Why do bent divers get 100% O2?

I understand that one of the recommendations for divers who are bent or
think they might be is 100% O2 on the surface. From what little
understanding I have of decompression theory, this seems like a bad idea.

Why? Well, as I understand it, the rate at which inert gases go into or
come out of solution in the bodily tissues depends on two things: First,
the nature of the tissue, and second, the difference between the partial
tension of the gas in the bloodstream and in the tissue. The partial
tension in the bloodstream is directly related to (i.e. pretty much "the
same as") the partial pressure in the lungs.

DCS results when the tissues have achieved a ptN2 significantly higher than
the ppN2 of the gas being aspirated. When this difference is large enough,
N2 comes out of solution too fast and forms bubbles.

A bent diver is in a situation where a ppN2 of .79 atm is low enough
relative to the ptN2 in some tissues that the difference results in
bubbling, right?

How, then, is it not harmful to lower the ppN2 still *further* (to
approximately 0 atm), by breathing 100% O2? It would seem that to reduce
bubbling it would be better to increase the ppN2, though breathing 100% N2
would clearly create another problem, and wouldn't increase ppN2 by all
that much.

Am I missing something, or is this just another example of how our
understanding of decompression theory is incomplete?

Shawn.

"Shawn Willden" <captainshawn@willden.org> wrote in message
news:kggMc.287$GA6.26962@news.uswest.net...
> I understand that one of the recommendations for divers who are bent or
> think they might be is 100% O2 on the surface.

It's a good question and I'll take a stab at it. There is some anti-ischemic effect
from high PO2 but I don't think that's the main reason.

Someone who is bent has bubbles of air (mostly nitrogen) floating in the blood and we
want to decrease the size of these bubbles as quickly as possible. To do that there
has to a concentration gradient for nitrogen. The greater the gradient the faster
the bubbles disappear and you get the fastest gradient by breathing as little
nitrogen as possible-- ie 100% O2.

The bubbles can be shrunk even faster by placing the victim in a hyperbaric chamber,
which compresses the bubbles mechanically through pressure.

Adam

Shawn Willden wrote:

> Am I missing something, or is this just another example of how our
> understanding of decompression theory is incomplete?

Answering my own post, here's a theory that just occurred to me:

It may be the case that although 100% O2 increases the likelihood of more
bubbling, it decreases the damage caused by bubbles that already exist by
improving the oxygenation of tissues whose normal blood flow is partially
blocked. I can see that mitigating damage that is already occurring may be
more important than preventing damage that may be caused by more bubbles
that may form due to lowering the ppN2.

That might also be a reason why in-water recompression is not recommended,
since although it will prevent formation of new bubbles, it's unlikely to
cause the existing bubbles to redissolve -- unless the bent diver is taken
deep, which poses all sorts of additional risks. So, with the rationale
that it's more important to treat the problem that exists rather than to
prevent the possible occurrence of additional problems, the recommendation
is to stay on the surface. (I'm ignoring all of the other risks of shallow
in-water recompression, such as loss of consciousness underwater, increased
mobility of smaller bubbles, etc.).

Does this make any sense? Are there other reaons?

Shawn.

The primary reason for giving the O2 post trauma as I understand it is to O2
saturate the blood so if there is any restriction/ short fall of blood
getting to some tissues, what blood does make there will hopefully carry
enough O2 to prevent those tissues from being oxygen starved and damaged,
especially nerve tissue. I may be missled.

"Shawn Willden" <captainshawn@willden.org> wrote in message
news:kggMc.287$GA6.26962@news.uswest.net...
> I understand that one of the recommendations for divers who are bent or
> think they might be is 100% O2 on the surface. From what little
> understanding I have of decompression theory, this seems like a bad idea.
>
> Why? Well, as I understand it, the rate at which inert gases go into or
> come out of solution in the bodily tissues depends on two things: First,
> the nature of the tissue, and second, the difference between the partial
> tension of the gas in the bloodstream and in the tissue. The partial
> tension in the bloodstream is directly related to (i.e. pretty much "the
> same as") the partial pressure in the lungs.
>
> DCS results when the tissues have achieved a ptN2 significantly higher
than
> the ppN2 of the gas being aspirated. When this difference is large
enough,
> N2 comes out of solution too fast and forms bubbles.
>
> A bent diver is in a situation where a ppN2 of .79 atm is low enough
> relative to the ptN2 in some tissues that the difference results in
> bubbling, right?
>
> How, then, is it not harmful to lower the ppN2 still *further* (to
> approximately 0 atm), by breathing 100% O2? It would seem that to reduce
> bubbling it would be better to increase the ppN2, though breathing 100% N2
> would clearly create another problem, and wouldn't increase ppN2 by all
> that much.
>
> Am I missing something, or is this just another example of how our
> understanding of decompression theory is incomplete?
>
> Shawn.

