Dive Pacific Iss 171 Oct- Nov 2019

Fig. 1 Advantageous counter-diffusion after a helium to
nitrogen switch during decompression. This situation
would likely apply in all tissues except the inner ear.
Fig. 2 Disadvantageous counter-diffusion in the inner
ear after a helium to nitrogen switch during decompression.
The fluid pools are represented by the darker blue
panel.
Terrifying instance
I had one particularly terrifying
experience with a buddy
suffering this problem during a
decompression from a 120m dive
when we still had two hours of
decompression to go. He was so
dizzy he could not open his eyes
and spent those hours coming off
the rebreather to vomit on open
circuit scuba, and going back on
the rebreather again. It was a
remarkable feat of diving skill and
endurance that we were able to
complete the decompression. The
puzzling thing is that, as in this
case, it is often only the inner ear
affected by the problem. Why only
the inner ear? Why aren’t other
organs affected at the same time?
Unique risk factor…
The answer is that the inner
ear has a unique risk factor for
developing excess supersaturation
during decompression from deep
dives: it is the one and only organ
in the body vulnerable to isobaric
counter-diffusion problems
(there’s a term you can drop into a
diving medicine conversation!).
In deep diving helium is typically
blended with oxygen and nitrogen
as ‘trimix’ and used for its
non-narcotic, low density properties.
In decompressing from deep
dives it has been common practice
to switch from helium breathing
to nitrogen breathing (air or
nitrox) during the decompression.
In theory, this should result in
faster reduction of tissue inert gas
because the helium being a light
molecule will diffuse from tissue
to blood faster than nitrogen will
diffuse from blood to tissue.
The principle is illustrated in
Figure 1 which shows a theoretical
tissue with a blood vessel
passing by just after a switch from
…normal scuba air divers are also vulnerable to inner ear
DCS, particularly if they venture down around the
25-30m depth much…
breathing primarily helium to
primarily nitrogen. The helium
diffuses rapidly into the blood
(large arrow) because there is
little helium there after the gas
switch to nitrogen breathing, and
nitrogen diffuses into the tissue
but more slowly (smaller arrow)
than the helium diffuses out. The
net effect is a faster reduction of
inert gas pressure in the tissue
and this process of the two gases
diffusing in opposite directions is
called isobaric counter-diffusion.
Figure 1. Advantageous counter-diffusion
after a helium to
nitrogen switch during decompression.
This situation would
likely apply in all tissues except
the inner ear.
…because of unique
anatomy
Unfortunately this does not quite
work for the inner ear because of
some unique anatomy. The inner
ear is tiny, but it contains several
relatively large pools of fluid that
can absorb and eliminate gas from
the blood – but only through the
sensitive neural tissues. These
pools of fluid have no blood supply
of their own. If this fluid has
absorbed a lot of helium during
the dive, and it only loses it slowly
through the tissue space, then a
switch to nitrogen will have the
opposite effect to that described
above. Yes, helium moves from
the tissue into the blood faster
than the nitrogen diffuses from
the blood into the tissue, but the
helium pressure in the tissue is
constantly topped up by helium
moving into the tissue from the
adjacent pools of fluid, and so the
helium pressure in the tissue does
not fall.
The net effect is that the diffusion
of nitrogen into the tissue will
transiently increase the dissolved
gas pressure in the inner ear, thus
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