Narcosis is something that all qualified divers should be aware of, have an understanding of how it can affect them as they descend deeper, and know what to do to reduce its influence- before and during a dive. The effects of narcosis down to 30m are easily manageable for most divers, and even at 40m, as long as a student can demonstrate that they are in control, an instructor will certify them to go to this depth autonomously. Any good diver should know their limits and be able to constrain themselves within their personal comfort zone.
All recreational diving agencies impose depth limits that have been developed to minimise risk in terms of inert gas uptake, oxygen exposure, gas consumption and narcosis. Technical diving also applies limits, but they become more complicated to manage below the recreational depth limits; probably why it’s called technical diving.. Gas management is crucial, and a variety of breathing gases are utilised to minimise oxygen exposure, optimise decompression, and reduce the effect of narcosis. Beyond 40m narcosis becomes increasingly debilitating, so our exposure to it must be minimised as much as possible. This process begins before even thinking of putting your fins on; it is an inherent part of the dive planning process.
Firstly, what is narcosis? The word comes from the Greek word narke, which roughly translates as ‘numb’. You may also have heard it described as ‘rapture of the deep’, or ‘the martini effect’. The exact mechanism is not completely understood, but the dominant theory is the so-called Meyer-Overton hypothesis, which states that the narcotic effect of a gas relates to its solubility in the lipid (fat) phase. In English this means that the greater the amount of gas dissolution in certain body tissues, the greater the narcotic effect that gas will have. When gases are dissolved in nerve membranes, they cause disruption in nerve transmission, particularly in the brain. The effects on the diver are variable, but become more and more incapacitating as depth continues to increase.
By measuring the partition coefficient (differences in solubility expressed as a ratio), the Relative Narcotic Potency (RNP) can be determined for a given gas, expressed as a number. Below shows the different RNPs for numerous gases:
Helium- 4.26 (least narcotic).
Xenon- 0.039 (most narcotic).
Argon is used for drysuit inflation only. Neon is used as a breathing gas, but only in deep commercial diving, as it is very expensive. Nitrogen is cheap and abundant, and is therefore one of the biggest culprits in determining how narced you will feel, especially as most people dive on air.
However, you may have noticed that I said ‘gases’. Whilst in the recreational diving community the effect is known as nitrogen narcosis, nitrogen is not the only gas that we breathe that has a narcotic effect. In technical diving it is either called ‘inert gas narcosis’, or simply (and correctly) ‘gas narcosis’. The oxygen that we breathe in, and carbon dioxide (CO2) that we produce as a by-product of respiration also have a narcotic effect. The level of influence that each individual gas has on the level of narcosis is poorly understood, but we do know that as divers we must try to mimise any kind of build-up of CO2 at all points of a dive, and also factor oxygen in to calculations to determine the Equivalent Narcotic Depth (END) when using trimix on deeper dives (compared with air at the deepest part of a dive).
During respiration, oxygen is metabolised by the cells and CO2 is produced, dissolving in the blood until it is expelled through the lungs. CO2 is narcotic in itself, and at high partial pressures has anaesthetic properties. The reason it is not used as an anaesthetic during surgery is because highly elevated partial pressures imbalance the acidity of the blood, which has been shown to induce seizures in humans and animals. CO2 is 25 times more lipid soluble than nitrogen, which, as previously mentioned has been shown to increase narcotic potency. Moreover, CO2 is a vasodilator, which means that more CO2, N2 and O2 can be delivered to tissues, notably, the brain. This can increase the chance of both narcosis and CNS oxygen toxicity. So it goes without saying that divers should at all times be thinking about preventing the build-up of CO2.
Other factors that determine how narced you might feel include:
- Cold water
- Alcohol in the body (including hangovers)
- Increased work of breathing
- Higher partial pressures of oxygen
- Feelings of vertigo (no visual reference in blue water)
- Turbidity and bad visibility (can be related to vertigo as above)
- Task loading
- Descent rate
So we have an idea of what narcosis is, but how do we tell when we are ‘narced’, or identify it in other divers? Symptoms include feelings of euphoria, light-headedness, perceptual narrowing, altered perceptions of time, anxiety and/or paranoia, and even numbness. It’s very common for divers to go to 30, 40 or even 50m and say that they felt nothing. Some of them may be telling the truth to the best of their ability, some of them are downright lying, and some may attribute other factors as to why they didn’t quite feel themselves. Symptoms can often be very subtle, such as it taking longer than normal to read your SPG or not immediately understanding common hand signals. I occasionally get a very pleasant tingle when I hit 40m that passes through my entire body then disappears as abruptly as it appeared. Another common occurrence, mainly on deep air technical dives (50-55m) is something called narcosis amnesia. A diver will complete the dive, get back on the boat and then turn to their buddy and say “what the hell happened on that dive!?” Not ideal when you spent a thousand pounds on a tech liveaboard to dive a world war 2 battleship at 60m.
