ppO2, FO2, and Washout

[tbone1004]

@Beau640 and @Michael Guerrero, took a while to read back through the last couple of pages and trying to figure out what was going on.
@Beau640 reread post #16.

I think you two are both trying to say the same thing but using the wrong gas to say it with. O2 is not the right gas to be using when talking about decompression imho. It is the gas we end up using to talk about decompression, but that doesn't necessarily make it the right one. We use O2 because that is the "third gas" in the mixes we breathe and we trying trying to get rid of the other two. You want to use as rich of an O2 mix as possible not because of pO2 or fO2 but because of lack of pN2/pHe or fN2/fHe.

Now, imho, you want to avoid using non 100% mixes as much as possible for the final deco gas. I understand there are times when that is not possible due to surge, but in general you want to get off inert gasses ASAP and get as shallow as possible ASAP. If you have to deco out on 80/20 then that's fine. I would switch when the 30ft stop shows up if it shows up, but I'd get as shallow as I can as soon as I can.

So TLDR, higher pO2's get you on the rich mixes faster which obviously accelerates the deco. The risk here is the rapidly ticking CNS clock so while I'm OK with switching to deco mixes as soon as you can, and I do, I also try to get as shallow as possible as soon as possible. I subscribe to standard gasses, so I'm not going to sit there and calculate a perfect mix for 27ft if that is the deco area because it might save me a few minutes but isn't worth the hassle. I'll stay on 50/50 and deal with it. Is it ideal? no, but it's not worth the hassle to have a weird mix. Someone hopefully will have a deco planner handy and can run a normal deco schedule with 50/50 vs O2 to check, but the important part is get shallow ASAP. DIR people obviously disagree with this, but I don't believe in deep stops, so that's my opinion.

 

[Michael Guerrero]

I was trying to scope this conversation to specifically talk about deco with O2 and our choices of where we stop when using O2. It wasn't meant to be a general discussion about decompression theory, gas choices, nor a discussion about gas kinetics per se.

I had hoped that the subject line of the post would make it clear that I'm specifically talking about the decisions we make when decompressing with O2, as they relate to FiO2 and ppO2. Based on that, I'll restate what I think one more time, hopefully in a way that makes what I'm driving at clear.

ppO2 and FiO2 are inherently linked, as ppO2 is the product of FiO2 and aveolar pressure. When considering what our O2 deco mix should be, we think about ppO2 mostly for safety reasons, because that allows us to have the highest FiO2 in our mix without (hopefully) risking ox tox. However, it is really the increased FiO2 that we're after, because the increased FiO2 is what displaces inspired inert gas, increasing the gradient between tissue tension and inspired inert gas partial pressure. Yes, reducing ambient/aveolar pressure also produces a gradient, but is outside the scope of this thread.

Thus, there is no inherent benefit of remaining at a higher ppO2 of inspired gas, if one can safely move to a lower ppO2, provided the FiO2 remains the same. Using EAN50 as an example, if we're clear to move to a 40ft stop, there is no inherent benefit to staying at 70ft to capitalize on a high ppO2 of 1.6 vs. moving to the shallower stop, which has a lower ppO2 but which increases the supersaturation/inspired inert gas gradient. Likewise with pure Oxygen. It does us no good to stay at the 20ft stop with a ppO2 of 1.6 if we can safely move to the 10ft stop, because we have maximized the inert gas gradient at that point.

I'm not factoring in bubbling or environmental conditions here that might make a 20ft stop more desirable. I am also trying only to compare decisions about ppO2 and the underlying FiO2, not anything about ascent rates or different mixes. The scope of this thread is purposely narrow to try and answer a specific question based on a discussion with another person. Hopefully what I've said above is clear.

 

[Beau640]

Looks like everything you said is correct if you assume what you said, "I'm not factoring in bubbling or environmental conditions here that might make a 20ft stop more desirable." You do need to factor in bubbling when you consider if there's a benefit to deco'ing at a higher ppO2. The, "Thus, there is no inherent benefit of remaining at a higher ppO2 of inspired gas, if one can safely move to a lower ppO2, provided the FiO2 remains the same," wouldn't be necessarily true.

I think it's chapter 5 if I'm remembering correctly in Deco for Divers that discusses the oxygen window and partial pressure vacancy. There's some thought that by deco'ing at a ppO2 of 1.6, you are maximizing your oxygen window and thus reducing micro bubble formation and helping shrink already formed bubbles. Oxygen window here would be defined as the difference in ppO2 between the arterial and venous side secondary to consumption. Various sources both in deco for divers, on this board, and through GUE discuss the possible benefit of this definition of the O2 window as it relates to bubble stabilization, formation, and dissolution. There may actually be a benefit from deco'ing at the higher ppO2. Others have also argued that deco'ing at 20ft instead, assuming you aren't worrying about CNS %, can help reduce bubble formation 2/2 the increased ambient pressure.

So for your question, if you are only focusing on time in the water/time to off gas, then what you're saying is true. But if you begin to consider other factors that would effect safety, then there would be benefits to being at the higher partial pressure of O2.

 

[Michael Guerrero]

See my previous posts for acknowledgment about the difference between arterial and venous pressures due to a high partial pressure. Again, it is a distracting detail not germane to the scope of the thread or my intent.