Venous occlusion plethysmography

Venous occlusion plethysmography (VOP) is a painless and bloodless examination to investigate vein function.
A strain gauge attached to the limb measures changes in volume when the venous outflow is suddenly occluded. The leg veins are first of all filled to the maximum by inflating a blood pressure cuff to prevent venous return. If the veins are healthy, they can additionally take up a certain volume of blood. It is referred to as the venous capacity and is normally 2.5-5ml/100ml tissue. When varicose veins are present, they are dilated and can hold more blood than normal.

The blood pressure cuff is then deflated and the venous blood flows out of the legs. Normal venous blood drainage is 35-100 ml/100ml tissue. Rapid drainage shows that the veins are patent (open). If a vein is occluded (blocked), by thrombus for example, the blood takes longer than normal to drain away.