Ted the Anesthesiologist asks:
>Bob,
>
> Are you sure you're not an anesthesiologist?
Nope just a chemist... *
> You got the alveolar gas equation right, even though you
> left out the effects of exhaled CO2 (it makes things worse.).
I also left out the mass flow rate of O2 needed, which determines
how much oxygen needs to enter the alveolae to maintain
blood saturation. The CO2 going out and the substantial
volume (at altitude) of O2 going in are what limit is to about
25K feet without pressure masks. But the previous email
was long enough with going into more equations.
> I would like to see a source for the
> overpressure limitation you cite.
As I remember it, there are two limits affecting pressure breathing:
1) Arterial embolism becomes
at risk at about 1.5 psi (10kPa) overpressure
according to the SCUBA texts.
http://www.scuba-doc.com/artgsemb.htm which says...
"Distention of the alveoli leads to rupture, alveolar leakage of gas,
and extravasation of the gas into the arterial circuit."
2) The muscles driving exhalation get tired quickly if they have to work
against an externally applied pressure. One report of pleural pressure
gives a maximum of about 16 cm H2O (12 Torr, 0.23 psi) for young
subjects during exercise breathing, and half that for old folks like us.
http://www.madsci.org/posts/archives/aug97/865350639.An.r.html
To test how much backpressure you can tolerate, exhale into a
tube that is immersed in water. How deep can you put the air outlet
and tolerate the work of exhaling? 2 feet = 1 psi.
With a pressure mask, the
breathing process is the opposite of what we are used to.
The pressure mask
pushes oxygen into the lungs when the muscles
relax, then the chest and abdominal muscles have to squeeze to
propel the exhaust stroke.
It occurs to me that that could be helped by pulsing the pressure mask
to allow easier exhalation. Probably such a mask exists, but I have
not used one.
-bob mackey
* BTW, here's some of the other recent chemistry projects I have
worked on:
2BLEVE
http://www.youtube.com/watch?v=l9vYE7B1_PU
http://www.youtube.com/watch?v=cm5VB7JaJWA
http://www.youtube.com/watch?v=EdKYGMaHNqY
Fire Vortex
http://www.youtube.com/watch?v=3rX4dESXWmQ
http://www.youtube.com/watch?v=XaX_Hm5Dl5E
http://www.youtube.com/watch?v=OtDqRX4duvE
http://www.youtube.com/watch?v=Txq-tHOMxAM
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