Pulse Oximetry
A Pulse Oximetry is a non- invasive and continuous method of assessing oxygen saturation in the arterial blood as well as heart rate (heart beats/minutes).
It is not a measure of partial pressure of both oxygen and carbon dioxide in the arterial blood which require invasive arterial blood gases sampling technique.
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Principle:
Pulse oximetry works on the principle of spectro-photometry. It contains a probe emitting light at the red and infra-red wavelength and a photodetector. It relies on the differing amount of light absorbed by the saturated and unsaturated Hemoglobin molecules. The percentage oxygen saturation in the blood is calculated from the ratio of these two forms of the molecule. The readings are very useful in the critically ill patients in the intensive care particularly in case of sudden deterioration of the clinical condition.
Sources of errors in pulse oximetry readings:
Carboxyhemoglobin can be measured as oxyhemoglobin causing falsely high reading.
Methemoglobinemia (increase blood methemoglobins level). The condition can under or over-estimate the arterial blood oxygen saturation under normal or low oxygen tension respectively.
Low saturation below 70 to 80% causes progressive inaccurate readings and falsely low measurements.
Cardiopulmonary bypass poor signal due to non-pulsatile blood flow.
Limb ischemia and shock states due to a low pulsatile flow as well as motion artifacts like tremors and fits cause falsely low reading.
Strong ambient sunlight, fluorescent and xenon lamps as well as inadequate probe position and skin contact cause a falsely low reading.
Nail polish especially blue cause a falsely low SpO2.
Magnetic resonance Imaging scanners may cause a falsely high reading.