Four-wire impedance measurement is used in which plethysmography method?

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

Answer: (B)

Four-wire impedance measurement is used in impedance plethysmography. By placing two electrodes to inject a small alternating current and two separate electrodes to sense the voltage, this setup minimizes the influence of skin-electrode contact resistance and lead resistance. The tissue impedance changes as blood volume in the limb varies with the heartbeat, so impedance decreases when the limb is more blood-filled and rises when it’s less. The resulting waveform reflects pulsatile blood flow and can be used to assess vascular status such as perfusion and edema.

Photoplethysmography relies on light absorption to detect volume changes, not electrical impedance. TcPO2 measures transcutaneous oxygen tension, not impedance. Volume plethysmography measures volume changes via cuff or air displacement rather than electrical impedance.