Vitalcare 506DN3, Service Manual

Page: 19 / 254

Criticare Systems, Inc. VitalCare
TM
506N3 Series Service Manual Page 1-7
Section 1 —Introduction
Nellcor Pulse Oximetry
Measurement (SpO
2
)
The 506N3 Series monitor is also available with Nellcor OxiMax
®
technology to measure blood oxygen saturation (SpO
2
).
Definition
The Nellcor OxiMax uses pulse oximetry to measure functional
oxygen saturation in the blood. Pulse oximetry works by applying an
OxiMax sensor to a pulsating arteriolar vascular bed, such as a finger
or toe. The OxiMax sensor contains a dual light source and a photo
detector.
Because a measurement of SpO
2
is dependent upon light from the
OxiMax sensor, excessive ambient light can interfere with this
measurement.
Criticare’s implementation of the OxiMax oximeter rounds down
SpO2 saturation values above 99.6% that might normally be reported
as 100% oxygen saturation in other implementations.
Method
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 OxiMax 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
nonpulsatile absorbers such as tissue, bone, and venous blood.
The display is updated at least once per second with the numeric
values that were calculated during the intervening period.
The plethysmographic pulse bar is not auto-gained. The amplitude
display of the plethysmographic pulse bar is proportional to the pulse
volume changes occurring in the tissue illuminated by the SpO
2
sensor.