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Rescue Life - User Manual Rev. 3.1
45
SpO
2
MONITORING
The SPO
2
Module measures functional oxygen saturation in the blood. The measurement
determines the oxygenated hemoglobin as a percentage of the hemoglobin that can
transport oxygen.
Pulse oximetry works by having light emitting
diodes pass red and infra-red light into
arteriolar vascular beds such as a finger or a
toe and having the light detected by a photo
detector afterwards.
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 ratio of light absorbed is translated into a measurement
of functional oxygen saturation (SpO
2
).
Pulse oximetry is based on two principles: that oxyhemoglobin and deoxyhemoglobin
differ in their absorption of red and infrared light (spectrophotometry), and that the
volume of arterial blood in tissue (and hence, light absorption by that blood) changes
during the pulse (plethysmography). A pulse oximeter 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 (LED) in
the oximetry sensor serve as light sources; a photo diode 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 pulse oximeter bases its SpO
2
measurements on the difference between maximum and minimum absorption
(measurements at systole and diastole). By doing so, it focuses on light absorption by
pulsatile arterial blood, eliminating the effects of non pulsatile absorbers such as tissue,
bone, and venous blood.
