Optimization of cardiac defibrillation by three-dimensional finiteelement modeling of the human thorax

The goal of this study was to determine the optimal electrode placement and size to minimize myocardial damage during defibrillation while rendering refractory a critical mass of cardiac tissue of 100%. For this purpose, the authors developed a 3D finite element model with 55,388 nodes, 50,913 hexahedral elements, and simulated 16 different organs and tissues, as well as the properties of the electrolyte. The model used a nonuniform mesh with an average spatial resolution of 0.8 cm in all three dimensions, To validate this model, the authors measured the voltage across 3-cm² Ag-AgCl electrodes when currents of 5 mA at 50 kHz were injected into a human subject's thorax through the same electrodes. For the same electrode placements and sizes and the same injected current, the finite element analysis produced results in good agreement with the experimental data. For the optimization of defibrillation, the authors tested 12 different electrode placements and seven different electrode sizes. The finite element analyses showed that the anterior-posterior electrode placement and an electrode size of about 90 cm² offered the least chance of potential myocardial damage and required a shock energy of less than 350 J for 5-ms defibrillation pulses to achieve 100% critical mass. For comparison. The average cross-sectional area of the heart is ≈48 cm², about half of the optimal area. A second best electrode placement was with the defibrillation electrodes on the midaxillary lines under the armpits. Although this placement had higher chances of producing cardiac damage, it required less shock energy to achieve 100% critical mass     

A nonlinear electrical-thermal model of the skin

Presents a model for the skin which accounts for both the nonlinearities and the asymmetries in its voltage-current characteristic. This model consists of an electrical submodel and a heat transfer submodel. The electrical submodel uses nonlinear devices in which some parameters depend on skin temperature. The heat transfer submodel models the heat exchange between the skin, the surrounding tissues, and the ambient medium and calculates the temperature of the skin to update the necessary parameters of the electrical submodel. The model is based on experiments designed to determine: (1) the dry skin voltage-current characteristic; (2) the changes in the skin breakdown voltage with location; (3) the moist skin voltage-current characteristic; (4) the changes in the voltage-current characteristic of the skin with duration after the onset of stimulation; and (5) the effect of skin temperature on its voltage-current characteristic. During these experiments the authors used 84-mm² square Ag-AgCl electrodes to apply sinusoidal voltage of 0.2 and 20 Hz. The simulations were performed using the Advanced Continuous Simulation Language (ACSL), capable of solving differential and integral equations with variable coefficients. The model predicted the skin behavior satisfactorily for a large range of amplitudes and frequencies. The authors found that the breakdown occurred when the energy delivered to the skin exceeded a threshold. Above this threshold the voltage-current characteristic of the skin became nonlinear and asymmetric and, in a real situation, the subject would experience an uncomfortable sensation which could rapidly develop into pain     

Modeling current density distributions during transcutaneouscardiac pacing

The authors developed a two-dimensional finite element model of a cross-section of the human thorax to study the current density distribution during transcutaneous cardiac pacing. The model comprises 964 nodes and 1,842 elements and accounted for the electrical properties of eight different tissues or organs and also simulated the anisotropies of the intercostal muscles. The finite element software employed was a version for electrokinetics problems of Finite Element for Heat Transfer (FEHT) and the authors assessed the effects upon the efficacy of transcutaneous cardiac pacing of several electrode placements and sizes. To minimize pain in the chest wall and still be able to capture the heart, the authors minimized the ratio, R, between the current density in the thoracic wall (which causes pain) and the current density in the heart wall (which captures the heart). The best placement of the negative electrode was over the cardiac apex. The best placement of the positive electrode was under the right scapula, although other placements mere nearly as good. The efficiency of pacing increased as electrode size increased up to 70 cm and showed little improvement for larger areas. Between different configurations of the precordial electrodes V1, V2, ···, V6 the most efficient configuration to pace with was V1 and V2 positive and V5 and V6 negative. A more efficient configuration uses an auxiliary electrode located at the right subscapular region     

Cardiac resynchronization therapy.

The rhythm of each heart beat is controlled by electrical pulses. These signals coordinate and synchronize the contraction of the atria and of the ventricles. They also provide the electrical sequence of activation that causes the ventricles to contract in a coordinated manner. In heart failure, this electrical sequence of activation is distorted A new surgical treatment can now be offered as an option to patients suffering from heart failure: cardiac resynchronization therapy (CRT). Implantable devices designed for CRT stimulate the two ventricles of the heart to beat at the same time by delivering tiny electrical pulses to both sides. This therapy is intended for patients with moderate to severe heart failure who also have ventricular dysynchrony, a condition in which the two ventricles are not beating synchronously. An example of a CRT device is a CRT pacemaker, which is a more advanced version of a standard pacemaker and is about the size of a pocket watch. Once implanted in the chest, the CRT pacemaker not only helps the heart keep a normal rhythm but improves contraction pattern in the left ventricle, improving the overall efficiency beneath the skin, usually below the collarbone. The performance of CRT devices was studied during the course of several clinical trials. The study showed that the CRT devices provide proven quality-of-life benefits to patients and funded hope for an increased life span.     

A fast pipelined VLSI adder for fast trigger decisions at the Superconducting Super Collider

We present a four 12-bit binary number adder proposed for use in the computation of the pipelined energy sums of data from the detectors at the Superconducting Super Collider (SSC). It was fabricated using a 1.2 μm N-well CMOS process. It comprises three 12-bit adders organized as a two-stage pipeline. To compute the final carry of each of the 12-bit adders, we used the Carry-Select technique applied to their 4-bit adder subcells. The 4-bit adders used the Carry-Lookahead method to compute their carries. In order to reduce the circuit area and to simplify the structure of this application specific integrated circuit (ASIC) we employed a Multiple-Output Domino Logic design style. The first stage of the pipeline (two adders) performs two 12-bit additions in parallel while the second stage (one adder) finishes up the previously started computation. The pipeline is driven using a two-phase clocking strategy by processing a single-phase external clock. We achieved an worst case throughput of 18 ns. In the best case the throughput was 16.5 ns. We included a built-in facility for testing the first stage of the pipeline. The area of the circuit is 1425×5510 μm², it has 76 pads, and it is packed in a 132 pin grid array (PGA). The transistor count is 6639. The dissipated power at a 18-ns clock period was ≈ 0.75 W. The circuit has been fabricated through the MOSIS service. We found an yield of ≈ 80% for a lot of 50 chips.     

The mosaic electrical characteristics of the skin

The authors constructed a suction microelectrode with a 200-μm internal diameter and performed several tests on two male subjects. It was found that the average skin impedance on the forearm was larger than the average impedance on the palm and that the ratio between the maximal and minimal skin impedance was larger for the forearm than for the palm. For both the magnitude and variance of skin impedance decreased with increasing stimulus frequency. The density of low-impedance points observed on the forearm and palm was consistent with the density of DC current pathways through the skin as indicated by traces left on 1-cm ² Ag electrodes. The ratio between the highest and lowest impedances decreased as temperature decreased. The authors were not able to break down the skin using the suction microelectrode. Tests suggest that breakdown is of thermal nature, and that the thermal capacitance of the saline in the suction microelectrode prevents the temperature of the underlying skin from increasing very rapidly, increasing the breakdown voltage