A patient breathing 100 percent oxygen has haemoglobin 15 g/dL, SaO2 100 percent, and PaO2 500 mmHg. His calculated arterial oxygen content is closest to:
- A 20 mL/dL, because dissolved oxygen contributes negligibly even at very high PaO2 ✓
- B 26 mL/dL, because dissolved oxygen adds substantially above a PaO2 of 100 mmHg
- C 30 mL/dL, because haemoglobin saturation exceeds 100 percent on pure oxygen
- D 15 mL/dL, because oxygen content equals half the haemoglobin concentration
Correct answer: A. 20 mL/dL, because dissolved oxygen contributes negligibly even at very high PaO2
Explanation
Arterial oxygen content equals 1.34 times haemoglobin times saturation plus 0.003 times PaO2. Here the bound component is 1.34 x 15 x 1.0, about 20.1 mL/dL, and the dissolved component is 0.003 x 500, only 1.5 mL/dL, giving roughly 21.6 mL/dL, closest to option A. Saturation cannot exceed 100 percent, killing option C, and the tiny solubility coefficient of oxygen means even a PaO2 of 500 mmHg adds little dissolved content.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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