In calculating arterial oxygen content (CaO2), which formula correctly represents CaO2?

In calculating arterial oxygen content (CaO2), which formula correctly represents CaO2?

• A. CaO2 = (Hb × 1.36 × SaO2) + (PaO2 × 0.003)
• B. CaO2 = (Hb × SaO2) + (PaO2)
• C. CaO2 = (Hb × 1.0 × SaO2) + (PaCO2 × 0.003)
• D. CaO2 = (Hb × 1.36 × SaO2) + (PaO2 × 0.01)

Answer: (A)

Oxygen content in arterial blood comes from two sources: the oxygen bound to hemoglobin and the oxygen dissolved in plasma. The bound portion is calculated by multiplying the hemoglobin concentration by how much oxygen each gram can carry and by how saturated the hemoglobin is with oxygen: Hb × 1.36 × SaO2. The dissolved portion is the amount of oxygen physically dissolved in plasma, which depends on the arterial PO2 and the solubility of oxygen in blood: PO2 × 0.003. Add these two parts together to get CaO2: CaO2 = (Hb × 1.36 × SaO2) + (PO2 × 0.003). This formula assumes Hb in g/dL and SaO2 expressed as a fraction (e.g., 0.97 for 97%), with PO2 in mmHg. If SaO2 is given as a percentage, it’s common to convert it to a fraction first (divide by 100), or equivalently use a coefficient adjusted for percent. Why the other forms aren’t correct: the dissolved oxygen term should use PO2 with the 0.003 coefficient, not a different number like 0.01; the bound portion uses a factor around 1.34–1.36 mL O2 per g Hb, not 1.0; and the equation should use PO2 rather than PaCO2, since PaCO2 does not contribute to arterial oxygen content.

Oxygen content in arterial blood comes from two sources: the oxygen bound to hemoglobin and the oxygen dissolved in plasma. The bound portion is calculated by multiplying the hemoglobin concentration by how much oxygen each gram can carry and by how saturated the hemoglobin is with oxygen: Hb × 1.36 × SaO2. The dissolved portion is the amount of oxygen physically dissolved in plasma, which depends on the arterial PO2 and the solubility of oxygen in blood: PO2 × 0.003. Add these two parts together to get CaO2: CaO2 = (Hb × 1.36 × SaO2) + (PO2 × 0.003).

This formula assumes Hb in g/dL and SaO2 expressed as a fraction (e.g., 0.97 for 97%), with PO2 in mmHg. If SaO2 is given as a percentage, it’s common to convert it to a fraction first (divide by 100), or equivalently use a coefficient adjusted for percent.

Why the other forms aren’t correct: the dissolved oxygen term should use PO2 with the 0.003 coefficient, not a different number like 0.01; the bound portion uses a factor around 1.34–1.36 mL O2 per g Hb, not 1.0; and the equation should use PO2 rather than PaCO2, since PaCO2 does not contribute to arterial oxygen content.