Monitoring living tissues by electrical impedance spectroscopy

Solving the experimental difficulties associated with measurement of the electrical impedance of living tissues gives access to valuable tissue compartment parameters which are sensed within seconds using minimally invasive, simple metallic electrodes. Extracellular conductivity and cell membrane capacitance can be followed over time under conditions of metabolic toxicity, perfusion loss and thermal stress in liver, brain cortex, and muscle, respectively. Application of this technique in burns therapy allows an accurate estimation of the severity of thermal injury to skeletal muscle, supporting predictions on tissue survival.     

Theory and cardiac applications of electrical impedance measurements

The methodology of the two-electrode, four-electrode, and guard-ring techniques is presented following a brief history of impedance plethysmography. The theoretical basis for predicting the sampling fields for conductivity and volume changes is presented. Theoretical and experimental studies of the sampling field associated with various electrode arrays are reviewed. With this background, the use of impedance plethysmography for cardiac monitoring and diagnosis is reviewed. The basic methodology is presented and models used to interpret the signal are reviewed. Theoretical and experimental studies of what is sampled are summarized. The accuracy of impedance stroke volume estimates is evaluated by surveying the results of human studies and examining critical animal studies. The usefulness of impedance cardiography for ventricular performance evaluation is also reviewed. Additional uses for cardiopulmonary diagnosis are briefly presented.     

A mathematical model for the polarization impedance of cardiac pacemaker electrodes

A mathematical model for the linear properties of the polarization impedance of cardiac pacemaker electrodes is presented. This model reproduces frequency and time domain behavior of the electrodes in physiological saline. The model chosen is a special case of a more general model used to describe dielectric relaxation behavior. Equations are presented for the dependence on frequency of polarization resistance, capacitance and tanδ, and for the potential difference across the interface vs. time in response to a current step. Frequency domain measurements were made with an impedance bridge. Time domain measurements were made with constant current pulses delivered to the electrodes immersed in saline. The experimental results exhibit the same behavior as predicted from the model, with good agreement between theory and experiment. Features proposed by this model are also exhibited by other metal electrodes.     

An electrical impedance index to distinguish between normal and cancerous tissues

Bioimpedance measurements have previously shown that cancerous tissues in many cases have a higher characteristic frequency than normal tissues. This paper tries to find an index which is sensitive to variation in the characteristic frequency but is independent of the impedance value itself. Calculated indexes from several publications in most cases show values larger than onefor cancerous tissues and less than one for normal tissues.     

Ionic basis of electrical activity in cardiac tissues.

Cardiac electrical events are described in terms of membrane physiology. The concept that cardiac membranes possess specific ionic channels controlled by gates bearing electrical charges is discussed. When open, these channels permit ions to cross the membrane, giving rise to passive inward (depolarizing) and outward (repolarizing) currents. Two different inward and four or five different outward currents appear to be responsible for the development of cardiac electrical activity; both inward currents appear to be controlled by activation and inactivation variables, whereas outward currents are essentially controlled by activation variables and/or inward-going rectifiers. The potential range in which the different currents activate and inactivate (or are limited by inward-going rectification), and the kinetics of activation and inactivation processes explain the development of electrical activity in normal cardiac tissues and in partially depolarized fibers. In addition to passive ionic currents, electrogenic active transport participates in the development of electrical phenomena. The conductance of the membrane for potassium ions and the electrical coupling between cardiac cells depend on the intracellular concentration of calcium ions.     

An electrical impedance index to distinguish between normal and cancerous tissues.

Bioimpedance measurements have previously shown that cancerous tissues in many cases have a higher characteristic frequency than normal tissues. This paper tries to find an index which is sensitive to variation in the characteristic frequency but is independent of the impedance value itself. Calculated indexes from several publications in most cases show values larger than one for cancerous tissues and less than one for normal tissues.     

Simulation method for cardiac stroke volume estimation by intracardiac electrical impedance measurement

Using the electrical impedance measurement technique to investigate stroke volume estimation, three models of the ventricle were simulated. A four-electrode impedance catheter was used; two electrodes to set up an electric field in the model and the other two to measure the potential difference. A new approach, itself an application of the quasi-static case of a method used to solve electromagnetic field problems, was used to solve the electric field in the model. The behaviour of the estimation is examined with respect to the electrode configuration on the catheter and to catheter location with respect to the ventricle walls. Cardiac stroke volume estimation was found to be robust to catheter location generating a 10 per cent error for an offset of 40 per cent of the catheter from the chamber axis and rotation of 20° with respect to the axis. The electrode configuration has a dominant effect on the sensitivity and accuracy of the estimation. Certain configurations gave high accuracy, whereas in others high sensitivity was found with lower accuracy. This led to the conclusion that the electrode configuration should be carefully chosen according to the desired criteria.     

Effect of skin impedance on image quality and variability in electrical impedance tomography: a model study

A computer simulation is used to investigate the relationship between skin impedance and image artefacts in electrical impedance tomography. Sets of electrode impedance are generated with a pseudo-random distribution and used to introduce errors in boundary voltage measurements. To simplify the analysis, the non-idealities in the current injection circuit are replaced by a fixed common-mode error term. The boundary voltages are reconstructed into images and inspected. Where the simulated skin impedance remains constant between measurements, large impedances (>2kΩ) do not cause significant degradation of the image. Where the skin impedances ‘drift’ between measurements, a drift of 5% from a starting impedance of 100Ω is sufficient to cause significant image distortion. If the skin impedances vary randomly between measurements, they have to be less than 10 Ω to allow satisfactory images. Skin impedances are typically 100–200 Ω at 50 kHz on unprepared skin. These values are sufficient to cause image distortion if they drift over time. It is concluded that the patient's skin should be abraded to reduce impedance, and measurements should be avoided in the first 10 min after electrode placement.     

