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Nitrogen narcosis, "Raptures of the Deep" (as the late Jacques Costeau eloquently described it) or Inert Gas Narcosis to be strictly correct, is a diving malady that most experienced divers will be familiar with. These terms refer to a reversible, global impairment of central nervous system function caused by the increased partial pressure of certain inert gases in the brain. The exact underlying mechanisms are complex and not fully understood but the effects are predictable and may contribute to diving accidents.
We all learnt as novice divers the quaint Martini’s Law, describing the effect of descending on air being analogous to drinking one Martini for every 10msw. After 40m most divers might expect some mild sense of euphoria (or anxiety), after 60m the effects can be quite disabling, and deeper than this progressive impairment to point of unconsciousness may occur. Hence one of the reasons for the recommended max recreational diving depth of 37m on air.
Some divers seem more or less affected by narcosis, and some factors seem important in this regard. Experience at depth and with task loading enables divers to better cope with narcosis. A slow descent may also lessen the effects. Conversely, inexperienced divers in cold, dark water who are task loaded may expect worsening narcosis. Any sudden increase in exertion at depth, especially with an associated increase in CO2 levels may precipitate the sudden onset of disabling narcosis. Pre-dive state (dehydration, hangover, tiredness or anxiety) may also contribute.
Many divers will not recognize that narcosis is present during a dive but the effects are consistent and quantifiable. However when a problem arises on the dive such as entanglement, a low air situation or buddy separation, narcosis may suddenly be obvious and will decrease the diver’s ability to extricate themselves from difficulty. In other words the diver’s margin of safety has been significantly reduced.
Is tolerance to narcosis a genuine phenomenon? Many divers will state that repeated deep dives in a short time frame will make them less prone to narcosis. It appears that the "tolerance" seen is more a behavioural adaptation than a true physiological effect. Divers become familiar and comfortable with diving at depth but the narcosis can still be demonstrated to be present. And again, when trouble arises during the dive and new problem solving decisions are required, the diver’s abilities will be diminished. This is why automatic responses from thorough training are so important in technical diving.
Most inert gases have a narcotic potential which primarily seems related to their solubility in lipids (brain tissue is very fatty). In order of least narcotic (and lipid soluble) to most are:
Helium – Hydrogen – Nitrogen – CO2 – Xenon
Hence divers doing deeper dives will substitute nitrogen with helium to avoid or lessen narcosis. Conversely, xenon can be used as a general anaesthetic agent! Of course there are many other issues which come into play at these depths but this is a very effective strategy to reduce narcosis.
Next time your diving deep, stop and take stock of how you feel. Dives like the new site The Ulonga (in South Australia) contain all the risk factors for a good dose of narcosis, so take care! |
