Impedance Plethysmography

Noninvasive test for deep venous thrombosis

leg pain
DVT. BSIP/UIG/Getty Images

Impedance plethysmography is a non-invasive test used to help diagnose deep venous thrombosis.

Deep venous thrombosis, or DVT, is a condition in which blood clots develop in one or more of the the major veins of the leg. These blood clots can occlude the leg veins, leading to severe swelling and pain in the affected leg. Even more significantly, DVT is the leading cause of pulmonary embolus, a potentially life-threatening condition.

Because treatment with anticoagulants (blood thinners) helps relieve DVT and greatly reduces the risk of pulmonary embolus, making the correct diagnosis is important when DVT is present.

Diagnosing DVT can be relatively challenging, however, because the symptoms of DVT are often indistinguishable from other kinds of conditions affecting the legs, including infections and joint conditions.

Diagnosing DVT

The “gold standard” test for diagnosing DVT is the venogram, an invasive study in which dye is injected into a leg vein, and x-rays are taken to visualize the dye as it moves through the veins. An obstruction caused by a DVT can be visualized fairly readily. However, performing a venogram is relatively cumbersome, can be painful, and carries a certain amount of risk (such as a risk of infection.)

What Is Impedance Plethysmography?

Impedance plethysmography is a non-invasive method of diagnosing DVT.

“Plethysmography” is a test that estimates the volume of a body part, and “impedance” is a measure of resistance to an electrical current. So impedance plethysmography is a technique that measures electrical resistance through leg tissue to estimate the volume of the leg.

When DVT is present, a blood clot in one of the leg veins partially blocks blood from leaving the leg and returning to the heart.

This partial blockage increases the amount of blood that remains in the leg -- and therefore, increases the volume of the leg. Impedance plethysmography is a method of detecting this increased leg volume, and of determining that the increased volume is caused by a blockage in a leg vein.

Impedance plethysmography is performed by placing a cuff (like a blood pressure cuff) around the thigh, and inflating it. This inflation compresses the veins and prevents blood from returning to the heart. A tiny electrical current is then administered to electrodes attached to the calf, which is detected by other electrodes on the thigh. The electrical resistance is measured to estimate the volume of the leg.

Once this measurement is made, the cuff is deflated, allowing the "trapped" blood to flow back out of the leg. The calf volume measurement is then repeated. If no DVT is present, the leg volume drops quickly.

However, if DVT is present, the normal drop in leg volume will be significantly diminished -- indicating that the leg veins are partially obstructed by a blood clot.

This finding indicates that DVT is present.

How Useful Is Impedance Plethysmography?

Studies have indicated that impedance plethysmography is often quite accurate in detecting the presence of DVT. However, the test needs to be performed by a well-trained person to yield accurate results, and certain conditions (such as a pelvic tumor, or the presence of heart failure or peripheral artery disease) can give a false positive result.

In recent years impedance plethysmography has largely been supplanted by another non-invasive technique for detecting DVT - compression ultrasound.

Compression ultrasound generally is not regarded as more accurate than impedance plethysmography. However, ultrasound equipment is today readily available in most hospitals and can be performed with a minimal amount of training - whereas equipment for plethysmography is not readily available and the test must be performed by experiencesubd operators. So compression ultrasound is usually the more convenient and readily available of these two noninvasive tests.


Huisman MV, Buller HR, ten Cate JW. Utility of impedance plethysmography in the diagnosis of recurrent deep-vein thrombosis. Arch Intern Med 1988; 148:681.

Prandoni P, Cogo A, Bernardi E, et al. A simple ultrasound approach for detection of recurrent proximal-vein thrombosis. Circulation 1993; 88:1730.

Qaseem A, Snow V, Barry P, et al. Current diagnosis of venous thromboembolism in primary care: a clinical practice guideline from the American Academy of Family Physicians and the American College of Physicians. Ann Intern Med 2007; 146:454.

Continue Reading