Covidien Nellcor GR101704, Service Manual

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Theory of Operations
3-2 Service Manual
3.3
Theoretical Principles
The monitoring system uses pulse oximetry to measure functional oxygen sat-
uration in the blood. Pulse oximetry works by applying a Nellcor™ sensor to a
pulsating arteriolar vascular bed, such as a finger or toe. The sensor contains a
dual light source and a photodetector.
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
).
Ambient conditions, sensor application, and patient conditions can influence
the ability of the monitoring system to accurately measure SpO
2
. Reference
Performance Considerations , p. 8-1.
Pulse oximetry is based on two principles: oxyhemoglobin and deoxyhemoglo-
bin differ in their absorption of red and infrared light (measured using spectro-
photometry), and the volume of arterial blood in tissue (and hence, light
absorption by that blood) changes during the pulse (registered using plethys-
mography). A monitoring system 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 sensor serve as light sources; a photo diode serves as the photo detector.
Since oxyhemoglobin and deoxyhemoglobin differ in light absorption, the
amount of red and infrared light absorbed by blood is related to hemoglobin
oxygen saturation.
The monitoring system uses the pulsatile nature of arterial flow to identify the
oxygen saturation of arterial hemoglobin. During systole, a new pulse of arte-
rial blood enters the vascular bed, and blood volume and light absorption
increase. During diastole, blood volume and light absorption reach their lowest
point. The monitoring system 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 nonpulsatile absorbers such as tissue, bone, and
venous blood.