Oxygen-Haemoglobin Dissociation Curve — FRCA Primary MCQ
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Correct answer: A — Increased PaCO2
Increased PaCO2 shifts the oxygen-haemoglobin dissociation curve to the right through the Bohr effect. Carbon dioxide increases hydrogen ion concentration and also forms carbamino compounds with haemoglobin, stabilising the deoxygenated T state. This increases P50, reduces haemoglobin's oxygen affinity and facilitates oxygen unloading in tissues. The other options shift the curve to the left: decreased 2,3-DPG reduces stabilisation of deoxyhaemoglobin; fetal haemoglobin binds 2,3-DPG less avidly than adult haemoglobin; increased pH favours the oxygenated R state; and decreased temperature increases oxygen affinity. Thus, E is the only defensible answer.
Reference: British Thoracic Society, BTS guideline for oxygen use in adults in healthcare and emergency settings, section 5.1.3: The oxyhaemoglobin dissociation curve and the Bohr effect, 2017. https://thorax.bmj.com/content/72/Suppl_1/ii1