[Transthoracic electrical impedance (author's transl)].

The measurement of transthoracic impedance is used by many authors to determine tidal volume and changes of intrathoracic fluid volume. In clinical diagnostics, too, this method has been introduced. Furthermore, the stroke volume of the heart is often said to be calculable from changes in impedance synchronous with the cardiac cycle. This paper investigates the theoretical foundations, and tries to estimate the extent by which the various measuring errors inherent in this method may falsify the results obtained. Measurements of the phase angle show that the errors occurring when using impedance instead of resistance are negligible. The determination of impedance is considerably influenced by breathing and body position. An analysis of these normally superimposing effects demonstrates the problematic nature of this method. It is shown that changes in intrathoracic fluid volume can be determined only with unchanged positioning of the electrodes, constant body position and elimination of the influence of breathing. Respiratory movements can be registered, whereas the influence of changes in the basic impedance on determining the stroke volume by impedance cardiography has not been fully researched.