From: dave@hpdstma.ptp.hp.com (Dave Waller)
Subject: Re: Fat not a contributing factor to the onset of DCS?
Organization: Hewlett Packard
Date: Thu, 8 Oct 1992 21:05:45 GMT

In article <1992Oct8.163520.5519@PA.dec.com>, fgz@kodak.pa.dec.com (Federico Genoese-Zerbi) writes:
|> 
|> Those who subscribe to this notion point to the fact that fat is
|> poorly perfused, and therefore is slow in offgassing (it's also slow
|> in ongassing -- but this part is often glossed over).

This is not true... The halftime is not ignored w.r.t. offgassing. However,
remember that the rate of on or offgassing is not dependent solely on the
halftime, but is a function of the halftime AND the ambient pressure.
This is why it is possible to ongas to the M0 for a theoretical compartment
much faster at depth than the corresponding time to completely offgas
on the surface -- the pressure gradient is substantially greater while
under water.

As such, a slow compartment can ongas to its M0 within a reasonable series
of dives, while the time it takes to remove this N2 can be many times greater
(indeed, it almost always will be) than the total bottom time.

|> Yet this fact
|> alone should not make for an added risk of DCS.  If the tissue's half
|> time is appropriately modeled by the tables or computer being used to
|> plan dive profiles, then it should be no more a risk factor than any
|> other slow (or fast) comparment.

Here is one of the basic misconceptions that leads to the analysis you
have provided, yet because of the faulty basis, your conclusions are
not useful.

Haldanean based computers model the entire body based on subdividing the
theoretical continuous range of tissue types by quantizing them into a
set of "compartments" with specific halftimes. However, there is no
correspondence to actual tissues, and as such, the approximation of
quantizing the infinite number of possible halftimes into a discrete
set of compartments cannot perfectly model the body.

Further, the model has no way of knowing what the distribution of these
tissues, and their relative proportion overall. As a result, the computer
can only give a general approximation of the physiological processes of 
N2 absorption. If a person is grossly out of proportion to the
average body characteristics that are being modeled by the computer, the
predictive results can be expected to similarly have an increased rate
of error.

For example, consider the fictitious case of a diver made up entirely of
the fastest tissues. Ofr all intents and purposes, this person can be
modeled by the fastest compartment. If this person were to take down
a typical dive computer and dive at 100 feet for a long time, incurring
a two hour decompression obligation on the computer, this information
would be grossly inaccurate, as this person could decompress in considerably
less time due to the absence of slower tissues.

Similarly, if a body is composed of a disproportionate amount of slow
tissues, the model no longer represents this diver very well, and the
individual can wind up bent when the computer says they're fine. Statistics
seem to confirm this hypothesis.

|> And, in the end, I find the experimental evidence that obesity is a
|> contributive factor DCS to be lacking, which is the ultimate test..

So you don't believe the statistical evidence? We are talking about a
statistically significant number here.

|> While it is true that DAN statistics do show correlation between being
|> overweight and being hit, other numbers contradict this. 

What other numbers?
 
|> First of all, the increased likelyhood is not geometric with
|> percentage of body fat. [rest of explanation elided for brevity]

One would not expect a first order relationship such as this. Remember that
there are alot of other tissues in the body, which must be considered as
well (DCS does not occur exclusively in fat tissues, thereby skewing the
statistics).

Further, human physiology is a very complex subject, and as such, simple
mathematical models such as the one you are proposing do not generally
apply. Heck, the simple mathematical model developed by Haldane is
turning out to be a gross simplification that at best approximates the
statistics, but may have nothing to do with the actual physiology.

|> The other forgotten factor is fitness.

It is not forgotten at all, rather, it is cited as a potential factor for increased risk of DCS, although there are no theories (to my knowlege)
that explain a correlation here, nor am I aware of any statistical
confirmation that there is indeed a connection.

|> It is possible to be
|> extrememly fit yet obese.

Well, I know quite a few Physicians that would argue against this point vigorously. I take issue with your use of the word "extremely", and I'm
not sure what you mean by "obese". There is a quantitative medical
definition of obese that is something like 20% or more over what is
expected normal body weight for an individual (I'm not sure of the
precise figure).

However, regardless of how much stamina and strengh an obese person may
have, which from what I can discern from your statement here are the
factors you are including in the definition of "fit", they are still
unfit in a variety of other ways that are related to their body fat and
not the strength or stamina of their muscles and cardiovascular system.

Even "fit" obese persons have higher blood pressure, greater risk for
plaque deposits in the arteries (atherosclerosis), and significant
marbling of fat in the muscle tissue throughout the body. These
characteristics do not qualify as "fit" in most medical experts opinions.

Further, high blood pressure may be an added risk factor for DCS as well.

|> But the majority of obese individuals are
|> not very fit.  And since the studies I've seen suggesting a link
|> between obesity and DCS do not eliminate fitness (which could've been
|> quantified rather than briefly mentionned as a "potential player") as
|> a confounding factor, they would seem exceedingly flawed.

In order to make this assertion, you must first define what "fit" means,
and then offer at least a hypothesis for how it can affect DCS
susceptability. Just such a hypothesis does exist for increased body
fat percentage, and the statistics seem to confirm this hypothesis as
valid.

What does "fit" mean, and how can it influence susceptability to DCS?

|> The final straw, in my opinion, is that women for any given fitness
|> level have a higher percentage of body fat than men.  Yet it has
|> become clear that, despite the grim warnings of the "fat is bad for
|> diving" zealots, women seem to have no added risk of DCS than men.

I'll have to check with DAN on this; last time I was exposed to this
kind of information, I was told that women do indeed have a statistically
higher rate of DCS.

-- 
Dave Waller
YMCA Instructor