Mailing List Archive Mailing List Archive
Richard --
One other point on mixed gas diving: any deco stop on a lower %
mix than the diver was using (eg, switching from helium mixes to
nitrox, or nitrox to O2) is equivalent to a higher deco stop on the
original mix. That adds a rule to your schedule: if any Pyle Rules
stop is a mix change, make the change at the end of the stop. (This
won't usually apply to caves, where "ascents" are more horizontal
than vertical, but also, since cave diving ascents ARE mostly gradual,
cavers are already following a nice substiture for your rules a lot of
the time. Cavers are pretty smart, except for the going into caves
part.)
BTW part of the exhaustion that follows deep dives with minimal
stops might be that the lungs take a hit anytime there are bubbles in the
blood, because that's where they are trapped. Lungs don't have
nerves, but the damage leaks chemicals into the blood that shift
the brain into exhaustion and repair mode.
Just thoughts. I'm an innocent in the woods (demographer), and
didn't give a thought to deco schedules not getting it quite right
until the nonsense about racing to deco stops on here awhile back
reminded me that the purpose of stopping is to keep partial gas
pressures in the tissues from getting too much above the gas
pressure in the blood. Your technique has to be right: coddle the
slowest tissue all the way up, fix the deco stop times to take care
of the rest.
For the models, I haven't looked at VPM, but the *fix* to compartment
models is simple. The models need to lower the allowable difference
between the (inert gas) tissue pressure and lung-blood (ambient)
pressure more than they do for slow tissue. The absolute difference
matters more than the ratio since it (absolute diff.) governs the amount
of pressure any bubbles in the tissue will exert (in bone, the amount of
squeeze). I guess that wipes the computers that allow faster ascents
at depth based on lung overexpansion being a greater risk near the
surface. That rule only fits short dive times.
Now that my neck is stuck out: I'm not an expert here. Anybody who
slows their ascents because I said to is an ok person in my book.
Not necessarily bright, but ok. This is a disclaimer, anything you do
except spill hot coffee in your lap is your responsibility. -- All that's
going on here is trying to figure out why following the rules still gets
people bent. And yes, if you make precautionary stops at depth you
will have to make longer deco stops higher up. That's ok. Beats
getting your swim bladder popped by some crazed ichthyologist,
unless you're a fish, of course.
Dave Smith
U Tx School of Public Health
Richard Pyle wrote (sorry, it's all good so you get it all in case you
missed it):
O.K., here's the deal -- 20 minutes after I posted the offer to send
anyone who was interested a copy of the full-length article on deep
safety decompression stops, I had 23 requests in my inbox. While trying
to zap everybody copies, 6 more requests came in. Finally, Bill Brooks
suggested I just post it to techdiver, and I think he's probably right.
Sooooo...my sincerest apologies to anyone who is not interested. If this
long message cost you some bucks on your service provider, please feel
free to send me a bill.
One more thing - I feel a little guilty for making this article freely
available just after it was published in DeepTech - so I would like to
STRONGLY encourage any of you who do not already subscribe to DeepTech to
do so -- it's a great magazine.
Aloha,
Rich
*******************************************************************
The Importance of Deep Safety Stops: Rethinking Ascent
Patterns From Decompression Dives.
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 fsw 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 DCI. 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 DCI 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 DCI. Thus, to
minimize the probability of DCI, 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 DCI 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. DCI 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 DCI. 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 DCI 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.
--
Send mail for the `techdiver' mailing list to `techdiver@terra.net'.
Send subscription/archive requests to `techdiver-request@terra.net'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]