DCS from O2 decompression

[Rutger]

British did tests on this as I recall.

They took goats from the maximum depth at saturation to surface at 75'min rate, and found that they could get bent from O2. However in 3/4 of the cases it was quickly self corrected (the body re-adsorbed it) and in the latter cases, standard deco treatment took care of it.

The thing is, you will get bend from Nitrogen and/or seize from O2 toxicity long before you get to a situation where you will bend from O2, so don't worry about it. Focus on avoiding Nitrogen DCS and you will be fine.

 

[wedivebc]

I have self one time a DCS symptomatic directly after change on O2 (6m)... I think it is possible to have a DCS from each gas in breathing gas and not only from "inert" gas like N2 or helium.
Have you the same expirience with O2 bends?
Thanks

Could you please describe the dive profile and symptoms you experienced?
As you are probably aware O2 DCS is virtually unheard of so any light you could shed on this incident may benefit others since it is such a rare occurrence.

thanks.

 

[boulderjohn]

I have some thoughts and questions that are related to this, I think. Maybe I should start a new thread.

Let's say we are doing a decompression profile that calls for stops on back gas at different levels and times prior to an O2 switch at 20 feet. We might perhaps finish an 8 minute stop at 30 feet and then ascend to 20 feet for an immediate O2 switch.

That seems to me to be a very large jump in the gradient, and I wonder how close this comes to M-value limits on some dives. I wonder why we don't continue on the back gas for a few minutes at 20 feet before switching to O2.

 

[PfcAJ]

The M-value refers to the maximum overpressure a 'compartment' can tolerate. 101% = bends (not really, but thats the assumption of the model). If we're diving a Buhlmann based ascent profile, after those 8 minutes are up, we can ascent to 20ft (at whatever ascent rate the profile calls for) and the overpressure will still be below the M-value.

Getting on o2 effectively ''pulls'' the inert gasses out of your body. So when you are at 20ft, you could be at say...80% of the m-value. The o2 switch doesn't change this value, it only accelerates the rate at which the inert gasses are pulled out.

 

[brockbr]

I have some thoughts and questions that are related to this, I think. Maybe I should start a new thread.

Let's say we are doing a decompression profile that calls for stops on back gas at different levels and times prior to an O2 switch at 20 feet. We might perhaps finish an 8 minute stop at 30 feet and then ascend to 20 feet for an immediate O2 switch.

That seems to me to be a very large jump in the gradient, and I wonder how close this comes to M-value limits on some dives. I wonder why we don't continue on the back gas for a few minutes at 20 feet before switching to O2.

John,
Most algorithms predict the gas switch. For example, in iDeco (which uses a Bühlmann 16 w/ GF) if the next deco stop's GF is 50%, the algorithm will keep you at your deeper depth until you would arrive at the shallower depth *on the new gas* at the prescribed GF.

You can see this effect when you cut tables from most algorithms. Leave out deco gases, and you'll have a smooth ascent curve. Add deco gases, and you'll see the stop right before the gas switch is extended. This allows for the compartments to off-gas the correct amount.

b.

 

[brockbr]

The M-value refers to the maximum overpressure a 'compartment' can tolerate. 101% = bends (not really, but thats the assumption of the model). If we're diving a Buhlmann based ascent profile, after those 8 minutes are up, we can ascent to 20ft (at whatever ascent rate the profile calls for) and the overpressure will still be below the M-value.

Getting on o2 effectively ''pulls'' the inert gasses out of your body. So when you are at 20ft, you could be at say...80% of the m-value. The o2 switch doesn't change this value, it only accelerates the rate at which the inert gasses are pulled out.

It does, in fact, change the parameters. This is because m-values & GF's are factored off pN2 (and pHe) of inspired gas compared to ambient. If the inspired gas changes from one depth to the next (a switch to O2 at 20ft), the inspired pN2 and pHe will be different.

(btw, this same area of gas switching is also what causes ICD)

b.

