Raptures of the (maybe not so) deep

Raptures of the (maybe not so) deep

By Lynn Taylor

In 1943, Jaques Yves Cousteau enlightened the world to perhaps the most entrancing discovery of our time – the world under the sea. In the second chapter of this famous book, ‘The Silent World’, his colleague Didi vividly describes his experience of ‘Raptures of the Deep’ during an aqualung dive to 210 feet (70m). This is how he describes the record dive as he passes the 100 ft (30m) knot ‘The damned rope does not hang straight. It slants off into the yellow soup. It slants more and more. I am anxious about that line, but I really feel wonderful. I have a queer feeling of beatitude. I am drunk and carefree. My ears buzz and my mouth tastes bitter. The current staggers me as if I had had too many drinks.’

Jaques Cousteau himself sums up his personal euphoria and the recognition of the dangerous impairment of judgement that it brings ‘I am personally quite receptive to nitrogen rapture. I like it and fear it like doom. It destroys the instinct of life.’

As far as I am aware this is the first documentation of ‘nitrogen narcosis’ -a euphoric, anaesthetic effect that nitrogen (or indeed any inert gas) has when breathed at depth. Ok, so we might not dive to 70m but many of us do dive to 30m on a regular basis. As such divers, we should be aware, not only of the symptoms, but also of the implications that ‘nitrogen narcosis’ can have on our ability to think clearly and respond quickly and appropriately to a situation. In this article, I will take you through a small ‘experiment’ looking at the effects of nitrogen narcosis on mental ability, relate that to ‘so what does that mean?’ implications, and touch on the theory behind these effects. The aim is to highlight the fact that diving at depth does impair judgement and should not be taken lightly.

A dry ‘chamber’ dive experiment

Many of us will believe that we are not greatly affected by nitrogen narcosis and are convinced that we think just as clearly at depth. Are our beliefs are correct? Or is there evidence to suggest the contrary? In November 1999, Surgeon Commander Dr Alison Drewry kindly gave permission for us to do a test dry dive in the Slark recompression chamber at Devonport Naval Base. Under the supervision of highly experienced technician, Basil Murphy, and with trained medical personnel on stand-by, the group was taken to an equivalent sea depth of 50m and given a series of problem solving exercises. Similar exercises were then repeated after surfacing.

The group was mixed in terms of diving experience (6 to 3000+ dives), level of training (open water to Technical Diving Instructor, and age (23 – 58 years), some regularly dived to depths in excess of 50m. Well-known names included Dave Moran, Pete Mesley and Kathy Tarrant, with Rachel Broadley and Helen Phillips representing the recreational diver, and myself.

I had been fortunate enough to have a similar experience earlier this year when I had visited our very own Kiwi Dr Simon Mitchell at his hyperbaric chamber in Brisbane. The offer of a (relatively) safe dry dive to 50m was a temptation I could not resist. At a depth of 50m I was given a mathematical logic problem to solve. Feeling giggly, euphoric and care free, I tried desperately hard to focus on the task in hand. I knew it was a problem I should be able to solve but it took me a good couple of minutes to get my head around the logical steps involved, to be able to calculate the answer. So how did the experience of the New Zealand divers compare?

Well, on the way down we all relaxed into a calm carefree state. Pete was first to crack a joke at 50m and we all laughed at the polar bear joke. A social ‘party’ atmosphere evolved. At 50m we were given a series of logic problems involving shapes, word matches and number sequences. Also a mathematical problem ‘If it takes 6 men 3 days to mow 9 meadows, how long will it take 4 men to mow 8 meadows?’ (Test yourself – it’s not that difficult!). Most people completed the logic problems within the time limit but with only 75% correct responses (range 62.5%-87.5%). No one got the mathematical problem correct (with the exception of me, who wrote the problem so that doesn’t really count!).

Given the tests again at the surface, everyone got 100% on the logic problems and all could work their way through the mathematical problem. Comments from the group revealed two common threads. Firstly, that they found it difficult to concentrate on the problem (often finding they had to re-read the question after getting half way through the sentence) and, secondly, that considerable mental effort was required to work out the logical steps needed to solve the problem.

Interpretation of results

Some divers believe that underwater their ability to think clearly and react is not impaired at depth. This may be true of a task that has become so ingrained in our memories that it has become a natural reaction. However, this experiment highlights some potentially hazardous effects that breathing air at an increased partial pressure has on our ability to think clearly and respond quickly. In an underwater situation, if the unexpected happens and solution thinking is required, the delay in processing of the information and deciding on the appropriate course of action could mean the difference between a successfully managed outcome, and a disaster developing.