"Shawn Willden" <captainshawn@willden.org> wrote in message
news:kggMc.287$GA6.26962@news.uswest.net...
> I understand that one of the recommendations for divers who are bent or
> think they might be is 100% O2 on the surface. From what little
> understanding I have of decompression theory, this seems like a bad idea.
>
> Why? Well, as I understand it, the rate at which inert gases go into or
> come out of solution in the bodily tissues depends on two things: First,
> the nature of the tissue, and second, the difference between the partial
> tension of the gas in the bloodstream and in the tissue. The partial
> tension in the bloodstream is directly related to (i.e. pretty much "the
> same as") the partial pressure in the lungs.
>
> DCS results when the tissues have achieved a ptN2 significantly higher than
> the ppN2 of the gas being aspirated. When this difference is large enough,
> N2 comes out of solution too fast and forms bubbles.
>
> A bent diver is in a situation where a ppN2 of .79 atm is low enough
> relative to the ptN2 in some tissues that the difference results in
> bubbling, right?
>
> How, then, is it not harmful to lower the ppN2 still *further* (to
> approximately 0 atm), by breathing 100% O2? It would seem that to reduce
> bubbling it would be better to increase the ppN2, though breathing 100% N2
> would clearly create another problem, and wouldn't increase ppN2 by all
> that much.
>
> Am I missing something, or is this just another example of how our
> understanding of decompression theory is incomplete?
>
> Shawn.

Your reasoning is incorrect. Whether the gas comes out of solution to form bubbles
does not depend on partial-pressure gradients. It depends on the solubility and how
much gas is dissolved in the blood, and the solubility depends on the hydrostatic
pressure. When the pressure drops the blood can become supersaturated like a soda
bottle and the gas can come out solution to form bubbles.

Adam

Shawn Willden wrote:

> I understand that one of the recommendations for divers who are bent or
> think they might be is 100% O2 on the surface. From what little
> understanding I have of decompression theory, this seems like a bad idea.
>
> Why? Well, as I understand it, the rate at which inert gases go into or
> come out of solution in the bodily tissues depends on two things: First,
> the nature of the tissue, and second, the difference between the partial
> tension of the gas in the bloodstream and in the tissue. The partial
> tension in the bloodstream is directly related to (i.e. pretty much "the
> same as") the partial pressure in the lungs.
>
> DCS results when the tissues have achieved a ptN2 significantly higher than
> the ppN2 of the gas being aspirated. When this difference is large enough,
> N2 comes out of solution too fast and forms bubbles.
>
> A bent diver is in a situation where a ppN2 of .79 atm is low enough
> relative to the ptN2 in some tissues that the difference results in
> bubbling, right?
>
> How, then, is it not harmful to lower the ppN2 still *further* (to
> approximately 0 atm), by breathing 100% O2? It would seem that to reduce
> bubbling it would be better to increase the ppN2, though breathing 100% N2
> would clearly create another problem, and wouldn't increase ppN2 by all
> that much.
>
> Am I missing something, or is this just another example of how our
> understanding of decompression theory is incomplete?
>
> Shawn.

Adam is correct. The N2 bubbles because the air pressure on the tissues
has dropped, not because there is too little N2 there. No matter how
much N2 you have in your blood, decreasing the N2 pressure while keeping
the overall air pressure the same won't cause bubbling.

The reason O2 is given is that you want to get that N2 diffused out of
the blood as fast as possible.. and the best way to do that is to
eliminate N2 from the air you're breathing.

Shawn Willden wrote:

> Adam Helberg wrote:
>
>
>>Your reasoning is incorrect. Whether the gas comes out of solution to form
>>bubbles does not depend on partial-pressure gradients.
>
>
> Thanks for the response.
>
> So does this mean that the rate at which gas goes into solution depends on
> partial pressure differentials, but not the rate at which it comes out of
> solution?

There's a difference between diffusing into and out of tissues and bubbling.

Diffusion, the rate at which dissolved gas enters or leaves a tissue, is
determined solely on partial pressure differential.

Having the gas bubble out of solution is not a funtion of diffusion,
though. It's a function of the tissue fluid being supersaturated for
the ambient pressure. When N2 bubbles out, there is too much dissolved
for the current ambient pressure, so it literally comes out of solution
- which is different from it being diffused across a membrane, into the
blood, or into the lungs.

