Biomedical subjects

D Panescu

Publications and source records attributed to D Panescu.

13 recordsLinked to original sources

PubMed1999

Radiofrequency multielectrode catheter ablation in the atrium.

We developed a temperature-controlled radiofrequency (RF) system which can ablate by delivering energy to up to six 12.5 mm long coil electrodes simultaneously. Temperature feedback was obtained from temperature sensors placed at each end of coil electrodes, in diametrically opposite positions. The coil electrodes were connected in parallel, via a set of electronic switches, to a 150 W 500 kHz temperature-controlled RF generator. Temperatures measured at all user-selected coil electrodes were processed by a microcontroller which sent the maximum value to the temperature input of the generator. The generator adjusted the delivered power to regulate the temperature at its input within a 5 degrees C interval about a user-defined set point. The microcontroller also activated the corresponding electronic switches so that temperatures at all selected electrodes were controlled within a 5 degrees C interval with respect to each other. Physical aspects of tissue heating were first analysed using finite element models and current density measurements. Results from these analyses also constituted design input. The performance of this system was studied in vitro and in vivo. In vitro, at set temperatures of 70 degrees C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90 degrees C were contiguous. The lesion length increased almost linearly with the number of electrodes. Power requirements to reach a set temperature were larger as more electrodes were driven by the generator. The system impedance decreased as more electrodes were connected in the ablation circuit and reached a low of 45.5 ohms with five coil electrodes in the circuit. In vivo, right atrial lesions were created in eight mongrel canines. The power needed to reach 70 degrees C set temperature varied between 15 and 114 W. The system impedance was 105+/-16 ohms, with one coil electrode in the circuit, and dropped to 75+/-12 ohms when two coil electrodes were simultaneously powered. The length and the width of the lesion set varied between 17.6+/-6.1 and 59.2+/-11.7 mm and 5.9+/-0.7 and 7.1+/-1.2 mm respectively. No sudden impedance rises occurred and 75% of the lesions were contiguous. From the set of contiguous lesions, 90% were potentially therapeutic as they were transmural and extended over the entire target region. The average total procedure and fluoroscopy times were 83.4 and 5.9 min respectively. We concluded that the system can safely perform long and contiguous lesions in canine right atria.

Animals↗

PubMed1997

Nonuniform heating during radiofrequency catheter ablation with long electrodes: monitoring the edge effect.

BACKGROUND: Long, narrow electrodes are being considered for radiofrequency ablation of atrial fibrillation; however, preliminary work revealed coagulum formation on the electrodes and lack of lesion continuity. This may be due to the "edge effect," which concentrates radiated energy at sharp geometric gradients. It is proposed that temperature sensors at electrode edges are preferable to a single centered sensor for temperature feedback and monitoring of long electrode geometries. METHODS AND RESULTS: A finite element model was used to predict the heating properties of new long electrode geometries. Sixteen dogs with atrial fibrillation underwent left and right atrial ablation using catheters with multiple 12.5-mm coil electrodes. Electrodes with a single thermistor were compared with electrodes with dual thermocouples placed at opposite ends and on opposing sides of the electrode. Power, temperature, and impedance were recorded for all lesions, and coagulum adhesion and magnitude were noted in a subset of lesions. Finite element analysis shows uneven heating, with the main heating concentrated at the electrode edges and a propensity toward temperatures >100 degrees C with single-thermistor feedback control. Ablations with dual thermocouple electrodes achieved higher measured temperatures at lower power levels than those that used single-thermistor electrodes. Impedance rises and coagulum adherence occurred less frequently with dual thermocouple electrodes than with single, centered thermistor electrodes (176 of 395 versus 9 of 425 lesions; P<.0001; 46 of 98 versus 7 of 150 lesions; P<.0001, respectively). CONCLUSIONS: Maximum heating from radiofrequency energy occurs at the electrode edges, particularly with long electrodes. The safety of temperature-feedback atrial ablation with these electrodes is significantly improved by monitoring temperatures at the edges.

