From: fhd@panix.com (Frank Deutschmann)
Subject: O2 Toxicity (was: dangerous misconception)
Date: Tue, 17 Nov 1992 04:24:26 GMT
Organization: PANIX Public Access Unix, NYC


From what I was taught, O2 toxicity is linked to partial pressure
(affected by gas mix and depth), time of exposure, and workload.  In
addition, the current thinking is that O2 toxicity is largely the
result of CO2 retention, so that the body's carbonate buffer system
also plays a role.  

As far as limits, NOAA has 1.6ATA for 45min as the maximum normal
exposure (which translates to 218fsw on air, at 21% O2); bot NOAA and
the US Navy call 2.0ATA O2 an exceptional exposure.  ANDI, in their
NITROX course, strongly recomends abiding by the 1.6ATA limit; there
is little value and much risk (usually) in exceeding it.

One of the things that has always amazed me is that the certifying
agencies spend no time on O2 toxicity in basic courses, and little
time on the symptoms of narcosis.  If they spent some time on the
rationale for the 130fsw limit, perhaps fewer people would be inclined
to push that limit without training (no flamefests, please :-).

Since no one else has listed the symptoms of CNS O2 toxicity (these
don't apply to whole-body toxicity, a different ball game entirely),
I'll do it here:

Vision -- primarily tunnel vision, but any disturbance is cause for concern
Ears+Euphoria -- ringing in the ears (tinnitus), distortion, etc
		 as well as less feeling of danger
Nausea 
Twitches -- muscle spasms, especially lips -- this is, in practice,
		the most common symptom
Irritability -- a suddenly higher anxiety level
Dizziness -- sudden feeling of vertigo

These are easily remembered by the acronym VENTID.

Note that the list of symptoms is close to the list of narcosis
symptoms (Vision distortion, tinnitus, irrationality, vertigo); this
means that if you are in a situation where O2 toxicity could be a
problem, the symptoms should probably be taken much more seriously
than you would otherwise take narcosis symptoms (in other words, take
the symptoms seriously on a hard-working dive near the 1.6ATA limit).

Note also that you may not necessairly experience much warning with O2
toxicity symptoms; it is important to pay close attention to the
slightest symptom, which may otherwise be ignored in the case of narcosis.

A word on narcosis: as narcosis impacts the higher reasoning centers of the
brain first, it is difficult to be objective when gaguing whether or
not you are narc'ed:  How many other people have had that feeling (at
as shallow as 80-90fsw) that they just looked at their gagues, but
can't for the life of them remember what they said, but yet you didn't
feel narc'ed?

For safety in situations where you are relying on a buddy (to get a
task accomplished, for example), it is useful to check your buddy's
level of "narc-ed-ness".  To check you buddy, a simple protocol of
hand signals can be used: one diver holds up some number of fingers,
and the other diver immediately repeats the signal, adding one to the
count of extended fingers (this is especially tough when five or ten
fingers are used).  While not a thorough test, the response speed and
accuracy can be some help in deciding when the narcosis has reached
dangerous levels.  I believe this technique is from B.Gilliam; its
real handy when diving with a new buddy (make sure you explain it
before hand), or when going to a new personal record depth.


-- 
-frank
(fhd@panix.com)