ppO2, FO2, and Washout

[Michael Guerrero]

I was talking with someone recently about deco and using O2. We had different ideas of how these things interplay and how they affect off-gassing.

My understanding after reading a lot of different material is that ppO2 is about safety, and FO2 is about off-gassing. If I want to maximize the benefits of accelerated decompression using O2, that involves increasing the fraction of it in my breathing gas, thereby offsetting the fraction of the inspired inert gasses present. The ppO2 really has no part to play in this transaction, as gasses come into and out of solution independent of each other.

ppO2 comes into play from a safety perspective, in that the higher the ppO2 for a given depth the more likely it will cause a toxicity or pulmonary issue. A ppO2 of 1.6 during deco represents the highest fraction of oxygen that is "safe" to breath at a depth, but remaining at a high ppO2 is not beneficial in and of itself if the fraction of O2 remains constant, outside of perhaps reducing the likelihood of bubbling through increasing the pressure gradient between arterial and venous blood.

If I want to maximize my inert gas washout, I should maximize the fraction of O2 I am breathing at any given depth. Thus, washout at 10 ft on 100% O2 is just as efficient as washout at 20 ft, with perhaps the exception of a greater likelihood of bubbling at 10 ft vs. 20 ft. It is therefore the fraction of O2, not the partial pressure, that matters.

 

[RainPilot]

My understanding is that its the P(inert) vs the P(tissue) that is the issue. The higher the difference between the PP(inert) of the gas you are breathing and the P(tissue) of the inert gas you have absorbed, the greater the efficiency of off gassing. With 100% O2, then you are correct as the shallower you get the faster you will offfgas and the F(inert) and the PP(inert are the same, zero. However, obviously you need to control that rate and balance off-gassing efficiently vs bubble formation / growth etc. Thus the practice of a 20' stop on O2 to allow the maximum gradients while slowing down bubbles with some depth, followed by a 10' stop to reduce PPO2 exposure a bit again allow some more offgassing before surfacing.

Once you move to a EANX with some percentage of N2 in, then the PPN2 will have an influence. Lets say you deco on 50% at 20m. The PPN2 will be 1.5 ATA, compared to a tissue N2 of (for sake of calculation) 1.6 ATA. With a 0.1 difference in pressure, the nitrogen is not going to be in a hurry to go anywhere, however the O2 window will still assist as you metabolize O2.

Now take that same bottle up to 10m. PPN2 is now 1 ATA with a 0.6 difference, the off gassing would then be quicker as the gradient is higher.

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

 

[Beau640]

You are right that the ppO2 and the FiO2 are linked.

The FiO2 is the fraction of inspired oxygen. It is the oxygen of the gas you are breathing. If I put someone on nasal cannula on land, depending on how much I turn up the flow I can increase the amount of FiO2 they are receiving. If I put someone on an O2 mask on land, I can attempt to reach 100% FiO2.

FiO2 in diving would be the O2 percentage in your cylinder since that is the only air that reaches your lungs (on land you suck in some ambient air and it mixes with the supplemental air). If you are diving a 100% O2 mix, your FiO2 would be 100%.

The ppO2 is the partial pressure of oxygen dissolved in the blood. Pressure differences is what determines when gas goes from blood to tissue or from tissue to blood.

In general, these two things are linked. As you would imagine, increasing the FiO2 of what you are breathing will increase the ppO2. However the ppO2 of a person for a given FiO2 is different for each individual as it also depends on the characteristics of your lungs and how easily oxygen can diffuse across them. It also depends on different metabolic characteristics such as your temperature, acid/base status, etc.

Someone else can explain how these are linked to deco. My knowledge is not advanced enough to supply an explanation I would find acceptable.

Source: I am a physician and regularly work with ventilators.

 

[w3dge]

In general, these two things are linked. As you would imagine, increasing the FiO2 of what you are breathing will increase the ppO2. However the ppO2 of a person for a given FiO2 is different for each individual as it also depends on the characteristics of your lungs and how easily oxygen can diffuse across them. It also depends on different metabolic characteristics such as your temperature, acid/base status, etc.

ppO2 (in these terms) is the partial pressure of oxygen in the gas you are breathing at a given ambient pressure (depth). So if you are breathing 100% O2 on the surface, your ppO2 is 1ATA. 10 meters, 2ATA etc etc.

To the OPs question - deco efficiency is essentially governed by the gradient between the number of molecules of the inert gas in the inspired gas, compared to the number of molecules dissolved in the tissues. The less N2 molecules in the breathing gas, the greater the gradient, the faster you off gas - to put it simply.

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.

If you take deco'ing on 50% O2, as you descend the ppO2 will obviously increase, increasing the number of O2 molecules inspired. But so will the ppN2, giving no net difference. Clearly in this case, the increased ppN2 will make the gradient shallower, decreasing deco efficiency.

 

[Michael Guerrero]

It is therefore the fraction of O2, not the partial pressure, that matters.

So then...this?

