The noble gases argon, krypton, and xenon are more anesthetic than nitrogen at a given pressure, and xenon has so much anesthetic activity that it is actually a usable anaesthetic at 80% concentration and normal atmospheric pressure. (Xenon has historically been too expensive to be used very much in practice, but xenon has been successfully used for surgical operations, and xenon anesthesia systems are still being proposed and designed).
Due to its perception-altering effects, the onset of nitrogen narcosis may be hard to recognize, its severity is unpredictable, and in scuba diving it can be fatal, as the result of illogical behaviour in a dangerous environment or in extreme cases from its own toxic effect. However, the cure for nitrogen narcosis is a simple one, as effects disappear within minutes upon ascending to shallower depths.
When more serious the diver may begin to feel invulnerable, disregarding normal safe diving practices. Other effects include vertigo, tingling and numbness of the lips, mouth and fingers, and extreme exhaustion. Paradoxically, badly affected divers may panic, sometimes remaining on the bottom, too exhausted to ascend. The syndrome may cause exhilaration, giddiness, extreme anxiety, depression, or paranoia, depending on the individual diver and the diver's medical or personal history. An early effect may be loss of near-visual accommodation, causing increased difficulty in close-accommodation reading of small numbers in middle-aged or older divers who already have any degree of presbyopia.
Tests have shown that all divers are affected by nitrogen narcosis, though some are less affected than others. Even though it is possible that some divers can cope better than others because of training (learning), or special breathing techniques, the underlying effects remain. As with alcohol, these effects are particularly dangerous because even for the same diver, they are not perfectly reproducible at the same depth.
Although narcosis is most commonly reported below 30 metres, it may be that the divers' cognition is affected before that, but that they are unaware of the changes. Even so there is no reliable method to predict the severity of the effect on an individual diver, and as noted the effect may vary from dive to dive (even on the same day).
Nitrogen narcosis has been compared with altitude sickness insofar as its variability (though not its symptoms); its effects depend on many factors, with variations between individuals. Excellent cardiovascular health is no protection and poor health is not necessarily a predictor. Thermal cold, stress, heavy work, fatigue, and carbon dioxide retention all increase the risk and severity of nitrogen narcosis. Nitrogen narcosis is known to be additive to even minimal alcohol intoxication, and also to the effects of other drugs such as marijuana (which is more likely than alcohol to have effects which last into a day of abstinence from use). Other sedative and analgesic drugs, such as opiate narcotics and benzodiazepines, add to nitrogen narcosis.
Similar to the mechanism of ethanol's effect, the change of inert gas solubility in the nerve cell membrane may cause altered ion permeability properties of the neural cell's lipid bilayers. The Meyer-Overton hypothesis holds that narcosis happens when the gas penetrates the lipids of the brain's nerve cells, causing direct mechanical interference with the transmission of signals from one nerve cell to another. More recently, specific types of chemically-gated receptors in nerve cells have been identified as being involved with anesthesia and narcosis, but the basic and most general underlying idea that nerve transmission is altered in many diffuse areas of the brain, as a result of presence of gas molecules dissolved in the nerve cell's fatty membranes, remains largely unchallenged.
The relation of depth to narcosis is sometimes informally known as "Martini's law". This is the idea that Narcosis results in the feeling of one martini for every 10 meters below 20 meters depth. This is a very rough guide, and not a substitute for an individual diver's known susceptibility, or for standard diving safety guides. Professional divers use such a calculation only as a rough guide to give new divers a metaphor for a situation they may be more familiar with. Formal dive training stresses that deep dives can be made only after a gradual training to gradually test the individual diver's sensitivity to increasing depths, with careful supervision and logging of reactions. Diving organizations such as GUE are careful to emphasize that such sessions are for the purpose of gaining experience in recognizing the onset symptoms of narcosis for a given diver (which tend to be somewhat more repeatable than for the average group of divers, just as is the case with symptoms of intoxication with alcohol). At the same time, GUE stresses that there is little scientific evidence that a diver can "train" to overcome any measure of narcosis at a given depth or become tolerant of it, in contrast to the body's ability to become tolerant of alcohol intoxication at a given level of alcohol in the blood.
Scuba organizations which train for depths beyond recreational depths (i.e., depths greater than 40 meters) tend to simply ban diving with gases that cause too high narcosis levels at depth in the average diver, and instead, to 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.
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 from alcohol at least 24 hours before diving, and longer for heavy drinking. Abstinence time needed for marijuana is unknown, but due to the much longer half-life of the active agent of this drug in the body, it is likely to be longer than for alcohol.
The second most straightforward way to avoid narcosis is to use gasses that limit or exclude nitrogen like trimix for deeper dives.
Specialist training is normally required for certification up to 30 metres (100 feet) on air, and this training should include a discussion of narcosis, its effects, and cure. Some diver training agencies offer speciality training to prepare recreational divers to go to depths of 40 metres (130 ft), often consisting of further theory and some practice in deep dives with close supervision.
While the individual diver often 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 much 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 which their exhaled bubbles make becomes different. Specialist training may help divers in identifying these personal onset signs, and these may then be used as a signal to ascend to shallower depths. Although it is sometimes true that narcosis interferes with judgment to prevent such decisions, this is by no means always the case.
Equivalent air depth is a commonly used way of expressing the narcotic effect of different breathing gases. Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect: for example, neon at a given pressure has a narcotic effect equivalent to nitrogen at 0.28 times that pressure, so in principle it should be usable at nearly four times the depth. Argon, on the other hand, has 2.33 times the narcotic effect of nitrogen, and is not suitable 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. Helium is the least intoxicating of the breathing gases, but at greater depths it can cause high pressure nervous syndrome, a still-mysterious but apparently unrelated phenomenon.
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