Respironics 920 Instruction And Service Manual page 43

SpO = O Hb + COHb + MetHb
2 2
SaO = 100 x O Hb /(100 - COHb - MetHb)
2 2
Where:
SpO = Respironics 920 determined and displayed
2
oxygen saturation in percent
O Hb = Fractional oxyhemoglobin saturation in
2
percent
COHb = Carboxyhemoglobin saturation in percent
MetHb = Methemoglobin saturation in percent
SaO = Functional oxygen saturation in percent
2
Example 1
O Hb = 96
2
COHb = 0.5
MetHb = 0.6
SpO = 97
2
SaO = 97.07
2
The mathematics are fixed in the pulse oximeter hardware and
software. Thus no field calibrations are needed or are they possible.
There are no adjustable parts within the pulse oximeter that affect
the calibration.
The function, f, does depend on the color of the LEDs producing the
light. These colors are fixed by the manufacturing processes and
material. The sensors are checked for correct operation before
shipping so no adjustment or calibration is needed or possible.
In summary, because the pulse oximeter does all critical
computations in software and there are no critical parts to drift; no
re-calibration is needed. Also, since the pulse oximeter reading
does not depend on the absolute light intensity, but only on the
fluctuations, then extremely thick skin, finger nails, colored skin,
etc., do not affect the readings.
35
Example 2
O Hb = 88
2
COHb = 8
MetHb = 2
SpO = 98
2
SaO = 97.78
2