Pulse oximetry

Continuous, non-invasive monitoring of arterial oxygen saturation by placement of a probe emitting red and near-infrared light over a pulse on a digit, ear lobe, cheek, or bridge of the nose. It is unaffected by skin pigmentation, hyperbilirubinaemia, or anaemia (unless profound).

Physics

The colour of blood varies with oxygen saturation due to the optical properties of the haem moiety. As the haemoglobin molecule gives up O2, it becomes less permeable to red light and takes on a blue tint. Saturation is determined spectrophotometrically by measuring the ‘blueness’, utilizing the ability of compounds to absorb light at a specific wavelength. The use of two wavelengths (650 and 940nm) permits the relative quantities of reduced and oxyhaemoglobin to be calculated, thereby determining saturation.

The arterial pulse is used to provide timepoints to allow subtraction of the constant absorption of light by tissue and venous blood. The accuracy of pulse oximetry is 9 2% at values above 70% SaO2.

Indications

• Continuous monitoring of arterial oxygen saturation.

Cautions

• 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 artefact and high levels of ambient lighting may affect readings.

• Erroneous signals 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) and indocyanine green, may affect SpO2 readings as may nail varnish.