Bioimpedance: Principles, Measurements, and Applications

Impedance measurements in living systems have been used for many decades in varied applications from impedance cardiography and plethysmography to impedance spectroscopy. Recently, the potential advantages of an admittance formulation has opened a new avenue of investigation. The article begins with the vector electric field origins of measured impedance and admittance in distributed current fields, including the relevant instrumentation, and then presents examples of the many applications of impedance and admittance measurements in vivo . Making these measurements in living systems introduces the electrochemical nature of electrode–electrolyte interfaces, an extremely important consideration in impedance measurements below approximately 50 kHz . Tetrapolar electrode approaches are necessary at those frequencies. The complex admittance properties of muscle make it possible to uniquely identify its contribution to bulk impedance or admittance measurements. The experimental examples illustrate the utilities of the various methods currently in use. Finally, we conclude with some comments on impedance spectroscopy.