Bioelectrical impedance techniques in medicine. Part I: Bioimpedance measurement. First section: general concepts

After a brief historical overview, the concept of electrical impedance is introduced as a principle of transduction calling attention to the possible mechanisms by which a physiological event may change impedance, i.e., by geometric, resistivity, and/or permittivity changes. Thereafter, since impedance measurements usually require the injection of current, its biological effects are discussed in order to establish the safety criteria. Finally, the elements found in an impedancimetric circuit and their respective nature are presented and described. The particular behavior of the biological impedance and the electrode/electrolyte interface appear immediately as strikingly important. The section ends with a bird's-eye view of the basic circuitry to measure impedance. Each subsection is closed by partial conclusions to underline the relevant concepts.