Relevance to the undersea environment

On the surface, the partial pressure of nitrogen (N2) in air is 0.79 ata and causes no noticeable narcosis. Doubling the pressure in a chamber has little effect but at 3 ata (20m/66ft) mental processing slows measurably. Learning is slowed and errors are made in simple tasks. Whilst many people don’t notice the impairment at 3 ata, most recognise some mental dulling at 4 ata (30m/99ft). Virtually everyone is affected at 45m/150ft. Deeper than that divers become unreliable in judgement and performance.

In water, narcosis behaves somewhat differently. In the absence of socialisation the diver is more likely to notice slowness of thinking, slowness of action, shortened attention span and reduced vigilance. The diver may forget well-learned procedures and be indifferent to personal safety. Divers have been known to offer their regulator to passing fish! Test yourself. If you have had appropriate deep diver training and you are experienced in deep diving, try giving your buddy a dive table calculation to do at depth. Time how long it takes, or even if they can do it! (If you are going to try this, keep a careful eye on your air gauge, remember, at 30m you will consume air approx four times as quickly as at the surface.)

DAN statistics In the most recent report on Australian Diving Deaths (1972-1993), nitrogen narcosis was recorded as a contributing factor in 14 of the 178 scuba deaths (8%). Their dive depths ranged from 35-65m for the 7 cave/sink-hole divers and 36-54m for the 6 wreck divers who died. (One unknown, body never recovered but last seen descending over a reef which bottomed out at 400m+). These are the tragic fatalities; subtle narcosis could have been a contributing factor in an unknown number of accidents of others who have survived.

Signs and symptoms of nitrogen narcosis:

10-30m (33-100ft) Mild impairment of performance. Mildly impaired reasoning. Mild euphoria.
30-50m (100-165ft) Delayed response to visual / auditory stimuli. Reasoning and immediate memory affected. Calculation errors. Wrong choices. Task fixation. Over confidence and sense of well-being. Anxiety.
50-70m (165-230ft) Impaired judgement. Confusion. Sleepiness. Severe delay in response to signals or instruction. Terror. Hallucinations in some.
70-90m (230-300ft) Poor concentration. Mental confusion. Loss of memory. Further decrease in judgement.
90m+ (300 ft)+ Hallucination. Increased intensity of vision and hearing. Sense of impending blackout. Manic or depressive state. Unconsciousness. Death.

(Ref: Adapted from Deeper into Diving by John Lippmann)

Consider the practical effects at the maximum recommend depths for recreational divers (30-40m). Reading our gauges may take a little longer than usual. Task fixation may mean we focus so hard on air pressure that we don’t notice our buoyancy and sink to deeper depths. Our reflexes and thoughts will be slowed and, whilst most problems can be solved if we have the time and remain calm, impaired judgement and slowed reactions could have dire consequences.

Other contributing factors

Individuals have different degrees of susceptibility and even within the same individual, identical dives can give very different effects. Other factors increase the effects of N2 narcosis: anxiety / inexperience, recent intake of alcohol / certain medications, fatigue, heavy physical work / exercise, cold water, rapid descent, poor visibility (reduced sensory input), task loading.

Mechanism of nitrogen narcosis

The exact process that produces narcosis is complex and not completely understood. The most commonly accepted theory relates to the effect that the increasing partial pressure of nitrogen (by breathing air at depth) has on the amount of nitrogen absorbed into nerve cells. This results in a slowed transmission of nerve impulses travelling to the brain. This most commonly effects nerve cells in the brain responsible for coordination and alertness. Remember all the effects are reversible and will be alleviated by ascending to shallower water.


Breathing nitrogen at depth results in more nitrogen being absorbed into the body. This results in a slowing of nerve impulses and impairment in the processing ability of the messages sent to and from the brain. These effects manifest themselves as impairments in mental performance and judgement. Whilst these effects may go largely unnoticed if everything goes well on a dive, if a situation arises which requires quick solution thinking, the impairments could cause mis-judgements or delayed actions. The consequences of which could be very serious. Awareness of the potential dangers and recognition of symptoms are the two main points which will hopefully have been conveyed in this article. The responsibility for how this information is utilised rests with the individual diver.

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