Freediving Narcosis: an interesting interplay of gases
Freediving in the deep blue by Daan Verhoeven

Freediving Narcosis: an interesting interplay of gases

Narcosis occurs on long and deep dives thanks to the increased pressure at depth. The consensus is that nitrogen is the largest culprit, however, CO2 may play a large role in freediving narcosis as well.

This article will give you the facts, and we’ll speculate on what they mean.

  • Special guest Walid Boudhiaf describes his experiences with narcosis
  • Walid is a 120 meter freediver with tons of experience at depth
  • Freediving narcosis could be triggered by different mechanisms
  • CO2 can cause anaesthesia, and impairs motor function
  • N2 can cause narcosis, and impairs memory, attention, and planning
  • CO2 could play a large role in deep freediving narcosis
  • The role of O2 toxicity is unknown, but may exacerbate narcosis
  • SPECIAL VIDEO: Watch a Variable Weight (VWT) dive of Walid Boudhiaf during a training session at Freediving World
  • Walid thinks that one of the major differences in SCUBA vs. freediving: NARCOSIS IS EXPERIENCED MAINLY DURING THE ASCENT.
  • Read why below!

Reading Time: 12 minutes.

What is narcosis?

Nitrogen Narcosis in Diving

Also known also the “narks”, “rapture of the deep” or “inert gas narcosis” is caused by a build up of nitrogen in our bodies. Nitrogen, at high partial pressures, can have an anaesthetic effect on the diver, which affects their consciousness. Kirkland et al., 2020 describe nitrogen narcosis as a “change in consciousness, neuromuscular function, and behavior brought on by breathing compressed inert gasses.”

Nitrogen narcosis happens to freedivers and SCUBA divers alike. It is more common for SCUBA divers, since they spend more time at depth. However, it is a concern for freedivers as well, especially for deeper divers.

Nitrogen is a major component of the air we breathe. Every breath we take contains about 80% nitrogen.

We know narcosis also affects freedivers in deep breath-hold dives. But is nitrogen the only culprit for freediving narcosis?

Thibault, deep freediver, hanging underwater on one breath.
Thibault Guignes is known to do one minute hangs at 100 meters of depth. All on one breath. this subjects the body to freediving narcosis. Photo by Luca Malaguti.

The gases we breathe

Air, on average, contains the following mixture of gases:

  • Nitrogen — 78 % (ppN2 = 0.78 bar at seal level)
  • Oxygen — 21 % (ppO2 = 0.21 bar at seal level)
  • Argon — 0.93 %
  • Carbon dioxide (CO2) — 0.04 % (ppCO2 = 0.0004 bar at sea level)
  • Trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor.

Nitrogen is an inert gas that does not react with other substances in the body. However, even as an inert gas it can influence the workings of our nervous system. Nitrogen’s influence is likely more physical than chemical.

Oxygen and carbon dioxide, of course, are ingredients for our metabolism. Carbon dioxide is transported in several ways in the body and can react with water to form bicarbonate. Oxygen is consumed during metabolism, producing carbon dioxide.

Partial pressure of inert gases

Gases react not according to their absolute concentration, but according to their partial pressure. The partial pressure of gases increases as we dive deeper. For example, at 10m the partial pressure of nitrogen is double that at the surface. At 20m, it is triple.

The partial pressures of O2 and CO2 during a freedive are more difficult to calculate, because these gases are reactive in the body. Thankfully we can make some inferences from research on scuba divers and general gas toxicity.

Let’s dig into what makes a gas an anesthetic in the first place.

A crash course on anesthetics

Anesthetics are drugs used to numb pain or consciousness. Gaseous anesthetics such as nitrous oxide are used to numb pain or consciousness in general, whereas other anesthetics may be used locally. For an anesthetic to work well, it must be lipid soluble and not blood-soluble.

