SpO
2
Monitoring
12-12
12.3 Masimo SpO
2
Module
NOTE
z This section is only applicable to the monitor equipped with a Masimo
SpO
2
module.
12.3.1 Principles of Operation
The pulse oximetry measurement module (Masimo Set, which is called MS-7
) is
based on three principles:
Oxyhemoglobin and deoxyhemoglobin differ in their absorption of red and
infrared light (spectrophotometry).
The volume of arterial blood in tissue and the light absorbed by the blood
changes during the pulse (plethysmography).
Arterio-venous shunting is highly variable, and the fluctuating absorbance by
venous blood is a major component of noise during the pulse.
The working principle of MS-7 uses is similar to the traditional SpO
2
module. It
calculates the SpO
2
value by passing red and infrared light into a capillary bed and
measuring changes in light absorption during the pulsatile cycle. The red and
infrared light-emitting diodes (LEDs) in oximetry sensors serve as the light sources,
and the photodiode serves as the photodetector.
Traditional pulse oximeter 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:
)660(/)660()660(Re DCACd =
)940(/)940()940( DCACIr
This traditional instrument then calculates the ratio of these two arterial pulse-added
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