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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