By measuring the changes in the impedance of human tissues to a weak alternating current, the cardiac impedance signal reflecting blood volume variations can be obtained. The human body is composed of numerous cells, and the electrical model of biological tissue can be represented by the equivalent circuit shown in Figure 2.
This model explains bioimpedance using three elements:
- Cm = is the parallel capacitance of the cell membrane
- Re = the resistance of the extracellular fluid
- Ri = the resistance of the intracellular fluid
These values do not represent a single cell, but the overall equivalent electrical properties of the entire tissue. This is known as the three‑element RC bioimpedance model.
The system uses the four-electrode method to measure human bioimpedance. The principle of this method is to apply an excitation current through one pair of electrodes, while extracting the voltage using another pair of electrodes.
As shown in Figure 3, four 3M Ag/AgCl gel electrodes are placed in the standard configuration for ICG acquisition.
Electrode roles:
- D1 & D4 (serve as the pair of excitation electrodes) → Inject high-frequency AC current into the human body
- D2 & D3 → Measure voltage across the thorax
Because the thoracic impedance varies with cardiac activity, the potential difference also changes accordingly. This varying potential signal is the ICG signal.
During signal acquisition, the optimal electrode placement is as follows:
• D1 excitation electrodes: attached to the depressions behind the left and right ears.
• D4 excitation electrode: placed at the upper abdomen.
• D2 sensing electrodes: symmetrically attached above both clavicles.
• D3 sensing electrodes: placed on both sides of the depression below the xiphoid process.
Related Article:
What is Impedance Cardiography (ICG) and how does it measure stroke volume?
How does impedance cardiography estimate stroke volume and LVET?