Transcutaneous monitoring of oxygen saturations is carried out using a device called a pulse oximeter. It is a simple, non-invasive test that can be used to estimate the amount of oxygenated haemoglobin in the blood. This monitoring can take place continuously, or on an intermittent basis. A probe is placed on a finger, toe, or earlobe. The probe contains a light source which transmits red and infrared wavelengths through a pulsating capillary bed. The light is transmitted through the tissue, and a photodetector on the other side measures the light which passes through. The monitoring unit analyses this, and works out the amount of light that has been absorbed by the oxygenated blood in the tissue. The amount of oxygenated blood is then displayed as a percentage saturation.
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