Pulse oximetry

Cyanosis can be difficult to detect and one objective measurement of oxygenation is pulse oximetry. Oximetry measures the percentage saturation of haemoglobin and this indirectly relates to PaO2 through the oxygen dissociation curve. This indirect relationship can lead to errors in clinical practice if one forgets that pulse oximetry measures saturation, not PaO2. Red and infrared light are transmitted through the tissues by a light-emitting diode. A photodetector picks up the transmitted light and the ratio of light emitted to light absorbed is translated into a percentage saturation value. Inaccurate pulse oximeter readings arise from:

• light interference

• motion artefact

• low perfusion states

• abnormal haemoglobin.

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