Expansion
Two wavelengths distinguish oxyhaemoglobin from deoxyhaemoglobin in pulsatile blood
Expansion
Two light-emitting diodes at 660 nm (red) and 940 nm (infrared) shine through tissue. Oxyhaemoglobin and deoxyhaemoglobin absorb these differently, and the ratio of absorbance gives saturation.
Only the pulsatile component is analysed, so venous and tissue absorbance are subtracted out.
Causes of inaccuracy
- Carboxyhaemoglobin absorbs like oxyhaemoglobin, giving a falsely high reading
- Methaemoglobin drives the reading towards 85 per cent regardless of true saturation
- Poor perfusion, hypothermia, vasoconstriction and shivering: unreliable signal
- Nail varnish, dyes such as methylene blue, and bright ambient light
- Severe anaemia, and darker skin pigmentation, which can overestimate at low saturations
Key limitations of principle
- Says nothing about ventilation or carbon dioxide; a patient on oxygen can be dangerously hypercapnic with a normal saturation
- Says nothing about oxygen content, so it is normal in profound anaemia
- On the flat part of the dissociation curve, large falls in oxygen tension produce little change in saturation