Elevated Oxygen Nitrox

Peter McGuinness · May 22, 2005

The recent poll on what gas is used as a default got me thinking about the why's and wherefore's. The two often-quoted reasons for using elevated oxygen mixes are extended bottom times at moderate depth and safety. The safety arguments break down into greater margin of safety against DCS and reduced narcosis.

The greater margin argument seems to be well proven and amply documented but I had not seen any references to primary data on the narcosis argument (that would be controlled experiments looking at the incidence of narcosis while varying the O2 percentage in the mix) so I looked around and could find only unverified references claiming both that narcosis is reduced by the use of nitrox, and that it is not affected because of increased oxygen toxicity , or additive oxygen narcosis (which I had never heard of).

Here are a few examples:

from http://www.deep-six.com/page74.htm

Many gases are intoxicating when mixed with 21% oxygen. It is pressure-dependent. Also, the more a gas dissolves in fat the more narcotic it is. Nitrous oxide has a narcotic affect at sea level, and that is why you get "drunk" in the dentist's chair. Xenon dissolves in fat over 20 times better than nitrogen . It is far more narcotic and has been used for anethesia. The gases neon and argon do not produce intoxication at 1 atmosphere but will do it under a pressure less than 100' of seawater, argon having more potential than neon. Nitrogen is less intoxicating than neon. Oxygen has a narcotic effect similar to nitrogen, so breathing nitrox does not change the outcome

http://www.sdm.scot.nhs.uk/scuba_diving/

contains both claims in the same article (talk about hedging)

factors include:

* Alcohol.
* Carbon dioxide (CO2) retention.
* Inspired O2 more than 100kPa, possibly due to the additive narcotic effect of oxygen.

and then:

Because Nitrox contains less nitrogen than air, the effects of narcosis are reduced. However, because the oxygen concentration is higher there is an increased risk of oxygen toxicity.

from http://www.americandivecenter.com/nitrox/preview_p04.htm

Note: Reduced narcosis at depth is not one of the claims for Nitrox. Logic suggests that with less Nitrogen in the mix, there would be a reduced narcotic experience at depth. However, Oxygen is also a narcotic gas. The net effect is that the Narcotic experience does not change when diving Nitrox.

I would really like to hear from someone who knows, or can give a pointer to a primary source which can settle the question. After all, if we are being persuaded to part with extra $$$ in the name of safety, it would be great if it actually worked as advertised.

It's easy to get lost in a maze of opinion in a discussion like this, so I am asking for information on primary sources, which I define (as above, but more fully here) as the first hand reports of controlled experiments which look at the variation in incidence, onset and severity of narcosis with changes in the proportion of oxygen to nitrogen in compressed Nitrox. (I am not looking for information on helium).

Peter

liberato · May 22, 2005

Peter McGuinness:
...The safety arguments break down into greater margin of safety against DCS and reduced narcosis.

The greater margin argument seems to be well proven and amply documented...

I don't know about the narcosis element but I would dispute your claim that there is any statistically significant benefit as far as margin of safety against DCS. And that is only because the chance of DCS on a NDL dive with air is already so small that any theoretical improvement offered by EANx is lost in the margin of error noise.

Rick Murchison · May 22, 2005

Here is the most straightforward explanation on the subject I've found:

Narcotic Properties of Oxygen: Based on study of animals and humans, oxygen is a narcotic gas. Based on lipid solubility, oxygen should be twice as narcotic as nitrogen, as oxygen is twice as soluble. However, as several individuals have pointed out, the increase in tissue partial pressure of oxygen is limited as the oxygen is metabolized. This means that for any given increase in the inspired partial pressure of oxygen, the change in tissue partial pressure is less. Study of the narcotic properties of oxygen is limited by oxygen seizures; so study of oxygen narcosis in humans have been limited to lower partial pressures where "narcosis" is more subtle and difficult to test for. In animals, treatment to suppress seizures allows exposure to very high PO2's, and exposure to 11 ATA O2 produces anaesthesia in mice
1. In humans, testing for psychomotor performance indicates that 1.65 ATA N2 is about equal to 6.3ATA O2 in narcotic effect
2. Measurements of change in brain PO2 under hyperbaric conditions are limited in number, and also limited to animal studies. In animals, exposure to 2.34ATA O2 produced a brain PO2 of 291 mmHg (1 ATA = 760 mmHg)
3. Assuming a normal partial pressure of CO2 of 40 mmHg, alveolar PO2 would be 1738 mmHg, so the ratio between alveolar and brain PO2 is 219/1738 = 0.17, which means that the rise in brain PO2 is only about 1/6 the rise in inspired PO2. If we now apply the correction for lipid solubility, (twice as soluble/narcotic), oxygen should be about 1/3 (0.17 x 2 = 0.34) as narcotic. This takes into account both the lipid solubility and the change in brain PO2. In the human study cited above, oxygen was 1.65/6.3 = 0.26 as potent as nitrogen. The calculated (0.34) and measured (0.26) values are in reasonable agreement. J.E. Brian M.D.

