Electrical impedance measurements have been applied to the study of biologic systems for nearly 200 years. Indeed, the history of continuously flowing electricity began with Luigi Galvani's famous experiments on bioelectricity at the University of Bologna (1,2). It was not until the 1870s however, that Hermann Müller in Konigsberg/Zürich discovered the capacitive properties of tissue and the anisotropy of muscle conductance based on alternating current measurements. In 1864, James C. Maxwell contrived his now famous equations by specifically calculating the resistance of a homogeneous suspension of uniform spheres as a function of their volume concentration (1). In 1928, Kenneth S. Cole expanded on Maxwell's model by determining the impedance of a suspension of capacitively coated spheres over a range of frequencies.
Several additional and important developments occurred before the start of World War II. Rudolph Hoebers studied the conductivity of blood and found it to be dependent on the stimulation frequency. Simultaneously, the electrical properties of proteins and amino acids were discovered and extensively studied by Oncley, Fricke, and Wyman (3). These contributions lead to further developments in the science of biophysics and electrophysiology.
Bioimpedance research accelerated after WorldWar II. In 1950, Nyboer et al. launched an investigation into thoracic electrical bioimpedance (TEB) as an alternative to invasive methods of measuring cardiac function and published a novel method termed ''Impedance Plethysmography'' (4,5). However, Kubicek and Patterson were credited with the development of the original TEB system in conjunction with the National Aeronautics and Space Administration in the mid-1960s (6). This device was designed to monitor stroke volume (SV) and cardiac output (CO) noninvasively during space flight. In addition, Djordjevich and Sadove coined the term''electrohemodynamics'' in 1981 to describe a science that relates the theories of fluid mechanics and elasticity to the continuous impedance signal and to the time variations of arterial blood pressure (7). Jan Baan et al. introduced the impedance or conductance catheter technique to measure real time chamber volume in the mid-1980s (8). This technique revolutionized the study of cardiovascular mechanics in both the laboratory and clinical settings by making the study of ventricular-pressure volume relationships practical.
More recently, bioimpedance applications have continued to expand, especially in the area of implantable devices. Modern pacemakers and defibrillators routinely use biompedance measurements to verify pacing lead performance and position, monitor minute ventilation and thoracic fluid content, and optimize programmable device features such as pacing rate and AV delay in a closed-loop fashion (9-11).
The terms bioimpedance or tissue impedance describe both the resistive and reactive components of tissue at the applied stimulus frequency. The capacitive reactive components of the measured tissue impedance change at higher frequencies due to the relative conductive properties of tissue fluids and cellular membranes. Bioimpedance methods can be categorized into two areas: impedance plethysmography and impedance cardiography.
Impedance plethysmography, by definition, refers to the measurement of a volume change in a heterogeneous tissue segment using electrical impedance in which the changing impedance waveform (∆Z) is used to determine cardiac, respiration, and peripheral volume change as a function of time. In contrast, impedance cardiography is a subdivison of impedance plethysmography that focuses on the measurement of cardiac stroke volume and widely uses the first derivative (dZ/dt) of the changing impedance waveform (∆Z) to monitor fiducial time element points such as cardiac valve opening and closing. Both methods primarily use a single low frequency stimulus current (<100 kHz) where most of the elements in the current paths are primarily resistive. Techniques such as impedance plethysmography and impedance cardiography primarily depend on resistive rather than reactive components of the blood impedance. Thus, applications using low frequency stimulus current to primarily measure the resistive component of bioimpedance will be categorized in this article as resistive applications of bioimpedance.
The second general category of bioimpedance measurement involves estimation of fluid volume distributions such as intracellular and extracellular volume, percent body fat vs. percent muscle mass, and cell and tissue viability. This area primarily employs a multifrequency stimulus current bandwidth (>1000 Hz) where most of the elements in the current path contain significant resistive and reactive components. Thus, applications using high frequency stimulus current to measure the resistive and reactive components of bioimpedance will be categorized as reactive applications of bioimpedance.