ppO2, FO2, and Washout

[Michael Guerrero]

Kevrumbo

I think you might have been responding to another post in this thread than my question Kev. I think last time I asked a similar question you dug up a thread for 4 or 5 years ago where a guy explained something similar to what I'm saying. I searched but couldn't find that thread.

 

[Kevrumbo]

I think you might have been responding to another post in this thread than my question Kev. I think last time I asked a similar question you dug up a thread for 4 or 5 years ago where a guy explained something similar to what I'm saying. I searched but couldn't find that thread.

Here it is (abridged from the DecoStop in way back in 2004):

Originally Posted by Deepstops2003:

. . .But when you are doing accelerated deco on 100% O2, the PP deficit in the alveloar sacs is as high as you can drive it; indeed the inspired gas has ZERO inert gas in it - on purpose. . . Thus, you will offgas on 100% O2 equally well at 10' as you will at 20'.

Where people "get it wrong" is the GI3 pronouncements about high PO2 "spikes" being somehow good for decompression. That's not really the case - what's going on is that you're reducing the inert gas inspired PP when you make the switch, and coming off a low PO2 your lungs are not loaded up due to pulmonary toxicity. Therefore, your gas exchange is at an optimum - at least for a few minutes. This is not due to the "oxygen window" per-se; the total PP of all gasses inspired and present in the plasma at a given depth, assuming saturation (e.g. you're in equilibrium) is a constant. . . and your metabolic consumption, and thus the moles of O2 consumed by the body is constant across depth change, varying only with workload. . .

The misunderstanding of how this works leads people to say things like:

Quote:
Originally Posted by Chickdiver
It has nothing to do with holding a 10' stop. Deco is done at 20' because the gas gradient ("oxygen window") is most open, and offgassing most efficient.

Nope. There is no difference between a 20' stop and 10' stop, assuming both are made on 100%, in regards to the speed of offgassing. Zero. There is no inert gas in the breathing mix, so the gradient is the same for both. A fraction with a zero as the numerator always produces a zero result, irrespective of the denominator.

Think about it Heather (Chickdiver). You've got inert gas in your tissues; the only thing that matters in terms of decompression is the gradient between the inert gas tensions in your tissues and that in your lungs, since the gas must diffuse out of your system and into the lungs to be expelled. Oxygen, being metabolically active, plays no part in this; other than the toxicity issues you can ignore it.

The converse of this is why Nitrox gives you more no-stop time than air does. The total pressure absolute is not what controls diffusion - it is the PP of the inert gas and its speed of diffusion (e.g. Nitrogen .vs. Helium) that does that. If the total pressure absolute controlled diffusion then breathing Nitrox would offer no benefit in no-stop times, and enriched gas would offer no decompression benefit.

If you're on 100% whether you make your last stop at 20', 10', 2' or 0' (you breathe the O2 on the surface for a while) makes no difference in terms of how quickly you will offgas, since the inspired gas contains no inert gas of any kind. Therefore, the gradient is the same, for all intents and purposes, at all three depths, as equilibrium is a zero PP of Nitrogen (and Helium, if you were diving Trimix.) Critical tensions, however, may be exceeded at shallower depth if you haven't spent enough time doing the initial profile of the deco schedule! [i.g. Worst case omitting for whatever reason, doing the deco stop profile at depth on intermediate deco mixes like 50% starting at 21m/70' for example].

Indeed, if you were to breathe pure O2 at 20' for a very long period of time (assuming that you didn't tox or suffer pulmonary effects) you would effectively "de-nitogenate" your body. You'd do the same thing at 10', 5', 2' and 0', and at roughly the same rate, as the PP differential across the lungs is what matters. . .

The downside to doing the last stop at 20' is that you have to watch CNS loading carefully, as others have mentioned.
_____
Indeed, doing the entire stop at 20' has some undesireable qualities, which the following will illustrate.

