Oximetry pitfalls — CO & methaemoglobinaemia
What it means
A pulse oximeter reads oxyhaemoglobin and is fooled by abnormal haemoglobins. Carbon monoxide poisoning: CO binds Hb 240× tighter than O₂ (carboxyhaemoglobin) → real tissue hypoxia but SpO₂ and PaO₂ read falsely NORMAL (dissolved O₂ is unaffected); diagnose on a co-oximeter (carboxyhaemoglobin level), treat with high-flow 100% O₂ (hyperbaric if severe). Methaemoglobinaemia (dapsone, local anaesthetics, nitrates): Fe³⁺ can't carry O₂ → SpO₂ stuck around 85% regardless of O₂; chocolate-brown blood; treat with methylene blue.
Worked example
A housefire survivor with a headache, SpO₂ 100% on 15 L but genuine hypoxic symptoms → measure CARBOXYHAEMOGLOBIN (pulse oximetry is falsely reassuring).
In the exam
A man rescued from a house fire has a dull headache; SpO₂ reads 100% on high-flow oxygen and PaO₂ is normal, yet he is clearly unwell — the most useful measurement is not the oximeter.
What settles it
CO poisoning: normal SpO₂/PaO₂ but low true saturation (co-oximetry needed). Methaemoglobinaemia: SpO₂ fixed ~85%. Pulse oximetry misleads in both.
Classically confused with
Ankle–brachial pressure index (ABPI)
Source: StatPearls — Carbon monoxide toxicity