O2 Narcotic

[JSC]

Dr. Deco,

Let me start off by explaining I am not a technical diver (no tri-mix, nitrox or deco courses -yet) and this is for information only not actual use.

I am however, a curious engineer who dives a lot. I have been reading a lot on partial pressures and narcosis and am trying to figure out how O2 Narcosis is taken in to account in calculating your END. Can you answer this simply or point me in the direction of a good source. I have figured out that in a tri-mix you can increase helium and decrease nitrogen to make up for the O2 narcosis as well as the nitrogen narcosis; the O2 narcosis is still a mystery to me -not that I fully understand nitrogen narcosis either.

I assume that all gasses are narcotic at some depth depending on how "heavy" (not sure if that is the correct word) they are. Are there equations to figure out narcosis based on this or is it based off some theory and then trial and error?

Thank you,

JSC

 

[Rick Murchison]

"Narcosis" describes a "narcotic" effect -
Oxygen presents as more of a CNS poison than a narcotic at high partial pressures, hence the term "Oxygen Toxicity" to describe the most prevalent symptoms of too much oxygen.
The "narcotic" effect of other gases seems to be related to their relative ease of absorption in fat, which points at a modus operendi involving the myelin sheath, but to my knowledge the effect is still largely a mystery at the physical chemistry level.
Docs?
Rick

 

[Dr Deco]

Dear JSC:

Oxygen is generally feared more for its toxic effects than for its narcotic ones. We had a FORUM discussion on OXYGEN NARCOSIS a few months ago in this Dr. Deco column. Divers are able to reliably detect narcosis at partial pressures of nitrogen far less than those required to produce reliable scientific results. Thousands of divers claim that high pO2 nitrox results in less narcosis than air at the same depth. Peter Bennett, PhD (in "The Physiology and Medicine of Diving" ) states regarding oxygen narcosis, "…evidence available … indicates that raised pressures of oxygen alone can indeed produce narcotic effects and anesthesia." William Hamilton, PhD, (in Bove and Davis ”Diving Medicine"), "Oxygen probably has about the same narcotic potency as does nitrogen."

The use of oxygen in diving is to eliminate as much as possible the need to breathe an inert gas. Oxygen is quite a reactive substance, even at its concentration of 21% in the air we breathe. Scavengers are needed to remove “reactive oxygen intermediates” (e.g., free radicals) from the cells. The concentration of these substances (e.g., catalase, super oxide dismutase, etc) is limited and, while our normal atmosphere can be handled, the system is overwhelmed when oxygen at pressure is encountered.

The protection by the “scavengers” is time dependent. The time is shorter, the higher the oxygen concentration in the tissues. Thus, we see concentration (depth) and duration (bottom or exposure times) restrictions to prevent carrying a time bomb inside of us :boom:.

Dr Deco

 

[JSC]

Dr Deco,

Sorry I missed the original post on this, I appreciate your taking the time to answer the question (link to the article did not work). I also appreciate your explaining oxygen toxicity- I understood the limits of oxygen but not the reasons.

Let me try and summarize what I got out of both posts on oxygen narcosis. Please be warned that I may have strayed a little at the end; I did not do well in physics and chemistry.

There may be some oxygen narcosis at some partial pressure; since oxygen toxicity will typically happen at a lower partial pressure this is your limiting factor while diving. For example, when diving, you try to keep your pO2 less than or equal to 1.4 and you try and keep your pN2 less than 3.1 (Equivalent Narcotic Depth <END> of 96'). Hypothetically, if you assume that nitrogen and oxygen are equally narcotic- "Oxygen probably has about the same narcotic potency as does nitrogen." (William Hamilton, Ph.D., in Bove and Davis ”Diving Medicine")- then you will never get close to a pO2 of 3.1 without toxicity. This said a pO2 of 1.4 would be equivalent to an END of 25'.

This now begs the question for me: how do the two interact? If they are equally narcotic, then at the surface do they have narcotic pO2N2 of 1? This would then mean that a pO2N2 greater than 3.93 (96'on air) would be narcotic and you would want to add a filler gas to reduce either one (realistically nitrogen)- hence tri-mix? Then nitrogen may not be narcotic at a pN2 of 3.1 but at a lesser pN2 and when you remove Nitrogen from a mix and add a filler gas you are also compensating for the oxygen as well. Of course this would all be blown out of the water if Nitrox were truly less narcotic than air- this I have no idea about because I have not dived Nitrox.

