ppO2, FO2, and Washout

[Beau640]

Someone else will probably do a much better job at explaining this than I could, but it's really hard to explain without pictures/charts. I'd recommend besides taking the proper deco procedures course which will cover all of this, reading the book deco for divers by mark powell. Excellent easy to read book on decompression theory and why we deco the way we do.

Also check out Dive Rites post Gradient Factors | Dive Rite

 

[tbone1004]

Just to clarify, from an off-gassing perspective (not bubble formation, waves, etc.), is there any benefit to remaining at 20ft (1.6) on 100% in your example vs. moving to 10ft (1.3) on the same 100% O2? In other words, is there any inherent benefit from off-gassing at a high ppO2 vs. a lower ppO2 if the fraction of O2 is the same?

My understanding is that there isn't because you have already maximized the gradient associated with fraction of O2 in your breathing gas.

no because the gradient across the tissues is the same, but like @hroark2112 said, you go up too soon and you'll bend yourself. I leave my Petrel set for a 10ft final stop and while I don't always have the ability to go up to 10ft, I will try to get as close to it as possible for CNS counting

 

[high/baric]

Just to clarify, from an off-gassing perspective (not bubble formation, waves, etc.), is there any benefit to remaining at 20ft (1.6) on 100% in your example vs. moving to 10ft (1.3) on the same 100% O2? In other words, is there any inherent benefit from off-gassing at a high ppO2 vs. a lower ppO2 if the fraction of O2 is the same?

My understanding is that there isn't because you have already maximized the gradient associated with fraction of O2 in your breathing gas.

Just to clear one thing up, since you keep coming back to the oxygen fraction and ppO2. From a technical perspective, when off-gassing nitrogen the oxygen fraction of the breathing mix is completely irrelevant. The reason people use 100% oxygen for decompression is obviously because we need oxygen to live, with the added benefit that our bodies metabolise it and so it doesn't build up like inert gasses. If you could breathe for example 100% helium or 100% hydrogen it would be just as effective at off-gassing nitrogen, although you would be replacing it with another inert gas, to some extent. Only the partial pressure of nitrogen and more specifically the difference in nitrogen partial pressures matters, as some people have mentioned and well explained.

Short (and very simple) answer: the most effective way to off-gas purely nitrogen is to breathe any gas without nitrogen, the fraction or pp of oxygen in the mix is in itself irrelevant. Thanks to our physiology, breathing 100% oxygen is just the most convenient way to do this.

Edited for clarity.

 

[Kevrumbo]

The maximum ppO2 that you can be exposed to knowingly and with consent is 2.8 in dry Hyperbaric Oxygen Therapy.

In other words, the only reason to be exposed to a higher ambient pressure than 1.6 ATA on pure Oxygen is if you were undergoing a Treatment Table 6 in a Recompression Chamber for DCS (a ppO2 of 2.8 ATA at 18m/60'), squeezing pathological bubbles to 70% of their original acute symptom-causing size. For Arterial Gas Embolism Treatment Table 6A, it would still be a ppO2 of 2.8 ATA, but breathing Nitrox 47% at a much higher ambient pressure of 6 ATA (50m/165'), squeezing those bubbles to 55% -or nearly half- of their original potential stroke causing/blood vessel occluding size.

Controversially, if you choose to electively undergo riskier In-Water-Recompression for DCS type l, the modified Australian IWR method has you breathing pure O2 at 9m/30' for a ppO2 of 1.9 ATA (10 minutes on O2 with 5 minutes Air breaks), squeezing the symptom causing bubbles to 80% of their original size.

Finally with regards to Oxygen Toxicity and CO2: physical exertion with associated labored & inefficient breathing through a Scuba regulator impairs the normal increase in ventilation of the lungs which is how we expel excess CO2. If we don't ventilate the lungs enough, and therefore don't get rid of the CO2 we are producing, then body CO2 levels rise (CO2 retention). When CO2 levels rise, this markedly increases the circulation of blood through the brain. And so breathing pure Oxygen or a significantly high FiO2 at depth concurrently with a CO2 retention cycle therefore results in a higher delivery of oxygen to the brain tissue --obviously, this increases the chances of Ox-Tox symptoms/seizures. So the takeaway point is to be relaxed as best you can with nominal respiration when you make your switch to a high FiO2 deco gas.

 

[Michael Guerrero]

no because the gradient across the tissues is the same...

I'm pretty sure this is the right answer, thanks TBone. I had a post on here a while ago but I couldn't find it.

 

[Michael Guerrero]

I think that the FO2 is very important but its also inextricably linked to PPO2.

This is a true statement because ppO2 is a product of FO2 and depth. What I'm saying is that ppO2 is not directly involved in washout efficiency. Instead, it's the FO2, which serves to displace the FN2 and FHe in a breathing mix, that is directly involved in washout efficiency. This is true because a greater FO2 necessarily means a lower FN2/FHe at a given depth and therefore a lower inspired ppN2/ppHe, resulting in a larger gradient between the tissue pressure and the pressure in the arterial blood, leading to a transfer of inert gas into the bloodstream and elimination through the lungs.

 

[Michael Guerrero]

If you take deco'ing on 100% O2, there are zero N2 molecules in that gas. Therefore, it doesn't matter what the ppO2 of that gas is (will obviously vary with your depth) the (N2) gradient will remain the same.

I would have also accepted "yes."

 

[Michael Guerrero]

The Fi(of whatever gas) only has to do with the rate of gas exchange/off-gassing/on-gassing in that it increases the partial pressure of your gas as the atmospheric pressure increases.

I'm having a hard time answering your initial question because FiO2 and ppO2 are different things. Related, but different.

Sorry, I thought I made clear in my original post that FO2 and ppO2 are different. I think what you said right above that was "yes."

 

[Michael Guerrero]

Gareth

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

 

[Michael Guerrero]

It depends on how much nitrogen is in your blood/tissues when you get to your 20' stop. Progressing too quickly to 10' can cause too much nitrogen to leave your tissues & blood and potentially cause DCS.

I specifically said not bubbles in the post above. You're talking about bubbles. I agree 100% that you need to do an adequate amount of deco at a particular ambient pressure to avoid critical supersaturation and exceeding m-values. The question I was asking was in relation to a higher or lower ppO2, but I could have clarified that you could safely move to a 10ft stop without exceeding any m-values.