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Section 10: Introduction and System Description
10.5 NIBP PROCESSING
10.6 SPO
PROCESSING
2
10.7 TEMPERATURE PROCESSING
10-6
The patient' s respiration is detected by using two of the three leads of the ECG
electrodes and cable. A low-level excitation signal is applied to these leads, and
the variation of the thoracic impedance caused by the breathing is sensed and
processed for display and measurement.
The NIBP processing uses an oscillometric technique to provide needed
measurements at selected intervals. This technique uses an inflatable
sphygmomanometer cuff similar to those used by clinicians in routine
measurements.
A motorized pump inflates the cuff to approximately 180 mmHg initially, at
which point the pressure effectively stops the flow of blood . Then, under
monitor control, the pressure in the cuff is gradually reduced, while a pressure
transducer detects the air pressure and transmits the parameter signal to the
NIBP input circuitry.
As the pressure is reduced, blood flows in the previously occluded artery, and
changes the measurements made by the transducer. The point at which
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.
Measurement of oxygen saturation in the blood uses a specrophotometry
technique. It is based on the facts that oxyhemoglobin and deoxyhemoglobin
differ in the 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 photodiode.
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.
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.
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