It looks like it is time to sign up for some more classes; I figure this is discussed in one of the books.

Thanks for all your help, if I am way of base feel free to correct me.

JSC

 

[danreind]

I've never looked terribly deeply into narcosis, particularly once I entered the world of trimix and never had to worry about having a clear head anymore, but I'll make a stab at a post anyways. I can see the confusion - if oxygen and nitrogen are equally potent then diving nitrox shouldn't affect your impairment since the narcosis would depend on the total pressure right?

However oxygen is not inert, in the tissue it is mostly burned off. For example you are now breathing 0.21 atm of oxygen but your tissues are only experiencing around 0.06 atm. Because of how hemoglobin works the tissue O2 level doesn't vary hugely until the inspired O2 rises above a certain level - higher than the nitrox limits of ~1.2-1.6 atm(depending on tissue blood flow and metabolism etc) . So my understanding is that oxygen is narcotic on a per atm basis, but because the oxygen pressure is lower where it counts - the nerves - that it is less narcotic than the same inspired pressure of nitrogen.

It's entirely anecdotal, completely uncontrolled and not the least bit scientific but my personal experience supports this idea. I've been rebreather diving on nitrox and found myself at depth with a ppO2 of 0.6 atm or so at the end of descent. I've noticed that a few moments after I press the O2 and get it up to 1.2-1.3 atm that the narcosis was noticeably reduced. Take it for what it's worth.

Cheers,

Dan Reinders

 

[turnerjd]

The usual rule taught in a nitrox class is that there is no difference in narcosis whatever mix you use.

In terms of calculation, I don't know of any reliable END calculation for different mixes, simply because no-one has measured the actual narcotic potential of oxygen. We know it is narcotic, from anecdotal evidence, but this is something very hard to put actual numbers on.

I would suspect down on the physicochemical level as well as the partial pressure, when you have more than 1 inert gas present other factors such as the gas soluability (probably measured using a water/octanol partition co-efficient) come into play. Some gasses such as helium are absorbed much faster (and eliminated faster) than others. There will be a gas solubility limit, and the loading of each gas in the body will change with the different solubilities and partial pressures. This is however, when there is more than 1 inert gas.

When there is only one inert gas (ie EANx, O2 and N2 only) this is not a problem as the oxygen is metabolised, so there is only the one gas to consider.

I don't have a phyical chemistry book here at work, but there are plenty of equations to explain how solubility can be changed by the presence of other competitive species.

I will have a re-read of my old degree textbooks and see what I can come up with for this.

I might, of course, be on completely the wrong track here, but there will surely be some interaction there, after all, partial pressures are just a measure of gas concentration, and when you have mixtures of chemicals all sorts of things start happening to their solubilities.

Jon T

 

[Dr Deco]

Dear JSC:

While I have never tried NITROX personally, many divers claim that it is less narcotic than air at the same depth. Beyond that anecdotal series, there does not exist a side-by-side comparison for these two mixes. Certainly in the pre-WW II days of diving, it was found that air was potentially dangerous at depths of 200 fsw and deeper. A switch to helium/oxygen was made. Breathing mixes in the final analysis are blended to attempt to reduce the total decompression time.

It certainly is true, as indicated by Dan, that the partial pressure of oxygen at the cellular level is lowered because of metabolism. Based on the lipid solubility, we could expect that oxygen would have the same narcotic potency as nitrogen if there was not a loss from cellular metabolism.

The change in solubility of gases that you suggested is probably not large when the species are not chemically reactive.

One could certainly expect variations in solubility if we were discussing ionizable molecules. There we would possibly find the common ion effect . The gas species do not ionize, and thus we can discard this rational.

Another variation could result from large changes in the activity coefficient of water (e.g., by the ionized species). I would expect that later to occur only in the case of gases such as carbon dioxide, ammonia, or hydrogen chloride (to name some common examples). While there are certainly gas-water interactions, these are not large when measured. The largest variation in solubility of gases between pure water and physiological fluids would probably result from the dissolved ions (primarily sodium chloride) in tissue fluids.

