Impedance Plethysmography Measurements
venous occlusion plethysmography (VOP)   pulse wave analysis

Impedance plethysmography is a measurement technique that measures the change in blood volume (venous blood volume as well as the pulsation of the arteries) for a specific body segment. As the blood volume changes, the electrical impedance (resistance) also changes. This electrical impedance is measured by passing a small amount of alternating current (AC) through the body segment. This technique is noninvasive because the amount of AC current is so small that the patient does not feel any sensation from the probe.

By measuring the changes in blood volume, doctors can detect blood flow disorders such as arterial occlusive diseases (and estimate severity), early stage arterioscleroses, functional blood flow disturbances, deep venous thromboses, migranes, and general arterial blood flow disturbances.

The rheoscreen® compact has been carefully developed to make the impedance plethysmography technique practical and easy to use for the health care professional. The rheoscreen compact has the best signal-to-noise ratio in the market today. The control process is designed to obtain the best reproducibility. Unlike many other devices, the entire pulse wave (detectable because of the compact's excellent resolution) is preserved so its other parameters can also be used for diagnostic tools.

How impedance plethysmography works in practice.

Measuring the impedance changes (and with them the blood volume changes) requires 4 electrodes as shown below. The two middle electrodes detect a voltage, and their placement defines the measurement segment. That means that the blood volume changes between these two electrodes are measured. The outer electrodes are used to emit the small imperceptible current required to measure the impedance. The placement of these outer electrodes is not critical. In former times special stripe electrodes were common. We have done a lot of scientific and electronics work to replace them with common ECG electrodes which we consider much easier and cheaper to use.

As described, the measured signal contains the venous blood changes as well as the arterial pulsation. The pulsations are much smaller and can be seperated due to their pulsatile character. Depending on the selected measurement program, the rheoscreen compact presents the venous changes or the arterial pulse wave.

Impedance plethysmography directly measures the relative blood volume in ml blood per 100ml tissue without requiring any mechanical gauge. The resolution of the method is only limited by the electronics used, giving us the possibility to create the high-resolution impedance technique we are famous for.

Venous Occlusion Plethysmography (VOP) is one of the most common measurement procedures that uses the impedance plethysmography technique.

The general VOP procedure involves applying a pressure cuff to the limb (thigh). The cuff is inflated above venous pressure but below diastolic pressure so that blood can flow through the arteries but not through the veins. That means blood can flow in via the arterials but not flow out through the veins and is therefore accumulated distal to the cuff in the calf and foot.

The blood volume is then examined to determine either the rate of inflow (arterial VOP), or the rate of outflow (venous VOP) after the pressure cuff is released.

The arterial VOP can be used to diagnose arterial obliterative processes, evaluate the haemodynamic quality of collateral circulation, and to quantify parameters for comparative studies such as pharmaceutical drug effectiveness.

The venous VOP can be used to diagnose deep venous thromboses, to evaluate postthrombotic syndrome functionality, and to determine other functional parameters of the venous system (drainage function, venous tone, etc.)

With VOP, other measurement methods are also common, for example the strain gauge plethysmography and pneumoplethysmography (with special pneumatic sensing cuffs). However, our high-resolution impedance plethysmography has a better resolution, delivers a better reproducibility, and provides the arterial pulse waves with the same sensor application--all without risking exposure to the toxic mercury in strain gauge sensors!

Arterial pulse wave has a very low amplitude and is superimposed on the venous blood volume changes. The small signal-to-noise ratio achieved by the rheoscreen® compact enables good detection of this valuable wave signal.

Pulse wave measurements are possible in many locations including the head (this measurement is called rheoencephalography) and extremities like the fingers.

The shape of the pulse wave is determined, in part, by the elasticity of the blood vessels. A trained technician can analyze the shape of the curve to get early information about a developing arterial vascular disease. The pulse wave analysis can often detect changes very early, and is noninasive, very easy to use, and one of the most economical techniques available today.

The rheoscreen® compact can perform the pulse wave analysis with the same device and even with the same electrodes as used for VOP. So the pulse wave examination can be used with very little additional expenditure.

This picture gives an impression how the measured pulse wave is changed by different diseases:

Pulse wave analysis can be used for:

• Diagnosis and early recognition of arterioscleroses, chronic and /or acute arterial vascular diseases, and functional circulatory disturbances.
• Monitoring at-risk patients such as diabetics, smokers, and patients older than 50 years.
• Postoperative check-ups after vascular surgical operations.
• Determination of the impact of pharmaceuticals on the vascular system.