This is an article written on another forum, but it is VERY worth reading so I thought that I'd post it here. I've put it in the medical forum because it seemed to fit best in here, however if a mod feels it's better placed elsewhere then feel free to move it.

The article is written by Mark Powell, one of the UK's top technical diving instructors (and author of the book Deco For Divers).

Narcosis - an introduction

Of all the physiological factors that affect us as technical divers Nitrogen Narcosis is the most common but also the most widely misunderstood. Narcosis occurs as we dive deeper and becomes more severe the deeper we go. It has several side effects all of which serve to impair our ability to carry out basic tasks. Narcosis was most famously described in Jacques Cousteau's book, The Silent World, where Cousteau describes the symptoms and named it the "Rapture of the Deep.

The cause of narcosis has not been definitively proven although it is widely believed to be caused by the anaesthetic properties of certain gases at raised partial pressures. Many divers incorrectly consider narcosis to be a black and white issue, either they are suffering from narcosis or they are not. Furthermore many divers claim that they have never suffered from narcosis. This shows a misunderstanding of the properties of narcosis and a lack of understanding of the symptoms.

If, as we believe, narcosis is caused by the anaesthetic properties of gases at increased partial pressure, then the level of narcosis experienced should be proportional to the depth. As the diver descends the narcosis should build up gradually. Those divers which claim never to have experienced narcosis have just never noticed the effects.

There are many effects of narcosis. The two most widely known are the extreme feelings of either euphoria or panic. In many ways the diver who experiences these feelings of panic and doom is lucky in that this type of narcosis is acting as a failsafe and preventing them going deeper and experiencing more narcosis. The diver who experiences euphoria is potentially at more risk as they are more likely to take risks or act dangerously. However, these two symptoms are not the only effects of narcosis.

There is a wide range of other effects. These may not always be as obvious as a feeling of overwhelming panic. Their symptoms can be much more subtle and so are not always immediately obvious. Divers suffering from narcosis often show a lack of judgement. They don’t always make the best decisions or in some cases take an inordinate amount of time to make what should be a simple decision. I once watched a diver on a wreck penetration course take several minutes deciding which of two points to use to make a tie off. Until pointed out later they didn’t realise that they had taken this long to make the decision.

Narcosis appears to affect our memory. Divers who report no other symptoms of narcosis frequently show a lack of memory of certain parts of the dive. I spoke to a diver a few years ago who had just done the same dive as me. I was on Trimix and they were on air. They confessed that despite a 20 minute bottom time they couldn’t remember any specific detail of the dive.

Tasks which are easy in shallow water for some reason tend to become more difficult at depth. Loss of dexterity or motor control is a frequent symptom of narcosis. On many occasions I have seen someone send up a DSMB from 10m in just a few seconds only to have exactly the same task take several minutes at depth.

Narcosis often causes perceptual narrowing or task fixation. Divers become obsessed with completing the task they have begun, even when other tasks have obviously become a much higher priority. Divers suffering from narcosis often respond slower than they would in shallower water. These extra seconds can make a vital difference at depth.

It is interesting that many of these additional symptoms of narcosis are not noticeable unless a problem or emergency occurs. If everything is going well then the fact that tasks take a little longer is no problem, especially as both parties are likely to forget many of the details of the dive anyway. However, narcosis become much more of an issue if a problem occurs. In this case the diver now has to assess the situation, make a judgement and act on it. All three of these are things that may be affected by their levels of narcosis. This means they are much less able to deal with a problem due to their level of narcosis.

We are lucky that the vast majority of dives do not involve an incident of any kind. During the dives that go well we can tolerate the level of narcosis that we experience. It is only when dives don’t go well that that level of narcosis becomes dangerous. Unfortunately I still haven’t been able to reliably identify in advance the dives when things will go well and those when an incident will occur. Until then I will remain wary of narcosis.

Divers that claim never to have experienced narcosis are focusing on the obvious symptoms and are ignoring, or forgetting, the more subtle symptoms. If you can feel that you are affected by narcosis then the symptoms probably started 10-15m shallower then when you noticed then.

For any given breathing mixture the level of narcosis is related to the depth of the dive, with technical diving we are almost by definition involved in deep diving. However there is more to it than that. Depth is only one of the factors involved. The environmental and personal factors also play a significant part in narcosis. Environmental factors can increase your susceptibility to narcosis and can increase the symptoms at any given depth. Visibility is one of the biggest factors in susceptibility to narcosis. Consider a dive where you have 20m visibility and plenty of ambient light but then a week later you do the same dive but this time the visibility is less than a meter and there is no ambient light. The second dive is much more likely to produce symptoms of narcosis than the first.

