12.3 Nellcor SpO2 Module
NOTE
•
This section is only applicable to the monitor equipped with a Nellcor SpO2
module.
12.3.1 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 SpO2 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 (SpO2).
•
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 SpO2 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 SpO2 measurements on the difference between maximum
and minimum absorption (i.e., measurements at systole and diastole). By doing so, it focuses
on light absorption by pulsatile arterial blood, eliminating the effects of nonpulsatile
absorbers such as tissue, bone, and venous blood.
12-8
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