P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
Discussion
IAN MACKNIGHT: Here's a very simple question. I understand the problem, and
thank you for making it so clear, but the obvious question is what does one do about
it?
SIMON MITCHELL: That's a good question. We've written a paper on this
accident, which is referenced in the manuscript and which does discuss the issue in
more detail. You do have to understand that David Shaw's accident occurred at an
extreme depth. (Editor’s note: see Technical Diving Conference DVD on DAN
Website for video.) But that's where technical divers are heading, so we need to
consider this issue. I think there are a couple of things that can be done. First,
recognize the signs of carbon dioxide toxicity and do something about it early, which
means rest early, stop doing what you're doing early. Reduce your work of breathing
in some way early. So bailing out might have helped, perhaps, if David could have
ventilated himself using his second stage purge button early. Once you get into effort
independent exhalation though, it may be too late for that. Nevertheless, trying to
stop the process early is one issue. Second, configure your equipment to make a lower
work of breathing a priority. David had a fine grade CO 2
absorbent. He could have
had a courser grade. There were also a few equipment configuration issues, which
probably increased the resistance to breathing. But all of those are things that got him
into the spiral. It's not a way of getting out of it unfortunately. Once you're in that
spiral, it's very difficult to get out of it. Third, one of the issues that's been raised with
me is whether flipping over onto his back and giving himself a positive static lung load
(Editorial note: David Shaw’s rebreather had a back-mounted counterlung) might
have helped. A positive static lung load is a bit like what we refer to in medicine as
constant positive airway pressure, where you provide a bit of pressure to try to keep
the airway open. That may have helped, but I don't know the answer. To summarize,
there's a few levels at which you can intervene, but most of them are in the lead-up to
the event rather than what you do about it if you get into that spiral. Do you have any
comments you want to make about that, Dan?
DAN WARKANDER: Probably he was too deep, and maybe you could have gotten
shallower early, or used gas with more helium, but it's too late once you're there. It's
not an easy situation.
SIMON MITCHELL: The issue of gas is interesting. I'm trying to remember, but I
think it was something 4:80 he was breathing at the bottom. So he had about 16%
nitrogen. His reason for having that was to try to ameliorate the high pressure nervous
syndrome (HPNS), which we're going to hear about later today. With more helium
his HPNS might have been worse. He might have been exchanging one problem for
another.
Technical Diving Conference Proceedings 35