Diving narcosis and laughing gas

I watched a programme the other day about a deep sea mystery. A strangely high number of experienced deep sea divers had been lost on diving trips in a particular bay, and no one seemed to know why. The presenter, being a decent diver himself, went for a dive in the bay and noticed that he could make out the sunlight shining through the water at the other end of an underwater tunnel. His conclusion was that the now deceased divers saw this light and thought they could swim through the tunnel to the other side. What wasn’t obvious to the divers was that this light was deceptively far away and they would have to swim very fast for a long time to make it to the other end of the tunnel before running out of oxygen. But what could cause these supposedly experienced divers to make such a rash, fatal decision?

Above sea level, nitrogen is a pretty boring gas – it makes up about 80% of the air around us and doesn’t normally do us any harm. However, a problem arises when we breathe it in under high pressure – such as when diving. Several gases, including nitrogen, carbon dioxide, and oxygen are normally dissolved in our bloodstream. When you dive deep underwater, the increase in pressure exerted on your body by the surrounding water causes more of these gases to dissolve into your blood through your lungs when you breathe from the gas tank (because going deep-sea diving without a gas tank would be an even less recommendable thing to do). In fact, for every 10m a diver descends, their blood holds an extra 1.5 litres of dissolved nitrogen.

All that extra nitrogen rushing round in the bloodstream has weird, wonderful, and incompletely understood effects on the brain, collectively known as nitrogen narcosis.

Nitrogen narcosis is experienced by all divers – to varying degrees – and feels essentially like being drunk. Because of this similarity, nitrogen narcosis is often referred to as the ‘Martini effect’. Divers liken every 10m below sea level as the equivalent of having one martini – meaning they feel increasingly intoxicated the deeper they get. Even at comparatively shallow depths (10-30m below the surface), a diver will become less co-ordinated and a bit giddy – 20m lower they’ll start making mistakes and bad decisions and may start laughing for no reason. At 50-70 metres, they may start experiencing hallucinations, sleepiness, terror, poor concentration and confusion, and at 90m they risk losing consciousness or even dying.

So, the worse symptoms of nitrogen narcosis aren’t exactly like getting drunk, because even a huge amount of alcohol doesn’t give people hallucinations (though some alcoholics experience hallucinations when withdrawing from alcohol). Actually, the closest similarity to nitrogen narcosis you can find on dry land is from breathing laughing gas, or nitrous oxide.

Nitrous oxide has been used by doctors to relax patients since 1794 and it is still used today as a form of pain relief for women during childbirth. It has been in the press a lot recently, dubbed ‘hippie crack’, as it’s often used recreationally (though usually not legally) for its mild hallucinogenic and euphoric ‘feel good’ effects, which have often been likened to nitrogen narcosis. So how does nitrous oxide affect the brain?

Although nitrous oxide is hugely understudied, there are several theories about how it can affect the brain. Because gases like nitrous oxide and nitrogen are really fat-soluble, they may interfere with cell membranes (which are made from fatty molecules) disrupting their normal function. In the case of brain cells, this may alter the way they communicate with one another. In addition, the dissolved gas molecules may directly bind to the receptors on the surface of brain and nerve cells. Nitrous oxide is used as a mild anaesthetic because it has been shown to block NMDA receptors – which normally ‘excite’ the brain – and because it activates potassium channels, which further suppress brain cell excitation. All this means is that brain activity is generally depressed and so users are more prone to making bad decisions or losing concentration.

As I mentioned before, nitrous oxide is also good for pain-relief, as it’s believed to activate opioid centres in the brain. When activated, the opioid system – the same one stimulated by drugs like heroin and morphine – then disinhibits certain adrenergic cells in the spinal cord, which dampen down any feelings of pain.

While there have been reports that nitrogen narcosis also decreases the perception of pain, it’s obviously difficult, and, well, not very practical to test the potential of high pressure deep sea diving on pain relief. Instead, what should be studied more are the effects of nitrous oxide on the nervous system. We’ve used the stuff for more than 200 years and yet the biology behind its uses and its dangers is still not fully understood. What’s more, the fact that people use nitrous oxide recreationally (and probably will continue to do so in spite of its non-legal status in many countries) means we really ought to know what its short and long term effects on the brain are. Unlike the mystery of the missing deep sea divers, the full extent of the ways in which nitrous oxide works remains unsolved.

Post by Natasha Bray