Anesthetics work by affecting the myelin sheet around nerves, essentially slowing or hampering the propagation stimuli. The stimuli can be related to for example pain, vision, or motor control. For example, you may not feel a pinch anymore, or it can become harder to use your hands.

Below you can see a graph with different gases, their oil/gas solubility (a proxy for lipid solubility) and their anesthetic potencies. You can see that inert gases fall on a straight line but carbon dioxide has a lower potency than expected. The reason for this is that carbon dioxide is not inert in the body. However, it is still an anesthetic with intermediate potency at the right concentration (or partial pressure).

Figure 2:
The Meyer-Overton correlation between lipid solubility and anesthetic potency.

The graph tells us that the oil/gas solubility is strongly correlated with anesthetic potency for inert gases. We need to inhale a lot of nitrogen to get “narced”. We need much less xenon, and hardly any methoxyflurane. CO2 falls off the curve since it is not inert.

As you might expect, this is going to get a little complicated.

Freediving in the deep blue by Daan Verhoeven
Freediving in the deep blue. Photo by Daan Verhoeven.

The graph above is constructed using exhaled gases. A person that exhales a tiny amount of methoxyflurane may be fully anesthized. But a person anesthized from CO2 would exhale less than we expect for the oil/lipid solubility of the gas.

The reason is that carbon dioxide is stored in the body in several ways. Only when our storage compartments are filling up, the gas starts to significantly affect our nerve cells.

How does CO2 affect us?

“Carbon dioxide is a narcotic gas capable of depressing awareness to the degree of total loss of consciousness.” – J. E. Brian Jr., M.D. (2004)

We have known for a long time that CO2 is a narcotic gas.

If you have ever overdone our CO2 cycling exercise in Longer and Deeper, that teaches you to reduce your breathing rate on a bicycle, you might have noticed. Push it too hard and your hands might start shaking (we do not recommend this). However, it would be incredibly difficult to pass out from CO2 during exercise at the surface.

It is much easier under pressure, suggests a 2016 article by Freiberger et al. Freiberger and his coworkers performed a comprehensive cognitive and motor test on 42 people breathing different gas mixtures. The mixtures had varying CO2, O2 and N2 concentrations and were administered at surface pressures as well as pressures corresponding to 40m depth and 60m depth.

Although the study did not include hypoxic conditions (it was aimed at SCUBA and tech divers rather than freedivers), it clearly shows the impairing contributions of different gases under pressure.

A final thought by Eric Fattah

It is possible that, during diving, CO2-induced dizziness could be mistaken for nitrogen narcosis.

Johnny E. Brian Jr., M.D., 2004

Most interestingly different gases have different effects, which appear to change with pressure. We won’t go into all the detail but here are some interesting snippets:

  • Motor performance is more influenced by CO2 than N2
  • High N2 partial pressure alone cannot explain narcosis
  • A decreased reaction time is impaired at depth mostly by N2, but at the surface mostly by CO2
  • Elevated N2 decreases our perception of narcosis

If you want to dig more into the findings I strongly suggest you find the Freiberger et al., 2004 article and read it.

My conclusion is that pure CO2 narcosis is absolutely possible. I can get pure CO2 narcosis from STATIC APNEA during a dry static CO2 table. Once I pass 10.0% CO2, then the symptoms I get are very similar to the type of narcosis I would get on 80m+ dives with fluid goggles and brain freeze and blackness.

Eric Fattah, Deeper Blue Forum, March 24, 2008

What happens at depth with freedivers and spearfishers

We always have to be a little careful when using research based on SCUBA. SCUBA divers breathe the same gas throughout a dive, unless they bring different mixes. On the other hand, freedivers have a reserve of changing concentrations of gas and move through the water column quickly:

  1. Freedivers increase CO2 during a dive
  2. They also decrease O2 during a dive
  3. The change in concentrations differs locally and systemically because of peripheral vasoconstriction

From what we know, CO2 narcosis is real, and likely affects us during a dive. CO2 and perhaps O2 exacerbate the narcosis caused by N2.

