Right-to-left shunt physiology — FRCA Primary MCQ
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Correct answer: E — Perfusion of non-ventilated alveoli
Marked hypoxaemia despite high FiO2 suggests right-to-left shunt, where blood perfuses non-ventilated alveoli and cannot be fully corrected by oxygen. Reduced inspired oxygen tension would be unlikely on high-flow oxygen with correct equipment. Carbon monoxide poisoning affects oxygen content and pulse oximetry more than PaO2. A normal PaCO2 can coexist with severe shunt because CO2 diffuses more readily and ventilation may increase.
Reference: West Respiratory Physiology; Nunn's Applied Respiratory Physiology