A patient with a right-to-left cardiac shunt has an oxygen saturation of 80 percent. Why does breathing 100 percent oxygen fail to correct the hypoxaemia?
Answer and explanation
Correct answer: A. Shunted blood bypasses ventilated alveoli
- Step 1: Hypoxaemia is caused by ventilation-perfusion mismatch, hypoventilation, diffusion impairment or shunt. Step 2. In a true shunt, some blood never contacts ventilated lung, so it stays desaturated, and well-oxygenated blood cannot compensate fully because haemoglobin is already nearly saturated. Answer: Shunted blood bypasses ventilated alveoli. Why not the others: Over-ventilation does not block uptake of oxygen. Affinity changes do not explain the failure to respond. Diffusion impairment responds to supplemental oxygen, unlike a true shunt.
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