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<< Lancair Builders' Mail List >>
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George Braly asks about the accuracy of the Nonin Oximeter. While I cannot
speak to that specific instrument's quality control, allow me to comment on
the technology.
Pulse oximetry works by passing two wavelengths of light through a perfused
tissue bed. The only difference between the presence and absence of a pulse
is the amount of arterial blood added to the bed. (All tissue factors
subtract out.) Therefore, the two lights, which measure hemoglobin with and
without oxygen, allow the instrument to evaluate the ratio of the two.
The next step is for the instrument to look up the measurement ratios in a
lookup table. This table was created by taking normal volunteers
(impoverished medical students), and having them breathe reduced oxygen air.
Arterial blood gas analyses calibrate the results. Since no one is willing
to take the SaO2 much below 75%, this is about the lower limit of the
experimental data. Using mathematical techniques, the curves are
extrapolated to zero. This becomes the table in the instrument.
As an aside, if you wiggle your finger while trying to get a measurement,
the instrument will read 85-86%. That is motion artifact, and not your SaO2.
It is a result of the numbers in the lookup table. Also, if your hands are
very cold, you may not have enough pulse in your fingertips for the
instrument to work. Also, bright light can occasionally interfere with the
measurement.
As you can see, the altitude at which the instrument is employed does not
figure into its operation. So if the Nonin uses the technology properly, it
ought to function well in flight.
Ted Noel MD
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Please send your photos and drawings to marvkaye@olsusa.com.
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