THE MEASUREMENT OF RESPIRATORY VOLUMES IN ANIMALS AND MAN WITH USE OF ELECTRICAL IMPEDANCE

The use of electrical impedance to measure respiration is based upon the close correlation found experimentally between changes in the respired volume and changes in transthoracic impedance. The method is elegant in its simplicity, requiring only the application of two or four electrodes to the surface of the thorax. This technique does not require direct contact with the airstream, and hence does not impose any restriction on breathing; neither does it impose restrictions on body movements as do many volume-determining devices such as spirometers and pneumotachometers. The method requires that a low-intensity high-frequency current be caused to flow through the thorax. This current may be applied directly by means of surface electrodes or may be a displacement current if the thorax is used as a dielectric between two capacitor plates. This latter technique is more difficult to apply. In practice, in order to detect the impedance change, either two or four electrodes are placed in direct contact with the thorax. Spirometry by the impedance method has received attention in recent years in the growing demand for the means of acquiring physiological data by noninvasive methods. Presently, its possible use in multiphasic screening programs is being investigated. It is the purpose of this paper to review briefly the work that has been reported in this field, emphasizing the latest studies that have been conducted in the authors’ laboratory.