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I have a question for mixed gas diving gurus or medical specialists.

During my mixed gas diving training, years ago, the instructor told us to
switch back and forth between Air (or Nitrox) and Trimix when making a gas
switch to avoid isobaric counter diffusion problems.  As well, Terrence
Tysall decided to use a transitional Trimix mixture to avoid the same type
of problem during his dive on the Edmund Fitzgerald.  My instructor stated
that inert gas isobaric counter diffusion could cause unconsciousness.

My past understanding (before I took 'mixed gas') of Inert Gas Isobaric
Counter Diffusion was that when switching from a 'slow' gas to a 'fast' gas,
it is possible to suffer from DCS because of counter diffusion of inert gas
in body tissues.  For example, if you breathe air for a couple of hours and
you switch to Heliox (which has no nitrogen), you should eliminate nitrogen
slower than helium gets in your tissue and therefore it is possible to have
tissues supersaturated with inert gas (without ascending).  Inner ear DCS
problems have occurred this way (vestibular DCS).  I got this information
years ago from an article of David Sawatzky, a discussion with Ronald Nishi
and other sources.

But I fail to understand how this might be a problem if you do not have time
to accumulate a lot of nitrogen.  I also fail to understand why it could be
a problem when switching back to Air or Nitrox from Trimix ('fast' gas to
'slow' gas).  And finally, I fail to understand what in this mechanism cause
unconsciousness.

I am aware that DCS due to inert gas isobaric counter diffusion may also
occur due to using a suit gas different from the breathing gas (if suit gas
is faster than breathing gas - or else, Argon would be a major problem for
skin bends).  My question is only about switches from one breathing gas to
another one.




Michel Therrien
m.therrien@ne*.qc*.ca*

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