At what true PaO2 value does the device begin to underestimate PaO2 due to hyperoxemia?

At what true PaO2 value does the device begin to underestimate PaO2 due to hyperoxemia?

• A. 90-100 mmHg
• B. 50-60 mmHg
• C. 120-140 mmHg
• D. 40-60 mmHg

Answer: (A)

The main idea is that some devices estimate arterial PaO2 from oxygen saturation (SpO2). The relationship between PaO2 and SpO2 is sigmoidal: it rises steeply at low PaO2, then quickly approaches a plateau as saturation nears 100%. Once SpO2 is already at or near 100%, further increases in PaO2 don’t change SpO2 much. If the device relies on SpO2 to infer PaO2, it loses sensitivity in this high–PaO2 (hyperoxemic) range and will report a PaO2 that is lower than the true value. In other words, the underestimation begins when PaO2 has entered the high end of the normal range, where SpO2 is saturated. For precise PaO2 during hyperoxemia, an arterial blood gas measurement is needed. At lower PaO2, the SpO2–PaO2 relationship is more dynamic, so the estimate is more reliable.

The main idea is that some devices estimate arterial PaO2 from oxygen saturation (SpO2). The relationship between PaO2 and SpO2 is sigmoidal: it rises steeply at low PaO2, then quickly approaches a plateau as saturation nears 100%. Once SpO2 is already at or near 100%, further increases in PaO2 don’t change SpO2 much. If the device relies on SpO2 to infer PaO2, it loses sensitivity in this high–PaO2 (hyperoxemic) range and will report a PaO2 that is lower than the true value. In other words, the underestimation begins when PaO2 has entered the high end of the normal range, where SpO2 is saturated. For precise PaO2 during hyperoxemia, an arterial blood gas measurement is needed. At lower PaO2, the SpO2–PaO2 relationship is more dynamic, so the estimate is more reliable.