Technical Discussion; Oximetry Overview - Nellcor OXIMAX N-595 Service Manual

T e c h n i c a l D i s c u s s i o n
Oximetry
Overview
N-595
The N-595 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 photo detector.
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
Because a measurement of SpO
excessive ambient light can interfere with this measurement.
Specific information about ambient conditions, sensor application, and patient
conditions is contained throughout this manual.
Pulse oximetry is based on two principles: that oxyhemoglobin and
deoxyhemoglobin differ in their absorption of red and infrared light (i.e.,
spectrophotometry), and that the volume of arterial blood in tissue (and hence,
light absorption by that blood) changes during the pulse (i.e.,
plethysmography). A pulse oximeter determines SpO
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 pulse oximeter 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
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.
is dependent upon light from the sensor,
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