Bottom line: While there may be some amelioration of the oxygen effect due to metabolic processes, It is best not to assume any reduced narcosis (from what you'd have with air) with Nitrox.
Rick

donooo · May 22, 2005

liberato:
I don't know about the narcosis element but I would dispute your claim that there is any statistically significant benefit as far as margin of safety against DCS. And that is only because the chance of DCS on a NDL dive with air is already so small that any theoretical improvement offered by EANx is lost in the margin of error noise.

I love that answer. From a personal perspective we can make it incredibly important. But statisticaly ?? If I all ready have virtually no chance, how much more should I pay to have less then no chance??

Thanks again, great answer, don O

Rick Murchison · May 22, 2005

liberato:
I don't know about the narcosis element but I would dispute your claim that there is any statistically significant benefit as far as margin of safety against DCS. And that is only because the chance of DCS on a NDL dive with air is already so small that any theoretical improvement offered by EANx is lost in the margin of error noise.

The problem is, of course, that the statistically significant statistic we demand can only be had if we have a group of divers willing to carry air dives to Nitrox ND limits. Comparing Nitrox dives made to air limits with air dives isn't likely to achieve any statistical significance because the "in limits" dive that produces symptoms on air will likely produce symptoms on Nitrox as well, because the anomoly that caused the hit is likely independent of the nitrogen load in the first place.
But if you want to try it (diving Nitrox NDL dives on air) to prove its statistical insignificance go for it - I'll do the Nitrox side of the experiment.

Rick

Peter McGuinness · May 22, 2005

Rick Murchison:
Here is the most straightforward explanation on the subject I've found:

Narcotic Properties of Oxygen: Based on study of animals and humans, oxygen is a narcotic gas. Based on lipid solubility, oxygen should be twice as narcotic as nitrogen, as oxygen is twice as soluble. However, as several individuals have pointed out, the increase in tissue partial pressure of oxygen is limited as the oxygen is metabolized. This means that for any given increase in the inspired partial pressure of oxygen, the change in tissue partial pressure is less. Study of the narcotic properties of oxygen is limited by oxygen seizures; so study of oxygen narcosis in humans have been limited to lower partial pressures where "narcosis" is more subtle and difficult to test for. In animals, treatment to suppress seizures allows exposure to very high PO2's, and exposure to 11 ATA O2 produces anaesthesia in mice
1. In humans, testing for psychomotor performance indicates that 1.65 ATA N2 is about equal to 6.3ATA O2 in narcotic effect
2. Measurements of change in brain PO2 under hyperbaric conditions are limited in number, and also limited to animal studies. In animals, exposure to 2.34ATA O2 produced a brain PO2 of 291 mmHg (1 ATA = 760 mmHg)
3. Assuming a normal partial pressure of CO2 of 40 mmHg, alveolar PO2 would be 1738 mmHg, so the ratio between alveolar and brain PO2 is 219/1738 = 0.17, which means that the rise in brain PO2 is only about 1/6 the rise in inspired PO2. If we now apply the correction for lipid solubility, (twice as soluble/narcotic), oxygen should be about 1/3 (0.17 x 2 = 0.34) as narcotic. This takes into account both the lipid solubility and the change in brain PO2. In the human study cited above, oxygen was 1.65/6.3 = 0.26 as potent as nitrogen. The calculated (0.34) and measured (0.26) values are in reasonable agreement. J.E. Brian M.D.

Bottom line: While there may be some amelioration of the oxygen effect due to metabolic processes, It is best not to assume any reduced narcosis (from what you'd have with air) with Nitrox.
Rick

Rick, could you send me a link to the source of that document?

The data points you give only allow a very partial understanding: I would like also to know the change over PP in each case (is the O2 curve steeper than the N2 curve at that point, for instance?), and whether the proportional change in PP is the relevant factor - this is because going from 21 to 32% O2 increases the PP of O2 by almost 50% but only decreases the the PP of N2 by 12% (very approximate figures).

Peter

Peter McGuinness · May 22, 2005

donooo:
I love that answer. From a personal perspective we can make it incredibly important. But statisticaly ?? If I all ready have virtually no chance, how much more should I pay to have less then no chance??

Thanks again, great answer, don O

Sure, but this is a different question. I can, of course, construct scenarios where certain individuals may want to take advantage of that extra margin, such as two buddies of very different physical characteristics or age, who may want to follow similar profiles and the buddy with high risk factors such as obesity or age may use nitrox for that purpose.

