Nitrogen narcosis

Nitrogen narcosis (also known as narcosis while diving, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high partial pressure. The Greek word (narkōsis), "the act of making numb", is derived from (narkē), "numbness, torpor", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness (alcohol intoxication), or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than . |Sense of impending blackout or of levitation | Dizziness, euphoria, manic or depressive states | Disorganization of the sense of time, changes in facial appearance | Unconsciousness, (approximate inspired partial pressure of nitrogen for anaesthesia is 33 atm) | Death

Causes

{| class="wikitable floatright" style="width:17.5em; margin-top:-0.5em;"

|+Some components of breathing gases and their relative narcotic potencies

! scope="col" | Gas

! scope="col" | Relative narcotic potency

| He || style="padding-left:2em;" | 0.045

| Ne || style="padding-left:2em;" | 0.3

| H2 || style="padding-left:2em;" | 0.6

| N2 || style="padding-left:2em;" | 1.0

| O2 || style="padding-left:2em;" | 1.7

| Ar || style="padding-left:2em;" | 2.3

| Kr || style="padding-left:2em;" | 7.1

| CO2 || style="padding-left:1.5em;" | 20.0

| Xe || style="padding-left:1.5em;" | 25.6

The cause of narcosis is related to the increased solubility of gases in body tissues, as a result of the elevated pressures at depth (Henry's law). It has been suggested that inert gases dissolving in the lipid bilayer of cell membranes cause narcosis. More recently, researchers have been looking at neurotransmitter receptor protein mechanisms as a possible cause of narcosis. The breathing gas mix entering the diver's lungs will have the same pressure as the surrounding water, known as the ambient pressure. After a change of depth, the partial pressure of inert gases in the blood passing through the brain catches up with ambient pressure within a minute or two, which results in a delayed change in narcotic effect after descending to a new depth. Rapid compression potentiates narcosis owing to carbon dioxide retention.

A divers' cognition may be affected on dives as shallow as , but the changes are not usually noticeable. There is no reliable method to predict the depth at which narcosis becomes noticeable, or the severity of the effect on an individual diver, as it may vary from dive to dive even on the same day.

Significant impairment due to narcosis is an increasing risk below depths of about , corresponding to an ambient pressure of about . Most sport scuba training organizations recommend depths of no more than because of the risk of narcosis. When breathing air at depths of  – an ambient pressure of about  – narcosis in most divers leads to hallucinations, loss of memory, and unconsciousness. A number of divers have died in attempts to set air depth records below . Because of these incidents, Guinness World Records no longer reports on this figure.

Narcosis has been compared with altitude sickness regarding its variability of onset (though not its symptoms); its effects depend on many factors, with variations between individuals. Thermal cold, stress, heavy work, fatigue, and carbon dioxide retention all increase the risk and severity of narcosis. Carbon dioxide has a high narcotic potential and also causes increased blood flow to the brain, increasing the effects of other gases. Increased risk of narcosis results from increasing the amount of carbon dioxide retained through heavy exercise, shallow or skip breathing, high work of breathing, or because of poor gas exchange in the lungs.

Prevention

thumb|Two [[diving cylinders being filled with heliox by blending helium from the brown cylinders with oxygen from the black and white cylinders.]]

The most straightforward way to avoid nitrogen narcosis is for a diver to limit the depth of dives. The other main preventive measure is properly informed selection/choice of which gas to use for the particular dive under consideration.

Since narcosis becomes more severe as depth increases, a diver keeping to shallower depths can avoid serious narcosis. Most recreational training agencies will only certify entry level divers to depths of , and at these depths narcosis does not present a significant risk. Further training is normally required for certification up to on air, and this training should include a discussion of narcosis, its effects, and management. Some diver training agencies offer specialized training to prepare recreational divers to go to depths of , often consisting of further theory and some practice in deep dives under close supervision. Scuba organizations that train for diving beyond recreational depths, may exclude diving with gases that cause too much narcosis at depth in the average diver (such as the typical widely used nitrox mixtures used for most recreational diving), and strongly encourage the use of other breathing gas mixes containing helium in place of some or all of the nitrogen in air – such as trimix and heliox – because helium has no narcotic effect. The use of these gases is considered to be technical diving and requires further training and certification.

While the individual diver cannot predict exactly at what depth the onset of narcosis will occur on a given day, the first symptoms of narcosis for any given diver are often more predictable and personal. For example, one diver may have trouble with eye focus (close accommodation for middle-aged divers), another may experience feelings of euphoria, and another feelings of claustrophobia. Some divers report that they have hearing changes, and that the sound their exhaled bubbles make becomes different. Specialist training may help divers to identify these personal onset signs, which may then be used as a signal to ascend to avoid the narcosis, although severe narcosis may interfere with the judgement necessary to take preventive action.

