Have you ever wondered why you need to pee as soon as you hit fifteen metres? Or why diving gives you a mouth that’s dry as the Sahara? Does effective equalising elude you? Or are you yet to get your head around the freaky phenomenon that is narcosis?

Ponder no longer. We have the answers.

The urge to pee is caused by immersion diuresis and is a result of the body's response to cold water. (Illustration by John Grainger)

The Science Behind The Narc

Narcosis. Some divers like it, some divers really don’t, and others claim they aren’t affected by it at all. For many, it’s simply a gentle buzz – the high without the low. But going deep to get the hit is a risky business.

The reported effects of narcosis are a variety of emotions and sensations that can manifest at a whole range of depths. Along with a sense of joy and wellbeing, narcosis can result in anxiety, fear, giddiness, euphoria, blackouts, and silly behaviour. Some divers even claim not to experience narcosis at all. Physical differences and susceptibility aside, never feeling a thing seems a little unlikely. But how does it work, physiologically?

Under pressure

Research is on-going and scientists are still divided over the exact causes of nitrogen narcosis. We do know, however, that it involves the effect of the “partial pressure” of the gas you are breathing. Under one atmosphere of normal air (i.e. on the surface, with nothing but that big blue thing above our heads) we are subjected to one atmosphere of pressure (which figures), or 1 ATM. With nitrogen making up around 21 percent of the air that we are breathing, the amount of this pressure that the nitrogen is responsible for exerting is 0.21 ATM. We call this the partial pressure.

As we take this air underwater, the partial pressure of the nitrogen increases as the ambient pressure increases. At 10 metres, the ambient pressure of the air we are breathing is 2 ATM and the nitrogen is now responsible for around 0.42 ATM of it. As we continue to descend, the pressure increases, and, as the partial pressure of the nitrogen goes up, so does its narcotic effect. But why?

According to the Meyer-Overton hypothesis, narcosis happens when the gas (nitrogen in this case but other inert gases can have the same effect) penetrates the lipids of the brain’s nerve cells causing interference with the transmission of signals from one nerve cell to another. Recently, specific types of receptors in nerve cells have been isolated as being involved with narcosis, but the general idea of nerve transmission being altered in areas of the brain, remains unchallenged.

Susceptibility varies from one individual to the next, but also from one day (or even one dive) to the next. Some factors that influence the effects of narcosis are hard work, cold water, alcohol (even the effects of a late night the day before), fear, descent rate, fatigue and illness, and medication, to name a few.

The Martini Effect

In diving circles, some people refer to this as the “Martini Effect” to express the amount of mental impairment that we can expect to occur at different depths. The exact depth at which narcosis starts to have an effect is not precise and is very much dependent on physiological differences, susceptibility, physical fitness and environmental factors. But thirty metres (100 feet) is a widely accepted, rough starting point for the initial sensations.

People liken being at 30 metres to the feeling of having had one martini on an empty stomach. The idea is that if you continue to descend past 30 metres, for every subsequent 10 to 15 metres the effect on mental impairment is the equivalent of having had one more martini.

In the same way that being a little drunk is unlikely to kill you, narcosis itself is not life threatening, but your actions or reactions due to your “inebriated” state could be. Three martinis on an empty stomach might not be lethal, but getting in your car could well be, endangering yourself and possibly others. Food for thought?

Read the rest of this article in Issue 5/2014, AA No.79 of Scuba Diver magazine by subscribing here or check out all of our publications here.

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