decompression physiologydecompression
sicknessone caseYount, 1988Bruce Weinke's articleJanuary, '96 issue of DeepTechDr. Peter Bennett's editorialLeMessurier and
HillsBennett, P.B. 1996Hamilton, B. and
G. Irvine. 1996LeMessurier,
D.H. and B.A. Hills. 1965Weinke, B. 1995Yount, D.E. 1988
Palau
'Twilight Zone' Expedition
BISHOP MUSEUM
An abridged version of
this article was originally in DeepTech, 5:64; and the full
version subsequently published in Cave Diving Group Newsletter,
121:2-5.
The
Importance of Deep Safety Stops:
Rethinking Ascent Patterns From Decompression Dives
by Richard L.
Pyle
Before I begin, let's make something perfectly clear: I am a
fish-nerd (i.e., an ichthyologist). For the purposes of this
commentary, that means two things. First, it means that I have
spent a lot of time underwater. Second, although I am I biologist
and understand quite a bit about animal physiology, I am not an
expert in decompression physiology. Keep
these two things in mind when you read what I have to say.
Back before the concept of "technical diving"
existed, I used to do more dives to depths of 180-220 feet than I
care to remember. Because of the tremendous sample size of dives,
I eventually began to notice a few patterns. Quite frequently
after these dives, I would feel some level of fatigue or malaise.
It was clear that these post-dive symptoms had more to do with
inert-gas loading than with physical exertion or thermal
exposure, because the symptoms would generally be much more
severe after spending less than an hour in the water for a
200-foot dive than they would after spending 4 to 6 hours at much
shallower depths.
The interesting thing was that these symptoms were not
terribly consistent. Sometimes I hardly felt any symptoms at all.
At other times I would be so sleepy after a dive that I would
find it difficult to stay awake on the drive home. I tried to
correlate the severity of symptoms with a wide variety of
factors, such as the magnitude of the exposure, the amount of
extra time I spent on the 10-foot decompression stop, the
strength of the current, the clarity of the water, water
temperature, how much sleep I had the night before, level of
dehydration...you name it...but none of these obvious factors
seemed to have anything to do with it. Finally I figured out what
it was - fish! Yup, that's right...on dives when I collected
fish, I had hardly any post-dive fatigue. On dives when I did not
catch anything, the symptoms would tend to be quite strong. I was
actually quite amazed by how consistent this correlation was.
The problem, though, was that it didn't make any sense. Why
would these symptoms have anything to do with catching fish? In
fact, I would expect more severe symptoms after fish-collecting
dives because my level of exertion while on the bottom during
those dives tended to be greater (chasing fish isn't always
easy). There was one other difference, though. You see, most
fishes have a gas-filled internal organ called a
"swimbladder" - basically a fish buoyancy compensator.
If a fish is brought straight to the surface from 200 feet, its
swimbladder would expand to about seven times its original size
and crush the other organs. Because I generally wanted to keep
the fishes I collected alive, I would need to stop at some point
during the ascent and temporarily insert a hypodermic needle into
their swimbladders, venting off the excess gas. Typically, the
depth at which I needed to do this was much deeper than my first
required decompression stop. For example, on an average 200-foot
dive, my first decompression stop would usually be somewhere in
the neighborhood of 50 feet, but the depth I needed to stop for
the fish would be around 125 feet. So, whenever I collected fish,
my ascent profile would include an extra 2-3 minute stop much
deeper than my first "required" decompression stop.
Unfortunately, this didn't make any sense either. When you think
only in terms of dissolved gas tensions in blood and tissues (as
virtually all decompression algorithms in use today do), you
would expect more decompression problems with the included deep
stops because more time is spent at a greater depth.
As someone who tends to have more faith in what actually
happens in the real world than what should happen according to
the theoretical world, I decided to start including the deep
stops on all of my decompression dives, whether or not I
collected fish. Guess what? My symptoms of fatigue virtually
disappeared altogether! It was nothing short of amazing! I mean I
actually started getting some work done during the afternoons and
evenings of days when I did a morning deep dive. I started
telling people about my amazing discovery, but was invariably met
with skepticism, and sometimes stern lectures from
"experts" about how this must be wrong.
