Which of the following best explains why a patient with a right-to-left intracardiac shunt does NOT correct their hypoxaemia with 100% inspired oxygen?

• A Pulmonary diffusion limitation is worsened by high FiO2
• B High FiO2 causes hypercapnia via Haldane effect, worsening ventilation-perfusion mismatch
• C Supplemental oxygen reduces hypoxic pulmonary vasoconstriction, increasing shunt fraction
• D Shunted blood bypasses alveoli entirely, so supplemental O2 cannot oxygenate it ✓

Explanation

In a true shunt (anatomical or absolute intrapulmonary shunt), blood traverses from right to left without contacting ventilated alveoli. Since haemoglobin in ventilated lung segments is already near-saturated at normal FiO2, adding 100% O2 increases dissolved O2 only minimally, insufficient to compensate for the shunted deoxygenated fraction. This produces the hallmark of a shunt: failure to correct PaO2 with high FiO2 (shunt is the only cause of this). V/Q mismatch, by contrast, does respond to supplemental O2.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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