Which plethysmography uses aluminum strip electrodes around the limb, thigh cuffs to inhibit venous flow, and low-level current to measure impedance with four-wire configuration?

• A. Photoplethysmography
• B. Volume plethysmography
• C. TcPO2
• D. Impedance plethysmography

Answer: (D)

Electrical impedance plethysmography measures limb blood volume by passing a small alternating current through the limb and sensing how impedance changes with the pulsatile arterial inflow. A four-wire configuration uses separate pairs of electrodes for delivering the current and for measuring the voltage, which reduces errors from electrode-skin contact and cable impedance. Inflating thigh cuffs to temporarily inhibit venous outflow amplifies the arterial pulse signal, making the impedance changes more pronounced and easier to interpret. Aluminum strip electrodes around the limb provide the contact points for both current delivery and voltage measurement, while the current is kept very low to ensure patient safety and comfort. This combination of four-wire impedance measurement with venous flow inhibition is the hallmark of impedance plethysmography, distinct from photoplethysmography (which uses light), TcPO2 (which measures tissue oxygen), or volume plethysmography that relies on different sensing principles.