Adding helium: some numbers to think about narcosis.

[dewdropsonrosa]

[Background]
This is a very rudimentary look at a single aspect of gas planning: the narcotic potential of the breathing medium. Gas narcosis (whether N2, O2, or CO2) impairs divers' cognitive functions, which poses safety concerns.

Understanding/mitigating the risks posed by narcosis is an important part of safe technical dive planning.

[Meyer-Overton Rule]
From the GUE Fundamentals workbook: "The Meyer-Overton rule predicts that the anesthetic potency of a gas is proportional to its lipid solubility," anesthetic potency being the degree to which narcosis is sedating you (!).

The rule evaluates the narcotic potential of diving gases on a 0-20 scale, 0 being least narcotic and 20 being most narcotic.

• Helium - 0.2/20
• Nitrogen - 1.0/20
• Oxygen - 1.7/20
• Carbon dioxide - 20/20

Obviously, too much of anything is a bad thing - but now you can see why "overbreathing" a CCR can be so deadly. Blackout isn't figurative jargon, it's a literal descriptor of being sedated into unconsciousness.

[The Helium Fix]
As the scale indicates, helium is 5x less narcotic than nitrogen, ~9x less narcotic than oxygen, and one hundred times less narcotic than carbon dioxide. Replacing a narcotic component of your inspired gas (N2, O2) with helium immediately reduces the narcotic potential. But by how much?

I ran the numbers to compare (A) a gas without helium to (B) a GUE standard gas of the same oxygen content that also contains helium. For ease of analysis, the PPN2 for a given depth is serving as the proxy for narcotic potential.

Caveats:

• As we know, each diver has a different level of susceptibility to narcosis. This level may vary between days, dives, and any other number of factors. This is always an unknown variable.
• Anecdotally, lots of people report awareness (which is not equivalent to onset) of their gas narcosis symptoms around a PPN2 of 3.

Fig. 1: EAN 21 versus Trimix 21/35.

Click here for uncompressed image.​

Diving the helium-containing mixture, the PPN2 at 100' (30m) has around half the narcotic potential of air.

Fig. 2: EAN30 versus Trimix 30/30 (Triox).

Click here for uncompressed image.
​

The advantage isn't quite as pronounced here (because of the additional 9% O2), but it still represents a considerable difference.

[Conclusion]

These charts only cover 0-100' (0-30m), the recreational/GUE Fundamentals diving range. Even in this limited dataset, the advantages emerge clearly. Extending them out to 150' (45m, TDI AN/DP max. depth) or to 170' (50m, GUE Tech 1 max. depth) would continue to show the benefit.

That exercise is left to the reader so that they do their own dive planning like a responsible adult.




[Final Caveats]

• Work-of-breathing (WOB) is a separate but related issue, but it's outside the scope of this post.
• Columns D, E, F, and G in each table were generated by formulas. It's possible that I made a mistake somewhere. If you see a glaring error that I didn't catch, please let me know so that I can update the image/post appropriately.

 

[dewdropsonrosa]

After a cup of tea (and some grad school insomnia), I decided that I liked the data as a graph:

Following the 1ATA = 1 martini rule, we can see nearly two drinks' of difference between EAN21 and 21/35 at 150' (45m). Know anybody who thinks two drinks make no difference behind the wheel?

 

[kensuf]

Great topic. I discuss this in CCR, Trimix, and AN/DP lectures so I can really stress how deadly CO2 is. Your approach is a little different than how I do it, I like to plot the narcotic potential of the gases themselves on a board, it's a steep upward J.

BTW -- I also include nitrous oxide in the mix, which is also in the meyer-overton whitepaper. I like to include nitrous because anyone that has ever experienced it at a dentist, or otherwise, can quickly see the narcotic potential of CO2 because of how close those two are compared to helium, oxygen, and nitrogen.