"Obviously," they would tell me, "you should get
out of deep water as quickly as possible to minimize additional
gas loading." Not being a person who enjoys confrontation, I
kept quiet about my practice of including these "deep
decompression stops". As the years passed, I became more and
more convinced of the value of these deep stops for reducing the
probability of decompression
sickness (DCS). In all cases where I had some sort of
post-dive symptoms, ranging from fatigue to shoulder pain to
quadriplegia in one case, it was on a dive
where I omitted the deep decompression stops.
As a scientist by profession, I feel a need to understand
mechanisms underlying observed phenomena. Consequently, I was
always bothered by the apparent paradox of my decompression
profiles. Then I saw a presentation by Dr. David Yount at the
1989 meeting of the American Academy of Underwater Sciences
(AAUS). For those of you who don't know who he is, Dr. Yount is a
professor of physics at the University of Hawaii, and one of the
creators of the "Varying-Permeability Model" (VPM) of
decompression calculation. This model takes into account the
presence of "micronuclei" (gas-phase bubbles in blood
and tissues) and factors that cause these bubbles to grow or
shrink during decompression. The upshot is that the VPM calls for
initial decompression stops that are much deeper than those
suggested by neo-Haldanian (i.e., "compartment-based")
decompression models. It finally started to make sense to me.
(For a good overview of the VPM, read Chapter 6 of Best
Publishing's Hyperbaric Medicine and Physiology; Yount, 1988.)
Since you already know I am not an expert in diving
physiology, let me explain what I believe is going on in terms
that educated divers should be able to understand. First, most
readers should be aware that intravascular bubbles are routinely
detected after the majority of dives - even "no
decompression" dives. The bubbles are there - they just
don't always lead to DCS symptoms. Now; most deep decompression
dives conducted by "technical" divers (as opposed to
commercial or military divers) are very-much sub- saturation
dives. In other words, they have relatively short bottom-times (I
would consider 2 hours at 300 feet a "short" bottom
time in this context). Depending on the depth and duration of the
dive, and the mixtures used, there is usually a relatively long
ascent "stretch" (or "pull") between the
bottom and the first decompression stop as calculated by any
theoretical compartment-based model. The shorter the bottom time,
the greater this ascent stretch is. Conventional mentality holds
that you should "get the hell out of deep water" as
quickly as possible to minimize additional gas loading. Many
people even believe that you should use faster ascent rates
during the deeper portions of the ascent. The point is, divers
are routinely making ascents with relatively dramatic drops in
ambient pressure in relatively short periods of time - just so
they can "get the hell out of deep water".
This, I believe, is where the problem is. Maybe it has to do
with the time required for blood to pass all the way through a
typical diver's circulatory system. Perhaps it has to do with
tiny bubbles being formed as blood passes through valves in the
heart, and growing large due to gas diffusion from the
surrounding blood. Whatever the physiological basis, I believe
that bubbles are being formed and/or are encouraged to grow in
size during the initial non-stop ascent from depth. I've learned
a lot about bubble physics over the last year, more than I want
to relate here - I'll leave that for someone who really
understands the subject. For now, suffice it to say that whether
or not a bubble will shrink or grow depends on many complex
factors, including the size of the bubble at any given moment.
Smaller bubbles are more apt to shrink during decompression;
larger bubbles are more apt to grow and possibly lead to DCS.
Thus, to minimize the probability of DCS, it is important to keep
the size of the bubbles small. Relatively rapid ascents from deep
water to the first required decompression stop do not help to
keep bubbles small! By slowing the initial ascent to the first
decompression stop, (e.g., by the inclusion of one or more deep
decompression stops), perhaps the bubbles are kept small enough
that they continue to shrink during the remainder of the
decompression stops.
If there is any truth to this, I suspect that the enormous
variability in incidence of DCS has more to do with the pattern
of ascent from the bottom to the first decompression stop, than
it has to do with the remainder of the decompression profile. DCS
is an extraordinarily complex phenomenon - more complex than even
the most advanced diving physiologists have been able to
elucidate. The unfortunate thing is that we will likely never
understand it entirely, largely because our bodies are incredibly
chaotic environments, and that level of chaos will hinder any
attempts to make predictions about how to avoid DCS. But I think
that we, as sub-saturation decompression divers, can
significantly reduce the probability of getting bent if we alter
the way we make our initial ascent from depth.