 

[PfcAJ]

My understanding of deco theory is a bit different than yours.

How can the maximum amount of overpressure in a 'compartment' change with different inspired gasses? Whether a dive is breathing 18% oxygen or 100% oxygen, the compartment is still at xx% of the m-value.

Now, breathing a gas with a high ppo2 will pull the dissolved gas out of the body, moving that compartment farther from the m-value, which is the goal of decompression.

Edit:
It will change, but only by virtue of the deco gas pulling out the dissolved gas

 

[gcbryan]

The M-value is based on the inspired gas currently being used up to the next stop. If you then change the gas it will effect the stop after that but not the current one. The algorithm makes sure you won't exceeded the M-value at your destination stop (at the given ascent rate). Changing to a higher pp02 at that point can't make you now exceed the M-value since it's not going to increase your ppN2.

Now if you're switching to a higher He mix then ICD is an issue but this is more or less outside the traditional algorithm and code is added on to alert to this condition.

So Pfc AJ is more or less correct as far as my understanding goes (other than describing hi ppO2 as "pulling" inert gas out of the body).

One gas isn't affecting the rate of another gas except in the sense that less nitrogen (no nitrogen) is currently being inspired and therefore won't have to be offgassed.

 

[boulderjohn]

The M-value is based on the inspired gas currently being used up to the next stop. If you then change the gas it will effect the stop after that but not the current one. The algorithm makes sure you won't exceeded the M-value at your destination stop (at the given ascent rate). Changing to a higher pp02 at that point can't make you now exceed the M-value since it's not going to increase your ppN2.

...
One gas isn't affecting the rate of another gas except in the sense that less nitrogen (no nitrogen) is currently being inspired and therefore won't have to be offgassed.

I believe the concern in ascent rates is keeping the gradient between the tissues in the body and the gas in the lungs at a safe level.

If we are at 99 fsw and we have been there long enough to saturate certain tissues, if we were to ascend immediately to the surface, we would have (putting it overly simply) 4 times as much nitrogen in the tissues as the air we are breathing. Consequently, the offgassing would be too fast and gas in solution could come out of solution and form bubbles. We therefore ascend more slowly and do stops so that enough nitrogen has a chance to come out of the tissues to keep the gradient at a safe level at shallower depths or at the surface. In short, we want to have the highest possible safe gradient between tissue nitrogen and inspired nitrogen so that we off gas quickly without forming new bubbles.

If we are doing a stop at 30 fsw, our tissues are off gassing in an attempt to reach equilibrium at roughly 2 ATA. They will not get there in the normal stop time. If we ascend to 20 feet and keep breathing the same gas, we have increased the gradient, since the air we are breathing has less nitrogen in it than it did at 30 feet, and we have thus sped up the off gassing process at a safe level.

If we switch to O2 at this point, we have gone in about 30 seconds from 2 ATA of N2 to 0 ATA of N2. That greatly increases the gradient and speeds up the off gassing--it does not just mean that there is no more nitrogen coming in that will need to be off gassed later. The question I was asking is how safe is this rapid change in gradient. Since so many people do it (including me), it must usually be OK. I guess I am just wondering how close we are cutting it and if it might not be safer to spend a few minutes on back gas at that depth.

 

[PfcAJ]

I said that high ppo2 ''pulls'' inert gasses out because of the concentration gradient between the blood and the lungs.

During decompression, more N2 and He is in the blood than in the inspired gas. Because there is a lower conentration of interts in the inspired gas (deco gas), the rate at which diffusion takes place is accelrated.

The diffusion rate slows as your blood and alveolar gasses approach equillibrium.

In a nutshell, if you have more N2 (or He, or both) dissolved in your blood than is in your deco gas, the N2 will diffuse into the area of lower concentration (your lungs, and then subsequently exhaled).

So there are two mechanisms at work. Decompression via pressure gradient and decompression via concentration gradient. M-values refer to pressure gradient, o2 window refferes to concentration gradient.