Calculating deco and mix

[texdiveguy]

A no-stop dive by definition has no required decompression. So shifting to a 50/50 ean mix for an ascent from a no-stop dive really does nothing at all. The only time using an additional decompression mix is warranted is when there is required decompression that you want to reduce the time for.

Cheers

JDS

Joel,,, this is ScubaBoard dude...PROVE IT!

:mooner:

 

[TSandM]

Actually, Joel, I think the OP wanted to know if he could reduce surface interval or no-fly time by doing his ascents on enriched mixes, which he probably can, since the same time spent underwater breathing backgas or deco gas will result in different tissue N2 tensions on surfacing. But it isn't worth it, in terms of expense, hassle factor, or the very real risk of oxygen toxicity.

 

[do it easy]

Yikes, you don't believe the world is flat too do you? Just kidding.

Try this:

"Gas Exchange, Partial Pressure Gradients, and the Oxygen Window" by Johnny E Brian, Jr., M.D., Associate Professor, Department of Anesthesia, University of Iowa College of Medicine.

You won't just be able to skim that baby' in the bathroom, trust me.

I will expect a single page book report, double spaced.....(just kidding)

From that paper:

In theory, He off-gassing should not altered by breathing air, 50% nitrox or 100% O2 during decompression from a He dive. He elimination during air or O2 decompression from a He-based dive has been measured, and the decompression gas did not affect the rate or volume of expired He (4). In another study at 1 ATA, tissue N2 washout is not different during O2 or heliox breathing (3). Both studies are consistent with the physics of gas diffusion in solution, where the presence of a second non-metabolic gas does not slow diffusion of the first on metabolic gas. The reality is that at any given ambient pressure, regardless of the size of the oxygen window, as long as there is no inspired He, the rate of He off-gassing will be unchanged.

Breathing pure O2 will drive off inert gasses, but I still don't think that there is a synergistic effect from pure O2.

 

[Bismark]

From that paper:

Breathing pure O2 will drive off inert gasses, but I still don't think that there is a synergistic effect from pure O2.

Holy taking things out of context Batman! What about:

"If a gas mixture with less than 100% 02 were breathed, then some of the oxygen window would be occupied by the inert gas.For example, if a 50% or 80% nitrox mixture were breathed at 20 fsw then N2 would occupy some of the partial pressure of the oxygen window. How much of the window would be occupied depends on the tissue N2 partial pressure, which will determine how much N2 would diffuse from blood into tissue and the resulting pvN2. The oxygen window would be reduced by an amount equal to the PvN2"

Your quote immediately follows the authors comments regarding most current decompression models where he states that they are perfusion-limited models, which he pretty clearly states are incomplete. Moreover, if you read the page with your quote and the preceeding page again I think you will find that their is pretty strong evidence in favour of perfusion actually being an important part of the whole picture. Thus, your quote taken out of this context is incorrect.

Moreover, my take on "...as long as there is no inspired He, the rate of He off-gassing will be unchanged." doesn't mean the 02 window is a non-event but rather that the He component of a given mix will not change, however the N2 component of what you have been breathing will. After all, we aren't just trying to remove He from our body when we have been breathing 21/35 or any gas with an N2 component.

Thats my take on it anyways, maybe an anesthestist could chime in here.

I bet you don't believe in Santa or the Easter bunny either

 

[do it easy]

Your quote immediately follows the authors comments regarding most current decompression models where he states that they are perfusion-limited models, which he pretty clearly states are incomplete. Moreover, if you read the page with your quote and the preceeding page again I think you will find that their is pretty strong evidence in favour of perfusion actually being an important part of the whole picture. Thus, your quote taken out of this context is incorrect.

In that same quote, he also references some actual studies that presumably support the off-gassing component of the incomplete perfusion models.

Moreover, my take on "...as long as there is no inspired He, the rate of He off-gassing will be unchanged." doesn't mean the 02 window is a non-event but rather that the He component of a given mix will not change, however the N2 component of what you have been breathing will. After all, we aren't just trying to remove He from our body when we have been breathing 21/35 or any gas with an N2 component.

Thats my take on it anyways, maybe an anesthestist could chime in here.

I bet you don't believe in Santa or the Easter bunny either

Of course- breathing pure O2 reduces the inert gases, He and N, simultaneously. I don't disagree with those benefits, I just don't think the O2 window is a magical event that sucks the inert gas out of your body. You could breathe 13% O2 and 87% argon at the 20' stop and the deco would be the same, although I don't believe that enough to actually try it Perhaps we are just splitting hairs over semantics?

Bottom line is that breathing pure O2 works for decompression, but I don't believe it's a one-hit-wonder gas- you still have to put the time in.

 

[limeyx]

In that same quote, he also references some actual studies that presumably support the off-gassing component of the incomplete perfusion models.


Of course- breathing pure O2 reduces the inert gases, He and N, simultaneously. I don't disagree with those benefits, I just don't think the O2 window is a magical event that sucks the inert gas out of your body. You could breathe 13% O2 and 87% argon at the 20' stop and the deco would be the same, although I don't believe that enough to actually try it Perhaps we are just splitting hairs over semantics?

Bottom line is that breathing pure O2 works for decompression, but I don't believe it's a one-hit-wonder gas- you still have to put the time in.

well, according to me (extremely poor) understanding of the O2 window (and I am not sure I agree with it either, or even have it right)

1) it is nothing to do with Pure O2
2) the argon would not give the same benefit because you dont metabolize it/dissolve in plasma/haemoglobin type stuff. or something like that.

I *think* the O2 window theory goes something like that the N2 and He can escape quicker than just by the gradient itself as the O2 creates a vacancy due to the way it is processed in the body, which allows a greater "gap" between the intert gases in the body, and the gas you will exhale.

