Electrophysiologic principles and theory of stroke volume determination by thoracic electrical bioimpedance.

Thoracic electrical bioimpedance (TEB) is a harmless, noninvasive, user-friendly technology with wide patient acceptance. Stroke volume (SV) determination is important because it helps to define oxygen transport. Measurement of SV by TEB is rooted in concrete, basic electrical theory, as well as in theoretical models of electrical behavior of the human thorax and great thoracic vessels. This article is concerned with basic electrical theory as applied to TEB, signal acquisition, and the origin of the thoracic cardiogenic impedance pulse (delta Z). The appendix of the chapter features a more extensive overview of alternating current theory as applied to electrical bioimpedance.

Problems involved in temperature measurements using EIT.

In cancer therapy, hyperthermic treatment by microwaves requires a non-invasive and reliable method for measuring the temperature distribution inside the body. EIT seems to be able to evaluate the temperature-dependent tissue impedance for delivering the temperature profile in a cross-section of the body. Assuming a temperature coefficient of the resistivity of an electrolyte of about -2% degrees C-1 and temperature measurement to an accuracy of 0.5 degree C, the error in impedance measurement must be lower than 1%. Irrespective of the accuracy of the tomographic measuring system itself, a problem arises from the fact that the fluid content in the tissue as well as the fluid distribution between the extracellular and the intracellular compartment change with temperature. Measurements of the impedance spectra of skeletal muscle and tumours of rats during hyperthermic treatment deliver very different temperature coefficients of the resistivity from -1.3% degree C-1 to -3% degree C-1, thus questioning the feasibility of the EIT as a temperature measuring method. However, changes in the tissue caused by hyperthermia (e.g., fluid shifts, development of oedema and membrane disintegration) can be detected.

Tissue impedance spectra and the appropriate frequencies for EIT.

The complex impedance of each kind of tissue depends on the frequency in a characteristic manner. Using appropriate measuring frequencies, EIT can provide a differentiating insight into the interior of a body. Therefore, a knowledge of the tissue impedance spectra of various organs is essential for choosing the appropriate frequencies. The impedance data of various tissues in different states (normal, altered by ischaemia or cancerous) show that the characterizing differences occur at frequencies below 500 kHz and down to a few kilohertz. Moreover, the spectra show that the imaginary component of impedance essentially contributes to the characterization of the kind and state of a tissue, even though the dissipative and reactive components are connected by the Kramers-Kronig relations. The course of a dispersion and the position in the frequency range, determined by the distribution of the time constants in the tissue, are clearly presented by the imaginary component. Tomographic imaging combined with spectroscopy for tissue characterization requires a frequency range of at least 10-800 kHz. The upper frequency limit depends on the fluid content of the tissue under investigation.

EIT using magnitude and phase in an extended frequency range.

The electrical impedance is a characteristic tissue property that can be used for imaging cross sections of the body. The full information contained in the complex tissue impedance can be utilized if not only the real part Re(Z) or the magnitude of the impedance but also the imaginary part Im(Z) or the phase is considered. Impedance measurements provide information about tissue structure, particularly extracellular space and cell membranes. Therefore, an electrical impedance tomograph was constructed which uses alternatively the real component, the imaginary component, the magnitude or the phase in an extended frequency range. The components are evaluated by digital correlation. The device allows state-different or frequency-different (almost static) imaging. 16 electrodes are used. Image reconstruction is arrived at by a back-projection algorithm. For frequency-different imaging the measured imaginary part values can be used after normalization (division by the measuring frequency); instead of the phase values, the quotients Im(Z)/Re(Z) are taken and divided by the actual frequency, representing time constants of the tissue. Frequency-different measurements on a tank filled with saline containing a metallic rod and an insulator show in a very illustrative manner the impedance of the metal/electrolyte boundary layer (phenomenon of electrode polarization). The first in vivo measurements are very promising, state-different as well as frequency-different images of the human thorax represent, for example, the lungs with higher contrast using the phase than using the magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)