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Yup, if you assume that bubbles are already present
at the point of gas switch, because bubble formation
is basically unrelated to decompression issues (except
for cases of gross tension differential), the
whole gradient-participation theory goes right to hell.

In which case, it's all in the lungs, baby.

But it ain't so simple. The question begs: what's "gross"?

So I think I can boil my previously verbose line of thought 
down to one question: 

When considering tissue tension (NOT bubbles), is it really 
absolute, unrelated to the dissolved percentages of its 
component gasses?

-Will 
"Off-gassing is such a natural thing, it's just
too bad it's still considered rude in public."

On Mon, 22 Dec 1997, Richard Pyle wrote: 
> > "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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