BIOELECTRICAL-IMPEDANCE TECHNIQUES IN MEDICINE .2. MONITORING OF PHYSIOLOGICAL EVENTS BY IMPEDANCE

The measurement of a physiological event caused by a change in dimensi on, conductivity, or permittivity can be easily carried out by the imp edance technique, requiring only the application of two or more electr odes, which are easy to apply. In some cases, the impedance is transfo rmed into its resistive and reactive components, in others the total i mpedance is measured. In certain cases only a change in impedance, wit h or without separation into its components, contains enough informati on to be correlated to the physiological event. Recent measurements of physiological data by impedance techniques have reemphasized the valu e of the painless and harmless acquisition from human and animal subje cts in such diverse domains as manned spacecraft, nutrition, and elect rical impedance imaging. This part attempts to present all the numerou s experiments performed on humans to estimate changes in volume, orien tation, and distribution of fluids and tissues accompanying physiologi cal activity. The main sections concern the respiratory system, the ca rdiovascular system, the brain, the total body impedance, muscle and s kin impedance, and bacteriometry.