Impedance plethysmography is a non-invasive diagnostic technique that measures changes in electrical impedance (resistance to electrical current) in a limb, most commonly the leg, to assess blood volume changes within the veins. Because blood has a different electrical conductivity from surrounding tissue, changes in blood volume alter the measured impedance; these changes are recorded to detect abnormalities in blood flow or venous filling.
The technique was historically used to detect deep vein thrombosis (DVT) in the legs, where a blood clot reduces or blocks venous outflow, causing abnormal impedance changes during testing. However, impedance plethysmography has been largely superseded by compression ultrasonography (Doppler ultrasound) for DVT diagnosis, which provides higher sensitivity and specificity and allows direct visualisation of the veins.
Plethysmography in a broader sense is used in other vascular assessments: pulse volume recording (arterial plethysmography) measures arterial pulse waveforms to detect peripheral arterial disease, and photoplethysmography (PPG) uses light to detect blood volume changes and underpins the technology in pulse oximeters and smartwatch heart rate monitors.
In the context of cardiac arrest, DVT is an important complication of prolonged critical care admission due to immobility and venous stasis. While modern vascular assessment relies primarily on Doppler ultrasound, impedance plethysmography may be referenced in older clinical literature or used in settings where ultrasound is unavailable.
« Back to Glossary Index