SpO
2
Monitoring
12-23
12.4 Nellcor SpO
2
Module
NOTE
z This section is only applicable to the monitor equipped with a Nellcor
SpO
2
module.
12.4.1 Principles of Operation
Bone, tissue, pigmentation, and venous vessels normally absorb a constant amount
of light over time. The arteriolar bed normally pulsates and absorbs variable
amounts of light during the pulsations. The Nellcor SpO
2
module uses pulse
oximetry to measure functional oxygen saturation in the blood. Pulse oximetry
works by applying a sensor to a pulsating arteriolar vascular bed, such as a finger or
toe. The sensor contains a dual light source and a photodetector. The ratio of light
absorbed is translated into a measurement of functional oxygen saturation (SpO
2
).
îš„ Oximetry Overview
Pulse oximetry is based on two principles:
1. Oxyhemoglobin and deoxyhemoglobin differ in their absorption of red and
infrared light (i.e., spectrophotometry).
2. The volume of arterial blood in tissue (and hence, light absorption by that blood)
changes during the pulse (i.e., plethysmography).
A monitor determines SpO
2
by passing red and infrared light into an arteriolar bed
and measuring changes in light absorption during the pulsatile cycle. Red and
infrared low-voltage light-emitting diodes (LEDs) in the oximetry sensor serve as
light sources; a photodiode serves as the photo detector.
Because oxyhemoglobin and deoxyhemoglobin differ in light absorption, the
amount of red and infrared light absorbed by blood is related to hemoglobin oxygen
saturation. To identify the oxygen saturation of arterial hemoglobin, the monitor uses
the pulsatile nature of arterial flow. During systole, a new pulse of arterial blood
enters the vascular bed, and blood volume and light absorption increase. During
diastole, blood volume and light absorption reach their lowest point. The monitor
bases its SpO
2
measurements on the difference between maximum and minimum
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