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How using helium can reduce narcosis

It is becoming increasingly popular for divers to use helium based mixtures, usually called trimix, for deeper dives. The main reason for this is to reduce narcosis.

Koh Tao, Thailand

Of all the physiological factors that affect us as technical divers 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.

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. Those divers who 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.

Helium is considerably less narcotic then nitrogen and so if we replace some of the nitrogen with helium we are reducing the overall narcotic effect of the combined gas. This 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 afterwards.

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.

When we add helium into our breathing mix we can get a different set of gases. Commercial divers sometimes use a mixture of helium and oxygen which is known as heliox. However, cost considerations mean that, outside of the commercial world, divers will usually use a combination of oxygen, nitrogen and helium, known as trimix. In the same way that the general name nitrox can be used to describe any mixture of nitrogen and oxygen, the term trimix is used to describe any combination of oxygen, nitrogen and helium. Specific types of mixes are also given names as the characteristics of each are very different.

A trimix which has between 18 and 21 per cent oxygen is referred to as normoxic trimix. The term normoxic refers to ‘normal’ levels of oxygen. In other words, a similar level of oxygen to normal air. The helium content in normoxic trimix is likely to be between 21-45 per cent. This type of trimix is used to reduce narcosis in the 45-60m depth but does not change the oxygen levels significantly and so is limited to 60m by oxygen toxicity considerations.

For deeper dives the amount of oxygen in the mixture needs to be reduced and for this a hypoxic mixture is used. Hypoxic means a reduced level of oxygen such that the mixture will not sustain consciousness at the surface. Oxygen levels below 18 per cent are usually considered hypoxic for diving. The helium levels in a hypoxic trimix will be much higher as it is being used to reduce the amount of oxygen as well as being used to reduce the amount of nitrogen and so helium levels between 40-60 per cent are common.

In recent years it has become increasingly popular to add helium to the breathing mix for dives between 30-45m. In this case it is possible to use a weak normoxic trimix but it is also possible to have a higher level of oxygen, just as if diving nitrox, but with the addition of some helium. This would be called hyperoxic trimix, due to the higher than normal levels of oxygen, but this name is not commonly used in order to avoid confusion with hypoxic trimix. Hyper means high levels and hypo means low levels. Any trimix with a higher level of oxygen than air was originally called helitrox when NAUI launched the first training course into its use and TDI has stuck with this name. Other training agencies have used the term triox or Recreational Trimix to refer to the same thing. Helitrox will typically contain between 10-30 per cent helium.

There are, however, some downsides to use helium. These mostly stem from the physiological aspects of helium. Helium is a very small molecule and is a very light gas. Amongst other things this means that it conducts heat much faster then air. As a result, a diver using a helium based gas will feel colder than a diver using air or nitrox. For small helium percentages this is not too noticeable but as the helium percentage increases this becomes more and more noticeable and so for helium mixes above 20 per cent it is recommended that a separate suit inflation cylinder is used specifically for filling your drysuit. This suit inflation cylinder is usually worn either mounted on the side of the twinset, attached to the backplate or on the waistband. A simple first stage provides an inflator hose for suit inflation. No second stage is attached but instead an over pressure relief valve is attached in case of a problem with the first stage.

The physical propertied of helium also make decompression more complicated. This doesn’t necessarily mean that trimix decompression is longer or shorter, better or worse than nitrox, just different. The reason for this is that with trimix we are dealing with two inert gasses rather than just one. There may be times when the body is taking in one inert gas while releasing the other. In addition, the small size of the helium molecule meant that is a fast gas, in other words it on-gasses faster than nitrogen but also off-gasses faster. All of this combines to make trimix decompression more complicated than air or nitrox decompression.

When it comes to dive planning using trimix there are a number of options. It is possible to buy trimix decompression tables but there are so many possible combinations of trimix together with combinations of deco gasses that it is not a very practical option. Until recently many divers have used PC planning programs such as V-Planner, Proplanner or Decoplanner to plan their dives. These tools allow the diver to calculate a decompression schedule together with gas requirements and oxygen toxicity levels. A range of plans can be generated which can be used to cover a number of alternative scenarios as well as providing backup plans in case the dive is deeper or longer than anticipated. Together with a bottom timer this is a very easy and cost efficient way of planning trimix dives.

Increasingly we are seeing a range of dive computers that can handle trimix as well as air or nitrox. The VR3 was the first successful commercial trimix computer and this has now been joined by computers from a range of other manufacturers. Some manufacturers focus on the extreme end of the technical diving market whilst other manufacturers have produced computers aimed at those divers using normoxic trimix or helitrox. Suunto’s HeloO2 has brought trimix computers to the mass market and has done much to popularise this type of diving. These computers can be programmed with the details of your mixture together with any decompression gasses and will work out your decompression based on the combination of helium nitrogen and decompression gasses.

The last disadvantage of using trimix is the cost. There is no doubt that a trimix fill is more expensive than nitrox but when compared against the cost of the boat fees, petrol to get to the dive site, accommodation and a few drinks in the pub it becomes less significant. Many divers will happily spend £30 in the pub but would not spend the same amount on trimix. If that £30 can help you remember the dive then it seems good value and if it can help you deal with a potentially dangerous situation then it becomes incredibly good value.


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