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The amount of light reflected from beneath the skin surface will depend on the volume of blood that is present, and this will vary in a pulsatile manner. In theory this technique could be used to give some quantitative information about the blood supply to the extremities in vascular disease. However, its commonest use is simply as a pulse monitor in the operating theatre or high dependency unit.
The whole plethysmograph simply consists of a sensing head which is placed on a finger tip or on the ear lobe and contains a light source and photocell. The light source is often a conventional tungsten bulb which will provide a little heat to improve the local blood circulation. The reflected light is picked up by a photocell, which is usually a photoresistor. The probe is connected to a detector unit which senses the light fluctuations and presents these on a meter, cathode ray tube, or rate-meter.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details