10: Monitoring SpO
2
Understanding Pulse Oximetry
110
Understanding Pulse Oximetry
A pulse oximetry sensor sends light through patient tissue to a receiver on the other side of the
sensor. Light-emitting diodes transmit red and infrared light through the peripheral areas of the
body such as a finger. See Figure 58.
A photodetector positioned opposite the light emitting diodes compares the light absorbtion before
and after pulsation. The amount of light getting through reflects the blood flow in the aterioles. This
measurement of light absorbtion during pulsation is translated into an oxygen saturation percentage.
The SpO
2
value and wave are displayed.
WARNING: SpO
2
readings may be inaccurate in patients that:
• Are hypothermic
• Are receiving a photosensitive drug
• Are receiving vasoconstrictor medications
• Have poor circulation
NOTE: For accurate SpO
2
measurements, the following conditions must apply:
• The patient must have perfusion in that extremity.
• The light emitter and photodetector must be directly opposite each other.
• All of the light from the emitter must pass through the patient’s tissue.
• The sensor site should be free of vibration and excessive motion.
• The sensor cable and connector should be positioned away from power cables to avoid electrical
interference.
Figure 58 Pulse Oximetry Sensor
Light
Emitting
Diodes
Photodetector
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