That should be "higher pH caused by hyperventillation"

Someone who is underventillating (not breathing) gets a lower (acidotic)
pH. due to the buildup of CO2.

We hyperventillate patients on mechanical ventillators in the OR all the
time, and still rely on the oximeters. The readings correllate very well
to actual measured blood gases, taken from an artery. It would take a lot
of hyperventillating to shift the oxyhemoglobin dissociation curve enough to
be significant. (Core Temperature is probably as important, btw)

If one is not to rely upon an oximeter.....what other convenient mechanical
means of measuring hypoxia should we rely on? Certainly, if your
oximeter reading is in the high 90's, you are OK.

One should become familiar with one's baseline readings on the ground,
btw. Normals differ from person to person due to age, cardiovascular
health...etc. A healthy, normal person breathing 21% O2 (room air at
sea level) usually shows anywhere from 97 to 99 on an oximeter.

Also, it takes a good minute, often longer, of not breathing to see a
change in oximeter readings. Conversely, there is a lag in upswing of
the readings once you start breathing again. Breathing a slightly thinner
atmosphere (at altitude) should show a slowly dropping reading as altitude
is gained, and/or more time is spent at that higher altitude. (Time at
altitude relationship, as mentioned in the FAR's regarding Oxygen use while
flying)

I guess the lesson would be to familiarize yourself with these vagaries on
the ground before using/relying upon the instrument at altitude.

It's a good numerical safety check to have on board the aircraft, but like
they teach us in training.....don't forget to also check the patient!






"Kilo Charlie" wrote in message
news:eupYe.261468$E95.67794@fed1read01...

"Shawn" sdotcurry@bresnananotherdotnet wrote in message
...
W.J. (Bill) Dean (U.K.). wrote:
Peter Saundby has put the following posting on the U.K. u.r.a.s.:

Following the string on oxygen systems, pilots intending to fly at
altitude
should be very wary of relying upon oxymetry devices intended for
clinical
use to confirm in the air that they are adequately oxygenated.
Professor
John Ernsting, long of the RAF Institute of Aviation Medicine gave a
presentation on their limitations at the recent International Congress
of
Aerospace Medicine in Warsaw. These devices have been used by pilots of
unpressurised aircraft to monitor the adequacy of their oxygen supply or
to
assess the performance of emergency oxygen systems. The problem is that
when suffering a degree of hypoxia there is a tendency to hyperventilate
and
the consequent reduction of carbon dioxide will increase the stability
of
oxy-haemoglobin. While blood saturation appears adequate, too little
oxygen
will be released to the brain. This theory was confirmed by experiment
in a
chamber; therefore extreme caution should be exercised when using
oximetry
at altitude because it can offer a false assurance.

Lower pH, resulting from hyperventilation, decreases hemoglobin's
affinity for O2. The rest makes sense (higher affinity, higher measured
saturation level, and yet poor O2 delivery in tissues). Could it be
*hypo* ventilation. Do you have any references we could read.

Shawn

I agree with Shawn.....but still feel that an oxymeter is the best way of
providing info wrt your oxygen system working. I also think that within
the flight levels that most of us fly it is a clinically insignificant
difference. If your saturation is OK then there would be less drive to
hyperventilate and so it would be less of an influence. I'd like to see
the original data too.

Casey Lenox
KC
Phoenix