A question about the Partial Pressure of Oxygen

[FPDocMatt]

Steve's talking about CCRs, which are the only diving equipment at present which can maintain a constant ppO2.

Ah, I see.

On a CCR, you are rebreathing the gas in the loop until or unless you inject some more gas, so the ppO2 will slowly fall as you metabolize the oxygen.

Makes sense.

In addition, if you are at .21 at 33 feet, the ppO2 will fall as you ascend, and you'd be at .1 by the time you got close to the surface -- UNLESS you add O2.

Does the CCR have the capability of adding more oxygen? I know there is some kind of rebreather setup that involves adding more oxygen as needed. Of course, I understand you wouldn't do this just to ascend, if your ppO2 were 1.2 so you didn't have to.

There are three considerations in choosing diving gases: Oxygen toxicity, narcosis, and decompression. The more oxygen you can have in your breathing mixture, the less decompression you have to do. The choice of other gases to use in a breathing mixture is not very broad -- there aren't that many gases from which to choose, and many of them are toxic. Of the ones which aren't poisonous or anesthetic, many are narcotic. It has come down to nitrogen and helium because they are available, nontoxic, relatively affordable, and helium is virtually non-narcotic. Although there are controversies about the comparative behavior of nitrogen and helium in decompression, I think it's legitimate to say that both require attention to ascents rates and times. Minimizing inert gas makes for shorter deco, but raises oxygen toxicity risk. GUE, the agency through which I have some of my training, chooses to keep a relatively low ppO2 (1.2 or lower) for long exposures, but most of us will tolerate 1.4 for shorter dives, and 1.6 on deco, where there is very little exertion and the exposures are generally fairly short.

That's very clear, thank-you. It also explains some of the historical stuff I read in the Navy Diving Manual. I need to re-read that part, I bet it'll make a lot more sense now.

So you increase the oxygen as much as safely possible in order to decrease decompression time.

Is this ppO2 not going above 1.4 (or 1.6 for short periods) what's behind the Maximum Operating Depth (MOD) of enriched air nitrox? The MOD would then be the depth at which the ppO2 of your mixture reached or approached 1.4?

I've heard that oxygen causes narcosis as well, but people talk as if increasing oxygen and decreasing nitrogen reduced narcosis.

My understanding of why you don't just use heliox at depth instead of trimix is:

1. Helium needs longer decompression times than nitrogen.
2. You're more likely to get chilled breathing helium.
3. Helium is more expensive.
4. Helium causes high pressure nervous syndrome.​

Is this correct?

I must say I find all this gas stuff fascinating.

 

[FPDocMatt]

And just one other point that was mentioned about oxygen being used to reduce narcosis. There is some thought than O2 itself is narcotic and as an MD you know the effect it has on nervous or upset patients. It calms them so simply reducing the N2 with O2 may not have the desired effect of reducing narcosis but may have none or possibly add to it.

Sounds like this hasn't been definitively determined. Some people talk as if increasing the percentage of oxygen reduced narcosis, so I presume the jury's still out.

He though does require much greater attention to ascent rates and it also has a greater heat transfer efficiency. Ie you get colder faster breathing helium. I also never use back gas with He for drysuit inflation. Being so light there is the possibility of isobaric counterdiffusion if you use it as a suit gas. The theory is that it can actually be absorbed through the skin while you are trying to off gas it, if used as a suit gas.

The references I've been reading say that because helium conducts heat so well, you need to use argon in your drysuit to stay warm. I presume the helium chills you when you breathe it, but would chill you even more if you used it in your drysuit.

I just found out there's such a thing as a heated wet suit. Man, I could have used that in Gibraltar last month!

 

[Jim Lapenta]

Helium off gasses faster than Nitrogen. About 4 times as fast.

Using He shortens deco times. Helium doesn't actually chill when you breath it unless you mean it chills you. The gas does not chill. It has less ability to retain heat and therefore transfers it from your body much faster than a "heavy" inert gas like Argon.

HPNS is a consideration when using He but we are talking extreme depths there. ie 450? -500 feet or more.

And yes the jury is still out on the narcotic effects of O2 at depth.

