A 22-year-old patient with Hb 15 g/dL has pre-induction PaO2 120 mm Hg with SaO2 100%, and post-induction PaO2 150 mm Hg with the same saturation. How has the oxygen content changed?

A 22-year-old patient with Hb 15 g/dL has pre-induction PaO2 120 mm Hg with SaO2 100%, and post-induction PaO2 150 mm Hg with the same saturation. How has the oxygen content changed?

• A. Increased by 10%
• B. Increased by 5%
• C. Increased by less than 1%
• D. Cannot be determined without pH

Answer: (C)

Oxygen content in blood comes from two parts: the amount bound to hemoglobin and the small amount dissolved in plasma. The bound portion is calculated as Hb × 1.34 × SaO2, and the dissolved portion as PaO2 × 0.003. With Hb 15 g/dL and SaO2 at 100%, the bound O2 is 15 × 1.34 × 1.0 = 20.1 mL O2 per dL in both states. The dissolved portion changes with PaO2: 120 mm Hg gives 0.36 mL/dL, while 150 mm Hg gives 0.45 mL/dL. So pre-induction CaO2 ≈ 20.46 mL/dL, post-induction CaO2 ≈ 20.55 mL/dL. The increase is about 0.09 mL/dL, which is roughly 0.4–0.5% of the total CaO2—less than 1%. Therefore, oxygen content increases by less than 1%.

Oxygen content in blood comes from two parts: the amount bound to hemoglobin and the small amount dissolved in plasma. The bound portion is calculated as Hb × 1.34 × SaO2, and the dissolved portion as PaO2 × 0.003. With Hb 15 g/dL and SaO2 at 100%, the bound O2 is 15 × 1.34 × 1.0 = 20.1 mL O2 per dL in both states. The dissolved portion changes with PaO2: 120 mm Hg gives 0.36 mL/dL, while 150 mm Hg gives 0.45 mL/dL. So pre-induction CaO2 ≈ 20.46 mL/dL, post-induction CaO2 ≈ 20.55 mL/dL. The increase is about 0.09 mL/dL, which is roughly 0.4–0.5% of the total CaO2—less than 1%. Therefore, oxygen content increases by less than 1%.