At 09:10 AM 12/30/99 -0500, Paltz, Art wrote:
>Can you elaborate on Oxygen Exposure?  I remember you saying many times that
>OTU's and CNS clock's are BS.  How do you track this?  What about repetitive
>dives?
>>
Art,

This question requires a far longer response than would be practical right
now. I will see what I can put together in the near future but lets start
with this general outline. Oxygen Exposure is problematic for several
reasons including:
1) High degree of variability in oxygen tolerance
2) Inconsistent measurements and difficulty assessing oxygen tolerance
3) Poor understanding of oxygen toxicity mechanisms

In general the biggest problem with oxygen relates to the significant
variability between individuals and within one individual over time. Plots
of PO2 and time look more like a wide bore shotgun blast than sensible
graph. Subjects would often manage huge tolerances (sometimes several
hours) one day and then tox in a matter of minutes the next day.
Unfortunately tolerances would vary so dramatically that there was not any
notable success in establishing a trend such as increased or decreased
tolerance. 

Most oxygen tolerance testing was done with pure oxygen commonly at 30,60,
and 90'. Many individuals managed amazing tolerances that make current
limits seem ridiculously conservative. However, other individuals
experienced problems very early in the tolerance time limits. Actually
measuring tolerance proved to be a significant problem with exposure
testing while early tests often used time limits based upon actual toxic
events (ie seizures) and later studies limited time to what has become the
commonly recognized "symptoms" known by the acronym VENTID. As a result
some individuals that managed significant times when pushed to seizure were
then limited by the occurrence of a symptom perhaps prematurely. The
occurrence of symptoms did seem to at least loosely relate to toxicity
incidence but unreliably and with many complications. This complication in
the measuring process further skews what one might refer to as the "actual"
toxicity time limits


Furthermore, our inability to get a handle on the mechanisms behind oxygen
toxicity confuse limiting its impact and/or measuring the time limits. For
example, consider immersion in water and its role in the development in
oxygen toxicity. Merely being immersed (as opposed to being in a dry
chamber) increases the risk of oxygen toxicity. Being immersed in hot or
cold water (either pole is similar in impact) decreases tolerance. Numerous
factors affect one's likelihood of succumbing to toxicity but our
understanding of these mechanisms and our ability to manipulate them is a
very inexact process and highly problematic

The preceding issues are important in considering oxygen toxicity
manifestations and in discussing the "oxygen clock". This clock "works" by
relating the chosen maximum time at a given PO2 (ie 45@1.6) to a percentage
of accumulated time at this PO2. Of course, the success or failure of this
clock relates to the accuracy of the time limit which is confused by
susceptibility. The 100% (ie 45min at 1.6) limit suggests that as you
exceed this parameter your risk of oxygen toxicity markedly increases.
Actually it is a bit more complicated than this but in general this is the
idea. However, on our dives we regularly exceed several thousand percent
and have pushed into the 10,000% range leaving these numbers to seem
meaningless. While they are not meaningless and are in fact a good but very
general rule of thumb it is important to see these "limits" for what they
are- a forced best guess based on highly variable and conflicting data.
Realistically it seems that these limits are probably overly conservative
for the vast majority, good for some, and not enough for a very few.
However, given the risk (ie seizure and likely drowning) it is prudent for
most people to stay near the limits. These "limits" are really more of a
range that should be used as a generally sensible guideline. 

The problem in technical diving is that an ardent belief in these numbers
leads people to assume that they must save the extra 5 or 10% off their
clock by using odd gas mixtures. It is very likely that divers are best
served by regular breaks from oxygen (for deco benefit and extended O2
tolerance) and the conservative use of PO2 (such as 1.4 or less for
diving). Oxygen tolerance limits are in many ways similar to Decompression
limits in that they are likely too conservative for many and highly
variable for most individuals. Furthermore, violating these "limits" may
increase the risk of an unpleasant outcome or more likely will produce no
noticeable impact. However, the risk of violation (which includes pain and
death) is likely not worthwhile for the vast majority of divers. These
ranges should be viewed with respect but also an understanding of their
history and an appreciation for the variation that leads some divers to
reevaluate their role in technical diving.

