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STRAIN GAUGE PLETHYSMOGRAPHY
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This involves placing a circular band around the leg which responds to changes in volume. As the leg swells, the band is stretched and its conductivity altered. In a normal limb, the application of a tourniquet around the upper thigh will result in leg swelling. This swelling is detected by the strain gauge around the lower part of the leg. There is a slow but progressive change which corresponds to the swelling of the leg. When the tourniquet is released, blood flows from the leg rapidly. This rapid flow of blood from the leg results in a rapid return to the previous conductivity. If the venous outflow is obstructed, for example, with a deep vein thrombosis, then there will be a very slow return to the normal conductivity, indicating the presence of venous obstruction.