Divers that feel euphoric will usually indulge in the feeling and forget where they are and what they’re doing, which obviously includes such tasks as checking gas and runtimes! Paranoid divers on the other hand, usually obsess over their gas supply, as if fighting to stay in control. Paranoia however, is usually accompanied by anxiety, which can easily lead to panic. So don’t assume when your buddy is constantly checking their gas and asking you for yours that they are fine!
Divers that deny feeling narced are in my opinion, the most dangerous kind of diver, as this complete lack of self-awareness/sense of denial is usually accompanied by an inner belief that they can do anything, and that the laws of physics don’t apply to them. They have the potential to push beyond their limits by simply not recognising that their coordination and judgement is way slower than at the surface. If they push deeper than they have been trained to, they may suddenly find themselves (and their buddy) so overwhelmed by narcosis that they are unable to help themselves, and that’s assuming that they don’t have an issue to deal with during that portion of the dive, they are not already low on gas, and they haven’t had a CNS hit!
Regardless of any obvious symptoms, your brain is slower, your reaction times and reflexes are slower, your coordination is impaired, and your ability to problem solve is decreased. This gets worse and worse as depth is increased.
Minimising the effects of narcosis starts at the planning stage. How deep do you want to go and for how long? What are the conditions like? Cold water? Possibility of currents? Likelihood of the weather changing whilst you are underwater (bad timing with tide changes)? These are the factors that will influence your gas choice, decompression obligation (including deep stops), maximum operating depths and maximum partial pressures for oxygen and nitrogen. Technical divers have a maximum PPO2 of 1.4 on the bottom portion of a dive, and 1.6 on the decompression stops. But depending on the conditions, depth and duration of the dive, and duration of deco at high partial pressures of O2, this may be lowered to increase conservatism. For an extended range dive the maximum depth is 55m (with TDI), which gives a PPN2 of 5.13, which is pretty darn high. It is generally accepted that a partial pressure of 3.16 (30m) will be noticeable. So if you plan on going deeper than that you can use trimix with the quantity of helium being chosen to offset some of the nitrogen and keep the equivalent narcotic depth to 30m.
Other factors that can help to minimise narcosis include getting enough rest the night before a dive, being hydrated, not being hungover, and being as relaxed as possible before, and during the dive.
On deeper dives, use of a high performance regulator that reduces the work of breathing becomes critical to reducing CO2 build up. CO2 is a dense gas in relation to nitrogen and helium. This effectively means that on very deep dives the respiratory response is not effective in getting rid of enough CO2 with each breath. Little research has been done in this regard, but experiments conducted by the US Navy suggests that 30m is all that is required for the density of CO2 to be high enough that the effort required to breathe reduces effective respiratory ventilation. Devices with increased dead air spaces, such as rebreathers and full face masks also increase CO2. So exertion at depth is the last thing you want to be doing, both in terms of increased narcosis and higher risk of hypercapnia.
At the beginning of a dive it’s a good idea to descend in a controlled manner, using some kind of shot line to give you a visual reference, and stopping for 20-30 seconds at certain points during the descent so that you can acclimatise to that level of narcosis before continuing on, rather than dropping like a stone and suddenly being hit by a wall of fuzziness!
Whilst you cannot build up a tolerance for narcosis, you can learn to cope with how it affects you. This means doing lots of dives of gradually increasing depth, and not going deeper until you are sure that you can function at that depth and deal with any issues that might arise, such as an out of gas situation or a catastrophic gas loss. If you don’t think you can… don’t do that dive.
The advice given to recreational divers if they experience narcosis and don’t like the feeling is to ascend to a shallower depth, and the feeling should go away. For some people this is true, the intensity of the narcosis will be reduced, but for many people there is a delay beforehand. For technical dives it doesn’t quite work in the same way. The amount of inert gas dissolved is greater, due to the longer bottom time, so the delay in reducing narcosis after ascending can be longer. I’ve done countless dives to 50m and felt really narced, but at a level that is manageable for me. On one occasion both myself and my buddy heard a high pitched engine noise independently in our heads at the bottom of a wreck at 55m, which wasn’t real (narcosis or CNS symptoms?). Whilst doing my deco stops I felt as narced at 12m as I did at 50, and it wasn’t until I switched to O2 at 6m that I started to feel normal again!
But perhaps the best thing you can do to minimise narcosis is to exercise good judgement and never go beyond your training and experience. You may well have been certified as an extended range diver and been to 55m during your course, but maybe you hated every second on the bottom but persevered because you really wanted the cert. If you hated it, why would you put yourself through that again? Perhaps it’s time to think about doing the TDI trimix course?
Richard Devanney, 22nd April 2015.