Some of you may now be thinking "But he said he's not an
expert in diving physiology - why should I believe him?" If
you are thinking this, then good - that's exactly what I want you
to think because you shouldn't trust just me. So, why don't you
dig up your September '95 issue of DeepTech (Issue 3) and read Bruce Weinke's article? I know it covers
some pretty sophisticated stuff, but you should keep re- reading
it until you do understand it. Why don't you call up aquaCorps
and order audio tape number 9 ("Bubble Decompression
Strategies") from the tek.95 conference, and listen to Eric
Maiken explain a few things about gas physics that you probably
didn't know before. While you're at it, why don't you order the
audio tape from the "Understanding Trimix Tables"
session at the recent tek.96 conference? You can listen to Andre
Galerne (arguably the "father of trimix") talk about
how the incidence of DCS was reduced dramatically when they
included an extra deep decompression stop over and above what was
required by the tables. On the same tape you can listen to
Jean-Pierre Imbert of COMEX (the French commercial diving
operation which conducts some of the world's deepest dives) talk
about a whole new way of looking at decompression profiles which
includes initial stops that are much deeper than what most tables
call for. Why don't you ask George Irvine what he meant when he
said he includes "three or four short deep stops into the
plan prior to using the first stop recommended by each of the
[decompression] programs" in the January, '96 issue of DeepTech
(Issue 4)? If that's not enough, then check out Dr. Peter Bennett's editorial in the
January/February 1996 Alert Diver magazine; he's talking about
basically the same thing in the context of recreational diving.
If you really want to read an eye-opening article, see if you can
find the report on the habits of diving fishermen in the Torres
Strait by LeMessurier and
Hills (listed in the References section at the end of this
article). The lists goes on and on. The point is, I don't seem to
be the only one advocating deep decompression stops.
Are you still skeptical? Let me ask you this: Do you believe
that so-called "safety stops" after so-called "no-
decompression" dives are useful in reducing probability of
DCI? If not, then you should take a look at the statistics
compiled by Diver's Alert Network. If so, then you are already
doing "deep stops" on your "no-decompression"
dives. If it makes you feel better, then call the extra deep
decompression stops "deep safety stops" which you do
before you ascend to your first "required"
decompression stop. Think about it this way: Your first
"required" decompression stop is functionally
equivalent to the surface on a dive that is taken to the absolute
maximum limit of the "no-decompression" bottom time.
Wouldn't you think that "safety stops" on
"no-decompression" dives would be most important after
a dive made all the way to the "no- decompression"
limit?
Some of you may be thinking, "I already make safety stops
on my decompression dives - I always stop 10 or 20 feet deeper
than my first required stop." While this is a step in the
right direction, it is not what I am talking about here.
"Why not?", you ask, "I do my safety stops on
no-decompression dives at 20 feet. Why shouldn't I do my deep
safety stops 20 feet below my first required ceiling?" I'll
tell you why - because the safety stops have to do with
preventing bubble growth, and bubble growth is in part a function
of a change in ambient pressure, not a function of linear feet.
Suppose that, after a dive to 75 feet, you make a safety stop at
20 feet. Well, the ambient pressure at sea level is 1 ATA. The
ambient pressure at 75 feet is about 3.3 ATA. The ambient
pressure at your 20-foot safety stop is 1.6 ATA - which
represents roughly the midpoint in pressure between 3.3 ATA and 1
ATA. Now, suppose you're on a dive to 200 feet (about 7 ATA) and
your first required decompression stop is 50 feet (about 2.5
ATA). The ambient pressure midpoint between these two depths is
4.75 ATA, or a little less than 125 feet. Thus, on this dive you
would want to make your deep safety stop at about 125 feet -
exactly the depth I used to stop to stick a hypodermic needle in
my little fishies.
But of course, the physics and physiology are much more
complex than this. It may be that ambient pressure mid- points
are not the ideal depth for safety-stops - in fact, I can tell
you with near certainty that they are not. From what I understand
of bubble-based decompression models, initial decompression stops
should be a function of absolute ambient pressure changes, rather
than proportional ambient pressure changes, and thus should be
even deeper than the ambient pressure mid-point for most of our
decompression dives. Unfortunately, I seriously doubt that
decompression computers will begin incorporating bubble-based
decompression algorithms, at least not in their complete form.
