SaO2
Summary
SaO2 is the percentage of hemoglobin oxygen saturation in arterial blood, normally 95-100%. Reflects the proportion of hemoglobin bound to oxygen, determined by the sigmoidal oxygen-hemoglobin dissociation curve.
Detail
The relationship between PaO2 and SaO2 is sigmoidal: at PaO2 60 mmHg, SaO2 ~90%; at PaO2 >=80 mmHg, SaO2 ~95-100%. Rightward shifts (increased temperature, decreased pH, increased 2,3-DPG) decrease oxygen affinity, facilitating tissue offloading. Leftward shifts increase affinity. Pulse oximetry estimates SaO2 non-invasively but fails in CO poisoning and dysfunctional hemoglobins. Oxygen content (CaO2) depends on SaO2 and hemoglobin: CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003).
Sources
- First Aid for the USMLE Step 1
- West's Respiratory Physiology
- Pathoma
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