GE Corometrics 126, Service Manual

Page: 132 / 440

4-74 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: MSpO
2
Board
The Masimo SET MS-5 pulse oximeter as well as traditional pulse oximetry
determines SpO2 by passing red and infrared light into a capillary bed and
measuring changes in light absorption during the pulsatile cycle. Red and infrared
light-emitting diodes (LEDs) in oximetry sensors serve as the light sources, a
photodiode serves as the photodetector.
Traditional pulse oximetry assumes that all pulsations in the light absorbance signal
are caused by oscillations in the arterial blood volume. This assumes that the blood
flow in the region of the sensor passes entirely through the capillary bed rather than
through any arterio-venous shunts. The traditional pulse oximeter calculates the
ratio of pulsatile absorbance (AC) to the mean absorbance (DC) at each of two
wavelengths, 660 nm and 940 nm:
S(660) = AC(660)/DC(660)
S(940) = AC(940)/DC(940)
The oximeter then calculates the ratio of these two arterial pulse-added absorbance
signals:
R = S(660)/S(940)
This value of R is used to find the saturation SpO2 in a look-up table built into the
oximeter's software. The values in the look-up table are based upon human blood
studies against a laboratory co-oximeter on healthy adult volunteers in induced
hypoxia studies.
The Masimo SET MS-5 pulse oximeter assumes that arterio-venous shunting is
highly variable and that fluctuating absorbance by venous blood is the major
component of noise during the pulse. MS-5 decomposes S(660) and S(940) into an
arterial signal plus a noise component and calculates the ratio of the arterial signals
without the noise:
S(660) = S1 + N1
S(940) = S2 + N2
R = S1/S2
Again, R is the ratio of two arterial pulse-added absorbance signals and its value is
used to find the saturation SpO2 in an empirically derived equation into the
oximeter's software. The values in the empirically derived equation are based upon
human blood studies against a laboratory co-oximeter on healthy adult volunteers in
induced hypoxia studies.
The above equations are combined and a noise reference (N') is determined:
N' = S(660) - S(940) x R
If there is no noise N' = 0: then S(660) = S(940) x R which is the same relationship
for the traditional pulse oximeter.
The equation for the noise reference is based on the value of R, the value being
sought to determine the SpO2. The MS-5 software sweeps through possible values
of R that correspond to SpO2 values between 1% and 100% and generates an N'
value for each of these R values. The S(660) and S(940) signals are processed with
each possible N' noise reference through an adaptive correlation canceler (ACC)
which yields an output power for each possible value of R (i.e., each possible SpO2
from 1% to 100%). The result is a Discrete Saturation Transform (DST™) plot of