Some interesting things came out of the GUE conference. Nothing was
really a surprise to me, but what really interested me is that there are
studies that are starting to prove out what we've believed to be correct.

Most of the hyperbaric studies and knowledge are centered around air and
recreational diving. The volume of information around helium and the
style of diving that DIR/GUE supports is paltry in comparison. So I was
heartened to hear of some of the work that David Doolette has been doing
for the Navy. As a side note, if you have the chance to talk
to him, ask him about tuna wrangling.

Anyway, back to the topic at hand .... one of the studies he did was on
N2 and He uptake and washout. No one has really done a study
on this within humans. He commented that after he set up and ran the
study he "understood why no one else had done it." (due to the
difficulty in setting it up)

The result was quite interesting. Up until that point most people had
operated under the assumption that the uptake of He occurred much faster
than N2 due to its molecular properties (smaller size and increased
diffusion rate). The oft-quoted factor was that He ongassed about 2.4
times as fast as N2, so the theory was that our inert gas loading would
be much higher with He. What David found was that the uptake of He was
roughly the same as N2 in all critical areas of the body (brain, heart,
spine, etc), as well as most other areas of the body. He didn't
specifically state his theory on this, but my guess is that even though
the helium atom moves three times as fast as the nitrogen molecule in
terms of its diffusion rate, the nitrogen molecule can cross the cell
membrane more rapidly as it has a higher permeability constant. Or
something to that effect. Bear in mind that's just a guess on my part.

However the overall washout was faster with He than N2. While he didn't
expound on it much, the impression I got was that he believed it was due
to the lower solubility of He in fatty tissue.

I asked David if he would send me the paper he wrote on this so I could
get more details. He said he would, so hopefully I'll have that
sometime soon. <note - I've since gotten a couple of David's papers - I haven't digested them yet, but will come back with more after I read them>

Anyway, it shows that in addition to the numerous other properties of He
that we like, it truly is a friendlier deco gas (and this doesn't even
talk about the body's and blood's reaction to high pressure N2 vs He) ....

There was also a discussion about isobaric counter-diffusion. One thing
I learned from that is that GI's explanation of it sounds less offensive
when done with a German accent. :-P (Michael Waldbrenner so kindly
related it to us) But anyway, the short version is that ICD can
actually occur but not in any application we'd ever see in diving.
You'd have to be completely saturated with He, using a heliox mix and
then switch to a pure nitrox gas without ascending (note that switching
to pure O2 is not a problem). Thus we'd never see it because even if
you were using heliox, you'd be ascending from depth before you switched
to an N2 based gas, and even with that you'd still have a good portion
of He in your mix.

In a partially-related vein, there was a question about using helium in
deco mixes. There haven't been any actual studies on this, but JJ said
that he found that with longer dives there was a clear (positive)
difference using a higher-helium mix. The same theory should apply to
shorter dives but he said he's noticed less of a benefit there.
Personally I continue to use He even in my 50% bottle, in part because
of the theoretical advantage, but also in part because the bottle rides
better that way.....

More from the conference later ...

-Dave