• As only two wavelengths are used, pulse oximetry measures functional rather than fractional oxyhaemoglobin saturation. Erroneously high readings are given with carboxyhaemoglobin and methaemoglobin.

• With poor peripheral perfusion or intense vasoconstriction the reading may be inaccurate ('fail soft') or, in newer models, absent ('fail hard').

• Motion artefacts and high levels of ambient lighting may affect readings.

• Erroneous signal may be produced by significant venous pulsation from tricuspid regurgitation or venous congestion. Venous pulsatility accounts for differences between ear and finger SpO2 in the same subject.

• Ensure a good LED signal indicator or a pulse waveform (if available) is seen on the monitor.

• Vital dyes (e.g. methylthioninium chloride (methylene blue), indocyanine green) may affect SpO2 readings.


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