System Processing Description
90 N5500 Service Manual
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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 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 monitor 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.
Automatic Calibration
Because light absorption by hemoglobin is wavelength dependent and because the mean
wavelength of LEDs varies, an oximeter must know the mean wavelength of the
sensor’ s red LED to accurately measure SpO
2
. During manufacturing, the mean
wavelength of the red LED is encoded in a resistor in the sensor.
During monitoring, the instrument’ s software reads this resistor value and selects
coefficients that are appropriate for the wavelength of that individual sensor’ s red LED;
these coefficients are then used to determine SpO
2
. This resistor value is read when the
monitor turned on, periodically thereafter, and each time a new sensor is connected.
Additionally, to compensate for differences in tissue thickness, the light intensity of the
sensor’ s LEDs is adjusted automatically.
Measured versus Calculated Saturation
The measured SpO
2
value from an oximeter may differ from the saturation value that is
calculated from a blood gas partial pressure of oxygen (PO
2
). This usually occurs
because the calculated saturation was not appropriately corrected for the effects of
variables that shift the relationship between PO
2
and saturation: pH, temperature, partial
pressure of carbon dioxide (PCO
2
), 2, 3-DPG, and fetal hemoglobin.
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PO
2
( m m H g )
pH
Temperature
PCO 2
2,3-DPG
Fetal Hb
pH
Temperature
PCO2
2,3-DPG
Figure 25. Oxyhemoglobin Dissociation Curve