Current can also be a major factor in bringing on narcosis. If you are fighting against a current and breathing faster than usual due to working hard to swim down a shotline then you are at a higher risk of experiencing narcosis. Minor equipment problems can also induce narcosis. A slight equipment problem which, in itself will not cause any issues, may be enough to induce narcosis. This is related to other psychological causes of narcosis. Concern over the dive, diving with unfamiliar equipment or unfamiliar buddies, cold, drugs, fatigue, stress, motion sickness and motion sickness medications have all been linked to an increase in the likelihood of narcosis.

All of this means that the depth in itself is not the only factor that determines your level of narcosis. As such it’s impossible to draw an arbitrary line where you can say air/nitrox is safe at this depth but no deeper.

It is clear that psychological factors affect narcosis. There have been a number of studies which have attempted to show the psychological aspect of narcosis. Tom Mount and Dr Gilbert Milner carried out a study in 1965 that demonstrated that divers tend to experience a level of narcosis that is consistent with the level they expect to experience. A more recent study carried out for the HSE by the Diving Diseases Research Centre and Plymouth University supported the importance of psychological aspects in addition to the bio-physical impact of narcosis. One of the conclusions from this study was that narcosis is not simply an objective measurable phenomenon; it also has a subjective facet.

Until very recently it was commonly accepted that the use of Nitrox would reduce narcosis. On the face of it this seems to make sense. If an increased partial pressure of nitrogen causes narcosis then if we replace some of the nitrogen in the breathing mix with oxygen we will reduce the partial pressure of nitrogen at a given depth. The majority of nitrox courses taught exactly this reasoning until quite recently. However, it is now believed that it’s not just nitrogen that causes narcosis but that different gases result in varying levels of narcosis. Nitrogen has a high level of narcosis but is not the only narcotic gas.

As there is no definitive explanation for the causes of narcosis it is difficult to prove which gases have more or less potential to cause narcosis. The best estimate for the levels of narcosis is derived from a theory that says the level of narcosis caused by an individual gas is related to the solubility of that gas in a fatty substance. This is known as the Meyer-Overton hypothesis. Using this measure oxygen should be more narcotic than nitrogen. If this is the case then Nitrox will not reduce our levels of narcosis as we are just replacing one narcotic gas with another.

It would be nice if we could prove this argument one way or the other by comparing the narcotic effects of air and Nitrox. Unfortunately it’s not that easy. In order to ensure that we had a measurable level of narcosis we would need to be at a significant depth. At these depths the risks of CNS oxygen toxicity mean that we would have to reduce the amount the oxygen in the breathing mixture to the point where it would be too small to be able to distinguish between the effects of air and nitrox. The result is that the question of whether Oxygen is more or less narcotic than nitrogen can generate some interesting discussions but is effectively irrelevant for recreational technical divers.

As we dive deeper the effects of narcosis become more and more significant. We have seen that using Nitrox does not help in reducing narcosis. Furthermore the increased levels of oxygen limit the depth that we can dive using Nitrox without risking CNS oxygen toxicity. So for deep diving we must look at another solution.

We know that different gasses have different narcotic properties and so the best solution is to find a gas that is considerably less narcotic than either nitrogen or oxygen and use this to replace some of the nitrogen in the breathing mix. Helium and Neon both have properties that predict that they would be considerably less narcotic then nitrogen and experiments have shown this to be the case. Neon is prohibitively expensive and so Helium, though still expensive, has been used as the gas of choice for deeper diving.

Helium is considerably less narcotic then nitrogen and so as we replace some of the nitrogen with helium we are reducing the overall narcotic effect of the combined gas. As we increase the amount of helium in the mixture, and so further reduce the amount of nitrogen present, we further reduce the narcotic level of the overall gas.

Commercial and military divers often replace all of the nitrogen in their breathing mixture and just use a mixture of helium and oxygen. This is known as Heliox. This produces virtually no narcosis but due to the cost of helium is a very expensive option. Recreational technical divers tend to use a mixture of Helium, Oxygen and Nitrogen, known as Trimix. By adjusting the level of the three gases the diver can select a mixture that has the desired level of narcosis.

A trimix diver can perform a dive to 80m but can choose his breathing mixture so that they experience a level of narcosis that is the same as if they were breathing air at 35m. On a subsequent, deeper dive to 90m they may be 10m deeper but can choose a breathing mix that still gives the same level of narcosis. This is known as the Equivalent Narcotic Depth (END). In this case a Trimix diver at 90m may be experiencing less narcosis than a recreational diver at 40m on air.

The reduction in narcosis introduces a number of advantages. A clearer head allows the diver to enjoy the dive and actually remember what they see down there. There is little point in exploring a wreck if you don’t remember the experience after.

In addition the reduction in narcosis removes the lack of judgement, loss of coordination and inability to resolve problems. This can give the technical diver a huge safety advantage. As they go deeper, and the risks increase, they can help to reduce those risks by reducing their level of narcosis. With Trimix relatively easily available these days there is really no reason for divers to risk diving deep on air and inducing symptoms of nitrogen narcosis.