In a patient with massive pulmonary embolism, arterial blood gas shows PaO2 48 mmHg, PaCO2 28 mmHg. The hypoxaemia does NOT correct with 100% oxygen. The predominant mechanism of hypoxaemia is:

• A Hypoventilation
• B Diffusion impairment
• C Increased oxygen consumption
• D True intracardiac right-to-left shunt (patent foramen ovale opened by elevated RV pressure) ✓

Explanation

Hypoxaemia unresponsive to 100% oxygen (shunt fraction physiology) in massive PE occurs when elevated right atrial/ventricular pressure reopens a patent foramen ovale, creating a true anatomical right-to-left shunt. In pure V/Q mismatch, 100% O2 usually corrects hypoxaemia. Hypoventilation would cause hypercapnia, not hypocapnia. Diffusion impairment is fully correctable with supplemental oxygen.

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

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