Go run Vplanner, set up a dive to 250 ' on 16/50 for 20 minutes, with 50% and 100% deco mixes. Set the last stop to 20'. At +2 conservatism this produces the following profile:

V-Planner 3.40 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM-B
DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2
Dec to 200ft (4) on Trimix 16.0/50.0, 50ft/min descent.
Dec to 250ft (4) on Trimix 16.0/50.0, 60ft/min descent.
Level 250ft 15:10 (20) on Trimix 16.0/50.0, 1.37 ppO2, 89ft ead, 108ft end
Asc to 180ft (22) on Trimix 16.0/50.0, -30ft/min ascent.
Stop at 180ft 0:40 (23) on Trimix 16.0/50.0, 1.03 ppO2, 59ft ead, 73ft end
Stop at 160ft 1:00 (24) on Trimix 16.0/50.0, 0.93 ppO2, 50ft ead, 63ft end
Stop at 140ft 3:00 (27) on Trimix 16.0/50.0, 0.84 ppO2, 41ft ead, 53ft end
Stop at 120ft 2:00 (29) on Trimix 16.0/50.0, 0.74 ppO2, 33ft ead, 43ft end
Stop at 110ft 2:00 (31) on Trimix 16.0/50.0, 0.69 ppO2, 29ft ead, 38ft end
Stop at 100ft 2:00 (33) on Trimix 16.0/50.0, 0.64 ppO2, 24ft ead, 33ft end
Stop at 90ft 3:00 (36) on Trimix 16.0/50.0, 0.60 ppO2, 20ft ead, 28ft end
Stop at 80ft 4:00 (40) on Trimix 16.0/50.0, 0.55 ppO2, 16ft ead, 23ft end
Stop at 70ft 2:00 (42) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 60ft 3:00 (45) on Nitrox 50.0, 1.41 ppO2, 26ft ead
Stop at 50ft 4:00 (49) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (54) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (61) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 26:00 (87) on Oxygen, 1.60 ppO2, 0ft ead
Asc to sfc. (87) on Oxygen, -30ft/min ascent.
Off gassing starts at 191.8ft
OTU's this dive: 116
CNS Total: 83.0%

Now this ain't too cool, because those CNS numbers are a bit high. I've also (intentionally) turned off backgas breaks to make a point; you'd want them on, of course, for a real dive with this profile.

Now let's tell the software that we want our last stop at 10'.

V-Planner 3.40 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM-B
DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2
Dec to 200ft (4) on Trimix 16.0/50.0, 50ft/min descent.
Dec to 250ft (4) on Trimix 16.0/50.0, 60ft/min descent.
Level 250ft 15:10 (20) on Trimix 16.0/50.0, 1.37 ppO2, 89ft ead, 108ft end
Asc to 180ft (22) on Trimix 16.0/50.0, -30ft/min ascent.
Stop at 180ft 0:40 (23) on Trimix 16.0/50.0, 1.03 ppO2, 59ft ead, 73ft end
Stop at 160ft 1:00 (24) on Trimix 16.0/50.0, 0.93 ppO2, 50ft ead, 63ft end
Stop at 140ft 3:00 (27) on Trimix 16.0/50.0, 0.84 ppO2, 41ft ead, 53ft end
Stop at 120ft 2:00 (29) on Trimix 16.0/50.0, 0.74 ppO2, 33ft ead, 43ft end
Stop at 110ft 2:00 (31) on Trimix 16.0/50.0, 0.69 ppO2, 29ft ead, 38ft end
Stop at 100ft 2:00 (33) on Trimix 16.0/50.0, 0.64 ppO2, 24ft ead, 33ft end
Stop at 90ft 3:00 (36) on Trimix 16.0/50.0, 0.60 ppO2, 20ft ead, 28ft end
Stop at 80ft 4:00 (40) on Trimix 16.0/50.0, 0.55 ppO2, 16ft ead, 23ft end
Stop at 70ft 2:00 (42) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 60ft 3:00 (45) on Nitrox 50.0, 1.41 ppO2, 26ft ead
Stop at 50ft 4:00 (49) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (54) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (61) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 10:00 (71) on Oxygen, 1.60 ppO2, 0ft ead
Stop at 10ft 16:00 (87) on Oxygen, 1.30 ppO2, 0ft ead
Asc to sfc. (87) on Oxygen, -30ft/min ascent.
Off gassing starts at 191.8ft
OTU's this dive: 108
CNS Total: 56.3%

Notice anything interesting?

