User:RexxS/Narcosis (diving)

Narcosis while diving, commonly called nitrogen narcosis or inert gas narcosis, is a reversible alteration in consciousness in scuba divers at depth. It produces a state similar to alcohol intoxication or nitrous oxide inhalation. It occurs to some small extent at any depth, but in most cases does not become noticeable until greater depths, usually from 30 to 40 meters (100 to 130 feet).

Apart from helium, all gases that can be breathed have a narcotic effect, which is greater as the lipid solubility of the gas increases. At the depth increases, the effects of narcosis may become hazardous as the diver becomes increasingly impaired. Although divers can learn to cope with this impairment, it is not possible to develop a tolerance. Predicting the depth at which narcosis will affect a diver is difficult as susceptibility varies widely from dive to dive and amongst individuals.

Narcosis is completely reversed by ascending to a shallower depth and produces no after effects. For this reason, narcosis while diving in open water rarely develops into a serious problem as long as the diver is aware of its symptoms and ascends to manage it. Diving beyond 40 m requires extra training and the use of a gas mixture containing helium is recommended.

Classification
Narcosis results from breathing gases under elevated pressure and may be classified by the principal gas involved. All of the noble gases, except helium, as well as nitrogen, oxygen and hydrogen cause a decrement in mental function but their effect on psychomotor function (processes affecting the coordination of sensory or cognitive processes and motor activity) varies widely, while the effects of carbon dioxide consistently result in decreased mental and psychomotor function. The noble gases argon, krypton, and xenon are more narcotic 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 it has been successfully used for surgical operations, and xenon anesthesia systems are still being proposed and designed.

Signs and symptoms
Due to its perception-altering effects, the onset of narcosis may be hard to recognize. At its most benign, narcosis results in relief of anxiety and a feeling of tranquillity and mastery of the environment. These effects are similar to both alcohol and familiar benzodiazepine drugs such as Valium (diazepam) and Xanax (alprazolam). Such effects are not harmful unless immediate dangers are not recognized or addressed. 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.

The most dangerous aspects of narcosis are the loss of decision-making ability, loss of focus, impaired judgement and multi-tasking and coordination. Other effects include vertigo, tingling and numbness of the lips, mouth and fingers, and exhaustion. The syndrome may cause exhilaration, giddiness, extreme anxiety, depression, or paranoia, depending on the individual diver and the diver's medical or personal history. When more serious the diver may begin to feel invulnerable, disregarding normal safe diving practices. Paradoxically, badly affected divers may panic, sometimes remaining on the bottom, too exhausted to ascend.

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.

Causes
The cause of narcosis is related to the increased solubility of gases in body tissues, occurring at elevated pressures (Henry's law). The breathing gas mix entering the diver's lungs will have the same pressure as the surrounding water, known as the ambient pressure. For any given depth, the pressure of gases in the blood passing through the brain catches up with ambient pressure within a minute or two and this produces a delay in narcotic effect after coming to a new depth.

Although narcosis is most commonly reported below 30 metres, the divers' cognition is affected before that, but they are usually unaware of the changes. Even so there is no reliable method to predict the severity of the effect on an individual diver, as the effect may vary from dive to dive (even on the same day). Significant impairment due to narcosis is an increasing risk below depths of about 30 m or an ambient pressure of about 4 bar. Most sport scuba training organisations recommend depths of no more than 40 m because of risk of narcosis. When breathing air at depths of 90 m - an ambient pressure of about 10 bar - narcosis in most divers leads to hallucinations and unconsciousness. A number of divers have died in attempts to set air depth records below 120 m; because of these incidents the Guinness Book of World Records no longer reports on this figure.

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. 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 narcosis.

The vertical speed of the diver's descent does have some minor effect on narcosis, because of the delay related to the rate at which gas can pass from the lungs into the blood and thence to the brain. Although some experienced divers have recommended maintaining a constant vertical speed of descent, thereby avoiding sudden changes of depth which would cause an irregular solution of gas in blood and delay in pressure adaptation, there is no scientific evidence that rate of descent makes a difference to the eventual effect.

Mechanism
The precise mechanism is not well understood, but it appears to be a direct effect of gas dissolving into nerve membranes and causing temporary disruption in nerve transmissions. While the effect was first observed with nitrogen (in air), other gases including argon, krypton, and hydrogen also cause very similar effects under higher than atmospheric pressure. Some of these effects have been suggested as due to antagonism at NMDA receptors and potentiation of GABAA receptors, similar to the mechanism of nonpolar anesthetics such diethyl ether or ethylene. However, their reproduction by the very chemically inactive gas argon makes them unlikely to be a strictly "chemical" bonding to receptors in the usual sense of a chemical bond. An indirect physical effect (such as a change in membrane volume) would therefore be needed in some cases for a chemical effect on nerve cell ligand-gated ion channels.

Similar to the mechanism of ethanol's effect, the increase of gas solubility in the nerve cell membrane may cause altered ion permeability properties of the neural cell's lipid bilayers. It has been found that the partial pressure of a gas required to cause a measured degree of impairment correlates well with the lipid solubility of the gas: the greater the solubility, the less partial pressure needed. An early theory, the Meyer-Overton hypothesis suggested 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.

Diagnosis
Although the symptoms described may be caused by other factors during a dive, the presence of any of these symptoms should imply narcosis. Alleviation of the effects upon ascending to a shallower depth is sufficient to confirm the diagnosis.

Prevention
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 Global Underwater Explorers (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.

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 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 judgement to prevent such decisions, this is by no means always the case.

The most straightforward way to avoid nitrogen narcosis is for a diver to limit the depth of dives. If narcosis does occur, the effects disappear almost immediately upon ascending to a shallower depth. As narcosis gets worse with increasing depth, a diver keeping to shallower depths can avoid serious narcosis. Most recreational dive schools will only certify basic divers to depths of 18 m, and at these depths narcosis does not present a large risk.

Further training is normally required for certification up to 30 m 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 m, often consisting of further theory and some practice in deep dives with close supervision.

Some diving organisations teach their divers to 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.

Scuba organizations which train for depths beyond recreational depths, commonly considered depths greater than 40 m), 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 and heliox because helium has no narcotic potential. The use of these gases forms part of technical diving and requires further training and certification.

Equivalent narcotic depth (END) 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. 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 which influences the choice of gas mixture; the risk of decompression sickness and oxygen toxicity, cost, and other factors are also important.

Because of similar and additive effects, divers should avoid sedating medications and drugs, such as marijuana 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 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.

Management
The management of narcosis is to ascend to shallower depths; the effects then disappear within minutes.

Prognosis
Narcosis is potentially one of the most dangerous conditions to affect the scuba diver at depth. The effects of narcosis are entirely reversible by ascending 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 illogical behaviour in a dangerous environment.

Epidemiology
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 manage better than others because of training (learning) to cope with impairment, the underlying effects remain. These effects are particularly dangerous because even for the same diver, they are not perfectly reproducible at the same depth.

History
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."

Jacques Cousteau famously described it as "l’ivresse des grandes profondeurs" or the "rapture of the deep".

The first report of anaesthetic potency being related to lipid solubility was published by H. H. Meyer in 1899, entitled "Zur Theorie der Alkoholnarkose". Two years later a similar theory was published independently by Overton.

Originally thought to be caused by nitrogen, it was found that other gases could cause the same effects. For the inert gases 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.

The National Oceanic and Atmospheric Administration (NOAA) Diving Manual was revised to recommend treating oxygen as if it were as narcotic as nitrogen, following research by Lambersten et al. 1977,1978.