon dioxide, Case and Haldane noted this as early as 1941 and reported that the combined effects of nitrogen and carbon dioxide in studies at 300 fsw with inspired CO2 percentages were much more severe than either gas alone.
Another somewhat hidden effect of CO2 is the enhancement of the effects of nitrogen narcosis by exercise or work during diving as shown again by the work of Adolfson. This indicated significant increases in decrements of psychometric tests in working divers compared to resting divers at 100, 200 and 300 fsw.
Oxygen Narcosis
Since technical diving involves use of nitrox and other mixed gases, there is considerable interest, if not controversy, on whether oxygen is a narcotic. Oxygen is different from the inert gases as it is metabolized in the body. Nevertheless, oxygen narcosis has indeed been reported in humans and animals.
In a human study oxygen was found to be only 0.26 as potent as nitrogen. The lower effect is no doubt due to its being metabolized and much lower levels actually occurring in the brain compared to the lungs.
No significant differences were found between nitrox and air. This in spite of many claims by divers breathing nitrox that they feel less narcosis – a possible placebo effect.
So in conclusion, in regard to oxygen narcosis, yes, oxygen can exert a narcotic effect, but it is less than predicted by its lipid solubility, almost certainly because oxygen is metabolized and tissue PO2 does not equilibrate with the PO2 in the lungs at the oxygen pressures that can be safely used in diving. Thus, in practical terms, oxygen narcosis is much less of a problem for technical diving than would arise from the use of nitrogen at significant depths. To overcome the potential for nitrogen narcosis, deep divers do use oxygen-helium mixtures as an alternative. However, this then introduces the problem of the high pressure nervous syndrome
Mechanisms of Nitrogen Narcosis
Briefly, it is considered to be similar to general anesthesia by gaseous anesthetics. This is based on the Meyer-Overton hypothesis “All gaseous or volatile substances induce narcosis if they penetrate the cell lipids in a definite molar concentration”.
In this regard there is still a strong relationship between solubility in lipids (fat) and narcotic potency. This was later confirmed in mice, newts and isolated nerves. This led Keith Miller at Harvard to propose the “Critical Volume Theory,” i.e., “Anesthetics expand a critical hydrophobic molecular site and pressure contracts this.”
In simple terms, a “critical hydrophobic molecular site” represents structures in cell membranes that are important for conduction of the nerve impulses upon which normal function of our nervous system depends.
If these structures are physically distorted by absorption of large numbers of gas molecules, then conduction of nerve impulses can be impeded, manifesting as the cognitive impairment we call narcosis. This distortion can be reversed, i.e. the membrane structures can be returned to their original size or configuration by the application of high pressures; hence the fascinating phenomenon of “pressure reversal.”
Interestingly, this theory also implies that the application of pressure in the absence of membrane expansion by a narcotic gas would compress those same important membrane structures below their normal size or configuration, once again producing an interruption of normal function that is usually called HPNS.Perhaps all that can be said is that at 100 fsw the narcosis is minimal, but becomes progressively worse with increasing depth and by 200 fsw can be a real issue as regards safety. In practice, virtually all of the technical diving training agencies encourage the use of helium mixtures for dives beyond 150 feet for making narcosis manageable.
