11
PRINCIPLES OF OPERATION
Operating Principles
OPERATING PRINCIPLES
Pulse oximetry is based on two principles: that oxyhemoglobin
and deoxyhemoglobin differ in their absorption of red and
infrared light (that is, spectrophotometry), and that the volume
of arterial blood in tissue (and hence, light absorption by that
blood) changes during the pulse (that is, plethysmography). A
pulse oximeter 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-
power light-emitting diodes (LEDs) in the oximetry sensor
serve as light sources; a photodiode serves as the
photodetector.
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 (that is,
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.
C-LOCK ECG Synchronization
C-LOCK ECG synchronization: read through motion to provide
valid readings for many types of motion. During C-LOCK
signal processing, the monitor requires two signals that reflect
cardiac activity: the electrical pulse from the ECG and the
optical pulse from the oximetry sensor.
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