Nitrox: More, Less or Equally Narcotic as Air

[Blackwood]

At boulderjohn's behest, I'm trying to move the hijack from this thread.

Here are the posts relevant to this new thread:

Whilst we are all busy hijacking this post, I was recently reminded of another controversial issue about Nitrox that has yet to be resolved by a definitive study - whether Nitrox assists in reducing narcosis.

It seems to be an article of faith in North America that Nitrox does not reduce narcosis. Equally it seems to be an article of faith in Europe that it does. It is certainly an article of faith with Rhone Man that it doesn't really make that much difference either way because you can't really use Nitrox at a depth where this would make a significant difference. But if we are going to hijack this thread, let's do it right...

I'm always up for a good challenge!

I often dive nitrox in the 100-130' range, and that is where I have observed narcosis symptoms, on both air and nitrox.

The answer to that is NO...

Narcosis is an effect of how 'heavy' the gas is, the molecular weight. In nitrox you replace nitrogen, a fairly heavy gas with oxygen an equally heavy gas. Trimix reduces narcosis because it uses helium a much lighter gas. If I remember correctly, it has to do with short circuiting the synapses but don't quote me on that.

Bruce

Nitrox does not prevent narcosis, you have not removed the narcotic - Nitrogen. It is just diluted down. To really affect the narcosis malidy you have to go to a mix that has no, or very little of any of the narcotic gases like HeliOx or a trimix with a small Nitrogen % to stave off HPNS. Most of the noble gases like Nitrogen are narcotic at higher pressures and will cause some narcosis when present.

Nitrogen isn't a noble gas. It's not chemically inert. It is narcotic.

Helium is a noble gas. It is chemically inert. It is not narcotic.

Oh, Lordie... let's get this stuff straight now, before it gets really confusing.
"Noble" gasses are those with "full" electron "layers" and are therefore chemically inert. This has nothing to do with narcosis, however.
Nitrogen (N₂), though not a noble gas and not chemically inert, is "physiologically inert" in that it isn't changed chemically by human body processes. It is dissolved and released as N₂ and doesn't change while it's in the body.
Compressed gas narcosis, commonly called "nitrogen" narcosis, is a property of all gasses; some are just more narcotic than others. The Narcotic properties of a gas appear to be fairly closely correlated with the gas's solubility in lipids (fats & oils). Helium, for example, is practically insoluble in lipids and has very little narcotic effect, while Argon - another noble gas - is very soluble in lipids and is very narcotic. Nitrogen is in between. Just exactly why gasses that are "inert" cause narcosis isn't well understood, but it appears they somehow change the conductivity - and therefore the ability to transmit impulses - in nerves (or in the myelin sheath) - by physical (as opposed to chemical) interference.
Looking at Oxygen, it's about twice as soluble in lipids as nitrogen, so one might infer that Oxygen should therefore be about twice as narcotic as Nitrogen and that Nitrox should be a bit more narcotic than air... but... since oxygen is not inert, and is both metabolized in the body and scavenged by hemoglobin, nerve tissue titers (concentrations) are typically about a sixth what would be expected if oxygen were inert like Nitrogen. So, perhaps we should expect about a third the narcotic effect from oxygen as we do from nitrogen, and Nitrox should be slightly less narcotic than air
Personally, I like the wording in SSI's Nitrox materials: "It is best not to assume any reduction in narcosis" when moving to Nitrox from air.
Rick

It is not that absolute. All gases have some narcotic effect. Helium's effect is much less than nitrogen's. Oxygen's is supposedly similar to nitrogen's.

Neon is less narcotic than helium, and it really would be the mix gas of choice were it no so expensive.

Hydrogen (not a noble gas) is better than neon, and it would be really great in a mix were it not for its tendency to have an unhappy relationship with oxygen. It can be used (and has been used) in mixes with the oxygen less than 4%.

However, all of this, including why they have this effect, is not well understood and open for debate. That is why it is possible for Europe to have a different belief than America. (I had never heard that before--this thread is the only place I have heard it.) It would not be right for those who have been taught that nitrox is less narcotic than air to blithely dismiss the arguments to the contrary. What we were taught is really still at the hypothesis stage, and it is possible that it is wrong.

For more details, see Mark Powell's Deco for Divers, pages 171-176.

If you read Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis, the summary of a study of gas narcosis experimentally investigated in a hyperbaric chamber, you'll come across this conclusion:

"... for producing equivalent degrees of decrement in mental function, the rise in nitrogen pressure has to be 3 to 4 times greater than that in oxygen pressure and, hence, that oxygen is 3 to 4 times as potent a narcotic as nitrogen."

