Nitrogen narcosis
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Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in SCUBA divers at depths beyond 30m. Jacques Cousteau famously described it as the "rapture of the deep". Its precise mechanism is not well understood, but it appears to be a direct effect of high nitrogen pressure on nerve transmission. While it was first observed with nitrogen (in air), other gases including argon and hydrogen also cause very similar effects under high enough pressure. Xenon is actually a usable anaesthetic at atmospheric pressure.
The onset is hard to recognize, its severity is unpredictable, and it can kill, from its own toxic effect or due to the resulting illogical behaviour. However the cure for nitrogen narcosis is a simple one, as effects disappear upon ascending to shallower waters.
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Effects
Along with the bends, which in contrast are a risk during the ascent of a dive, narcosis is one of the most dangerous conditions to affect the SCUBA diver. The most dangerous aspects of narcosis are the loss of decision-making ability, and impaired judgment and coordination. The diver may start to feel invulnerable, disregarding normal safe diving practices. Frequently divers throw away their masks and accelerate their descent to excessive depths. Divers may not even be aware they are impaired. Other effects include vertigo, tingling and numbness of the lips, mouth and fingers, and extreme exhaustion. Affected divers may panic, sometimes remaining on the bottom, too exhausted to ascend. The syndrome may also cause exhilaration, giddiness, extreme anxiety, depression, or paranoia.
Though some divers seem to be able to cope with the symptoms and even claim to be less susceptible than others, tests have shown that all divers are affected by nitrogen narcosis. Even though it is possible that some divers can cope better than others because of acclimation, training, or special breathing techniques, the effects remain.
Risk factors
When breathing air, serious impairment due to narcosis starts at depths of about 30 metres (100 feet) or a nitrogen partial pressure of 3.2 bar. At depths of 90 metres (300 feet) or nitrogen partial pressure of 8 bar nitrogen narcosis leads to hallucinations and unconsciousness.
Although narcosis is most commonly reported below 30 meters, there is no reliable method to predict the severity of the effect on an individual diver. Similar to altitude sickness, its effects depend on many factors, with variations between individuals and even from time to time in the same individual. Excellent cardiovascular health is no protection and poor health is not necessarily a predictor. Cold, stress, heavy work, fatigue, and carbon dioxide retention all increase the risk and severity of nitrogen narcosis.
Mechanism
One of the important factors leading to its occurrence seems to be the vertical speed of the diver's descent. Pressure increases as the diver descends, but nitrogen disolves more slowly than other gases in blood.
Similar to the mechanism of alcohol's effect, this change may cause altered permeability properties of neural cell lipid bilayers. The Meyer-Overton hypothesis states that narcosis happens when the gas penetrates the lipids of the brain's nerve cells. Here it apparently interferes with the transmission of signals from one nerve cell to another.
The relation of depth to narcosis is informally known as "Martini's law": It's like one martini per 10 meters below 20 meters. This is a very rough guide, which can never be a substitute for the real diving safety rules. Professional divers never suggest such calculation attempts, stressing instead that deep dives can be made only after a gradual training to increasing depths, and always with a linear vertical speed.
The mechanism of the narcosis is related to the solubility of nitrogen in the blood occurring at elevated atmospheric pressures. Once it leaves the tank and enters the diver's lungs it will have the same pressure as the surrounding water, the ambient pressure. Although some experienced divers recommend a constant vertical speed, avoiding sudden changes of inclination which would cause an irregular solution of gas in blood due to a "delay" of pressures adapting, this is not supported by scientific evidence.
Mitigation
Good divers frequently check their mental state while immersed using the "thumbs test". The two companions regularly show each other their fingers. One shows a number of fingers (e.g. 2), and then the other must respond by showing back one more or one less (i.e. 3 or 1), depending on previous agreement. If either of them botches the arithmetic, they should suspect narcosis.
Because of similar and additive effects, divers should avoid sedating medications and drugs, such as marijuana and alcohol before any dive. In addition to dehydration increasing the risk for decompression illness, a hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely. Experts recommend total abstinence at least 24 hours before diving, and longer for heavy drinking. Alcohol combined with nitrogen is likely to bring on a far more serious case of narcosis. This is yet another good reason for not drinking and diving.
Avoidance and cure
The most straightforward way to avoid nitrogen narcosis is for a diver to limit the depth of his/her dives. If narcosis does occur, the effects disappear almost immediately upon ascending to a shallower depth. As narcosis only occurs during deep dives, a diver keeping to shallower depths can avoid narcosis entirely. Most recreational dive schools will only certify divers to depths of 18 metres (60 feet) for their basic training, and at these depths narcosis does not present a risk. Specialist training is normally required for certification up to 30 metres (100 feet), and this training should include a discussion of narcosis, its effects, and cure.
Other gases
In technical diving, breathing gases such as trimix and heliox are used to reduce nitrogen nacrosis by replacing a portion of nitrogen in the gas mixture with helium, reducing the partial pressure of nitrogen at depth.
Equivalent air depth is a commonly used way of expressing the narcotic effect of different breathing gases. Standard tables list conversion factors for narcotic effect: for example, neon at a fixed pressure has a narcotic effect equivalent to nitrogen at 0.23 times the pressure, so in principle it should be usable at four times the depth. Some gases have other dangerous effects whenbreathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity. Helium is the least intoxicating of the breathing gases, but it can cause high pressure nervous syndrome a still-mysterious but apparently unrelated phenomenon.
The choice of gas mixture is influenced by many factors other than inert gas narcosis; decompression sickness, oxygen toxicity, cost, and other factors are also important.
References
- Nitrogen Narcosis (http://www.deep-six.com/page74.htm) from Diving with deep-six by by George D. Campbell, III. (viewed 10 Apr 2005)
- Exotic diving gases (http://www.techdiver.ws/exotic_gases.shtml) by Matti Anttila. (viewed 10 Apr 2005)
External links
ScubaDoc's web page on marijuana and diving (http://scuba-doc.com/marij.html)
ScubaDoc's web page on alcohol and diving (http://scuba-doc.com/alch.htm)bg:Азотна наркоза