Philips M3921A, Service Manual

Page: 139 / 150

Theory of Operation and System Architecture
Chapter 10 - Training Program 127
oscillation increases sharply is defined as systolic pressure. As the cuff
continues to deflate, oscillation amplitude increases to a maximum, and then
decreases. The peak oscillation amplitude is defined as the mean arterial
pressure. The point at which the system detects a rapid decrease in oscillation
is defined as the diastolic pressure.
SpO
2
Processing
Measurement of oxygen saturation in the blood uses a specrophotometry
technique. It is based on the facts that oxyhemoglobin and deoxyhemoglobin
differ in their absorption of red and infrared light, and that the volume of
arterial blood in tissue changes during the pulse.
Using these facts, a pulse oximeter passes red and infrared light into an
arteriolar bed and measures changes in light absorption during the pulsatile
cycle. The light sources are red and infrared light emitting diodes (LEDs),
while the detection is accomplished by a photo diode.
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 both blood volume and light absorption increase.
During diastole, blood volume and light absorption reach their lowest point.
The measurement is based upon the difference between maximum and
minimum absorption, focusing on the pulsatile arterial blood.
In addition to the oximetry function, the input signals may be used to calculate
heart rate.
Temperature
Processing
Measurement of patient temperature is accomplished by processing the signal
from a probe containing a resistor whose impedance is temperature
dependent. The class of such components is called thermistor.
The A3 is designed to accept the signals from electrically isolated Series 400
probes manufactured by Yellow Springs Incorporated. Interchangeable
probes in this series may be used for esophageal, rectal, skin or surface, or
airway temperature measurement. Probes are furnished with a standard 10-
feet lead. Extension leads are available.
The signal from the probe is conditioned by the monitor input circuitry,
processed, and used to drive the numeric display.