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11.4 Nellcor SpO
2
Module
NOTE
z This section is only applicable to the monitor equipped with a Nellcor SpO2
module.
11.4.1 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
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.
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