From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: rec.aviation.student,sci.med
Subject: Re: Lack of Oxygen (Hypoxia)
Date: 29 Mar 1999 01:50:18 GMT

In <36FE92FE.D10385A0@interlog.com> Happy Dog <happydog@interlog.com>
writes:

>Jeff Johnson wrote:
>
>> They have a medical device called a pulse oxymeter(sp?) which they use
>> to measure body oxygen levels during surgery involving general
>> anesthesia. Just clips around one finger. That would be a neat device
>> to couple into a flight monitoring system for an aircraft, eh?
>
>I don't think that it would help much. The effects of altitude vary
>between individuals. Experience will tell you about personal effects.
>Your blood O2 levels will vary directly with altitude and, changes in
>your physiology notwithstanding, will be the same each time you go up.
>I'd be interested in hearing more about this though so I've cross posted
>to sci.med. Maybe one of the docs there like Harris (hint) can offer some
>insight from experience. I'm still not sure how one becomes aclimatized
>to altitude. Is it analagous to getting used to behaving sober after a
>few drinks? Some people get better with practice and some are comelete
>buffoons after two martini's.
>hd


   A pulse oxymeter sounds like a natural in a pilot's monitoring, but
the problem is where to put it.  The fingers are out because the things
are bulky.  They do have skin patches, but they are uncomfortable and
expensive ($50 each).  I'm sure that g forces causing skin blood flow
problems would totally mess them up, so they'd only be useful in
"normal" low g flying.  But all that's beside the real point, which is
that I have no doubt the mask system O2 delivery % (partial pressure)
is directly monitored anyway at the mask in front of the pilot's mouth,
so there's little point in monitoring the pilot's blood: these are all
completely healthy men, and if you know what the partial pressure of
oxygen they breathe is, you will know what the saturation of their
blood is, without having to measure it.  And that in turn will tell you
what their mental status should be, because you KNOW them from having
tested them at low partial pressures and under various other stresses.
These guys will all have been though centrifuges and decompression
chambers and what not with all this tested a zillion times, so the
aerospace docs know who's physiologically funny, and who isn't.  Those
who don't stand it all very well wash out before they bother to waste
money training them at all.

  Adaption to altitude involves a whole lot of factors, some of which
are understood and some of which aren't.  But it takes months and is
the kind of thing they do in preparation to climb Everest (spending a
lot of time hiking on the Tibetan plateau, etc).   None of it applies
to individuals going from normal oxygen to extreme hypoxia (pressures
less than half normal) in a few minutes.  So far as I can tell, that's
something you can neither train for (beyond being in good general
aerobic shape), or adapt to. How you stand it is pretty much written
into your genes, and individuals vary widely.  The ones unusually
sensitive don't make it into fighter pilot school, either.  The rest
can only make sure they haven't had any alcohol for a few days, and are
in good aerobic fitness shape.

                                      Steve Harris, M.D.

From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: rec.aviation.student,sci.med
Subject: Re: Lack of Oxygen (Hypoxia)
Date: 29 Mar 1999 02:15:16 GMT

In <clw-2803991526420001@i48-17-32.pdx.du.teleport.com>
clw@teleport.com writes:
>
>In article <36FE92FE.D10385A0@interlog.com>, happydog@interlog.com
wrote:
>
>> Jeff Johnson wrote:
>>
>> > They have a medical device called a pulse oxymeter(sp?) which they
>> > use to measure body oxygen levels during surgery involving general
>> > anesthesia. Just clips around one finger. That would be a neat device
>> > to couple into a flight monitoring system for an aircraft, eh?
>>
>> I don't think that it would help much. The effects of altitude vary
>> between individuals. Experience will tell you about personal effects.
>> Your blood O2 levels will vary directly with altitude and, changes in
>> your physiology notwithstanding, will be the same each time you go up.
>> I'd be interested in hearing more about this though so I've cross
>> posted to sci.med. Maybe one of the docs there like Harris (hint) can
>> offer some insight from experience. I'm still not sure how one becomes
>> aclimatized to altitude.
>
>This is a combination of increased red blood cell mass (ie, greater
>oxygen carrying capacity of the blood) and increased cardiac output to
>attempt to deliver more oxygen carrying blood per minute and increased
>minute ventilation of the lungs and perhaps cellular (mitochondrial
>adaptations to working in a situation of lower oxygen partial pressures
>and gradients).
>
>The blood oxygen content (tension or saturation) is dependent on the
>partial pressure of oxygen in the alveolar gas. At altitude, the total
>barometric pressure in low, and even though the percent of oxygen in the
>air is still about 21%, there are fewer molecules of oxygen in a given
>volume of air. Thus, as inhaled gas mixes with the residual alveolar gas,
>the partial pressure of oxygen is much lower than the 100 millimeters of
>mercury that we have at sea level. Thus, after the first few molecules of
>oxygen are taken up by the blood, the partial pressure of oxygen falls
>rapidly. Thus, the hemoglobin at altitude will have a lower partial
>pressure of oxygen, and all the hemoblobin will not be fully saturated.
>But if you have more hemoglobin mass in the blood and you pump it around
>faster, the total amount of oxygen available to the tissues approaches
>that delivered at sea level. It is just that the tissues will be working
>at lower oxygen partial pressures and thus the gradients to move oxygen
>from capillaries and through the tissues to the cellular mitochondria
>(where the oxygen is used) will be lower.
>
>The above is superficial and incomplete.
>
>First get a basic medical physiology text and then one of the
>Anesthesiology texts to more fully understand oxygen uptake, transport
>and tissue utilization.