Until then, we decompression divers need a simpler method - a
rule of thumb to follow that doesn't require the processing power
of an electronic computer. Perhaps the ideal method would be
simply to slow down the ascent rate during the deep portion of
the ascent. Unfortunately, this is rather difficult to do -
especially in open water. Instead, I think you should include one
or more discrete, short-duration stops to break up those long
ascents. Whether or not it is physiologically correct, you should
think of them as pit-stops to allow your body to "catch
up" with the changing ambient pressure.
Here is my method for incorporating deep safety stops:
1) Calculate a decompression profile for the
dive you wish to do, using whatever software you normally use.
2) Take the distance between the bottom
portion of the dive (at the time you begin your ascent) and the
first "required" decompression stop, and find the
midpoint. You can use the ambient pressure midpoint if you want,
but for most dives in the "technical" diving range, the
linear distance midpoint will be close enough and is easier to
calculate. This depth will be your first deep safety stop, and
the stop should be about 2-3 minutes in duration.
3) Re-calculate the decompression profile by
including the deep safety stop in the profile (most software will
allow for multi-level profile calculations).
4) If the distance between your first deep
safety stop and your first "required" stop is greater
than 30 feet, then add a second deep safety stop at the midpoint
between the first deep safety stop and the first required stop.
5) Repeat as necessary until there is less
than 30 feet between your last deep safety stop and the first
required safety stop.
For example, suppose you want to do a trimix dive to 300 feet,
and your desktop software says that your first
"required" decompression stop is 100 feet. You should
recalculate the profile by adding short (2-minute) stops at 200
feet, 150 feet, and 125 feet. Of course, since your computer
software assumes that you are still on-gassing during these
stops, the rest of the calculated decompression time will be
slightly longer than it would have been if you did not include
the stops. However, in my experience and apparently in the
experience of many others, the reduction in probability of DCI
will far outweigh the costs of doing the extra hang time. In
fact, I'd be willing to wager that the advantages of deep safety
stops are so large that you could actually reduce the total
decompression time (by doing shorter shallow stops) and still
have a lower probability of getting bent - but until someone can
provide more evidence to support that contention, you should
definitely play it safe and do the extra decompression time. One
final point. As anyone who reads my posts on the internet diving
forums already knows, I am a strong advocate of personal
responsibility in diving. If you choose to follow my suggestions
and include deep safety stops on your decompression dives, then
that's swell. If you decide to continue following your
computer-generated decompression profiles, that's fine too. But
whatever you do, you are completely and entirely responsible for
whatever happens to you underwater! You are a terrestrial mammal
for crying out loud - you have no business going underwater in
the first place. If you cannot accept the responsibility, then
stay out of the water. If you get bent after a dive on which you
have included deep safety stops by my suggested method, then it
was your own fault for being stupid enough to listen to
decompression advice from a fish nerd!
References:
Bennett, P.B. 1996.
Rate of ascent revisited. Alert Diver, January/February 1996: 2.
Hamilton, B. and
G. Irvine. 1996. A hard look at
decompression software. DeepTech, No. 4 (January 1996): 19- 23
LeMessurier,
D.H. and B.A. Hills. 1965.
Decompression sickness: A thermodynamic approach arising from a
study of Torres Strait diving techniques. Scientific Results of
Marine Biological Research. Nr. 48: Essays in Marine Physiology,
OSLO Universitetsforlaget: 54-84.
Weinke, B. 1995.
The reduced gradient bubble model and phase mechanics. DeepTech,
No. 3 (September 1995): 29-37.
Yount, D.E. 1988.
Chapter 6. Theoretical considerations of Safe Decompression. In:
Hyperbaric Medicine and Physiology (Y-C Lin and A.K.C. Niu,
eds.), Best Publishing Co., San Pedro, pp. 69-97.
I would like to thank Eric Maiken for explaining bubble
physics to me and for adding some theoretical foundation to my
silly ideas.
Palau
'Twilight Zone' Expedition
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