Hypothetical question

[Dominik_E]

Also, as said several times, the Haldane/Workman/Buehlmann line of reasoning is only of limited help here. We know that divers form bubbles all the time without ever crossing any M-line.

I do black holes for a job, not medicine or biology or deco model development. I do however have to say I am not sure if any expert on those fields would be able to drop in here and simply clarify this issue (of "could bubbles form on ascent in this case?"). From reading the literature, to me this seems a case of lack of fundamental understanding of the exact process/location of microbubble formation, and a lack of data in the relevant regime.

And please everyone, let us not forget that this part of the discussion is primarily of theoretical interest to understand the process. As far as I know and see, the question never was if these bubbles should greatly worry a diver that has to board a real plane. I think we are all very clear that there is an overwhelming probability these bubbles would be cleared. But once bubbles would be there, the safety of the diver is just a very high probability. Not any longer a trivial mathematical certainty ("ppN2 not higher than while playing tennis"). That's the point of the whole argument to me...

 

[dmaziuk]

Edit of entire post since I found the study I was fuzzily half-remembering:

http://www.ultimatedivelog.com/articles/25.pdf

Cool, though not the one I had in mind. I tried again, from home, and failed again, but I found one I feel I need to share with the class:

Viagra - curse or blessing for a diver?

 

[RyanT]

Here are two thought experiments to better illustrate this.

1) You have a cylinder with a piston and some amount of dissolved N2 in solution, and a gas-liquid interface, in Henry's law equilibrium. Let's say the PPN2 is 0.79. The piston has the cylinder pressurized to 5 ATA. You suddenly drop the piston, resulting in a sudden drop in pressure to 2 ATA. Gas will come out of solution, even though the PPN2 is the (arbitrary) value found in surface air.

2) Lets say that oxygen toxicity doesn't exist, or that we are doing this with some animal model with no CNS to make it an issue. This is just about bubble formation. You are at 330 FSW (11 ATA), breathing EAN93, which gives you a PPN2 of 0.79, the same as on the surface. You suddenly ascend with no stops to just below the surface. Do you feel that there is no decompression risk, no bubble formation? Remember, your tissues don't "know" what is on the surface. They just have experienced essentially an explosive decompression, and they have N2 in them.

@doctormike, let me preface my response with what you said "I'm (also) not a deco expert.." so If I'm wrong here, I hope someone corrects me!

I don't think pressure itself is really the issue. Gases move into and out of tissues by diffusion. The bigger the concentration gradient, the faster they move. What pressure does is just change the concentration gradient. You on gas at depth because you are inspiring inert gas at a higher concentration than what exists in your tissues.

So in your two thought experiments, I think the answer would be no, you would not get bubble formation. Let's make a caveat for scenario number 1. I'm assuming there is a layer of gas above the liquid and that layer of gas has a PN2 of 0.79. Now in both scenarios, even though you are changing pressures, you haven't changed the concentration gradient. So yes, you can change the pressure, but now, there is no concentration gradient for the gases to follow.

Again, if I've really missed something here, someone correct it!

 

[Dominik_E]

RyanT, the point is that before surfacing to the layer of ppN2=0.79 (the atmosphere of the Earth), you will be for a short while also at 1bar ambient, but still be breathing EAN 60. There is a gradient between breathing EAN 60 at 10 meters, and breathing EAN 60 at 0.1 meters...

 

[TrimixToo]

@doctormike, let me preface my response with what you said "I'm (also) not a deco expert.." so If I'm wrong here, I hope someone corrects me!

I don't think pressure itself is really the issue. Gases move into and out of tissues by diffusion. The bigger the concentration gradient, the faster they move. What pressure does is just change the concentration gradient. You on gas at depth because you are inspiring inert gas at a higher concentration than what exists in your tissues.

So in your two thought experiments, I think the answer would be no, you would not get bubble formation. Let's make a caveat for scenario number 1. I'm assuming there is a layer of gas above the liquid and that layer of gas has a PN2 of 0.79. Now in both scenarios, even though you are changing pressures, you haven't changed the concentration gradient. So yes, you can change the pressure, but now, there is no concentration gradient for the gases to follow.

Again, if I've really missed something here, someone correct it!

Going a bit out on a limb here, but I think we all tend to focus on bubbles in the blood. I don't know any reason why bubbles could not form in the tissues themselves. Diffusion would not be involved in that process.

 

[Storker]

Total pressure (sum of all partial pressures, Dalton's law) and ambient pressure are the same thing. I said it was the change in ambient (or total) pressure that caused a DCS risk, not the absolute amount of ambient (or total) pressure, which is what I though you were claiming.

What I'm claiming is that absolute amount of ambient (or total) pressure is quite irrelevant. Partial pressure vs tissue tension, on the other hand...

 

[RyanT]

Going a bit out on a limb here, but I think we all tend to focus on bubbles in the blood. I don't know any reason why bubbles could not form in the tissues themselves. Diffusion would not be involved in that process.

The physics of bubble formation is a bit beyond my expertise, for example, I don't really understand why nuclei generate bubbles. But the movement of a gas from solution (e.g. intracellular fluid) into a bubble is diffusion.

 

[doctormike]

What I'm claiming is that absolute amount of ambient (or total) pressure is quite irrelevant.

Correct. The CHANGE (specifically, the decrease) in ambient pressure is relevant to decompression stress. Not the absolute amount. No argument there.

 

[Storker]

for example, I don't really understand why nuclei generate bubbles.

Nuclei don't generate bubbles, but they initiate them. Without a nucleus where bubble formation can start, there could be significant supersaturation without any bubbles forming.

Similar phenomena: supercooling and superheating. A very good example of supercooling is those rechargeable hand warmers. It's a plastic bag with an extremely supercooled sodium acetate solution and a small metal disk. When you bend ("click") the disk, you initiate the freezing of the supercooled liquid. After use, you immerse the plastic bag in boiling water, melting the solidified solution. If all crystals have been dissolved, you can cool the bag and get the contents back to its supercooled state. If there's just one single crystal left when you take out the bag to cool it, it'll solidify when you cool it. Just one single nucleus. But the nucleus doesn't generate freezing, that's caused by the cooling. It just initiates freezing.

 

[Storker]

The CHANGE (specifically, the decrease) in ambient pressure is relevant to decompression stress.

No. The CHANGE (specifically, the decrease) in partial pressure is relevant to decompression stress