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Will,
> "Inert Gas Counterdiffusion", as the current
> theory holds, is thought to be a cause of DCS incidence
> that occurs when switching from mixes high in Helium to
> Nitrogen-based mixes. The theory runs that
> the sudden on-gassing of the Nitrogen somehow
> magically interferes with the off-gassing
> of the Helium, and bubbles form.
I guess I didn't pick up on this during our private communications, but
my understanding of IGCD involves no magic. Specifically, it involves
"fast" diluent in a gas space (lung or middle ear), and "slow" diluent in
the blood (either dissolved, or in the form of micornuclei).
My understanding of the theory is this:
Assume the blood is heavily loaded with N2 (our "slow" gas), say after
an air or nitrox dive. Bubbles in the blood/tissues would be composed
primarily of the "slow" N2. Now the diver switches to a helium-based
breathing mixture. The helium (our "fast" gas) diffuses into the blood
at a fast rate, and the N2 diffuses out of the blood into the lungs at a
slower rate. The net result is a sudden increase in total inert gas load
(N2 & He) in the blood. When we consider "microbubbles" already in the
blood, a similar thing happens. The "fast" helium diffuses into the
bubbles faster than the "slow" nitrogen diffuses out of the bubbles.
This results in a net increase of gas molecules within the bubbles,
leading to a net increase in the size of the bubbles. The point, however,
is that it occurs when we switch from a "slow" gas to a "fast" gas; the
effect would not occur when switching from a "fast" gas to a "slow" gas
(i.e., helium to nitrogen during an ascent).
Where have we seen bends that has been attributed to IGCD? One place is
in decompression chambers filled with helium in the surrounding
atmosphere. If a diver ascending from a air/nitrox dive enters a
decompression chamber filled with helium, a bad case of IGCD likely
results - even if the diver continues to breathe air/nitrox. This is
because the helium from the surrounding atmosphere diffuses through the
skin into the blood rapidly, leading to the net growth of bubbles (yet
another reason not to put helium in the ol' drysuit).
Another place we've seen a few cases of bends attributed to IGCD is when
commercial divers have switched from heliox to nitrox/air during
ascent/deco. How can this happen, given the above description?
Well...as it turns out, symptoms of such cases seem to always be
"vestibular" in nature (nausea, loss of balance, disorientation, etc).
In other words, bends in the inner ear. The theory in this case, I
believe, is that the diver's middle ear is filled with heliox following
the commercial dive. Switching to air/nitrox on ascend begins to
saturate the blood with N2. Throughout most of the body, the total gas
load is dropping, because helium is likely leaving the body faster than
N2 is entering it. However, in the middle ear, we have a supply of
heliox. The blood going to the inner ear is hammered first with N2 from
the lungs, then with He from the middle ear. Net result: increase in
total inert gas load, growth of bubbles in the inner ear, vestibular
bends.
Now, let's take a look at "technical" diving practices. At the surface,
we've got a fairly steady dissolved PN2 of 0.79, and effectively no
helium in our blood. We begin our descent on air/nitrox "travel" gas
until we reach the helium depths. At that point, we switch to trimix.
We have the potential for IGCD-induced bubble growth at this point in the
dive, because we've just switched from a slow diluent to a fast diluent.
However, the reasons we seldom (if ever) observe bends symptoms during
this portion of the dive include:
1) Most of us switch to trimix, so the helium content is not as large as it
would be if we had switched to heliox.
2) Our blood is likely not saturated with N2 at ambient pressure at the
time of gas switch (two or three minutes during a rapid descent is
probably insufficient time for large quantities of N2 to populate our
bodies). Thus, the total gas load is insufficient to lead to significant
bubble growth.
3) Our tissues are barely saturated at all above a PN2 of 0.79, which
leaves a large fluid "reservoire" in our bodies to absorb and distribute
the additional dissolved gas.
4) After switching to trimix, we generally continue the descent, which
means ambient pressure continues to increase, further discouraging bubble
growth.
It might be argured that this represents a reason why we should switch
from travel gas to bottom gas early during the descent (perhaps even at
the start of the descent), to minimize the potential for N2 loading prior
to switching to a helium-based breathing gas. However, if we continue
following this dive, I think perhaps it will make sense why this would
not be such a hot idea (switching to bottom gas early).
