Surface coverage effects on defibrillation impedance for transvenous electrodes

Pendekanti, R.; Henriquez, Craig S.; Tomassoni, Gery; Miner, W. H.; Fain, Eric; Hoffmann, D.; Wolf, Patrick D.

doi:10.1007/bf02684851

Cited by 4 publications

References 11 publications

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2007

J Interv Card Electrophysiol

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While no simple electrical descriptor provides a good measure of defibrillation efficacy, the waveform parameters that most directly influence defibrillation are voltage and duration. Voltage is a critical parameter for defibrillation because its spatial derivative defines the electrical field that interacts with the heart. Similarly, waveform duration is a critical parameter because the shock interacts with the heart for the duration of the waveform. Shock energy is the most often cited metric of shock strength and an ICD's capacity to defibrillate, but it is not a direct measure of shock effectiveness. Despite the physiological complexities of defibrillation, a simple approach in which the heart is modeled as passive resistor-capacitor (RC) network has proved useful for predicting efficient defibrillation waveforms. The model makes two assumptions: (1) The goal of both a monophasic shock and the first phase of a biphasic shock is to maximize the voltage change in the membrane at the end of the shock for a given stored energy. (2) The goal of the second phase of a biphasic shock is to discharge the membrane back to the zero potential, removing the charge deposited by the first phase. This model predicts that the optimal waveform rises in an exponential upward curve, but such an ascending waveform is difficult to generate efficiently. ICDs use electronically efficient capacitive-discharge waveforms, which require truncation for effective defibrillation. Even with optimal truncation, capacitive-discharge waveforms require more voltage and energy to achieve the same membrane voltage than do square waves and ascending waveforms. In ICDs, the value of the shock output capacitance is a key intermediary in establishing the relationship between stored energy-the key determinant of ICD size-and waveform voltage as a function of time, the key determinant of defibrillation efficacy. The RC model predicts that, for capacitive-discharge waveforms, stored energy is minimized when the ICD's system time constant taus equals the cell membrane time constant taum, where taus is the product of the output capacitance and the resistance of the defibrillation pathway. Since the goal of phase two is to reverse the membrane charging effect of phase one, there is no advantage to additional waveform phases. The voltages and capacitances used in commercial ICDs vary widely, resulting in substantial disparities in waveform parameters. The development of present biphasic waveforms in the 1990s resulted in marked improvements in defibrillation efficacy. It is unlikely that substantial improvement in defibrillation efficacy will be achieved without radical changes in waveform design.

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Current density distributions, field distributions and impedance analysis of segmented deep brain stimulation electrodes

2005

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Deep brain stimulation (DBS) electrodes are designed to stimulate specific areas of the brain. The most widely used DBS electrode has a linear array of 4 cylindrical contacts that can be selectively turned on depending on the placement of the electrode and the specific area of the brain to be stimulated. The efficacy of DBS therapy can be improved by localizing the current delivery into specific populations of neurons and by increasing the power efficiency through a suitable choice of electrode geometrical characteristics. We investigated segmented electrode designs created by sectioning each cylindrical contact into multiple rings. Prototypes of these designs, made with different materials and larger dimensions than those of clinical DBS electrodes, were evaluated in vitro and in simulation. A finite element model was developed to study the effects of varying the electrode characteristics on the current density and field distributions in an idealized electrolytic medium and in vitro experiments were conducted to measure the electrode impedance. The current density over the electrode surface increased towards the edges of the electrode, and multiple edges increased the non-uniformity of the current density profile. The edge effects were more pronounced over the end segments than over the central segments. Segmented electrodes generated larger magnitudes of the second spatial difference of the extracellular potentials, and thus required lower stimulation intensities to achieve the same level of neuronal activation as solid electrodes. For a fixed electrode conductive area, increasing the number of segments (edges) decreased the impedance compared to a single solid electrode, because the average current density over the segments increased. Edge effects played a critical role in determining the current density distributions, neuronal excitation patterns, and impedance of cylindrical electrodes, and segmented electrodes provide a means to increase the efficiency of DBS.

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Surface coverage effects on defibrillation impedance for transvenous electrodes

Cited by 4 publications

References 11 publications

Lessons for the Clinical Implant

Lessons for the Clinical Implant

Optimizing defibrillation waveforms for ICDs

Current density distributions, field distributions and impedance analysis of segmented deep brain stimulation electrodes

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