Ben Hallert wrote:
I think the other posters have it right. The specific concept behind
it, if I remember my science fiction books correctly, is 'partial
pressure'. With the open cycle (the one that hooks to your nose),
there's an open path direct to the low pressure area through your
mouth. The oxygen enters your lungs through osmosis, and if the
pressure on the inside of the hemoglobin in your pulmonary capillary is
greater then the pressure of the O2 against it, it just won't enter.

Partial pressures of the gas in question DO define how oxygentation and
ventilation work, BUT the term you are looking for is "pressure
gradient", the difference between the partial pressures in question.

Another part of the problem may be that at high enough altitudes, CO2
may no longer be effective at triggering the breathing impulse.

Not hardly. In the body, arterial blood leaving the lungs has a partial
pressure of CO in the 35-45 mmHG range. In a "mixed venous sample" which
is in the blood returning to the lungs, this value is even higher. The
majority of healthy individuals use CO2 as the determining factor in
their breathing depth and rate, and this does not vary with altitude.

I think the partial pressure issue is probably more relevant.

If someone here is a doctor or actually KNOWS the answer, feel free to
tell me where I pooched it up. My education comes from the likes of
Del Rey Publishing and Baen Books, not John Hopkins.

Ben Hallert
PP-ASEL


I'm not a doc, just an ICU and ER nurse, Paramedic, and former
firefighter. I didn't go to John's Hopkins either.

Other posts elsewhere in the thread raised other questions I wanted to
address:

With a cannula, the limitation is due to the volume of oxygen that is
useable. In a cannula, once your "nose" is filled with free flowing
oxygen, the excess spills over into the mouth, and also out into the
environment. Increasing the flow rate past the point where the "nose"
fills between breaths only increases the waste.

A pulsed regulator (someone called it a "pusher") is helpful in this
regard because the regulator gives HIGH flow, but only during a breath,
so its OFF when not inhaling. Cuts waste WAY down. May give you a little
extra altitude as well. These devices were originally developed for home
health care type patients to extend their supply of oxygen on their
transport bottles, allowing them to get out of the house longer.

A mask gets its effect from having a larger area being filled with
oxygen between breaths (the entire mouth, and then the area under the
mask). For aviation use, a reservoir mask allows you to get an even
larger charge of oxygen per breath.

Those of you who are talking about "pressure breathing" need to
understand that the pressures involved are VERY miniscule. The pressures
that these "pressure breathing" regulators use is measured in
CENTIMETERS of water, which corresponds to a SMALL fraction of a single
PSI. Anything more than that would cause the lung to pop (barotrauma).
The pressures involved contribute LITTLE to the pressure differential.

What is different about the pressure breathing setup is that NORMALLY,
you WORK to breathe in, and relax to exhale. With this setup, your
inhalation is assisted (slightly, its called "pressure support" in the
medical community) and you have to WORK to exhale.

The tanks in question carry in the neighborhood of 1800-2000 psi. They
are routinely filled to 10% over (2000-2200 psi), and when they are
tested I believe the value is 150%. Adding 15 psi by taking the tank
into outer space (yea.. REALLY at altitude) is a miniscule thing in this
pressure equation. The altitude restrictions are a function of the
delivery device, not the storage tank.

Dave