Safe diving,
JJ






ent:	Wednesday, December 29, 1999 2:26 PM
>	To:	Shimell, David (shimell)
>	Cc:	QUEST@GU*.CO*; techdiver
>	Subject:	Re: SAMPLE DECO DIVE - 220 FOR 25

>
>	I ignore the first dive - in this case you have mushed the bubbles
>	anyway. After enough time, I ignore any first dive other than as
>regards
>	the oxygen exposure - that is where the risk lies. 
>
>	Shimell, David (shimell) wrote:
>	> 
>	> George
>	> 
>	> Here's another question.
>	> 
>	> Say you do your 220' for 25 and then ascend to do a multi-level
>dive to say
>	> 90' for 25.  How would you calculate the deco for this?  I recall
>you once
>	> talked about superimposing one table on another but I had it on my
>list of
>	> things to look at.
>	> 
>	> David Shimell
>	> Email: shimell@se*.co* <mailto:shimell@se*.co*>
>	> Project Manager, IBM NUMA-Q, Sequent Computer Systems Limited,
>	> Weybridge Business Park, Addlestone Road, Weybridge, Surrey, KT15
>2UF, UK
>	> registered in England and Wales under company number: 1999363,
>registered
>	> office as above
>	> 
>	> -----Original Message-----
>	> From:   kirvine@sa*.ne* [SMTP:kirvine@sa*.ne*]
>	> Sent:   Friday, December 24, 1999 12:43 PM
>	> To:     QUEST@GU*.CO*
>	> Cc:     techdiver
>	> Subject:        SAMPLE DECO DIVE - 220 FOR 25
>	> 
>	> Let's run through a sample dive. I will throw out the parameters
>and
>	> the standeard deco on one side, and my changes and why on the
>other ,and
>	> then we can discuss it.
>	> 
>	>  We can then build the dive into a longer dive, a deeper dive and
>we can
>	> add more gasses and more stituations, and then the equipment to do
>it.
>	> 
>	>  Let's start with this and get the discussion going that way. I
>need all
>	> questions, no matter how sophisticated, so we can get out the
>rock.
>	> 
>	> ***PROFILE 220 FOR 25                   CHANGES
>	> ***GAS 16% OXYGEN  50% HELIUM   1.2 PPO2
>	>                                 85 AED
>	> 
>	>          REASON FOR GAS CHOICE: the more heluim, the better . It
>is
>	> easier to breathe at depth, and it is easier to decompress from,
>	> contrary to what you have been told elsewhere. The reduced narc is
>	> obvious. I chose the 50% heliuum for this , but the more the
>better. The
>	> oxygen ppo2 should be kept intentionally low. The reason is that
>you do
>	> not want to unnecessarily deplete brain chemistry to pick up
>"perceived"
>	> deco advantage, expescially in light of the fact that we have
>boosted
>	> the helium which reduces the nitrogen damage and loading, and thus
>the
>	> deco requirement. While inert gas is inert gas for purposes of
>this
>	> discussion ( or said another way, the oxygen window is the oxygen
>	> window) the reality is that helium changes the physiological
>factors
>	> that are as real in deco as the straight compartment loading and
>	> unloading factors. Very important to us in our diving, and more so
>as
>	> the dives get more severe or repetitive. We also do not want to
>burn the
>	> shit out of our lungs with high PPO2's either, and keep in mind
>that
>	> depending on the bottom time, we are going to be forced to expose
>	> ourselves ot elevated oxygen to decompress.
>	> 
>	> It is critical in multiday exposure that you reduce the oxygen
>dose, and

