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Convert between PaO2 and SpO2/SaO2 using the oxyhemoglobin dissociation curve
Enter either PaO2 OR SpO2 to calculate the other
*Approximate values at normal pH and temperature
Helps unload O2 at tissues
Hb holds onto O2 more tightly
A sigmoid curve showing the relationship between PaO2 and hemoglobin oxygen saturation. At low PaO2, small changes cause big saturation changes. Above ~70 mmHg, the curve flattens.
It explains why SpO2 drops rapidly below certain PaO2 levels. It also shows how fever, acidosis, etc. can improve oxygen delivery to tissues (right shift).
SpO2 is measured by pulse oximetry (non-invasive). SaO2 is measured from an arterial blood gas (invasive). They usually correlate well but can differ in certain conditions.
The PaO2 at which hemoglobin is 50% saturated. Normal is ~26.6 mmHg. Lower P50 = left shift (Hb binds O2 tighter). Higher P50 = right shift (Hb releases O2 easier).
In COPD patients with chronic hypercapnia, high O2 can suppress respiratory drive. Conservative targets (88-92%) prevent CO2 retention while maintaining adequate oxygenation.
Get ABG when: SpO2 unreliable, need PaCO2/pH, assessing acid-base status, CO poisoning suspected, or managing mechanical ventilation.
The "90-60 rule": SpO2 ~90% corresponds to PaO2 ~60 mmHg. This is the inflection point where small drops in PaO2 cause steep falls in saturation - the "cliff" of the curve.