>
> I would have thought those processes were mirrored. Any idea what the
> physical reason for the difference might be? Does solubility of a gas vary
> with total pressure or partial pressure? My old college chemistry textbook
> doesn't say, unfortunately. It addresses mixed gases and solubility of
> gases, but not solubility of mixed gases
>
> Thanks,
>
> Shawn.

Shawn Willden wrote:


> Why? Well, as I understand it, the rate at which inert gases go into or
> come out of solution in the bodily tissues depends on two things: First,
> the nature of the tissue, and second, the difference between the partial
> tension of the gas in the bloodstream and in the tissue. The partial
> tension in the bloodstream is directly related to (i.e. pretty much "the
> same as") the partial pressure in the lungs.

Adam and John gave you most of the answer, and I'll try and cover the rest. The stuff
you said above is pretty much accurate, but there's at least one thing that's
incorrect. As Adam pointed out, it's the ambient pressure that determines the
saturation point, and therefore the possibility of bubbling, and that's what affects
gases being *dissolved.* Relative partial pressures (or tensions, as the case may
be), OTOH, determine how the gases *diffuse*.

Those gas laws you learned in high school chemistry and scuba class were figured out
by guys who where trying to figure out the *natural* sciences of physics and
chemistry, and they didn't plan on unnatural gas switches from air to tri-mix, or
100% O2. By breathing something other than air you're creating an unnatural
situation, so you can't expect laws about natural processes to properly explain the
specific workings. We had a discussion on this point perhaps 18 months ago. A bit of
detective work with Google should scare it up.


> DCS results when the tissues have achieved a ptN2 significantly higher than
> the ppN2 of the gas being aspirated. When this difference is large enough,
> N2 comes out of solution too fast and forms bubbles.

This is the part where a literal interpretation of the gas laws has mislead you into
being completely wrong. If reducung the partial tension in the bloodstream caused
bubbling, then every trauma victim who got 100% O2 from the EMT's would also be bent
as well as broken when they got to the hospital. As already pointed out, a gas switch
increases the rate of diffusion, therefore reducing the N2 (whether it's still
dissolved or already in bubbles), but the increased partial tension of the O2
supplies the necessary pressure to keep the dissolved N2 dissolved.

Hope that explains it well enough.

--
Steve

The above can be construed as personal opinion in the absence of a reasonable
belief that it was intended as a statement of fact.

If you want a reply to reach me, remove the SPAMTRAP from the address.

Adam Helberg wrote:

> Your reasoning is incorrect. Whether the gas comes out of solution to form
> bubbles does not depend on partial-pressure gradients.

Thanks for the response.

So does this mean that the rate at which gas goes into solution depends on
partial pressure differentials, but not the rate at which it comes out of
solution?

I would have thought those processes were mirrored. Any idea what the
physical reason for the difference might be? Does solubility of a gas vary
with total pressure or partial pressure? My old college chemistry textbook
doesn't say, unfortunately. It addresses mixed gases and solubility of
gases, but not solubility of mixed gases

Thanks,

Shawn.

Shawn Willden wrote:
> I understand that one of the recommendations for divers who are bent or
> think they might be is 100% O2 on the surface. From what little
> understanding I have of decompression theory, this seems like a bad idea.
>
> Why? Well, as I understand it, the rate at which inert gases go into or
> come out of solution in the bodily tissues depends on two things: First,
> the nature of the tissue, and second, the difference between the partial
> tension of the gas in the bloodstream and in the tissue. The partial
> tension in the bloodstream is directly related to (i.e. pretty much "the
> same as") the partial pressure in the lungs.
>
> DCS results when the tissues have achieved a ptN2 significantly higher than
> the ppN2 of the gas being aspirated. When this difference is large enough,
> N2 comes out of solution too fast and forms bubbles.
>
> A bent diver is in a situation where a ppN2 of .79 atm is low enough
> relative to the ptN2 in some tissues that the difference results in
> bubbling, right?

The bubbleing isn't the result of ppN2 to ptN2 but of ppN2 to pA ( or in
the theory, the 'M' value of a 'compartment').


>
> How, then, is it not harmful to lower the ppN2 still *further* (to
> approximately 0 atm), by breathing 100% O2? It would seem that to reduce
> bubbling it would be better to increase the ppN2, though breathing 100% N2
> would clearly create another problem, and wouldn't increase ppN2 by all
> that much.
>
> Am I missing something, or is this just another example of how our
> understanding of decompression theory is incomplete?
>
> Shawn.

how fast a gass is transported out of the body is effected by the
ppinspired to ppT. lowwering the ppN2 but keeping the pA the same speeds
up N2 offgassing. the same reason it's used for deco in water.