First, our CNS is now well within reasonable limits. The insult to your body has been decreased. This is a good thing, all-up.

Note that the 10' and 20' stops add to the original 26 minutes.

The offgassing gradient on your 100% has not changed with the reconfiguration of the software to use a 10' stop!

That is because you have changed nothing in terms of the gradient of dissolved inert gas to inspired gas, because you can't make the inspired inert gas percentage less than zero!

Also note that the CNS exposure is cut by one third by making the last stop at 10'. This is because you are severely penalized for high PO2s, and that 26 minutes on 100% at a 1.6 PO2 is "expensive" in terms of CNS loading.

The latter profile is actually preferrable for this reason IF you can hold the 10' stop. Where its dangerous is if you can't - and find yourself at 5' due to surge. During some part of that 10' time an incursion above 10' could result in taking a DCS hit.

But the latter profile gives you an interesting option that many people miss in this debate about "which way do I run these profiles, with a 20' or 10' final stop?"

Do you see what it is?

You should. Go back and look at it again before you read on....

.............................................

Let's say you plan the dive on the latter table, but you encounter 4-6' seas when you get to the dive site. Holding a 10' stop is going to be difficult! So what? Make the final stop at 15' instead of 10'. It changes NOTHING. Indeed, once the first 10 minutes are gone at 20', you can be anywhere between 10-20' for the remainder of the deco time and you have changed nothing in terms of the total obligation you must serve, as the gradient, once on 100% O2, does not change with depth.

This is because the PP of the inert gas you are inspiring is zero. As such, the gradient does not change with depth. Therefore, the controls on your required position in the water column are the PPO2 toxicity limit (20') and the leading tissue's overpressure. Anywhere between those two points produces the same amount of offgassing over a given amount of time, and is perfectly fine. . .

 

[Gareth J]

You didn't even attempt to answer my question Gareth.

Michael

There seemed to be a lot of confusion over terminology, percentage mix, po2 and toxicity issues around oxygen. Before attempting to answer any questions, I thought it important to clarify what we where talking about.

I would second the recommendation regarding the Deco for Divers book, I believe there is a new issue than the one I own.

In simple terms you are attempting to flush out the Nitrogen.
The higher the pressure gradient between the Nitrogen in the tissue and the Nitrogen in the blood the faster this will occur.
Achieving he pressure gradient can be achieved in two ways;
1. Reducing the ambient pressure (Ascending).
2. Reducing the Nitrogen in the blood.

Obviously, reducing the ambient pressure is the preferred and fastest approach. EXCEPT, that you want the Nitrogen to remain in solution, not return to its gaseous state whilst still in the tissue or circulation system, i.e. uncontrolled decompression. The limiting factor is what decompression models refer to as the M value, the pressure differential where a gas dissolved in solution returns to its gaseous state. Differing gases have differing M values. It should also be noted hat decompression models assign differing M values for differing tissues.
If the M values allow it you ascend.

Because we are aware that we will be limited by an M value during the ascent phase, we apply the second approach as well, we change the gases in the blood stream to increase the pressure differential to flush the Nitrogen (or other gases) out of the tissue. The simplest approach is to use oxygen, unfortunately we also need to minimise the bodies exposure to high partial pressures of O2 to avoid fitting, and minimise long term exposure to avoid other complications, including pneumonia. Oxygen does have one advantage is that the tissues metabolise O2, which reduces the risk of tissue damage, and decompression complications related to the O2 itself.

