SaO2 = oxygen saturation measured from an arterial sample
SpO2 = oxygen saturation measured from a pulse oximeter
PO2 = the 'partial pressure' of oxygen measured in kilopascals (kPa)
A precise normal SaO2 is not known, but in a healthy adult at sea level it is between 95 - 98%.
A normal PaCO2 is within 4.6 - 6.1 kPa (or 34 - 46 mmHg) and any PaCO2 of > 6.1 kPa should be considered abnormal.