 

[Beau640]

In the terms that you are using it, at the surface, ppO2 or N2 or any gas would equal the FiO2/N2/whatever gas. The partial pressure of a gas you are breathing is the fraction of that gas multiplied by the atm pressure. Breathing from a cylinder at the surface at 1atm, the ppO2 of a 50% O2 mix would be 0.5. The FiO2 would be 50%. 2 values essentially the same but expressed in different units.

However as you descend, these values would change and no longer equal each other. The FiO2 will always remain the same as it is expressed as a percentage of the gas you are breathing in and that percentage will not change with pressure. However the partial pressure will. At 3atm, the ppO2 of that same gas would now be 1.5.

As I was describing above, this is related to but not equal to the partial pressure of gas in the blood. The ppO2 of the blood will be effected by many things such as activity, metabolic state, temperature, altitude, etc. The pp(of whatever gas) difference between the lungs and the blood will determine the diffusion rate and gas exchange however the partial pressures of these two compartments are not actually the same. They are always attempting to equalize to each other, but they do not quickly become the same value. However for simplicity, safety, and how all of the dive tables were calculated, we assume that the ppO2 of what we breath at the pressure for the depth you are at as it enters your lungs is the ppO2 of O2 in your blood. This is not exactly true but it's close enough and since everything has been developed and calculated this way, it's what we use.

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.

 

[Gareth J]

A nitrox course would explain a lot of the terminology and physics. Granted accelerated decompression is a more advanced course. Before using Nitrox I strongly recommend you attend a Nitrox course - and stay within its limitations.

I will try to explain the basics

FO2 (Fraction of Oxygen).
This what is in the breathing mix. i.e.
Air has a fraction of oxygen of 21%.
Nitrox 36 has a fraction of oxygen of 36%.
Nitrox 32 has a fraction of oxygen of 32%.
Nitrox 50 has a fraction of oxygen of 50%
100% oxygen has a fraction of oxygen of 100%

Oxygen is VERY VERY Toxic at high partial pressures. The Human body CANNOT tolerate low partial pressures of oxygen or high partial pressures of oxygen. Low partial pressures result in blackout, and potentially death (from starving the body of oxygen). High pressures will result in fitting, blackout and potentially death (from overdosing on oxygen).
In diving terms, the result of blackout, or fitting is normally drowning.


When divers talk of Partial Pressure (PP or P). They are talking about the combined effect of the fraction of a particular gas in the breathing mix and the ambient pressure.
We could talk about the partial pressure of Nitrogen (PN) or the partial pressure of Helium (PH) or the partial pressure of Oxygen (PO2).
With air the fraction of oxygen is 21%, at atmosphere the PO2 is 0.21bar
at 10m (33ft) the PO2 is 0.41bar
at 20m (66ft) the PO2 is 0.61bar
with Nitrox 50, the fraction of O2 is 50%, at atmosphere the PO2 is 0.5bar
at 10m (33ft) the PO2 is 1bar
at 20m (66f) the PO2 is 1.5bar
at 30m (99ft) the PO2 is 2bar (This toxic and would likely result in fitting and death).

Whilst Nitrox (high oxygen mixes) have definite advantages with either reducing Decompression obligation or accelerating decompression. O2 and depth can be deadly.

Nitrox is a great gas for divers, it can increase safety considerably. But a safe Nitrox gas at one depth is deadly at another. Education is the key. I strongly recommend a nitrox course.

Even after a basic nitrox course a lot of the issues of O2 in breathing mixes are still not taught.

O2 is toxic in two particular ways, the effects of high PO2 on the central nervous system are normally well known by divers. In sport diving, the safe PO2 limit (partial pressure of O2) is generally set at 1.4bar. In technical diving, this can be raised to 1.6 bar. But it is a combination of both time, and PO2 which are the issue, added to this huge variable of an individuals physiology day by day.
Another issue is long term exposure of high PO2 result in damage to the lungs, ears and eyes. PO2 induced myopia was an issue in the early days of technical diving, especially rebreathers.

With diving, a little knowledge can be deadly, this is why each course puts limits on the fraction of O2 that can be used, and the maximum depth for that qualification that should not be exceeded.

We now have a set of reliable rules and systems for managing the risk. These are taught progressively on each course.
CNS exposure can be measured, we have safe diving limits for CNS that we know will avoid adverse reactions for 99% of divers. Long term exposure can be measured (UPTD or OTU dependent on the organisation), again we have sensible limits for these. We also have workable rules for 'washout' of O2 after diving.


Again I would strongly recommend that you attend a Nitrox course, and stay within the limitation of what you are taught.
Those of us who have seen a diver fitting in the water would rather not have to repeat the experience. In the early days we lost far to may people through lack of knowledge and lack of access to courses. There is no reason this should happen now, other than fool hardyness.

Gareth

 

[Michael Guerrero]

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

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.

 

[hroark2112]

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.

 

[RainPilot]

The deeper stop has the benefit of a higher ambient so bubble formation should be suppressed. If yo reverse the situation is there any benefit to going shallower? Other than OTU exposure obviously