Also known as an impedance plethysmogram, a plethysmogram is a noninvasive test to measure the blood flow in the veins of the arms or legs in order to detect and assess the presence of any blockages. The test is rarely used today, having been replaced by more advanced vein imaging tests, such as the Doppler ultrasound.

During the plethysmogram, the limb to be tested is hooked up to a machine called a plethysmograph. This machine uses electrodes to send a small electrical current through the area to be tested. A blood pressure cuff (sphygmomanometer) is then wrapped around the upper arm or upper leg and inflated for up to two minutes, then rapidly deflated. Meanwhile, the plethysmograph measures the resistance, or impedance, to the electrical current in the limb. The idea is to measure the blood volume in the limb by measuring how well it conducts electricity.

Following deflation, the process is repeated on the normal limb for comparison. Both limbs will probably be tested two or three times total, so the entire test generally takes about 30 to 45 minutes. Afterward, patients can immediately return to their usual daily activities. Depending on the results of the test, additional tests may be ordered to determine the cause and location of whatever blockages (e.g., blood clots) may have been found by the plethysmogram.