Doug Chapman wrote:

> I think many of the posts are looking too deep into this subject of
> hypoxia at altitude. It is very clear that hypoxia can occur at a
> sufficient altitude on air, or Nitrox for that matter. However when
> the diver descends from the surface, the O2 partial pressure will
> increase and move out of the hypoxic range. When the diver returns
> to the surface he or she may again enter the hypoxic range. This is not
> a mystery.


	So, are you suggesting that the altitude must be hypoxic before 
you dive in order to become hypoxic after the dive?  This would make sense.
A couple of days ago it was also suggested this is true. (followed below)


>         If you are breathing a high PO2, and ascend rapidly to the point
> where the PO2 drops significantly, it is possible to get a "reversed" O2
> gradient. O2 can start to move out of your body and you can become
> hypoxic. This is what happens to breath-hold divers as the PO2 in their
> lungs falls during ascent after a long dive: shallow-water blackout.
>         The key is that the the PO2 has to drop below thw .16ata or so
> threshold...as in the long-staying breath hold divers ascent. I don't
> understand how it would work when changing from one breathable medium to
> another, as in an ascent from any dive.
>                                                Bernie Woolfrey
>

Doug also wrote:

> Some of the posts are suggesting a physiologic phenomenon which occurs
> at altitude due to the higher percentage of O2 in Nitrox which causes a
> hypoxic condition after the dive. If this was true, you would think it
> would also occur at sealevel with Nitrox, and it very well may as there
> are many physiologic processes which occur during dive that are not
> well understood, especially by layman. I certainly don't know.

	I agree, I would think that if a differential in ppO2 would cause 
hypoxia, then it would also happen at sea level.  This would be altitude 
independent!  If a higher rate of hypoxia does occur after nitrox diving 
at altitude (which I have no reason to disbelieve), then there has to be 
some physiologic process involved.  Tonight, I have to catch up in my 
Environmental Biochemistry reading which we are discussing hemoglobin and 
partial pressures.  I will try to look for an answer and maybe see if my 
professor has an opinion. Maybe I can find more information on Doug's 
suggestion he posted recently when he quoted Dr. Buhlmann. (Followed below)

> However I suspect you are looking for an answer to another question
> rather than atmospheric O2 partial pressure requirements to sustain
> proper body function. I did find an interesting bit of information from
> Dr. Buhlmann. He states "The adaptation to high altitude mainly
> concerns the erythropoisesis with the development of polyglobulia, with
> which the circulatory system's capacity to transport oxygen is
> increased. The affinity of hemoglobin for oxygen decreases somewhat,
> which causes an increase in the PO2 in tissues." Go figure!
> Unfortunately Dr. Buhlmann is dead and can't comment.
 
 
	Doug also suggested that it might be possible to be hypoxic after 
nitrox diving at sea level.  Well, if this is true, I would think it 
would be caused by the same process.  Now what kinda evidence would 
support this subject.  I personally don't think this would be possible in 
the amount of time one would spend underwater.  Wouldn't the in water 
time have to be unbelievable before this could happen?
	This may be off track, but any takers!!!

Aloha
Tony

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