Nellcor Oximax NPB-40, Service Manual

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NPB-40 75
T e c h n i c a l D i s c u s s i o n
Oximetry Overview
The NPB-40 uses pulse oximetry to measure functional oxygen saturation in the
blood. Pulse oximetry works by applying an
O
XI
M
AX
sensor to a pulsating
arteriolar vascular bed, such as a finger or toe. The O
XI
M
AX
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
2
).
Because a measurement of SpO
2
is dependent upon light from the O
XI
M
AX
sensor,
excessive ambient light can interfere with this measurement.
Specific information about ambient conditions, O
XI
M
AX
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
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
O
XI
M
AX
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
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.
Functional versus Fractional Saturation
This pulse oximeter measures functional saturation -- oxygenated hemoglobin
expressed as a percentage of the hemoglobin that can transport oxygen. It does not
detect significant amounts of dysfunctional hemoglobin, such as
carboxyhemoglobin or methemoglobin. In contrast, hemoximeters such as the
IL482 report fractional saturation
-- oxygenated hemoglobin expressed as a