A 30-year-old man has chronic hypoxaemia with PaO2 of 58 mmHg on room air. Breathing 100% oxygen for 20 minutes raises his PaO2 only to 70 mmHg. Contrast echocardiography shows bubbles crossing into the left atrium. The refractory hypoxaemia is best explained by:
- A Ventilation-perfusion inequality
- B Right-to-left shunt ✓
- C Diffusion impairment
- D Hypoventilation
Correct answer: B. Right-to-left shunt
Explanation
Shunted venous blood never contacts ventilated alveoli, so even a FiO2 of 1.0 cannot raise its oxygen content appreciably, since haemoglobin is already nearly saturated and dissolved oxygen adds little. Pure hypoventilation, diffusion impairment and V/Q mismatch all correct substantially on 100% oxygen, which is precisely what fails here. Bubble passage on contrast echo confirms an intracardiac or pulmonary arteriovenous shunt.
Reference: West's Respiratory Physiology: The Essentials, 10th ed.
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