Deep dives should be made only after a gradual work-up to test the individual diver's sensitivity to increasing depths, taking note of reactions. Scientific evidence does not show that a diver can develop a resistance to the effects of narcosis at a given depth or become tolerant of it.

Equivalent narcotic depth (END) is a commonly used way of expressing the narcotic effect of different breathing gases. The National Oceanic and Atmospheric Administration (NOAA) Diving Manual now states that oxygen and nitrogen should be considered equally narcotic. Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect of other gases. For example, hydrogen at a given pressure has a narcotic effect equivalent to nitrogen at 0.55 times that pressure, so in principle it should be usable at more than twice the depth. Argon, however, has 2.33 times the narcotic effect of nitrogen, and is a poor choice as a breathing gas for diving (it is used as a drysuit inflation gas, owing to its low thermal conductivity). Some gases have other dangerous effects when breathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity. Although helium is the least intoxicating of the breathing gases, at greater depths it can cause high-pressure nervous syndrome, a still mysterious but apparently unrelated phenomenon. Inert gas narcosis is only one factor influencing the choice of gas mixture; the risks of decompression sickness and oxygen toxicity, work of breathing, cost, and other factors are also important.

Because of similar and additive effects, divers should avoid sedating medications and drugs, such as cannabis and alcohol before any dive. A hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely. Experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs.

Prognosis and epidemiology

Narcosis is potentially one of the most dangerous conditions to affect the scuba diver below about . Except for occasional amnesia of events at depth, the effects of narcosis are entirely removed on ascent and therefore pose no problem in themselves, even for repeated, chronic or acute exposure. Nevertheless, the severity of narcosis is unpredictable and it can be fatal while diving, as the result of inappropriate behavior in a dangerous environment.

Tests have shown that all divers are affected by nitrogen narcosis, though some experience lesser effects than others. Even though it is possible that some divers can manage better than others because of learning to cope with the subjective impairment, the underlying behavioral effects remain. These effects are particularly dangerous because a diver may feel they are not experiencing narcosis, yet still be affected by it.

History

thumb|342px|class=skin-invert-image|alt=graph with logarithmic scales showing a close inverse correlation between "Potency of anesthetic drug" and "Olive oil:gas partition coefficient" for 17 different agents|Both Meyer and Overton discovered that the narcotic potency of an anesthetic can generally be predicted from its solubility in oil. Minimum Alveolar Concentration is an inverse indicator of anaesthetic potency.

French researcher Victor T. Junod was the first to describe symptoms of narcosis in 1834, noting "the functions of the brain are activated, imagination is lively, thoughts have a peculiar charm and, in some persons, symptoms of intoxication are present." Junod suggested that narcosis resulted from pressure causing increased blood flow and hence stimulating nerve centers. Walter Moxon (1836–1886), a prominent Victorian physician, hypothesized in 1881 that pressure forced blood to inaccessible parts of the body and the stagnant blood then resulted in emotional changes. The first report of anesthetic potency being related to lipid solubility was published by Hans H. Meyer in 1899, entitled Zur Theorie der Alkoholnarkose. Two years later a similar theory was published independently by Charles Ernest Overton. What became known as the Meyer-Overton hypothesis may be illustrated by a graph comparing narcotic potency with solubility in oil.

In 1939, Albert R. Behnke and O. D. Yarborough demonstrated that gases other than nitrogen also could cause narcosis. For an inert gas the narcotic potency was found to be proportional to its lipid solubility. As hydrogen has only 0.55 the solubility of nitrogen, deep diving experiments using hydrox were conducted by Arne Zetterström between 1943 and 1945. Jacques-Yves Cousteau in 1953 famously described it as "l'ivresse des grandes profondeurs" or the "rapture of the deep".

Further research into the possible mechanisms of narcosis by anesthetic action led to the "minimum alveolar concentration" concept in 1965. This measures the relative concentration of different gases required to prevent motor response in 50% of subjects in response to stimulus, and shows similar results for anesthetic potency as the measurements of lipid solubility. The (NOAA) Diving Manual was revised to recommend treating oxygen as if it were as narcotic as nitrogen, following research by Christian J. Lambertsen et al. in 1977 and 1978, but this hypothesis has been challenged by more recent work.

References

Notes

Sources

External links

Undersea and Hyperbaric Medical Society Scientific body, publications about nitrogen narcosis.
Diving Diseases Research Centre (DDRC) UK charity dedicated to treatment of diving diseases.
ScubaDoc's overview of marijuana and diving.
ScubaDoc's overview of alcohol and diving.