A 28-year-old man admitted after near-drowning has persistent hypoxemia. On FiO2 = 1.0 for 20 minutes, his PaO2 rises only from 55 to 65 mmHg. Chest radiograph shows bilateral infiltrates. Which mechanism best explains the poor response to pure oxygen?
- A Hypoventilation
- B Diffusion impairment
- C Intrapulmonary shunt with perfusion of non-ventilated alveoli ✓
- D Low inspired PO2
Correct answer: C. Intrapulmonary shunt with perfusion of non-ventilated alveoli
Explanation
Blood perfusing completely collapsed or fluid-filled alveoli never contacts any gas, so raising FiO2 to 1.0 adds no oxygen to that shunted stream. Because even 100 percent oxygen cannot raise the end-capillary PO2 of shunted blood above mixed venous levels, the PaO2 rises minimally. Hypoventilation, diffusion impairment and low inspired PO2 all respond briskly to supplemental oxygen, which distinguishes them from true shunt.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
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