A patient with a large left-to-right intracardiac shunt producing a true right-to-left admixture is placed on 100% inspired oxygen. After 20 minutes the PaO2 rises only modestly and the large A-a gradient persists. The best explanation is:
- A Shunted venous blood mixes with end-capillary blood whose oxygen content cannot rise much further because hemoglobin is already nearly saturated ✓
- B Alveolar hypoventilation under the oxygen mask offsets the added oxygen
- C Diffusion impairment worsens at high inspired oxygen fractions
- D High FiO2 causes reflex pulmonary vasoconstriction of shunt vessels
Correct answer: A. Shunted venous blood mixes with end-capillary blood whose oxygen content cannot rise much further because hemoglobin is already nearly saturated
Explanation
In true shunt, deoxygenated venous blood bypasses ventilated alveoli entirely. Breathing 100% oxygen can add only dissolved oxygen to the end-capillary blood, a few mL per dL, because hemoglobin saturation is already essentially complete. Mixing this blood with fully shunted venous blood leaves a persistently low PaO2 and a fixed A-a gradient. Hypoventilation and diffusion defects are largely corrected by 100% oxygen, unlike shunt.
Reference: West's Respiratory Physiology: The Essentials, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.