How Narcotic Is Oxygen?

Steve

This post has me thinking on this topic one again; http://www.rebreatherworld.com/62291-post30.html

I have been wandering around the internet and reading the 2 hyperbaric medical texts that I own and as best I can tell the scientific data available do NOT support the conclusion that oxygen is as narcotic as nitrogen. The information I have found also does not and can not show that oxygen has no narcotic effect. There are a good number of simple explanations that support that oxygen is narcotic but they just smack of bad science and a limited understanding of what is going on. Basically, divers stating they know what is what when they really ought to be diving rather than playing scientist.

With my limited knowledge of biochemistry (I failed this subject at uni) I can’t track down an explanation of why gases are narcotic. I can understand some link between, the gas’ density, molecular weight and lipid solubility but a ‘rule’ escapes me. I understand enough to think that the physical properties of oxygen should not be used to support a claim that oxygen is as narcotic as nitrogen as this is just one part of the equation.

Can one of the boffins help me out please?

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wedivebc

without applying any scientific data or knowledge on the subject I know from personal experience while operating my CCR as semi closed mode at 100ft and letting my PPO2 fall to .5 then after a few minutes going back to 1.3 I felt an immediate increase in perception and focus. I have since repeated this experiment with similar results. As the O2 increases the head clears. That was enough to convince me.

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Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

fubari

Well, I read that as Dave saying:
.5 PO2 gave him less focus.
1.3 PO2 gave him more focus.

assuming air diluent,
.5 PO2 / 4ata -> .125 FO2 -> .875 FN
1.3 PO2 / 4ata -> .325 FO2 -> .675 FN

If O2 and N were equally narcotic, then
Dave would have felt similar (or same)
levels of focus at either PO2.

So... at about 4atas, Dave felt that
a pound of oxygen was gentler on
his focus than a pound of nitrogen.
(I know, I know, but "pound" is easier
to type than "an equivalent amount of..." )

Or, that oxygen is relatively less narcotic
than nitrogen, if you will.

How much less? I don't know - it will be interesting
to see if someone knows actual numbers.
(Hmm... how does one measure narcosis?
Reaction times? Memory tests? I'd be surprised
if there is an device like a brethalyzer for
blood-alchohol content).
John G.

decoweenie

John,

Got it now, thanks. Didn't have my coffee yet this morning when I was reading that...

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E-man

wedivebc's experiment isnt surprising. 3.5 ppN2 is the point where most divers feel nitrogen narcosis (or its been said so) at po2 .5 @4 ATA he was at 3.5 pN2 = feeling nac'd...at 1.3 po2 he was at 2.7 pN2

bendomatic

Steve here's some reading i just found which seems to explain some of the hypotheses on the mechanism of anaesthesia and o2. Haven't read it all myself, so not sure if its what you're really after. It presents hypotheses and experimental design to test them, but no data.

Mechanism of Anesthetic Action: Oxygen Pathway Perturbation Hypothesis

http://arxiv.org/html/physics/0101083

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Simon Mitchell

Steve,

This is a hoary old chestnut that keeps rearing its ugly head. At various times over the years I have reviewed the literature on the matter fairly thoroughly and it must be said that the anecdotal discussions that have taken place in this thread reflect the "definitive" literature quite accurately.

Nobody really knows why gases produce narcotic effects, and that includes the anaesthetic gases that I use every day. There are many theories which are essentially irrelevant to this discussion and I won't go into them here. However, we do know that narcotic potential appears directly related to the lipid solubility of the gas.

As you imply, it seems that divers armed with this knowledge go to physicochemical text books and discover that oxygen is more lipid soluble than nitrogen and conclude that it must therefore be more narcotic than nitrogen. However, oxygen is a metabolic gas that is consumed constantly in tissues. In addition, the interaction between oxygen dissolved in plasma and bound to haemoglobin creates a "buffering effect" which, when combined with the consumption of oxygen in tissues, largely prevents a linear increase in tissue PO2 as the inspired PO2 increases... up to a point. That point potentially comes somewhere around an inspired PO2 of 2.4 ATA depending on the subject's level of activity and tissue oxygen consumption. Beyond that sort of PO2 (which potentially provides sufficient oxygen dissolved in plasma to meet all metabolic needs), there is the potential for oxygen tensions in tissues to rise linearly with any further increase in PO2. That sort of situation could even result in the possibility of "oxygen bends", but obviously, we don't expose ourselves to those sort's of PO2s during diving.

The point of all this is that whereas tissue inert gas (nitrogen) tensions will, in theory, increase linearly as the inspired Pgas increases, it is much less clear what happens to oxygen. It will vary from tissue to tissue according to a number of factors, but it won't be a linear increase in the range of PO2s commonly used in diving. It is therefore quite invalid to imply that oxygen is as or more narcotic than nitrogen on the basis of lipid solubility alone, because the tissue Pgas is just as important.

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.

Warm regards,

Simon M

Steve

Thanks Simon. I was feeling a bit dumb by not being able to understand how the narcotic process works and you have left me feeling better. I'm happy to place it in the too hard basket and look forward to an article apearing in New Scientist in the future that talks about a clever chap that has it all worked out.

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dteubner

I thought I'd repost this -
Oxygen is carried in blood in 2 ways, bound to haemoglobin and dissolved in the blood. The haemoglobin in 100mls of normal blood carries about 20ml of oxygen and the dissolved component is 0.003ml per mmHg O2 per 100mls of blood.

You use the dissolved component first. Your brain uses about a quarter of the available oxygen breathing normobaric air, so you have about 15mls of oxygen per 100mls blood in the veins which equates to a partial pressure of oxygen of about 40mmHg. The venous and tissue partial pressures are roughly the same

If you are diving at a setpoint of 1.4, each 100mls of your blood contains a bit more oxygen than usual (20mls bound to haemoglobin and about 3.2 mls dissolved). If you are using the same amount of oxygen then there will be about 18mls O2/100mls blood, which means the haemoglobin is about 90% saturated and the partial pressure of O2 is about 75mmHg.

If oxygen was as narcotic as nitrogen then you can see why the contribution fo oxygen to the narcosis is negligable, and why narcosis worsens as the PO2 falls.

Say you are at 40m with air dil and a setpoint of 1.4. The partial pressure of nitrogen in the brain is 3.6 ATA (or 2736mmHg) and the partial pressure of oxygen in the brain is about .1 ATA (75mmHg) so the total "narcotic load" is 3.7 ATA.

Now say your setpoint changes to 0.2 (for whatever reason) The partial pressure of nitrogen in the brain is now 4.8 ATA (or 3648mmHg) and the partial pressure of oxygen in the brain is about .05ATA (40mmHg) so the total "narcotic load" is 4.85 ATA - much worse.

As an aside the Hyperbaric Technicians and Nurses Assosciation in Australia has kindly agreed to fund my little study about oxygen narcosis - stay tuned,

Mixaddict

Simon,

My own experience has been that I see no discernable difference between the narcosis level using Nitrox vs. air. Of course I dive Nitrox where appropriate, but not for anti-narcosis reasons. Call this simply anecdotal if you must, but after having dove the same dives back to back with Nitrox and then air, I can feel absolutely no difference in the Narcosis level.

As I mentioned earlier, Nitrox has all sorts of other benefits, I just don't buy into the anti-narcosis property theory.

Just my $.02
Randy

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Randy Thornton (MixAddict)
Inspiration, Evolution,Hammerhead & Sentinel
CCR Instructor