>	> we will show how to do that in each decompression.
>	> 
>	> STANDARD DECO FROM A PROGRAM            REAL LIFE DECO
>	> 
>	> DEPTH   TIME    GAS                     DEPTH   TIME    GAS
>	>                                         160-120 1 MINUTE EACH ON
>BG
>	> 110     1       16/50                   110     1
>	> 100     3                               100     1
>	> 90      4                               90      1
>	> 80      5                               80      1
>	> 70      3       50/50                   70      5       50/50
>	> 60      4                               60      2
>	> 50      5                               50      3
>	> 40      9                               40      5
>	> 30      11                              30      8
>	> 20      17      OXYGEN                  20      13      OXYGEN
>	> 10      26                              20-0    8
>	> 
>	> TOTAL 88                                TOTAL   60
>	> 
>	>                     REASONS FOR CHANGES
>	> 
>	> 1) we start our deco at 80% of the profile in atmospheres, or in
>this
>	> case , around 160 feet. We are not changing gases yet, so no
>reason to
>	> sit on these stops. The ascent rate is 30 fpm, or the equivalent
>of a 20
>	> second stop every ten feet from the bottom, so we are really only
>adding
>	> 40 seconds per "stop" from 80% of the profile. This actually
>"maxes out"
>	> at about 5 minutes per "deep stop" in anything approaching
>saturations,
>	> which I call 150 minutes for the purposes of decomprssion reality
>as
>	> opposed to trying to decompress a whale. In a long dive, we also
>use the
>	> first deco gas at 80% of the profile . Not in play here.
>	> 
>	> 2) moving up we do not extend these stops per Bulhmann, since we
>have
>	> already moved the "ceiling" quite a long ways above us, and are
>not yet
>	> really pressing the gradient ( in percentage or relative terms, ie
>the
>	> ratio of the atmospheric change deep a opposed to shallow). We
>need to
>	> get to a deco gas first and spend some time, so......
>	> 
>	> 3) we "SIT" on the 70 foot stop for quite a bit longer than is
>suggested
>	> by theory. The reason is we want to use that wide open oxygen
>window for
>	> all it is worth down there to both clear the slate as much as
>possible,
>	> and to move that ceiling again quite a bit so that we can
>abbreviate the
>	> stops above as their ppo2 declines, rather than lengthening them
>as
>	> either Bulhmann or bubble mechanics would indicate - real life
>says we
>	> are correct. In fact, I have SKIPPED up to 100 minutes of our 40
>foot
>	> stops with no repercussions, and Lucy Ho can vouch for me on this
>( and
>	> the doppler says so). The other reason is that we want to give the
>blood
>	> a chance to totally circulate with the new gas. While it hits the
>key
>	> body parts immediately ( spine, heart, brain), it takes a couple
>of
>	> minutes to get to everything properly. We do not want to hammer
>high
>	> popo2's, we want to use them to our advanntage.
>	> 
>	> 3) since we have done our deep stops and our high ppo2's, both
>moving

>	> our ceiling and shortening our upper time, we can go ahead and
>press the
>	> gradient as it condenses, and shorten the next few stops and get
>to that
>	> oxygen.
>	> 
>	> 4) the oxygen does not have to be hit too ahrd. I like 12-13
>minute
>	> stints with 6-8 minutes off. The reasons are that it takes almost
>no
>	> time to saturate with oxygen at any ppo2 once the immediate
>offgassing
>	> slows - a few minutes - and the damage starts to accrue after
>about
>	> 16-20 minutes in our experience, depending of course on the
>dosages
>	> already taken on the way up to this point.  The swelling of the
>lung
>	> tissue picks up after a few minutes, and the gas transfer process
>is
>	> severely hampered. Vasoconstriction gets worse, and you are
>pissing in
>	> the wind honking on the oxygen. You need to break to back gas, or
>in
>	> this case , just move up and get out.
>	> 
>	> 5) slow final ascent - the last move up to the surface is one of
>the
>	> most critical. No matter how long you sit on a stop with the
>window wide
>	> open, there will still be gas that will not be displaced by this
>method.
>	> When you increase the gradient by trying to surface, the last of
>the gas
>	> comes out rapidly and does so in bubble form. You must do a slow
>ascent
>	> to reduce the risk of this, and for long dive it is one foot per
>minute
>	> to the surface, for this dive it is more like 2.5 - 3 fpm.
>	> 
>	> 6) at the surface, sit still for a bit and try not to exert for
>about 30
>	> minutes thereafter. Bubbles forming at this time will now grow as
>they
>	> pick up offgassing nitrogen ( the helium is long gone) and will
>become
>	> problematic later, and severely so if you have a shunt that can
>then be
>	> opened by the in increasing pressure on the cappillary beds of the
>	> lungs.
>	> 
>	> OK - let's take it from here and get the discusion going.  Please
>do not
>	> copy back the whole message or it will become a mail bonb - just
>cut and
>	> paste the parts you want to discuss, and lets discuss one item per
>	> email, SVP. When we expand this to a long dive, then I will cross
>copy
>	> it to WKPP@eg*.co*, and we will bring in the big guns ( Rose,
>Mee
>	> ,et al).
>	> 
>	> Let's go .
>	> 
>	> --
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