Also remember, we don't really understand what is happening during decompression. The physiology is yet to be understood. The large number of decompression models reflect our lack of understanding of what is really happening.
Decompression models are based on a series of rules that have been developed through observation of effect. We know if you do X, Y is likely to occur, we understand that Y is the effect of bubbles in the tissue/circulation system. We know we cause this by reducing the ambient pressure and raising the pressure differential. We can demonstrate it in simple terms with a coke bottle. But we don't fully understand why the body reacts differently at different times. You can repeat the same dive a number of times with no adverse effect, then you suddenly get an adverse effect.
We know diver X appears to be able to do the dive without effect, but diver Z is bent immediately.
The solution to the unknowns has been to modify the M values, hence the proliferation of models, A, B, C etc as we adjust the model based on the number of bends we see. (Or create new models applying a different principles).
During the 70's models where actually tested on individuals, now this is not ethically acceptable, and we have a lot more data, so mathematical modeling is applied.

Gareth

 

[Michael Guerrero]

Anywhere between those two points produces the same amount of offgassing over a given amount of time, and is perfectly fine. . .

That's why you're my hero Kev. Do you have some kind of wayback machine? Who is Deepstops2003?

And thanks for posting this. Seriously. It should be a sticky in my opinion.

 

[Michael Guerrero]

Michael

There seemed to be a lot of confusion over terminology, percentage mix, po2 and toxicity issues around oxygen. Before attempting to answer any questions, I thought it important to clarify what we where talking about.

Gareth

Thanks Gareth. As far as Powell's book, I have it and have read it several times. I'm acutally Nitrox, Advanced Nitrox, Deco Procedures, and soon to be Trimix trained. I've also done a substantial amount of study outside of any formal course on physiology, gas kinetics, bubble vs. dissolved gas models, and how they relate to diving. I think if you go back and read my initial post you will see that my terminology was correctly used and that the question was actually straightforward. I think subsequent posts muddied the waters.

Anyway, Kev posted the link details I was looking for, which reaffirmed my initial assertion that there is no inherent benefit to a high ppO2 from an accelerated decompression perspective. It is the FO2, and therefore the fraction of inert gas displaced, that matters in influencing the rapidity of inert gas elimination.

So, all that said. ppO2 is about safety, and FO2 is about deco efficiency.

 

[Beau640]

If you are talking about 100% O2, then what you are saying is right. However it doesn't really apply to less than 100% O2 mixes.

There is a benefit from a high partial pressure of a gas from an accelerated decompression perspective. If you have actually read mark power's book multiple times like you assert, re-read pages 32 and 33.

The rate of off gassing of a particular gas is related to it's half time. Once a tissue compartment becomes supersaturated, it will off gas 50% of that supersaturation amount in it's half time essentially (poorly worded explanation I know). So for example, if a tissue compartment becomes supersaturated by 1 bar, in it's half time it will have reduced to 0.5 bar. An absolute difference of 0.5 bar. However if you ascend and supersaturate a compartment by 2 bars, it will reduce to 1 bar in it's half time, a difference of 1 bar.

To off gas, like Gareth said, you can either ascend to create an inert gas gradient (difference between tissue gas tension and inspired gas tension) or you can change the mix of gas you are breathing. If you are not changing the mix of gas you are breathing, your goal would be to get as shallow as possible without crossing the M line to increase the amount you are off gassing. If you are changing mixes to say 50% O2, you would still want to get as shallow as possible but keeping in mind you can't cross the M line, and you can't use a mix that would put you over 1.6 ppO2.

