Expansion
A pulse oximeter reads two wavelengths and can be badly misled
Expansion
How a pulse oximeter works: it compares absorption at two wavelengths, 660 nm and 940 nm, and assumes only oxyhaemoglobin and deoxyhaemoglobin are present. Co-oximetry on a blood gas uses multiple wavelengths and measures the other species directly.
Carbon monoxide poisoning
- Carboxyhaemoglobin absorbs almost identically to oxyhaemoglobin at 660 nm, so the pulse oximeter reads falsely normal or high
- The PaO2 is also normal, because dissolved oxygen is unaffected; it is the carrying capacity that has collapsed
- The diagnosis requires co-oximetry for carboxyhaemoglobin. Non-smokers are under 3 per cent, smokers up to 10
- The cherry red appearance is late and unreliable; headache, nausea, confusion and collective symptoms in a household are the practical clues
- Treatment is high flow oxygen, which shortens the half life from about 4 hours to under 1, with hyperbaric oxygen considered in severe cases
Methaemoglobinaemia
- Iron oxidised to the ferric state cannot bind oxygen, and it shifts the dissociation curve left, worsening delivery
- The pulse oximeter tends towards a fixed reading of about 85 per cent, regardless of the true value
- The saturation gap, between a calculated saturation from the blood gas and the pulse oximeter reading, is the giveaway
- Blood is chocolate brown and does not turn red on exposure to air
- Causes: dapsone, local anaesthetics (prilocaine, benzocaine), nitrates, nitrites, antimalarials, and congenital deficiency of cytochrome b5 reductase
- Treatment is methylene blue, avoided in G6PD deficiency, where it can cause haemolysis
Other causes of an unreliable oximeter reading: poor perfusion, motion, nail varnish, methylene blue itself, severe anaemia, and darker skin pigmentation, which is associated with overestimation of true saturation and consequently missed hypoxia.