Peter, this issue is a personal one between you and Gilliam and should be 
addressed as private email. I do not know Gilliam personally but 
although no one is writing a letter campaign re: him, clearly you are 
addressing over 500 divers on this list. Your points are well taken BUT 
get a life. Go diving. Try to save some of the 50,000 people slaughtered 
on our highways each year or clean up LA. A few dead divers is a drop in 
the bucket in the overall scheme of things. Rod

On Wed, 12 Jun 1996, Peter N.R. Heseltine wrote:

> June 12, 1996
> 
> Bret C. Gilliam, CEO
> Uwatec USA Inc.
> One Marcus Drive, Suite 201
> Greenville, SC 29615
> 
> Dear Mr. Gilliam:
> 
> Your letter dated June 4 arrived while I was overseas, so delaying my
> reply to your questions.
> 
> In our telephone conversation, I clearly explained my interests and
> background. Your letter indicates that you remain confused. As a physician
> and professor of medicine, I am called on to examine and treat divers, as
> well as advise them and other physicians on medical issues related to
> sport diving. A recreational diver for the past twenty years, I have taken
> advanced training with PADI and training in nitrox and extended range
> diving with PSA. As you note in your letter, I am in the process of
> completing training on the Drager/Uwatec Atlantis I rebreather, under the
> auspices of TDI.
> 
> Because of the growing popularity of nitrox sport diving, including the
> use of semi-closed rebreathers, more recreational divers than ever before
> are being exposed to partial pressures of oxygen greater than 1 ATA.
> Because of my interests, as noted above, I will continue to learn as much
> as I can about the medical aspects of such exposures. As part of that
> process, I am a member of the Undersea & Hyperbaric Medical Society and
> have corresponded with many to further my knowledge.
> 
> You statements on hyperbaric oxygen toxicity in your article in
> Undercurrent and in your letter to me are, at best, a oversimplification
> of a complex issue that involves physiology and risk analysis. You imply
> that the Navy and NOAA tables from which you derived your notes indicate
> no risk to the diver at pPO2 exposures of 1.6 ATA and less, unless the
> maximum exposure times of the tables have been exceeded.
> 
> As Tom Mount has recently written: The CNS % clock values are based on the
> data from Oxygen and the Diver by Donald, on the NOAA limits and on the
> work of Bill Hamilton. They are not an exact science. They are in theory a
> value where the AVERAGE population would be still safe once 100% of the
> clock has been used up. Beyond this point the AVERAGE diver would be
> entering a increased risk profile. The only problem is trying to define
> who is this average average diver and who is he (or she) today?
> 
> Your reference to Lanphier's publication is noted, but much additional
> work has been done since 1954, recently summarized in a publication by the
> US Naval Medical Research Institute: A model for predicting central
> nervous system toxicity from hyperbaric oxygen exposure to man.
> 
> >From this work we see, consistent with your opinion, that the time to
> onset of convulsions is often used as an index of CNS O2 toxicity and that
> a curve can be defined which describes the 10% probability of convulsions,
> relating time to onset with pPO2 at various pressures from 0.8 ATA and
> above. But, while the pPO2 vs time relationship is strong for pPO2 above 2
> ATA, it does not appear that the time of exposure is as consistent for
> pPO2s of less than 2 ATA. The work lists a number of published CNS O2
> toxicity incidents in immersed, exercised and not-exercised divers from
> the "new" and "old" literature at pPO2s ranging from 0.9 to 2.9 ATA. The
> relationship of these incidents to the exposures below 1.8 ATA have been
> thought by some to be insufficient to ascribe an accurate risk to such
> exposures, but it is not denied that they occur, even if they were
> probably exacerbated by other factors including increased CO2 or
> medications. Nevertheless, the Navy MRI model estimates the risk (at 0.6
> ATA and 1.1 ATA using 355 exposures) to be <4% and to increase with rising
> pPO2. Even if you do not accept the model's level of risk, your use of a
> time constant to predict or to exclude these stochastic events is not
> appropriate.
> 
> In fact, agencies such as your own (ITD) and experienced technical divers
> advise (as does the US Navy) to plan working dive pO2 exposures at a limit
> of 1.4, regardless of the duration of exposure. Some divers will increase
> this planned exposure to 1.6 ATA or even beyond with air breaks, during
> decompression. Commercial divers with whom I have corresponded, tend to
> plan their exposures at much lower pPO2s (0.5 ATA to 1.0), because they
> are more concerned about the lung toxicity that may be inherent in their
> longer working bottom times at higher pPO2s. For these reasons, I believe
> that your printed statement in Undercurrent is incomplete and consequently
> misleading.
> 
> You letter indicates that you also misunderstand the purpose of internet
> e-mail. It is a scientific forum for the rapid exchange of facts, ideas
> and opinions. No one, that I am aware, has started a letter writing
> campaign directed against you. I was disappointed by the intemperate ad
> homimem remarks you made in your June 4 letter to me. Your letter and it's
> tone were in sharp contrast to the very helpful reply I received from Mr.
> Mller at Drger in response to my enquiries and to the excellent service
> and courtesy extended to me as an Uwatec customer by the technical and
> support staff in Greenville over the past years.
> 
> As a physician and diver, I am committed to promoting safe diving and the
> thoughtful evaluation of new practices and techniques. To that end, I will
> continue to learn from all sources available to me and to engage in the
> dialogue that is the advancement of technology based science. Your letter
> mentions that you have received a copy of some physiologic parameters for
> operating semi-closed rebreathers that has been the subject of discussion
> between myself and others. Recognizing the exchange of opinions may be
> heated at times, I still value yours. If you are prepared to forego
> unwarranted personal attacks, I know that we would welcome your specific
> concerns about these parameters, which I see merely as starting points of
> discussion among the manufacturers and users of the new rebreathers.
> 
> I sincerely hope you will take up this challenge in the spirit it is
> intended, as you have done with many other challenges in your years of
> service to the sport of diving.
> 
> Sincerely,
> 
> 
> Peter N.R. Heseltine, M.D., F.A.C.P.
> Professor of Medicine, University of Southern California
> 
> cc:	Mr. M.J. Mller, Drgerwerk
> 	Mr. M. Pratt, OWHS
> 	Uwatec AG
> 	techdiver@terra.net
> 
> --
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> 

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