Everything you've been asserting is based on a 100% O2 mix. However with a lower mix it would be slightly different. The advantage to choosing a ppO2 of 1.6 rather than 1.4 when you are doing deco with a non 100% mix is that it allows you to choose the highest % O2 mix for the depth you will deco at. This would be the shallowest depth you can be close to the M line so that you can have the max inert gas gradient and have the highest efficiency with off gassing by taking advantage of both reductions in ambient pressure and reduction in N in your mix. With a non 100% O2 mix, your assertion that there is no relationship between ppO2 and off gassing efficiency is slightly wrong. It is true that the ppO2 itself does not relate to nitrogen off gassing. However maxing ppO2 at 1.6 in a non 100% mix allows you to get the highest O2 mix possible for your deco to have the highest inert gas gradient (essentially maxing ppO2 at 1.6 with non 100% O2 mix will give you the least amount of nitrogen in your breathing gas to help off gas quicker). If you sat at the M line and chose a gas mix with a ppO2 of 1.4, you would have more N in your gas and would off gas slower. If you had that same mix discussed before at 1.6 but then just tried to go shallower to 1.4, you would have crossed the M line and would be in danger (since you should have chose the highest mix for your deco depth at the shallowest you could safely be).

ppO2 is related to O2 safety. FiO2 when increased increases deco efficiency. However to say that ppO2 is not related at all to deco efficiency or a high ppO2 does not offer a benefit to deco is wrong if you are talking about less than 100% mixes. This comes into play when you have multiple deco stops rather than just one stop with 100% O2

 

[Michael Guerrero]

I don't think you're correct Beau640. ppO2 has nothing to do with M-values. M-values represent critical supersaturation thresholds of inert gas dissolved into tissues. ppO2s are based on fraction of inert gas in a breathing mix at a particular depth. You keep talking about ppO2, but the benefits you're really describing are fraction of O2. The fact that a certain ppO2 would be best for a given depth is because it's the highest fraction of O2 you can breathe "safely" at that particular depth. 100% Oxygen is actually the perfect gas to prove this because, as Kev posted above, you off-gas just as efficiently at a ppO2 of 1.3 vs. 1.6 when using pure Oxygen (assuming no interference from additional bubbling). I think that pretty clearly shows that it's the fraction of O2 which is important, not the ppO2.

You're kind of making a circular argument above, but the bottom line you get to is that a ppO2 of 1.6 is really about the safety of the oxygen content of a breathing gas, and that a ppO2 of less than 1.6 is sub-optimal for a given depth. However, the reason it's sub-optimal is because you could "safely" increase the fraction of oxygen in your breathing gas (only potentially, of course, based on the exposure you have already experienced or plan to--many rebreather divers will reduce their ppO2 to 1.3 or 1.0 for that very reason) to off-gas more quickly.

 

[Beau640]

I don't think you're getting it but at this point I can't explain it any other way. Your thought process and rationale is only focusing on 100% O2 mixes rather than understanding the physics and physiology that would be applicable to all O2 mixes. I never said that ppO2 effected the M value in any way. The M value is the M value and doesn't change. However when gas and deco planning, both the M value and what you plan to have your ppO2 at for your deco depth play into what FiO2 you choose. We are talking about dives that have multiple deco stops and require multiple gas mixes for decompression, not just a dive with a single 100% O2 deco tank.

Again, as Gareth said:

To off gas, you can either ascend to create an inert gas gradient (difference between tissue gas tension and inspired gas tension) or you can change the mix of gas you are breathing. If you are not changing the mix of gas you are breathing, your goal would be to get as shallow as possible without crossing the M line to increase the amount you are off gassing. If you are changing mixes to say 50% O2, you would still want to get as shallow as possible but keeping in mind you can't cross the M line, and you can't use a mix that would put you over 1.6 ppO2.

Again, re-read pages 32 and 33 and focus on understanding how a partial pressure of a gas relates to the inert gas gradient. We are making these choices to maximize the gradient for nitrogen without causing critical supersaturation or causing oxygen toxicity.

At this point I'm going to call in backup from @tbone1004 and @DevonDiver who both have tons more experience than me and are great at explaining things. I'm sure i've misspoken/incorrectly stated some things and caused some confusion and they would be better at potentially clarifying the core issue that you are getting at.

 

[w3dge]

I think he understands what you're trying to say perfectly, but you aren't explaining it too well.

 

[Beau640]

I know i'm not explaining it well! It's so much easier with pictures.