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At 12:46 PM -0400 6/2/98, Haine Adri wrote:
>Hi all,
>
>We are all well aware of the advantages of using EANx mixtures on our
>dives, provided we observe a reasonable max PPO2.
>I was wondering if excessive exposures to high levels of oxygen could
>also have some disadvantages on the human body. By excessive I mean
>diving 2/3 times a day, using EANx mixtures, with bottom times of 30-40
>minutes in the 1.3 - 1.4 ppO2 region, followed by decompression
>involving pure OXYGEN . If anyone knows of any studies being done,
>and/or published on this subject, could you please inform me
>therabouts?
Dear Haine,
Long term problems with O2 use involve tissue damage (mostly from oxygen
radicals... chemically charged forms of oxygen) this has been studied and
mostly involves debilitated, hospitalized patient's on high O2
concentrations for *DAYS* at a time. (premature newborns also are sensitive
and can develop a form of retinal eye damage called retrolental
fibroplasia)
While no doubt there are pulmonary lining cells injured on an individual
cell basis using diving gases, the rates and total amounts of such damage
(coupled to a normally rapid rate of cell turnover through normal repair)
hasn't been a clinical problem (for those who follow the time and depth
limits as outlined in their nitrox training) So, essentially it's the time
frame of the O2 use that is key.
However, I am concerned by your comment of keeping your ppO2 in the 1.3 to
1.4 range. This isn't consistent with your original "pledge" to "observe a
reasonable max ppO2" but is getting up there in the "contigency" levels.
The risk of neurotoxic effects is not a sharp line based soley on ppO2
("stay on this side of the line and you'll be safe!"). It seems to vary
from person to person and from day to day. Divers have been known to make a
hundred dives at some elevated ppO2 with no ill effects and then take a
neuro hit on the hundred and first dive at a *LOWER* ppO2. Frankly, we
don't know all there is to know on this topic, so over the years divers
have been *LOWERING* the max acceptable ppO2 just to give themselves an
ever greater safety margin.
The US Navy (based on their own research plus those of others have been
steadily reducing their max acceptable ppO2 to 1.3. (previously 1.4)
Keep in mind that 1.3 is the *MAX* acceptable. (according to the latest
Navy Diver's Manual, missions requiring ppO2's higher than 1.3 require
special waivers from paygrades a heck of a lot higher than mine! <g>) That
doesn't mean you have to run at that max level without a good reason! (Also
keep in mind that Navy divers dive with onsite chambers and dive medical
support... not to mention their physical condition is a damn sight better
than average)
The benefits of nitrox (in the appropriate setting!) have been well
documented. There are down sides but mostly for those who choose to ignore
or disregard the physiologic limits that we as divers and the training
agencies instruct.
Hope that is of some help to you.
BTW, Dr. Bill Hamilton (a dive physiologist who has *FORGOTTEN* more gas
physiology than I'll ever know!) has written a really superb manual on
Nitrox diving for NAUI which is worth a read even if your 're already
Nitrox certified. (and I swear I'm not getting a dime for this plug! <g>)
Robb W
====================================
CDR Robert B. Wolov, MC, (FS), USNR
Orthopedic Pathology / Aerospace Medicine
Dept. of Orthopedic Pathology
Armed Forces Institute of Pathology
Washington, DC 20306-6000
wolov@hi*.co* (preferred)
wolov@em*.af*.os*.mi*
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