So, rather, we should expect nitrox to be more narcotic than air.

Since I'm a big nerd, I'm going to suggest the equation:

Equivalent Narcotic Depth = [(FN2+3FO2)/1.42]*[D+33]-33


So at 100 feet, for example, EAN32 will be like air at 121 feet.

...I would be interested in hearing the explanation for those who think it is less narcotic (which, as I said, I had never heard before)...

Pool's open, and popcorn's on bj.

Note on my quoted post above, I'm simply rewriting the EAD formula with the assumption that O₂ is 3 times more narcotic than N₂ (and implicitly that He is not narcotic at all). To that end, I ratio the fractional content of the narcotic gases in one mix to that of Air where F_NARCOTIC = FN₂+3*FO₂ (e.g. F_NARCOTIC for air = .79+3[.21] = 1.42).

In retrospect - it was quite off the cuff- it may not make complete sense since you can't have a mixture made up of 140% of its constituents, but it's rather beside the point anyway.

 

[amascuba]

Here comes more controversy.

Lipid Solubility - The maximum concentration of a chemical that will dissolve in fatty substances. Lipid soluble substances are insoluble in water. They will very selectively disperse through the environment via uptake in living tissue.

Both Nitrogen and Oxygen are about the same. That is why some people think Oxygen is as much, if not more narcotic than Nitrogen. I'll see if I can find the numbers in a bit and post more information.

If you can measure narcotic effects based on lipid solubility numbers, then Nitrox has absolutely no affect on decreasing narcosis.

 

[Blackwood]

If you can measure narcotic effects based on lipid solubility numbers...

Personally, I never really bought that you could. My thought was "who cares how soluble something is if its presence doesn't affect anything?"

Then I read the study I linked to.

 

[Gene_Hobbs]

I think Simon sums up the available literature and my thoughts on the topic quite well here or here. Dr. Bennett also offered his similar opinion at the end of his talk on narcosis at the DAN Tech Conference last year.

 

[boulderjohn]

Originally Posted by Rhone Man
Whilst we are all busy hijacking this post, I was recently reminded of another controversial issue about Nitrox that has yet to be resolved by a definitive study - whether Nitrox assists in reducing narcosis.

It seems to be an article of faith in North America that Nitrox does not reduce narcosis. Equally it seems to be an article of faith in Europe that it does.

So, is there anyone from Europe (or anywhere else) reading this that does indeed hold it as an article of faith that nitrox reduces narcosis? If so, what are the reasons?

Please note that as I was quoted above, my question does not imply some sort of a line-in-the-sand challenge. I know that the mechanics of narcosis are not well understood and open for debate, and I assume that there must be a rationale for this belief. I am therefore genuinely interested in hearing it.

 

[boulderjohn]

People posted while I was typing. That is what I get for never learning to type properly.

OK, here is the key conclusion to the second citation from Gene:

My interpretation of the experimental literature is that oxygen does have some narcotic efffect, but that it is not as great as nitrogen. Consequently, increasing the fraction of oxygen in a nitrox mix would reduce narcosis, but increasing the fraction of oxygen in pure heliox might increase narcosis (marginally). This is consistent with the story related above.

This does indeed support the idea that nitrox is less narcotic than air, but the degree that this is true is not at all clear. Or am I reading this wrong?

 

[Cave Bum]

I have to admit, it's been years since I've done ANY diving on air. It's been either 32% or mix. Maybe I should get an air fill and see if I notice any difference on a dive at Ginnie. I just hate the extra deco I'm going to rack up, even with O2 at 20feet...

 

[boulderjohn]

I love this quote from Rickthediver in the TDS thread:

A truly scientific study that proves beyond doubt that oxygen may be more narcotic or less narcotic than nitrogen. Probably a lot less - but could be more.

 

[Steve SDI/TDI HQ]

Thanks for posting Gene... as always, a succinct and pointed review.

I think I have the topic for my next blog entry...

 

[Steve SDI/TDI HQ]

I have to admit, it's been years since I've done ANY diving on air. It's been either 32% or mix. Maybe I should get an air fill and see if I notice any difference on a dive at Ginnie. I just hate the extra deco I'm going to rack up, even with O2 at 20feet...

ANd when you switch to pure oxygen at 6 metres do you feel the narcotic hit?