medical electronics III

Impedance Plethysmography

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.