Spo2 is hemoglobin saturation, pao2 is plasma saturation
SpO2 is the Saturation (peripheral) of Haemoglobin with Oxygen, expressed as a percentage ration compared to fully saturated Haemoglobin (Hb) i.e. when the Haemoglobin (the main oxygen carrying component of blood, found in red blood cells) is fully loaded with oxygen, then is it 100% saturated. It is SpO2, because this is normally measured by placing a probe on the finger which shines red light of two different types through the finger, and a calculation is carried out based on how much of each type of light is absorbed, which tells us how much the Hb is loaded up or saturated.
PaO2 is the Partial pressure
(arterial) of oxygen in the blood. That is, if you took the oxygen gas in all the blood, and put it into a box by itself, what pressure would it exert on the walls. This is normally measured by using a special probe which uses electric currents to determine how much oxygen is actually present in the blood. It is normally expressed in mmHg (millimetres of Mercury), or (more commonly outside the USA) as kPa (kilopascals), which are units of pressure. 1 atmosphere of pressure is roughly 101kPa, which is roughly 760 mmHg or 760 Torr, which is roughly 1 bar. There are more precise values available on the internet for these. Whilst SpO2 and PaO2 are closely related, they do not actually measure the same thing
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