A mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography is discussed. The model includes the effects of discretization, shunt, and contact impedance. The complete model was validated by experiment. Bath resistivities of 284.0, 139.7, 62.3, and 29.5 Omega -cm were studied. Values of effective contact impedance used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 Omega -cm/sup 2/, respectively. Agreement between the calculated and experimentally measured values was excellent throughout the range of bath conductivities studied. It is desirable in electrical impedance imaging systems to model the observed voltages to the same precision as they are measured in order to be able to make the highest-resolution reconstructions of the internal conductivity that the measurement precision allows. The complete electrode model, which includes the effects of discretization of the current pattern, the shunt effect due to the highly conductive electrode material, and the effect of an effective contact impedance, allows calculation of the voltages due to any current pattern applied to a homogeneous resistivity field. >

Electrode models for electric current computed tomography

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

A capacitor-discharge implantable cardioverter defibrillator (ICD) has a relatively longer device life of greater that 5 years. The longer life of the ICD is achieved by selecting and arranging the internal components of the ICD to deliver a maximum defibrillation countershock optimized in terms of a minimum physiologically effective current (I pe ), rather than a minimum defibrillation threshold energy (DFT). As a result of the optimization in terms of a minimum effective current I pe , there is a significant decrease in the maximum electrical charge energy (E c ) that must be stored by the capacitor of the ICD to less than about 30 Joules, even though a higher safety margin is provided for by the device. Due to this decrease in the maximum E c , as well as corollary decreases in the effective capacitance value required for the capacitor and the net energy storage required of the battery, the overall displacement volume of the ICD is reduced to the point where subcutaneous implantation of the device in the pectoral region of human patients is practical. The size of the capacitor is reduced because the effective capacitance required can be less than about 125 μF. By optimizing both the charging time and the countershock duration for the smaller maximum E c , the size of the battery is reduced because the total energy storage capacity can be less than about 1.0 Amp-hours. In the preferred embodiment, the charging time for each defibrillation countershock is reduced to less than about 10 seconds and the pulse duration of the countershock is reduced to less than about 6 milliseconds.

Implantable cardioverter defibrillator having a smaller displacement volume

There are four criteria that must be considered when choosing material for an implanted electrode: (1) tissue response, (2) allergic response, (3) electrode-tissue impedance, and (4) radiographic visibility. This paper discusses these four criteria and identifies the materials that are the best candidates for such electrodes. For electrodes that make ohmic contact with tissues: gold, platinum, platinum-iridium, tungsten, and tantalum are good candidates. The preferred insulating materials are polyimide and glass. The characteristics of stimulator output circuits and the importance of the bidirectional waveform in relation to electrode decomposition are discussed. The paper concludes with an analysis, the design criteria, and the special properties and materials for capacitive recording and stimulating electrodes.

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Criteria for the selection of materials for implanted electrodes.

The effects of electric current applied to skin: A review for transdermal drug delivery

The resistive and capacitive properties of stainless-steel electrodes in contact with saline solutions of various concentrations were investigated over a frequency range extending from 20 to 10 kHz by using a variable-length conductivity cell. With a low current density, the series-equivalent resistance and capacitance of a single electrode-electrolyte interface were found to vary almost inversely as the square root of frequency. In the frequency range studied, it was found that the reactance was very nearly equal to the resistance and both varied almost inversely as the square root of the frequency, verifying Warburg's postulate. When current density was increased, resistance decreased and capacitance increased at all frequencies studied; the change in each was most apparent in the low frequency region. As a consequence, it can be stated that the impedance of an electrode-electrolyte interface decreases with increasing current density.

The impedance of stainless-steel electrodes.

The amount of energy, current and charge per gram of heart were determined on a series of 36 dogs. The data, which are examined on the basis of current duration (time constant), show that the three criteria are different. The optimum duration range using energy as a criterion is 0·03–3 ms. Minimum peak current was required with a 4 ms duration capacitor discharge pulse. There was no optimum duration for minimum charge. It was found that although short-duration current (0·02–3·0 ms) produced defibrillation, the high current required impaired the contractility of the ventricles. From this study, the optimum range of capacitor-discharge current for defibrillation is 0·3–4 ms. Estimates are presented on the amounts of energy, current and charge to defibrillate the ventricles of large animals.

Engineering and physiological considerations of direct capacitor-discharge ventricular defibrillation.

Experiments were conducted in man and the dog to determine the safe levels of sinusoidal alternating current which can be passed through the thorax. The sensation current threshold was determined over a frequency range of 20 Hz to 20 kHz in 8 normal adult males for two different electrode arrangements. The thresholds for vagal stimulation and ventricular fibrillation in the frequency range of 20 Hz to 3 kHz were determined in the dog for the same electrode arrangements, and the data obtained for fibrillation was extrapolated to estimate the comparable levels for man. The results indicate that the magnitudes of the current thresholds to produce these phenomena increase with an increase in frequency. Currents higher than 5 kHz in frequency are recommended for the measurement of physiological events by electrical impedance techniques.

Hazards in the use of low frequencies for the measurement of physiological events by impedance.

The impedance coefficient in ohms/litre obtained by means of bipolar impedance pneumography exhibits an inverse relationship with body weight in kilogrammes; this relationship appears valid on an inter-species basis and is probably also true from an intra-species point of view. The relationship is linear on log-log paper and nearly hyperbolic on a linear scale. A statistical regression analysis produced the expression log(ΔZ/ΔV)=2·656–1·08 logW with a Standard Error of Estimate of 0·24 and a correlation coefficientr=−0·976, where ΔZ/ΔV is the impedance coefficient in ohms/litre andW the body weight in kilogrammes. The impedance coefficient per kilogramme, obtained as the derivative of the expression above, is given by (log 490–2·08 logW) and describes with accuracy the change in sensitivity with different weights. For low values of body weightW, the impedance coefficient is very high; for high values ofW, the impedance coefficient is very low and, beyond 1000 kg, negligible signals are to be expected. This observation was verified in the elephant.

The law of impedance pneumography

An experimental investigation of the effect of vessel elasticity on Korotkoff-sound frequency has been conducted. A model was constructed to simulate conditions in the upper arm during blood-pressure determination. The experiment was performed using both cylindrical rubber tubes and fresh canine arteries. The modulus of elasticity of the specimens was measured using an apparatus for determining the pressure-volume relationship of distensible tubes. Sound waves produced by the specimens were recorded with an oscilloscope.

L. A. Geddes

Papers

The impedance of stainless-steel electrodes.

Engineering and physiological considerations of direct capacitor-discharge ventricular defibrillation.

Hazards in the use of low frequencies for the measurement of physiological events by impedance.

The law of impedance pneumography

The relationship between vessel-wall elasticity and Korotkoff-sound frequency.