During the bottom portion of the dive, we typically breathe some sort of
trimix. As we begin our ascent from depth, we typically switch over to
nitrox at 130fsw or so. We needn't fear IGCD-induced bends from the gas
switch, because we're going from a faster gas to a slower gas. But what
about vestibular (inner ear) bends? Don't we have to worry about that,
especially because commercial divers have occassionally suffered from it?
Well, that depends on what the gas composition inside our middle ear is.
Most comemrcial divers, I believe, begin breathing heliox at or near the
start of the descent (i.e., no "travel gas"). Thus, during the entire
descent, as the divers equalize their ears, they are filling their
middle ears with heliox. When they eventually switch back to N2 as a
breathing gas, the middle ear contains almost pure helium. BAM -
vestibular bends.
Because we "technical" divers breathe an air/nitrox "travel" gas during
the initial descent, we are initially flooding our middle ears with N2.
Only after we switch to trimix do we start adding helium, and even then,
it's part helium and part nitrogen. Thus, but the time we get to our
130-foot deco stop and switch back to air/nitrox, our middle ears
generally contain more N2 than Helium. (Now it suddenly doesn't make much
sense to swithc to bottom gas early-on during the descent).
Consequently, vestibular bends following a gas switch from bottom gas to
deco gas is very rare in technical divers.
> Most everyone agrees that there are really two major
> factors that determine bubble formation: gradient and
> absolute pressure. "Gradient" is the difference between
> the pressure of a gas dissolved in the blood and the
> partial pressure of that gas in the inspired mix. When
> the gradient gets too high, microbubbles form. These
> bubbles expand as the absolute pressure decreases, in
> the same way your BC volume expands while ascending.
There is also the gradient from the blood to the bubble that we need to
consider. This is *really* what causes the bubble to grow or shrink.
Growth of small bubbles is more dependant on this gradient than on
ambient pressure. Ambient pressure plays a proportionally larger role in
bubble size for bubbles that are already large (e.g., diver already
exhibiting DCI symptoms).
> Haldanean-based models use a mixture of gradient and
> pressure to determine a "safe" decompression curve, but
> don't specifically make the point that BUBBLES ARE FORMED
> BY HIGH GRADIENT, NOT ABSOLUTE PRESSURE CHANGES.
Actually, I think the bubbles are "formed" by processes unrelated to
diving. The diving stuff is more a function of how much those bubbles
will grow. Also, the critical gradient is not so much the lung/blood
gradient, as it is the blood/bubble gradient.
> Not so with Helium mixtures. When you do that deep switch
> from heliox or trimix to a nitrogen-based mixture, you
> are zapping the gradient to it's maximum in about five seconds,
> and you are doing it at the worst possible time, when the
> pressure of the dissolved helium is at its greatest. Plus
> you are usually ascending at the same time, which has got
> the helium gradient already reversed into "off-gassing" mode.
> Naturally, bubbles form.
I'm not so sure about this. I think their growth rate is actually probably
slowed at this point during the ascent, due to the "mirror" effect from IGCD
(bubbles/blood contain He, lungs contain N2, He exits bubbles/blood faster
than N2 enters bubbles/blood, so there is an initial shrinkage of bubbles at
the time of gas switch from trimix/heliox to air/nitrox.....except, of
course, in parts of the body where there may be helium-rich gas spaces, such
as the middle ear following a dive with a descent on heliox).
I don't mean to argue with you on this. As everyone should know, at this
point in our understanding of the relavent physics and physiology of
decompression, smart deco practices are more of an art than a science. I
don't want to sound as though I have all the answers - everything I've
written above is speculative. But at least it makes sense in the context
of gas physics, and it also seems to fit the observed data (bends
symptoms without concurrent ambient pressure changes tend to happen in
the skin when suddenly exposed to a helium atmosphere, and tend to happen
in the ears of commercial heliox divers during switches from heliox to
nitrox during deco).
Aloha,
Rich
Richard Pyle
Ichthyology, Bishop Museum deepreef@bi*.bi*.ha*.or*
1525 Bernice St. PH: (808) 848-4115
Honolulu, HI 96817-0916 FAX: (808) 841-8968
"The views are those of the sender and not of Bishop Museum"
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