It is important to be aware of the concerns that have emerged during the COVID-19 pandemic about the potential for pulse oximeters to produce inaccurate readings. This is why a SpO 2 reading should be considered as only being an estimate of the gold standard ABG measurement. It does not tell you what is happening downstream of the sensor in the vital organs, where O 2 is ‘downloaded’ from Hb and taken up and utilised by the cells.Īn arterial blood gas (ABG) measurement does give you this information and, therefore, provides a more accurate assessment of what is happening at a cellular level. However, it is important to understand that this measurement only tells you how much O 2 has been ‘uploaded’ to Hb in the lungs and is now travelling bound to Hb in the arterial circulation. Therefore, you can determine how much Hb is saturated with oxygen (O 2), which is presented as a percentage (SpO2). The fundamental principle behind pulse oximetry is that oxygenated and deoxygenated haemoglobin (Hb) absorb light of different wavelengths, so if you shine lights of these 2 wavelengths at Hb that is moving in a pulsatile manner, you can measure the proportion of oxygenated and deoxygenated Hb travelling within the arterial circulation.
You may also be interested in our OSCE guide to measuring basic observations (vital signs).
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