It is yet unknown whether the partial pressure of O2 negatively affects CO2 and N2 narcosis during a freedive. However, considering that it does so in SCUBA diving, this remains a possibility.

The most important thing to keep in mind is that impairment of attention and especially planning from nitrogen narcosis at depth impairs your decision making.

Narcosis experience with Walid Boudhiaf

Walid Boudhiaf is a world-class freediving athlete. He’s a Free Immersion (FIM) continental record holder and very deep freediver. His personal best is 120 meters deep in FIM. FIM tends to be rather slow and narcosis is a real concern.

Walid is also an instructor trainer and coach with tons experience in all disciplines. You can reach out to him for coaching on narcosis, equalization techniques, strength training for freediving and much more.

Visit Walid’s page here.

Walid’s experiences with narcosis

Walid dives deep enough that freediving narcosis presents a big challenge. He describes being “lost” on the surface without being hypoxic. He has been too narced to complete a proper surface protocol.

Vision plays a huge role too. Going down eyes closed versus eyes open gravely changes the outcome narcosis has on my dive.

Walid Boudhiaf
Walid Boudhiaf doing a 130m (426 feet) dive in Variable Weight (VWT). Although this dive is easy for him (equalization and breath-hold), he can still get influenced by narcosis. Video by Freediving World, Sharm El Sheikh.

Key factors affecting narcosis

For Walid, narcosis is related to a few key factors:

  • Total dive time,
  • CO2 build-up ,
  • Physical exertion,
  • Cold is a major factor as well,
  • Barometric pressure,
  • Darkness at depth, and
  • Stress and pre-dive “mental state”.

Walid feels it most on deep FIM dives since the dive time is longer and the diving itself is “slower”. He’s spending more time at depth.

When he does “shallow” hangs at 60m he also feels it.

Walid has noticed that narcosis really affects him during the ascent, and this could potentially be due to the increase in CO2 production in the body at depth. Walid thinks that this is one of the major differences in SCUBA vs. freediving: NARCOSIS IS EXPERIENCED MAINLY DURING THE ASCENT!

There’s definitely a difference in feeling and sensations of narcosis when doing a hang at 60m versus going very deep to 120 meters in FIM.

Darkness also affects him negatively. He now trains with fluid goggles rather than just a noseclip in order to see and remain lucid underwater. When he uses fluid goggles narcosis is far more manageable.

Effects of narcosis in competitions

In Walid’s experience, narcosis affects the “control” you have over a dive. Narcosis can strongly affect the progression of a freediver, and may go unnoticed. It is a serious obstacle.

Narcosis has affected him up to the surface protocol after some dives. That means narcosis could cost him a competition, and be the the difference between a red and white card.

Memory and narcosis

Walid has also noticed narcosis impacts the short-term memory of the dive. After a very long and deep dive, he’ll notice that he’s forgotten several details of the dive itself. Without remembering how a dive went, improving it becomes near impossible. It may also erode confidence.

Walid has enough experience to notice that for him, depths beyond 114-120 meters is a limit where beyond this things get “hectic” and narcosis gets very strong.

His solution in training?

Doing deep hangs to gauge the awareness of narcosis, learn to recognize it and remain in control. It’s a technique that may help to not become overwhelmed by the narcosis. It is unlikely however, that this technique limits narcosis.

We also know that narcosis really affects deep freedivers during the ascent and even at the surface. Training to adapt to this and focusing on a good surface protocol is challenging, but necessary for deep freediving.

Narcosis can definitely interfere with the progression of very deep freediving.

Walid Boudhiaf, -120 meter freediver.

References:

Luca Malaguti

Luca Malaguti is a former engineer turned freediving professional athlete and founded Sea to Sky Freediving. He lives in Vancouver, Canada among other places including Dahab, Dominica and Philippines.

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