Again the major problem with inert gases is their narcotic and solubility effects (to later produce bubbles), and with oxygen, we find toxicity is paramount.

Dr. Deco

 

[JSC]

I think I am starting to understand... but I still have a ways to go.

I apreciate all the time you guys have spent answering my questions.

JSC

 

[turnerjd]

OK, I had a read round this last night, and the problem so far that I have found is that nobody has managed to do the maths accurately for tertiary mixtures.

Binary mixtures have been explained for many years (Raoults law and another Henrys? Law, but A different Henry from the other one), and there is very little change in solubility in binary mixtures.

Now, due to energy considerations (Gibbs function of mixing etc..) there is only a fixed ammount of total gas that can be absorbed by anything. Another way of thinking of it is in terms of gas tensions within the tissue. I suspect (I'll have to go and look it up somewhere) that

Tt = T1 + T2 + T3

There Tt is the total tension of disolved (inert) gas, and T1-3 are the individual tensions). I am sure that Tt is fixed, (I'm sure this can be shortened for Nitrox diving to Tt = Tn2)

Really, for the non scientists, If we were able to survive breathing a mix of 2 inert gases (eg Nitrogen and helium), then at the surface we would be at a saturation level (relative to surface pressure). This saturation level will be for total inert gas in the body. This won't be that we are saturated with nitrogen, and also saturated for helium. We will be saturated for gas, of which a certain percentage is nitrogen, and the rest id helium.

Now, if we add O2 to the mix so we can survive, due to this being metabolised, we would reach pretty much the same situation in the tissues of the body, with a total level if inert gas, of which some is nitrogen, and some is helium.

No-one really knows about how more than one inert gas will interact when dissolved in the body. All the work that I know of has been done looking at nitrogen levels only, for decompression tables. Does any one, (especially the diving docs) know of any work done in this area?

For trimix diving, due to the faster absorption and elimination of helium (than nitrogen), this point has never really been researched.

To acurately calculate some form of narcotic potential (ENarcD) we would need to know rates of uptake of different gases, their narcotic power, their biological interactions, and how their different solubilities contribute to the total dissolved gas load.

We could probably experimentally measure the narcotic power of O2 (we can breath pure O2 for upto about 600 mins safely on the surface = UPTD of 600) using some standard scientific test.

We know which gasses are inert, and that only O2 has a biological interaction.

We could probably come up with some rough numbers for solubility (water / octanol partition coefficient)

We would just have to go and do some experiments on total gas loads, and mixes of inert gases and how their solubilities affect the total inert gas load, and loads of individual gases.

Research project anybody? Or am I barking up the wrong tree, it has been a long time since I did any real physical chemistry?

Jon T

 

[Dr Deco]

Dear turnerjd:

Actually there has been considerable research performed to determine how more than one inert gas will interact when dissolved in the body.

Gas Uptake and Elimination

While most recreational divers are acquainted with air diving, thesis work was performed several decades ago. In the second half of the 20th century, most work was done in the laboratory of mixed gas diving. It this case, I am speaking of helium-nitrogen-oxygen dives. This could be accomplished by two different means.

In the first case, this could take the form of the gases mixed together. Considerable work was performed at Duke University (North Carolina) and by Prof. Buhlmann (Switzerland). The LZ-H16 model attempts to model the transport of both helium and nitrogen simultaneously in tissues. Even though the half times are different (faster absorption and elimination of helium than nitrogen), this model appears to work successfully. It has been well tested in Prof. Buhlmann’s laboratory in Switzerland.

The other system is to administer the gases in a sequential fashion. This was originally done by End in the US in 1940, and later was done by Keller in Switzerland. Other work has been performed by Cabarrou and Fust in Germany. (I helped in some of this while I was in Germany, 1977-1980).

Narcosis

Since only nitrogen is really narcotic (helium is not and oxygen has a low partial pressure because of metabolism), we need only to know what is in the breathing mix since the rates of uptake of this gas in the brain is very fast. This fast rate is true for poorly soluble gases; soluble gases, such as ether, have a long time of onset and a long time to come off.


Dr Deco