DCS from O2 decompression

[gcbryan]

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

But it doesn't. The rate at which N2 comes out of your blood is not related to any other gas. Gases act independent of each other.

...
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.

The oxygen window, to the extent it has anything to do with decompression, refers to the reduced total pressure in your tissues as in less pressure so reduced bubble size but it has nothing to do with oxygen speeding up nitrogen offgassing.

High PP02 deco gasses work simply because you aren't inspiring more n2.

 

[gcbryan]

This isn't what's happening (it goes without saying...someone correct me if I'm wrong).

When you switch to O2 all that you have changed instantly is the O2 that is inspired. You haven't changed ambient pressure and you haven't instantly changed the tissue pressures.

You have changed total gas pressure so the oxygen window effect will supposedly help reduce bubble size to the extent of the oxygen window effect.

There's no more inerts in the breathing mixture so no more is being inspired but ambient pressure isn't changing which is the same as pressure in the lungs (to be accurate there is a slight reduction due to water vapor).

Offgassing is still controlled by half-lives. The rate has changed simply because you are now taking in less (or no N2) but the process is still controlled by half-lives.

...
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.

 

[wedivebc]

But it doesn't. The rate at which N2 comes out of your blood is not related to any other gas. Gases act independent of each other.

.

But it does, that is why high oxygen mixes accelerate deco is because the higher oxygen content creates a higher gas pressure gradient in relation to the inert gas than the total pressure gradient alone can achieve.

 

[wedivebc]

This isn't what's happening (it goes without saying...someone correct me....


Offgassing is still controlled by half-lives. The rate has changed simply because you are now taking in less (or no N2) but the process is still controlled by half-lives.

Yes but the half-life time is still pressure dependent. The time it takes to equalize is the same the partial pressure it equalizes to is less therefore faster exchange.

 

[Mr Carcharodon]

A Workman like m-value seems to assume a model that does not apply to oxygen because hemoglobin dominates oxygen transport.

1.5% of all oxygen being transported is in solution at sea level with the residual being bound to hemoglobin. Presumably the fraction in solution will increase with increasing ambient pressure during the dive. But even for a 60 m dive still only 10% of all the oxygen being transported is in solution. But venous blood is half saturated due to the oxygen consumed by metabolism. So during ascent oxygen will come out of solution and be picked up by unsaturated hemoglobin. And that assumes it is not just metabolized. Finally since venous blood is unsaturated there is a gradient to drive oxygen bubbles to shrink.

But I guess for rapid ascents oxygen bubbles could form.

 

[PfcAJ]

gcbryan, if o2 window only affects bubble size, why do we see gains mathematically in both bubble models and dissolved gas models? The answer? Because it affects both!

If you're looking to understand the mechanics of decompression theory, a sound understanding of diffusion, gradients, membranes, etc is needed (aka bio 1). Decompression isn't a simple thing, multiple actions are taking place simultaneously.

 

[gcbryan]

If you have a link to whatever you are referring to I'd be interested in looking at it.

You might find this link interesting as well.
http://www.scubaboard.com/forums/as...4754-oxygen-window-explanation-purpose-4.html
Read Dr. Deco's comments in post #33.

gcbryan, if o2 window only affects bubble size, why do we see gains mathematically in both bubble models and dissolved gas models? The answer? Because it affects both!
...

 

[deepmaxim]

HI DEEPMAXIM:

If you talk to Jurgend, ask him he remembers Mike Powell from our early days in Bonn and the DFVLR.

Ok, i ask him!

 

[deepmaxim]

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.

All deko time was ok, 9m ok, we come to 6m, change to oxygen, in 2-4min later ihave feel the pressure in schoulder, knees etc. 5-8min later pain in the same places. No DCS Type 1 symptomatic, only a DCS Type2

Dive:

Meters Time Runtime

140-145 - 30 - 36
91 - 1 - 41
90 - 6 - 47
87 - 1 - 48
84 - 1 - 49
81 - 1 - 50
78 - 1 - 51
75 - 1 - 52
72 - 1 - 53
69 - 1 - 54
66 - 1 - 55
63 - 3 - 58
60 - 2 - 60
57 - 2 - 62
54 - 4 - 66
51 - 4 - 70
48 - 4 - 74
45 - 6 - 80
42 - 2 - 82
39 - 6 - 88
36 - 5 - 93
33 - 7 - 100
30 - 10 - 110
27 - 10 - 120
24 - 15 - 135
21 - 15 - 150
18 - 17 - 167
15 - 23 - 190
12 - 32 - 222
9 - 48 - 270
6 - 135 - 405
3 - 30 - 435

Gases
Тх 9/70, 15/55, 30/40, 50/25, О2
Decoplanner 3 (GF Lo15 и GF Hi 100)

 

[boulderjohn]

The oxygen window, to the extent it has anything to do with decompression, refers to the reduced total pressure in your tissues as in less pressure so reduced bubble size but it has nothing to do with oxygen speeding up nitrogen offgassing.

This is very controversial. Some people agree with you. Some people disagree with you. I for one do not see how it can, but others (especially UTD theorists) are fervid in their belief that it does.

High PP02 deco gasses work simply because you aren't inspiring more n2.

That's what I was actually saying.

When you switch to O2 all that you have changed instantly is the O2 that is inspired. You haven't changed ambient pressure and you haven't instantly changed the tissue pressures.

When you yourself suggested above, when you immediately switch to 100% O2, you have instantly switched to 0% N2. It is the 0% N2 that is having the desired effect.

There's no more inerts in the breathing mixture so no more is being inspired but ambient pressure isn't changing which is the same as pressure in the lungs (to be accurate there is a slight reduction due to water vapor).

When there is no N2 in your lungs, then the ambient pressure is irrelevant as far as N2 is concerned. You have increased the pressure gradient between what is in your tissues (including blood) and the gas you are breathing, so the offgassing goes much faster.