My comment was only that the science is clear on this point, whereas I am not so sure on the narcosis issue.

Peter

wedivebc · May 22, 2005

If I may add a little anecdotal evidence to this topic. Since diving open circuit the gas choices are limited and unless one is to carry several different gasses at different depths and somehow analyze the very subjective degrees of intoxication, it is hard to come up with a valid opinion. After diving CCR I became very aware of narcosis differences in 2 gas breathing mix.
While conducting a SCR bailout skill in 100fsw I was required to breath my unit as a semi closed rebreather by exhaling through my nose then breathing off my unit for 4 breaths then repeating the cycle. This resulted in my po2 to drop to about .7 using air diluent. This equates to about 17% oxygen 83% nitrogen in the loop. The skill was easy to accomplish but after a few minutes of this it was time to switch back to normal mode. My po2 was increased to 1.3 or about 33% o2 and 67% n2. There was an immediate feeling of a fog being lifted off and a sharpness and focus that came on immediately. This is something I can recreate any time I dive closed circuit rebreather and although many say narcosis is subjective this has convinced me that O2 is either much less narcotic or not at all.

Rick Murchison · May 22, 2005

wedivebc:
If I may add a little anecdotal evidence to this topic. Since diving open circuit the gas choices are limited and unless one is to carry several different gasses at different depths and somehow analyze the very subjective degrees of intoxication, it is hard to come up with a valid opinion. After diving CCR I became very aware of narcosis differences in 2 gas breathing mix.
While conducting a SCR bailout skill in 100fsw I was required to breath my unit as a semi closed rebreather by exhaling through my nose then breathing off my unit for 4 breaths then repeating the cycle. This resulted in my po2 to drop to about .7 using air diluent. This equates to about 17% oxygen 83% nitrogen in the loop. The skill was easy to accomplish but after a few minutes of this it was time to switch back to normal mode. My po2 was increased to 1.3 or about 33% o2 and 67% n2. There was an immediate feeling of a fog being lifted off and a sharpness and focus that came on immediately. This is something I can recreate any time I dive closed circuit rebreather and although many say narcosis is subjective this has convinced me that O2 is either much less narcotic or not at all.

On the other hand... (DIR bubbas may leave the room now) I have done many dives using EAN32 as travel gas and air as bottom gas with a switch at 130', thus going from 32% to 21% O2 (68% to 79% N2) on the way down and from 21% to 32% on the way back up. I notice precisely zero change in alertness going in either direction at the switch. So for my money the narcotic effects of nitrogen and oxygen for me are interchangeable, and for trimix dives I figure my END at the PP of N2+O2.
Nor have I ever noticed any difference with changes to 50% or higher for deco, but then those are at such a low total gas pressure that narcotic effects are absent or nearly absent anyway. Which brings the point... since at "narcotic" depths the limiting percentage of oxygen is dictated by O2 toxicity anyway, is it really worth pursuing?
---
On the article I cited above - that's a copy I saved; I'll try to find the source, but if I remember correctly there wasn't any amplifying info there either.
Rick

Peter McGuinness · May 22, 2005

wedivebc:
If I may add a little anecdotal evidence to this topic. Since diving open circuit the gas choices are limited and unless one is to carry several different gasses at different depths and somehow analyze the very subjective degrees of intoxication, it is hard to come up with a valid opinion. After diving CCR I became very aware of narcosis differences in 2 gas breathing mix.
While conducting a SCR bailout skill in 100fsw I was required to breath my unit as a semi closed rebreather by exhaling through my nose then breathing off my unit for 4 breaths then repeating the cycle. This resulted in my po2 to drop to about .7 using air diluent. This equates to about 17% oxygen 83% nitrogen in the loop. The skill was easy to accomplish but after a few minutes of this it was time to switch back to normal mode. My po2 was increased to 1.3 or about 33% o2 and 67% n2. There was an immediate feeling of a fog being lifted off and a sharpness and focus that came on immediately. This is something I can recreate any time I dive closed circuit rebreather and although many say narcosis is subjective this has convinced me that O2 is either much less narcotic or not at all.

This is an interesting piece of experience, and your story is consistent with experiments done using depleted oxygen nitrox where narcosis was very pronounced. However, this also illustrates the dangers of anecdotal evidence: there is more than one thing going on in this experiment, the first is what you reported (an increase in pn2) but also, the increase is very rapid, equivalent to a very high descent rate, which was shown by the same researchers to result in much earlier onset of narcosis for given starting pn2 and to exaggerate its effects. Also, you experienced simultaneous rapid deoxygenation, which would cause a feeling similar to the one produced by standing up too quickly after bending over.

All said, I think it is unsafe to allow this experience to convince you of anything (except to be careful of getting narced by this drill)

Peter

Elevated Oxygen Nitrox

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