Venous Occlusion Plethysmography (VOP)
This method is a non-invasive function test to evaluate the venous system particularly deep veins in the legs to determine the venous capacity and to diagnose deep venous thrombosis. It also allows to evaluate the process of recanalization during therapy.
For this examination the patient is lying and the legs are in a lifted position (30-45°) over heart level so that the venous blood can flow out of the legs. The outflow of the blood should not be reduced by an incorrect patient positioning or tight cloths.
Cuffs are placed on the upper legs. After the veins of the legs are emptied the cuffs are inflated until a diastolic pressure of about 80 mmHg. Now the arterial blood can flow in but the venous blood cannot flow out anymore.
The inflow of the arterial blood and the filling of the veins are recorded. The veins are completely filled when the curve doesn´t rise any more (horizontal curve). The amplitude of this filling curve represents the venous capacity which is increased in case of varicose veins. For an accurate determination of venous capacity it is important that at the beginning the veins have been emptied completely.
After the filling phase has been completed the cuffs are deflated automatically as fast as possible so that the venous blood accumulated in the legs can flow out. The outflow of the blood is recorded and analysed. A prolonged outflow is an indicator for an outflow obstruction, such as deep venous thrombosis (DVT) or another obstruction.
During this course of examination in the lower legs blood volume changes take place which can be measured by Impedance Plethysmography (IPG) or strain gauges.
|VC||Venous Capacity||Blood volume accumulated in the leg during venous occlusion|
|VO||Venous Outflow||Velocity of blood volume change after opening the venous occlusion; this value can be defined for 2, 3 and 5 s after opening|
|AI||Arterial Inflow||Maximum velocity of blood volume change (filling) after venous occlusion|