 

[TSandM]

There is some thought than O2 itself is narcotic and as an MD you know the effect it has on nervous or upset patients. It calms them

I think in general, O2 calms people if they were hypoxic to begin with.

The contribution of oxygen to narcosis is unclear, because you really can't breathe a mixture without it Using the Meyer-Overton hypothesis, oygen SHOULD be moderately narcotic, but then again, as it is so poorly soluble in aqueous medium, and therefore very little of it is present in the dissolved form, it may not be of practical significance. Some technical divers choose to regard it as contributing to narcosis, and others don't. Hopefully, we all adjust our mixes until we're fairly sure our brains work

HPNS is really only an issue at significant depths -- I believe I recall it starts to be a real issue at about 600 feet. Warmth when breathing helium can be dealt with by using Argon or suit heaters. The real reason we all don't max out the helium percentage in our mixes is cost.

 

[Mr Carcharodon]

1. How is the metabolic use of the oxygen a factor? I metabolize the oxygen I breathe all day long, but the 0.21 atmospheres of oxygen available is quite adequate.

.

The metabolic use of oxygen facilitates decompression because total dissolved gas pressure drops in venous circulation. That happens because the by product of metabolism is carbon dioxide and it has 20x greater solubility than oxygen. When the total pressure drops the difference in pressure from tissues to blood goes up and of course the diffusion rate goes up too. The end result is faster decompression.

Of course as high ppO2 mixes displace inert gases that then do not have to be decompressed as well.

 

[DiveNav]

Helium off gasses faster than Nitrogen.

yes, but it also ON gasses faster then Nitrox

That is another reason why tech divers uses multiple gas mixtures.

Alberto (aka eDiver)

 

[FPDocMatt]

The metabolic use of oxygen facilitates decompression because total dissolved gas pressure drops in venous circulation. That happens because the by product of metabolism is carbon dioxide and it has 20x greater solubility than oxygen. When the total pressure drops the difference in pressure from tissues to blood goes up and of course the diffusion rate goes up too. The end result is faster decompression.

Of course as high ppO2 mixes displace inert gases that then do not have to be decompressed as well.

Hm. So with EAN32 for example, I'm breathing less nitrogen with each breath, so less nitrogen is getting dissolved in my body per unit time. But also, the total dissolved gas in venous circulation is lower, resulting in faster off-gassing.

This is not totally clear to me. I may be breathing a higher concentration of oxygen, but isn't my body using oxygen at the same rate as it normally uses oxygen? In which case, I would be either breathing more slowly or exhaling some of the oxygen because my body didn't need so much of it.

 

[TSandM]

Remember that CO2 is the determinant of respiratory minute volume . . . above the inflection point on the oxyhemoglobin dissociation curve, hypoxic drive really doesn't come into play.

The concept of diminished total gas tension in venous blood is one of the things that is referred to as the "oxygen window". It's questionable whether it really exists or influences offgassing at all. It's a controversial point.

 

[SeaCobra]

Helium off gasses faster than Nitrogen. About 4 times as fast.

Using He shortens deco times. Helium doesn't actually chill when you breath it unless you mean it chills you. The gas does not chill. It has less ability to retain heat and therefore transfers it from your body much faster than a "heavy" inert gas like Argon.

HPNS is a consideration when using He but we are talking extreme depths there. ie 450? -500 feet or more.

And yes the jury is still out on the narcotic effects of O2 at depth.

Helium diffusion rates are 2.65 times faster than Nitrogen. Can't say He itself has ever shortened my deco times; modern dive tables and O2 for deco has. He itself typically lengthens it.

 

[Mr Carcharodon]

The concept of diminished total gas tension in venous blood is one of the things that is referred to as the "oxygen window". It's questionable whether it really exists or influences offgassing at all. It's a controversial point.

Total gas tension in blood was measured over a century ago, and partial pressures of indivigual gasses more than fifty years ago. There reallly is a pressure difference due to metabolizing oxygen into carbon dioxde. That much is not speculation. The pressure difference certainly influces diffusion and impacts bubble diameter and diffusion out of bubbles. But how much and its impact on decompression schedule is speculation. So we are left with rules of thumb from divers who were human lab rats.