A patient with hypoxemia from a large intrapulmonary right-to-left shunt breathes 100% oxygen. What is the expected effect on the PaO2?
Answer and explanation
Correct answer: A. It rises only slightly
- Step 1: Shunted blood never reaches ventilated alveoli, so extra inspired oxygen cannot oxygenate it. Step 2. Only the small increase in dissolved oxygen in blood from ventilated units helps. Answer: it rises only slightly. Why not the others: a rise above 500 mmHg happens with V/Q mismatch, not shunt, and a shunt always widens the A-a gradient.
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