Nitrogen narcosis

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in scuba divers at depth. It occurs to some small extent at any depth, but in most cases doesn't become noticeable until deeper depths, usually from 30 to 40 meters. 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 nitrogen dissolving into nerve membranes and causing temporary disruption nerve transmission. 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 atmopheric pressure. Nitrous oxide (laughing gas) appears to exert its effect by this mechanism.

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.

Effects
Along with the bends, which is a risk during the ascent of a dive, narcosis is one of the most dangerous conditions to affect the scuba diver at depth. The most dangerous aspect of narcosis is the loss of decision-making ability, loss of focus, impaired judgment and multi tasking and coordination. At its most benign, nitrogen narcosis results in relief of anxiety and a feeling of tranquility 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 there are immediate dangers to be dealt with, and often they are not recognized.

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. Paradoxially, 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 acclimation, training, or special breathing techniques, some 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.

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, 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.

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 dissolves more slowly than other gases in blood.

Similar to the mechanism of ethanol'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": The feeling of 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. Other diving organisations such as GUE claim that a diver can never train to overcome narcosis, in the same way that you can not train to not get drunk when drinking. Instead, they ban diving with gases that cause too high narcosis levels at depth, and use trimix instead.

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
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.

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.

Avoidance and cure
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 metres (60 feet), and at these depths narcosis does not present a large risk.

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

Other gases
Breathing gases such as trimix and heliox are used in technical diving to reduce nitrogen narcosis by replacing a portion of nitrogen in the gas mixture with helium, thus 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 when breathed 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.

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.