Bioelectrical impedance techniques in medicine. Part II: Monitoring of physiological events by impedance.

The measurement of a physiological event caused by a change in dimension, conductivity, or permittivity can be easily carried out by the impedance technique, requiring only the application of two or more electrodes, which are easy to apply. In some cases, the impedance is transformed into its resistive and reactive components, in others the total impedance is measured. In certain cases only a change in impedance, with or without separation into its components, contains enough information to be correlated to the physiological event. Recent measurements of physiological data by impedance techniques have reemphasized the value of the painless and harmless acquisition from human and animal subjects in such diverse domains as manned spacecraft, nutrition, and electrical impedance imaging. This part attempts to present all the numerous experiments performed on humans to estimate changes in volume, orientation, and distribution of fluids and tissues accompanying physiological activity. The main sections concern the respiratory system, the cardiovascular system, the brain, the total body impedance, muscle and skin impedance, and bacteriometry.