Which measurement is used along with hemoglobin to estimate arterial oxygen content (CaO2)?

• A. PaO2
• B. Dissolved oxygen in plasma
• C. SaO2
• D. Hematocrit

Answer: (C)

Oxygen content in arterial blood reflects both what’s bound to hemoglobin and the small amount dissolved in plasma. The part bound to hemoglobin depends on how much hemoglobin you have and how saturated it is with oxygen, which is shown by the arterial oxygen saturation (SaO2). To estimate CaO2, you use the hemoglobin amount multiplied by its saturation (the bound portion), plus a small contribution from dissolved oxygen that comes from PaO2. So the measurement that goes with hemoglobin to estimate CaO2 is SaO2. If SaO2 drops, CaO2 drops even if hemoglobin is normal, highlighting why saturation is key for this calculation.