A temperature-sensitive cardiac pacemaker

An artificial cardiac pacemaker which is sensitive to the temperature of blood in the right atrium has been fabricated. For a temperature change of 20 degrees C the circuit achieves 90% of its final response within a period of 18 s. In the authors' opinion this is satisfactory since changes in blood temperature are generally small. Cardiac output in dogs rose from 2.37 +/- 0.65 to 4.54 +/- 1.15 l/min when the rate was increased from 202.6 beats/min(b.p.m.) at 37.6 degrees C to 231.6 b.p.m. at 41 degrees C. Cardiac output was found, from statistical observation, to be improved at temperatures over 39.6 degrees C.     

Dual-chamber cardiac pacemaker tester

To investigate the operation of dual-chamber cardiac pacemakers, a pacemaker tester is developed that is able to examine pacemaker parameters, demand modes and rate-adaptive modes. The tester generates simplified electrograms and responds to pacemaker pulses in a closed-loop simulation. The authors describe the hardware and the software algorithm of the tester. To demonstrate the capabilities of the tester, four tests are presented, together with their results on a rate-adaptive dual-chamber pacemaker.     

The immediate effects of some thiamine antagonists on the electrical activity of isolated cardiac tissues

Summary 1. The action of three thiamine antagonists (Neopyrithiamine, Oxythiamine, Amprolium) have been studied with intracellular microelectrodes on isolated heart tissue of rat, calf and sheep.2. All three antimetabolites decreased the excitability of the tissue, two (oxythiamine and amprolium) reduced the rate of spontaneous beat, all three increased the duration of the action potential.3. In Purkinje fibers oxythiamine and amprolium decreased the conduction velocity, in the atrium only oxythiamine decreased it.4. There is a marked difference of action between the excitable membrane of peripheral nerves (frog) and cardiac tissue. Neopyrithiamine has a strong action on nerve, but almost no action on cardiac tissue and for oxythiamine and amprolium the relation is reversed.

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Zusammenfassung 1. Die Wirkung von drei Aneurin-Antagonisten (Neopyrithiamin, Oxythiamin, Amprolium) wurde mit intracellulärer Mikroelektrode am isolierten Herzgewebe von Ratte, Kalb und Schaf untersucht.2. Alle drei Antimetaboliten vermindern die Erregbarkeit des Gewebes, zwei (Oxythiamin und Amprolium) verlängerten den spontanen Rhythmus und alle drei verlängerten die Dauer des Aktionspotentiales.3. In Purkinje-Fasern verminderten Oxythiamin und Amprolium die Leitungsgeschwindigkeit, am Vorhof war nur Oxythiamin wirksam.4. Zwischen der Wirkung der Antimetaboliten an der erregbaren Membran der peripheren Nerven (Frosch) und am Herzgewebe besteht ein grundlegender Unterschied. Neopyrithiamin hat eine starke Wirkung auf den peripheren Nerven, aber so gut wie keine Wirkung am Herzgewebe, und für Oxythiamin und Amprolium ist die Wirkungsbeziehung umgekehrt.

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With 4 Figures in the Text

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Dedicated to Prof. Dr. Alexander von Muralt on the occasion of his 60th birthday.     

Factors affecting electrode-gel-skin interface impedance in electrical impedance tomography

The magnitude, mismatch and temporal variations of the electrode-gel-skin interface impedance can cause problems in electrical impedance tomography (EIT) measurement. It is shown that at the high frequencies generally encountered in EIT the capacitive properties of the electrode interface, and especially those of the skin, are of primary importance. A wide range of techniques are reviewed that could possibly be used to minimise these problems. These techniques include the use of skin preparation, penetration enhancers, temperature and electrical impulses. Although several of these techniques appear very attractive, they are not without serious potential drawbacks. A combination of some of these techniques may well hold the key to success.     

Comparison of electrical field plethysmography with electrical impedance plethysmography

As a means for assessing cardiac function, electrical field plethysmography (EFP) has been shown to have some features quite different from electrical impedance plethysmography (EIP). Here the two techniques are compared by using the two systems simultaneously on a subject and also with independent use in different electrode configurations. The results conform with the view that EIP is related primarily to volumetric changes of the aorta, whereas EFP is affected predominantly by changes in cardiac dimensions and orientation. Because of this difference, the standard time differential formula used for EIP is not applicable for the computation of cardiac output from the EFP waveforms. An alternative method of computation based on the amplitude of the EFP waveform is suggested.     

A demand radiofrequency cardiac pacemaker

An implantable capsule containing circuitry to transmit the endocardial ECG and convert received radio-frequency pulses to constant current stimulus pulses is described. Inasmuch as the control unit of the pacemaker is located outside the body wall, different modes of pacing (self-synchronous,R-wave coupled, and asynchronous) are possible. Alterations in the capsule load impedance, coupled to the transmitting antenna, is the method of transmission. A signal to noise ratio of 10:1, with a 1 mv source ECG signal can be obtained through a 2 cm spacing between the implant and external coil. The endocardial ECG may be recorded and employed to reveal alterations in electrode geometry. The anode electrode is attached directly to the implanted capsule, providing a unit having a single flexible electrode. The assembly allows recording the endocardial ECG without a wire connection to the hospital utility system. Due to the constant current feature, placement of the external coils is less critical than radiofrequency coupled tissue stimulators previously described.