Rapture of the deep

Photo by Flickr user SteelCityHobbies. Click for sourceWhen scuba divers start swimming deep under water they can sometimes start feeling dreamy, light-headed and mentally fuzzy, an effect nicknamed ‘rapture of the deep’ but better known as nitrogen narcosis.

It is caused by changes in the way nitrogen, one of the gases in the divers’ air tank, dissolves in the body when under high pressure from the depth of the water.

No-one is quite sure exactly how it affects the brain, but many divers have noted the similarity between nitrogen narcosis and being drunk.

Psychologist Malcom Hobbs was intrigued by this connection and conducted a study [pdf], published last year in Undersea and Hyperbaric Medicine, to investigate the psychological similarity between the two states.

The experiment compared the subjective experience and effects on problem solving of alcohol and narcosis, but, also rather elegantly, looked at whether the two effects could be caused by a similar neurobiological process by seeing whether people with high alcohol tolerance also had a high narcosis tolerance.

Hobbs divided a group of divers into experienced and novice divers, as those with more experience should be more tolerant to narcosis, and made a further division between those who drank a lot of booze and those who drank very little, to look at differences in alcohol tolerance.

In the first experiment, they found the interesting effect where experienced divers adapted to the subjective effects of narcosis, but not the behavioural effects. While they felt more in control than novice divers, they actually weren’t. This chimes with an identical effect seen in heavy vs light drinkers.

But crucially, Hobbs also found that those affected to a greater degree by nitrogen narcosis are affected to a greater degree by alcohol on both subjective experiences and performance on the problem solving task (and vice versa), indicating that there is cross tolerance between the two states.

This suggests that they may affect the brain in similar ways. Although more research needs to be done on the actual neurobiology of the two states to be sure of the exact relationship, this study suggests that divers may indeed be ‘drunk’ when experiencing the rapture of the deep.

UPDATE: I just got emailed this interesting snippet by an experienced diver friend (thanks Ben!):

Something extra which happens with narcosis (which deviates from the alcohol analogy) is that, unless you’re already dead, the effects are completely reversible with no discernible side effects (eg hangover). One of the tricks divers use if they recognize narcosis (most often in their buddy than in themselves) is that ascending a few metres will often bring immediate clarity.

Even more interesting is that once clarity is achieved, descent back to the narcotic depth doesn’t necessarily bring back the narcotic effect of the nitrogen, which hasn’t really been explained yet. Theories abound regarding rate of descent and physiological effects of increasing ppN [partial pressure of nitrogen] and how it’s dissolved into various tissues.

Divers have known for years about this and have developed practical methods to deal with its effects (decreasing N content in breathing gases, replacing N with other inert gases etc). Actually, it’s known that oxygen also has a role to play in narcosis (as in nitrous oxide) but since some of it is metabolized, it’s effects are considerably less than the inert gas it accompanies.

I quite like the feeling of a little narcosis; but it does make time fly, and unfortunately time is the real enemy underwater!

pdf of full-text scientific paper.
Link to PubMed entry for same.

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