Dive Pacific Iss 171 Oct- Nov 2019

increasing any supersaturation that already exists and
potentially being the final straw that leads to bubble
formation. This process is illustrated in Figure 2.
One way of avoiding this problem is simply to avoid gas
switches during decompression, or taking care with
the depth at which you make such switches, but the
latter discussion is beyond the scope of this article.
Figure 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.
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Normal scuba air divers vulnerable too
Non-technical divers may be feeling smug that this
mechanism does not apply to them, but normal scuba
air divers are also vulnerable to inner ear DCS, particularly
if they venture down around the 25-30m depth
much. Dives to these depths (or deeper) are almost
certainly associated with a greater degree of bubble
formation in the venous blood on surfacing.
Some bubbles form in the veins relatively commonly
and rarely do us harm. But if you form a lot of venous
bubbles, and combine that with a means by which
these bubbles can avoid being filtered by the lung
capillaries, and enter the arterial circulation (such as a
patent foramen ovale (PFO)), then problems can arise.
Small venous inert gas bubbles entering the arteries
can distribute around the body in the blood. Problems
can arise if they find their way into the capillaries of
functionally important organs that remain supersaturated
with inert gas after the dive at the time these
bubbles arrive. The inert gas dissolved in the tissue
can then diffuse into the bubble and cause it to grow
and create problems – where normally it might not.
The inner ear tissue remains supersaturated with inert
gas for about 30 min after a dive, so that is a danger
period should any small bubbles arrive there in the
blood. It is no surprise that the typical inner ear DCS
case in a recreational air diver arises around 30 min
after an air dive to 25-30m, and that most of these
cases turn out to have a large PFO.
In summary
The reason we are seeing more of this fascinating but
distressing problem is almost certainly more divers
are becoming adventurous and going “tech” or pushing
their scuba air diving a little deeper than previously.
Inner ear DCS is treatable with recompression, so don’t
ignore severe dizziness/vertigo arising early after
diving (or during decompression from deep trimix
dives). It may be inner ear DCS and you need to contact
the DES line as early as possible.
Reference:
1. Azzopardi CP et al. Increasing prevalence of vestibulo-cochlear
decompression illness in Malta – an analysis
of hyperbaric treatment data from 1987 – 2017. Diving
Hyperb Med. 2019;49(3):161-6
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