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> I had some e-mail exchanges with Chris Parrett and Karl Huggins about this
> issue several months ago. In basic scuba training you are taught to
> consider TAD (time, air, depth). None of the tables actually take into
> account the *amount* of nitrogen you breath in, based on your size (i.e.,
> lung volume), work load etc. Oh, there are "J-factors" in almost every
> program that will empirically add minutes to your bottom time and/or deco
> for cold and work, but nowhere does any program I know of ask you your
> weight/height/body surface area that might be used to compute your actual
> dose/kg or dose/M2 of nitrogen.
Nor do they take into account factors that affect perfusion (i.e.,
exertion levels), gas-phase effects, blood clotting and complement
effects, differences in solubilities of different gases, level of
hydration, etc., all of which which may play even larger roles.
> If everyone is a 180lb 24 year old Navy diver doing exactly the same work,
> then this doesn't make much difference. The tables (or computer
> algorithms) are probably conservative enough that you are not placed at
> significant risk. Not too many divers get bent.
Tables sorta work most of the time, but they are grossly conservative for
many people on many dives, and are not conservative enough for many other
people on many other dives. Chaos ultimately rules, but I think we can do
generally better by making no-D times shorter, considering "safety stops"
as required deco stops (yes, even for recreational divers), and changing
the shape of decompression schedules from deep dives to include deeper
initial stops, and probably shorter shallow stops.
Another problem, of course, is that there is no clean line between "bent"
and "not bent" - it is a broad continuum from one extreme, defined in
terms of symptoms (subjective at best). As far as I'm concerned, given
the evidence of CNS lesions on people who have never reported DCS, the
line we draw between "bent" and "not bent" needs to shift more towards
the subtler symptoms end of the spectrum.
> But your note below
> reminds me that the explanation of why women may get bent more frequently
> than men has never been explained satisfactorily. It has even been
> suggested that men may be less likely to complain of pain (type II DCS)!.
> Right, check out your wife or friend next time they have baby!
I have no idea whether the staistics actually show that women are
statistically more likely to get bent than men, or whether this is
because statistically women have more body fat than men. I only brought
it up as an example of how statistics, which apply to populations, can be
grossly abused when extrapolated to individuals. It may well be that
women are more likely to get bent than men, and this may well be because
women tend to have more body fat. The abuse of statistics would be to
say that my rock-climber wife is more likely to get bent than the
couch-potato instructor on the grounds that women are more likely to get
bent than men because women, statistically, have more body fat.
Regardless of the specifics, the point is that statistics can tell us
what proportion of the population will be bent under given circumstances,
but they cannot calculate probabilities that a given individual will get
bent on a given dive (at least not with any shred of validity).
> Let's suppose that your couch-potato instructor has a SCR of 0.6 L/min: At
> the most simplistic level (not taking into account body fat composition or
> surface area, the latter is usually a better measure of drug toxicity
> resistance than weight), your wife would have to have a SCR of < 0.3 L/min
> just to get about the same N2 dose on a dive of the same profile. I am
> sure that her SCR and those of many other women are less than the men they
> dive with, but *that* much lower?
But we don't know anything about their blood chemistry, and we don't know
if they have PFO's, and we don't know a lot of other factors. So how do we
know that the factors outlined above are not dwarfed by the other factors
to the point that our calculations are meaningless? We don't. Even if we
could ultimately work out all the factors, it won't help us in diving
because it will be very unlikely that all the relavent factors can be
easily measured on a given indivudual on a given day. Some of the
factors may even change throughout the course of the dive. I suppose if
we could devise a portable MRI system in a housing that could scan our
bodies at molecular resolution, and if we could link that to a miniature
Cray computer, then we might actually be able to predict DCI with some
reasonable degree of confidence. Until such time, we're gonna be
flapping in the breeze.
> Obviously your wife isn't getting bent, so the tables "work"
She's not getting bent because she doesn't dive any more (she spends what
little free time she has rock climbing; or I should say, if she ever gets
any free time, she would rather go rock climbing than diving). The tables
seem to work for most of the people most of the time, but there are many
cases of people who get bent well within the tables, and MANY, MANY, MANY
cases of people who do not get bent diving way the F*** outside the
tables. And, of course, we must remember that the definition of "bent" by
the standards presently used is probably allowing more damage to take
place than most of us really want to deal with.
> , but they
> aren't working for some women and this may be particularly true of women
> (and men) doing repeat dives, multi-day diving or who have high SCRs
> because they are new to the sport. One recommendation has been to take a
> day off diving every third or fifth day. This is pretty expensive if
> you're paying for a live-aboard or a once-in-a-lifetime trip.
There are a lot of "patch" (in the computer software sense) solutions to
reducing liklihood of DCI. I think the best two are safety stops on "No-D"
dives, and deep stops on decompression dives (both of which probably work
in the same way). They are all just shot-gun approaches to fix the
fundamentally flawed (=enormously oversimplified) decompression schemes
in popular use today.
> To pay attention to this variable you need to know how much gas you are
> actually breathing, not just blowing off. So *now* with rebreathers you
> can do just that! (I knew I could work this in somehow) You could also
> make adjustments to the standard air tables too. Why don't we do this? Is
> this to discourage women from diving? Is it OK that they and smaller couch
> carrot men are exposed to an extra risk?
We don't do it because none of us really knows what's going on, and since
nobody knows what's going on (the most knowledgeable folks are the ones
who seem to least know what's going on - which means the lesser
knowledgable folks are just fooling themselves into believing they know
what's going on). Another reason we don't do it ("We", in the collective
sense of divers and people who advise divers on decompression) aren't
willing to go out on a liability limb by proposing modifications to
"community standards" that are based on something like 3.5 million hours
of testing. ( ;-) for those who caught that one).
> Safer diving through wiser physiology
Again, AMEN to that!
Aloha,
Rich
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