Helium - 0.015
Nitrogen - 0.052
Oxygen 0.11
CO2 - 1.34
Nitrous Oxide - 1.56

 

[Dominik_E]

Very interesting topic! I think it is however important to realise that while the Meyer-Overton-correlation is probably to good to be just coincidence, modern science does not consider lipid solubility as the one decisive factor in gas narcosis. It should thus be treated carefully I think. This is also true for the implied quite high narcotic potential of oxygen, which is very probably to some extent counterbalanced by metabolism.

 

[rhwestfall]

oh is this a good toopic! Thanks for starting it, and thanks to the contributors so far!

 

[Lorenzoid]

Very interesting topic! I think it is however important to realise that while the Meyer-Overton-correlation is probably to good to be just coincidence, modern science does not consider lipid solubility as the one decisive factor in gas narcosis. It should thus be treated carefully I think. This is also true for the implied quite high narcotic potential of oxygen, which is very probably to some extent counterbalanced by metabolism.

The quote from the GUE Fundamentals workbook, where GUE refers to it as the "Meyer-Overton rule," caught my attention. I have seen it referred to as the "Meyer-Overton hypothesis," and you refer to it as the "Meyer-Overton correlation." In my own words, my understanding is that it seems to hold true for some gases but may not hold true for all gases, and/or the mechanisms by which some gases are narcotic may be something other than lipid solubility.

 

[kensuf]

Anyone that claims they understand the actual mechanism behind how narcosis works is probably lying or ignorant. This episode of radiolab is a few years old, but it's pretty damn interesting.

 

[dewdropsonrosa]

This is a modified interpretation of the @kensuf chart of the lipid solubility values:

Landmarks on the solubility scale:

• Oxygen is ~7x more soluble than helium.
• N2O is ~14x more soluble than oxygen...
  • ...which works out to N2O being ~104x as soluble as helium.

Nitrous oxide is about 16% more narcotic than carbon dioxide (1.56/1.34 ≈ 1.16). Extending that proportion to the Meyer-Overton scale, nitrous oxide has a correlation of 23.2.

 

[Dominik_E]

The quote from the GUE Fundamentals workbook, where GUE refers to it as the "Meyer-Overton rule," caught my attention. I have seen it referred to as the "Meyer-Overton hypothesis," and you refer to it as the "Meyer-Overton correlation." In my own words, my understanding is that it seems to hold true for some gases but may not hold true for all gases, and/or the mechanisms by which some gases are narcotic may be something other than lipid solubility.

I think to call it a "rule" is very hard to defend in light of actual data. As said, for several gases there seems to be an established correlation between lipid solubility and narcotic potential, but it is not established that this goes back to the actual working mechanism of the narcosis, and thus could be trusted as a general rule.

 

[Dr Simon Mitchell]

I think to call it a "rule" is very hard to defend in light of actual data. As said, for several gases there seems to be an established correlation between lipid solubility and narcotic potential, but it is not established that this goes back to the actual working mechanism of the narcosis, and thus could be trusted as a general rule.

Hello,

This, and some of the other comments above, is/are pretty much on the money. Moderate to high lipid solubility is a requirement for a gas to exhibit narcotic / anaesthetic properties (hence the "correlation" mentioned above) , but there is abundant evidence that other less well characterized properties also play an important role. For example, in programs aiming to develop new anaesthetic gases some very lipid soluble agents have failed to produce anaesthetic effects.

One of my PhD students (Xavier Vrijdag - who is also a technical diver) is currently running a project funded by the US Navy Office of Naval Research. He is looking in detail at the EEG effects of nitrous oxide (increasing fractions at surface pressure), nitrogen (up to 6 ATA air), helium (up to 6 ATA 20:80 oxygen-helium), oxygen (up to 2.8 ATA) and CO2 (end tidal CO2 titrated upward at surface pressure, and up to 6ATA air) in an attempt to answer the perennial questions about relative narcotic potency of these gases. The use of detailed quantitative EEG analyses may also reveal something about mechanisms, or at least allow some conclusions to be drawn on whether they are the same. All the dives except the CO2 study are complete. We should have this conversation again in a year or so!

Simon M