    The above is a good start, however.  I might add for interest that
the single most important adaptation to extreme altitude (Mt. Everest)
is minute ventilation, which allows the partial pressure of CO2 to fall
low enough that alveolar gas has "room" for enough oxygen to keep you
barely alive (some respiratory physiologist MD climbed without oxygen
and actually took his own arterial blood gas samples on the summit of
Everest, and they are incredible).  Of course, any person at sea level
would die in a few minutes if taken to the summit of Everest, so it's a
lot more than that.  Still, what oxygen does for climbers is allow them
not to climb to the top of Everest (which can indeed be done without
oxygen tanks), but rather allow them to SLEEP at night.  Staying alive
at 29,000 feet requires you breathe like you're running a marathon,
just standing there.  People can't keep that up for more than a day.
So if you have no oxygen you go up and then come down fast, before you
have to sleep.  If for some reason you can't come down that fast, you
never do.

                                         Steve Harris, M.D.

From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: rec.aviation.student,sci.med
Subject: Re: Lack of Oxygen (Hypoxia)
Date: 29 Mar 1999 02:48:52 GMT

In <36febdeb.197987140@news2.ziplink.net> cgregory@gw-tech.com (Carey
Gregory) writes:

>Happy Dog <happydog@interlog.com> wrote:
>
>>Jeff Johnson wrote:
>>
>>> They have a medical device called a pulse oxymeter(sp?) which they use
>>> to measure body oxygen levels during surgery involving general
>>> anesthesia. Just clips around one finger. That would be a neat device
>>> to couple into a flight monitoring system for an aircraft, eh?
>>
>>I don't think that it would help much. The effects of altitude vary
>>between individuals. Experience will tell you about personal effects.
>>Your blood O2 levels will vary directly with altitude and, changes in
>>your physiology notwithstanding, will be the same each time you go up.
>
>You're right; it probably wouldn't help much.  Unless you're at
>extreme altitudes, a pulse oxymeter wouldn't tell you much because the
>O2 saturation of your blood doesn't change significantly.

   Well, sure it does.  The relationship is not linear (the Hb
saturation curve is S shaped and subject to movement over time even in
the same person), but it does change with altitude reasonably
sensitively.  For example, at 18,000 ft, the partial pressure of O2 is
about 74 Torr instead of the 159 you get in absolutely dry air at sea
level.  In your alveoli, due to CO2 and H2O, that goes down to 61 Torr
or so.  If you're a young guy with good lungs and an A-a gradient of 5,
you get 57 Torr in your blood.  That's a sat of around 89% instead of
what your normal would be at sea level (> 96%), and in practice it will
probably be a bit less than 89%, due to physiologic shunting of venous
blood which is more than unusually desaturated (due to having started
out with slightly less of a load).  If you're a normal 80 year old,
your A-a gradient might be 20, and then you'd be at 41 Torr, and now
your saturation on pulse oxymetry is down around 75%, which is that in
venous blood, so you're in real trouble if you don't hyperventolate.
An example of how dicey things get when you're on the steep part of the
Hb dissociation curve.