Wow, I probaby butchered that royally. I never honestly did understand it, I just happened to find that whatever basis it came from, the numbers and strageties we covered with GUE work for me, my teams and a lot of other people and are about as simple and easy to use as I think we are going to get. SO I use them

 

[wedivebc]

Yeah what is so magical about 1.6 ata of pure O2? For that matter what benefit do you get from pure O2 if you have to go back on air every 15 minutes or so to prevent your lungs from burning? I don't get some of these practices at all.

 

[Bismark]

well, according to me (extremely poor) understanding of the O2 window (and I am not sure I agree with it either, or even have it right)

1) it is nothing to do with Pure O2
2) the argon would not give the same benefit because you dont metabolize it/dissolve in plasma/haemoglobin type stuff. or something like that.

I *think* the O2 window theory goes something like that the N2 and He can escape quicker than just by the gradient itself as the O2 creates a vacancy due to the way it is processed in the body, which allows a greater "gap" between the intert gases in the body, and the gas you will exhale.

Pretty much bast**dizes, I mean sums it up

My understanding is that it works better for high pp02s and at a certain pp02, there is no great benefit. I am sure it would work better at pp02's higher than 1.6 but then we have to weigh the benefit against 02 tox. Hence, 50/50 at 70ft and then maybe 02 at 20'. My understanding is also that the more non-02 you take in, (because those inert gases are not metabolized at all) there is not as much "space" left for gases to leave your system with your exhalations. If you wondered why we (at least Lymex and I) don't spend much more than 4 minutes at 70' (using RD for Tech I level dives) it is because while you are ongassing some, the it takes about 3-4 minutes for your entire blood supply to transit your heart/lungs and after that, at that pp02, the oxygen window starts to close and your benefit of 02 window versus ongassing becomes one in favour of moving up and getting the pressure gradient back to work. Bumping your pp02 to 1.6 again at 20' with pure 02 effectively re-opens the 02 window and starts the process again.

I had this explained to me with the "milk delivery truck" analogy that may have originated with AG. Not sure though.

Lymex, I probably carved this up also but I am going to spend tomorrow (when I am supposed to be working) looking at this deeper. I will try to summarize it again tomorrow after a bunch more research and reading. As I was writing this I realized that I was not happy with my understanding of the concept either. Unless I can explain it clearly to others, I don't feel I have a firm grasp on it. I will check with some doctor/health professionals and get back to this post. Might take until Monday.

 

[Charlie99]

My understanding is not that there is anything magic about 1.6ata ppO2.

Breathing O2 or a high FO2 mix is simply a way of having a higher total ambient pressure to reduce bubble formation, while at the same time having lower inert partial pressures to enhance offgassing. The 1.6ata ppO2 comes into play because that limit beyond which most people feel that the oxtox risk exceeds any decompression benefit (except for dry habitat deco, where the consequences of oxtox are not as severe, so 2.0ata or higher is often used.)


What IS magic is the reduction in inert gas partial pressure you get when you have oxygen in the mix (in addition to the obviously beneficial aspects such as supporting life)

Or to put it another way --- since the body metabolises O2 it doesn't have to offgas it, and there isn't any decompression penalty associated with loading up on O2. So any partial pressure you can fill with O2 rather than an inert gas, you will be out ahead.

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Yet another way of saying it is that 1) bubble formation and DCS problems are caused by the relationship between the sum of inert partial pressures compared to the ambient pressure. 2) Offgassing of each inert gas is proportional to the difference between the tissue tensions of that gas, and the partial pressure of that gas in the breathing mix.

Having high ppO2 lets you have a higher ambient pressure (i.e. stay deeper) for the same levels of inspired inert gas pressures. Or alternatively, at the same depth, having the higher FO2 mix means that the partial pressure of the inert gases are lower and therefore offgassing is faster.


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To me, it has always seemed that the "oxygen window" explanation was just a shorthand or a lazy man's way to explain offgassing using high FO2 mixes.

 

[Bismark]

My understanding is not that there is anything magic about 1.6ata ppO2.

Breathing O2 or a high FO2 mix is simply a way of having a higher total ambient pressure to reduce bubble formation, while at the same time having lower inert partial pressures to enhance offgassing. The 1.6ata ppO2 comes into play because that limit beyond which most people feel that the oxtox risk exceeds any decompression benefit (except for dry habitat deco, where the consequences of oxtox are not as severe, so 2.0ata or higher is often used.)


What IS magic is the reduction in inert gas partial pressure you get when you have oxygen in the mix (in addition to the obviously beneficial aspects such as supporting life)

Or to put it another way --- since the body metabolises O2 it doesn't have to offgas it, and there isn't any decompression penalty associated with loading up on O2. So any partial pressure you can fill with O2 rather than an inert gas, you will be out ahead.

----------------

Yet another way of saying it is that 1) bubble formation and DCS problems are caused by the relationship between the sum of inert partial pressures compared to the ambient pressure. 2) Offgassing of each inert gas is proportional to the difference between the tissue tensions of that gas, and the partial pressure of that gas in the breathing mix.

Having high ppO2 lets you have a higher ambient pressure (i.e. stay deeper) for the same levels of inspired inert gas pressures. Or alternatively, at the same depth, having the higher FO2 mix means that the partial pressure of the inert gases are lower and therefore offgassing is faster.


-----------------------

To me, it has always seemed that the "oxygen window" explanation was just a shorthand or a lazy man's way to explain offgassing using high FO2 mixes.

From my take on it, your explanation is correct but incomplete. I will chime back in by monday with either some new stuff or my head on a platter.......