PHOTOPLETHYSMOGRAPH

Typical Number in Hospital: 3

Cost Bands: 2

References: 6,9

The name suggests that this device should measure volume by optical methods. In fact it is limited to detecting changes in blood perfusion in limbs and tissues. Light may be transmitted through a capillary bed such as in the ear lobe or finger tip. As arterial pulsations fill the capillary bed the changes in volume of the blood vessels modify the absorption, reflection and scattering of the light. This technique can be used to show the timing of events such as heart beats, but it is a poor measure of changes in volume and is very sensitive to motion artefacts.

A miniature tungsten lamp may be used as the light source but the heat generated causes vasodilation which alters the system being measured. An infrared light-emitting diode (LED) of a suitable colour (e.g. gallium arsenide LED) may produce a more accurate result. In their most simple form, photoplethysmographs are used for monitoring the pulse during anaesthesia or related test procedures.