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J. Babb (babb@sciences.sdsu.edu) wrote:

<Jibberish Med-Speak deleted>

: I've got a coupla questions:
:  I went into the hospital for a procedure and had an oximeter taped to my
: right index finger, afterwards I asked the doctor if I could have the
: sensor
: It had only ONE LED and Phototransistor, Not two. From this they got not
: only
: my pulse but blood oxygen level, too.
: The book referenced above is not available to me, is too expensive ($200!),
: and doesn't cover the one LED method anyway. Can anyone shed some light
: (other than 660 and 925nm, :-)   on this. 
: Jeff

Jeff,

Pulse oximetry works, in the 2 LED method, by looking at the absorbtion
raitos of the 2 frequencies of light.  Oxygenated hemoglobin will absorb
one of the frequencies, and non-oxygenated hemoglobin will absorb the other.
By comparing the amount of each that are absorbed, you can find the ratio of
one to the other, and derive from that the percent of available hemogloben
that is oxygenated.

Things get more complicated <of course>, when CO (Carbon Monoxide) enters
the picture, because it binds quite tightly to hemoglobin, but does not show
up with an optical pulse oximeter because it absorbs a totally different
frequency, I suppose.  So, you could be suffering from CO poisoning and still
have your Pulse Ox show, say, 97% which is about normal.  Even though you 
aren't getting enough O2 to your body, the pulse ox can't see the problem.

OK, that's how the 2-led system works.  Anyone have any idea about the 1-LED
system?  Unless perhaps it was two devices in one can?  How many leads did it
have on the photodetector?

Oh....the pulse is just done with an absolute amplitude going into an auto-
gain control, to give (usually) a bar-graph, sometimes a waveform, and
usually a 'beep' and a counter.  The AGC is needed due to the wide variance
in finger/earlobe thicknesses & so on.

Dave Hinz


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Dave Hinz  -  Opinions expressed are mine, not my employer's.  Obviously.
Ask me if I have an opinion on this! \ Don't blame me; I voted for Perot!
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