Impedance Plethysmography

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Impedance plethysmography

Definition

Impedance plethysmography, also called impedance test or blood flow or impedance phlebography , is a non-invasive test that uses electrical monitoring in the form of resistance (impedance) changes to measure blood flow in veins of the leg. Information from this test helps doctors detect deep vein thrombosis (blood clots or thrombophlebitis).

Purpose

Impedance plethysmography may be done in order to:

detect blood clots lodged in the deep veins of the leg
screen patients who are likely to have blood clots in the leg
detect the source of blood clots in the lungs (pulmonary emboli)

Accurate diagnosis of deep vein thrombosis (DVT) is critical because blood clots in the legs can lead to more serious problems. If a clot breaks loose from a leg vein, it may travel to the lungs and lodge in a blood vessel in the lungs. Blood clots are more likely to occur in people who have recently had leg injuries, surgery, cancer , or a long period of bed rest.

Precautions

Because this test is not invasive, it can be done on all patients and is easy to perform. However, the accuracy of the results is affected if the patient does not breathe normally or keep the leg muscles relaxed. Compression of the veins because of pelvic tumors or decreased blood flow, due to shock or any condition that reduces the amount of blood the heart pumps, may also change the test results. Both false-positives (e.g. when thrombi are non-occulsive) and false-negatives have been reported using this technique, which justifies repeated testing over a period of seven to ten days for patients with initial negative results. Success rates for this test have been estimated at anywhere from 65–66% to 92–98%.

Description

Using conductive jelly, the examiner strategically places two to four electrodes on the patient's calf (the four-electrode configuration yields a more uniform and precise current density and consequent measurement result). These electrodes are connected to an instrument called a plethysmograph, which records the changes in electrical resistance that occur during the test and produces a graph of the results.

The patient must lie down and raise one leg at a 30° angle so that the calf is above the level of the heart. The examiner then wraps a pressure cuff around the patient's thigh and inflates it to a pressure of 45–60 cm of water for 45 seconds. The plethysmograph records the electrical impedance changes that correspond to changes in the volume of blood in the vein at the time the pressure is exerted and again three seconds after the cuff is deflated. This procedure is repeated several times in both legs.

This test takes 30-45 minutes, costs an estimated $50-$100 (as of 2001), and results can be available within a few minutes.

Impedance plethysmography works by measuring the resistance to the transmission of electrical energy (impedance). This resistance is dependent upon the volume of blood flowing through the veins. By graphing the impedance, the doctor or technician can tell whether a clot is obstructing blood flow.

Preparation

Patients undergoing this test do not need to alter their diet, change their normal activities, or stop taking any medications. They will wear a surgical gown during the test and should be asked to urinate before the test starts. If keeping the legs elevated causes discomfort, mild pain medication can be given.

Aftercare

The patient may resume normal or postoperative activities after the test.

KEY TERMS

False negative —A test result that wrongly indicates a disease or condition is not present, when, in fact, it is.

False positive —A test result that wrongly indicates the a disease or condition is present, when, in fact, it is not.

Impedance —Denoted by Z, it is an expression of the opposition that an electronic component, circuit, or system presents to an alternating current (or direct current, in which case impedance equals resistance). Included in impedance are resistance, capacitance, and inductance. Impedance thus has both a real (resistance) and imaginary (phase) part.

Thrombophlebitis —Inflammation of a vein, associated with the formation of a blood clot.

Complications

Impedance plethysmography is painless and safe. It presents no risk to the patient.

Results

Normally, inflating the pressure cuff will cause a sharp rise in the pressure in the veins of the calf because blood flow is blocked. When the cuff is released, the pressure decreases rapidly as the blood flows away.

If a clot is present, the pressure in the calf veins will already be high. It does not become sharply higher when the pressure cuff is tightened. When the pressure cuff is deflated, the clot blocks the flow of blood out of the calf vein. The decrease in pressure is not as rapid as when no clot is present and the shape of the resulting graph is different, all of which is indicative of obstruction of major deep veins.

Health care team roles

Doctors, nurses, or well-trained technicians may perform all or part of the procedure, which includes application of electrodes and placement of cuffs as well as handling of the electronic equipment and analysis of results.

Training

Training for the procedure includes instruction on placement of electrodes and cuffs, facility with the electronic equipment, correct patient positioning during the procedure, and capability for accurate interpretation of resulting impedance graphs.

Resources

PERIODICALS

Aksamit, T. R. "Thromboembolism Occurrence and Diagnosis in the Medical Intensive Care Unit." Seminars in Thrombosis and Hemostasis 27, no. 1 (2001): 47-58.

Halek, J., "A Method of Local Skin Perfusion Detection." Journal of Medical Systems 24, no. 4 (2000): 257-264.

Kahn, S. R., L. Joseph, S. A. Grover, and J. R. Leclerc. "A Randomized Management Study of Impedance Plethysmography vs. Contrast Venography in Patients With a First Episode of Clinically Suspected Deep Vein Thrombosis." Thrombosis Research 102 (2001): 15-24.