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Hyperoxia test in cyanotic heart disease — MRCPCH TAS MCQ

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HardPhysiologyHyperoxia test in cyanotic heart diseaseMRCPCH TAS

A cyanosed newborn has oxygen saturation of 76% in air. After 10 minutes of high-flow oxygen, arterial PaO2 rises minimally and a murmur is heard. What is the most likely explanation?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Right-to-left shunting limiting pulmonary oxygen uptake

Failure of PaO2 to rise substantially with oxygen suggests cyanotic congenital heart disease with right-to-left shunting. Anaemia reduces oxygen content but PaO2 can still be normal or rise with supplemental oxygen. The pearl is that the hyperoxia principle distinguishes shunt physiology from many pulmonary causes of cyanosis.

Reference: Nelson Textbook of Pediatrics; Ganong's Review of Medical Physiology; RCPCH Theory examination syllabi