SpO
2
Monitoring
12-13
absorbance signals:
)940(/)660(Re IrdR =
This value of R is used to find the SpO
2
in a look-up table built into the instrument’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.
This MS-7 assumes that arterio-venous shunting is highly variable and that
fluctuating absorbance by venous blood is the major component of noise during the
pulse. The MS-7 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:
NrSrd +=)660(Re
NiSiIr +
)940(
SiSrR /=
Again, R is the ratio of two arterial pulse-added absorbance signals and its value is
used to find the saturation SpO
2
in an empirically derived equation into the 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:
RIrdN
−= )940()660(Re'
The equation for the noise reference is based on the value of R, the value being
seeked to determine the SpO
2
. This instrument’s software sweeps through possible
values of R that correspond to SpO
2
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 SpO
2
from 1% to 100%). The result is a Discrete Saturation Transform (DST™) plot of
relative output power versus possible SpO2 value as shown in the following figure
where R corresponds to SpO
2
= 97%:
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