Animals↗

PubMed1997

Intraventricular electrogram mapping and radiofrequency cardiac ablation for ventricular tachycardia.

Since its first use in the early 1980s, radiofrequency catheter ablation has gained acceptance as primary therapy for many cardiac rhythm disorders. This article reviews fundamentals of cardiac mapping and radiofrequency ablation and their clinical use for treatment of ventricular tachycardia. The review concludes that the use of radiofrequency ablation to cure ventricular tachycardia has consistently increased over the years, as better mapping and ablation tools have been made available to the medical community. Presently, high success and low complication rates are achieved only in patients with bundle branch, idiopathic, or monomorphic and stable ventricular tachycardias. The reviewed studies and reports suggest that, in order to increase the success rates in patients with ventricular tachycardias caused by coronary artery disease, mapping systems that can identify arrhythmogenic pathways more accurately and more efficiently and ablation devices capable of generating larger lesions are needed.

Catheter Ablation↗

PubMed1995

Optimisation of transcutaneous cardiac pacing by three-dimensional finite element modelling of the human thorax.

The goal of the study is to determine by finite element analysis (FE) the optimal electrode placement, size and electrolyte resistivity that minimise the pain experienced by patients during successful transcutaneous cardiac pacing (TCP). The three-dimensional FE model generated for this purpose has 55,388 nodes, 50,913 hexahedral elements and simulated 16 different organs and tissues, as well as the properties of the electrolyte. The model uses a non-uniform mesh with an average spatial resolution of 0.8 cm in all three dimensions. To validate this model, the voltage across 3 cm2 Ag-AgCl electrodes is measured when currents of 5 mA at 50 kHz are injected into a subject's thorax through the same electrodes. For the same electrode placements and sizes and the same injected current, the FE analysis produced results in good agreement with the experimental data. The optimisation analysis tested seven different electrode placements, five different electrode sizes and six different electrolyte resistivities. The analysis indicates that the anterior-posterior electrode placement, electrode sizes of about 90 cm2 and electrolytes with resistivity of about 800 omega.cm yield the most uniform current distribution through the skin, thus having the best chances to minimise the pain delivered to the patient during successful TCP. The anterior-anterior electrode placement is the second most efficient.

Cardiac Pacing, Artificial↗

PubMed1995

Optimization of cardiac defibrillation by three-dimensional finite element 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, we developed a 3-D 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, we 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, we 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 approximately 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.

Computer Simulation↗

PubMed1995

Three-dimensional finite element analysis of current density and temperature distributions during radio-frequency ablation.

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. We used validated three-dimensional finite element models to perform these analyses. We found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density decreased more slowly with distance from the electrode surface. We analyzed the effects of three tissue-electrode angles: 0, 45, and 90 degrees. More power was needed to reach a maximal tissue temperature of 95 degrees C after 120 s when the electrode-tissue angle was 45 degrees. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45 degrees than for 0 and 90 degrees. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.

Animals↗

PubMed1995

A database of cardiac arrhythmias.

OBJECTIVE: To describe a database of cardiac arrhythmia recordings, useful for the development and testing of ECG rhythm processing or monitoring algorithms and devices. METHODS: The raw data were acquired within the Wisconsin-Dane County emergency medical technician-defibrillation program and contained emergency rhythm recordings of an average length of 30 minutes. The raw data were integrated into a software platform designed for the annotation and visualization of the recordings. RESULTS: Currently the database contains the following arrhythmia episodes: ventricular fibrillation (56), asystole (65), electromechanical dissociation (31), and other arrhythmias (42). The software, resident on personal computers, also can transmit any of the database recordings, through a digital-to-analog converter board, to a device under test. CONCLUSIONS: The database technique described will provide a useful means of objectively assessing electronic devices for their ability to detect arrhythmias. The database is unique in that it contains lengthy episodes of arrhythmias. The database will be extended to include additional cases.

Arrhythmias, Cardiac↗

PubMed1994

Measurement of ventricular volume from blood conductance using two-dimensional finite element analysis.

We used finite element analysis to study the relationship between the intraventricular blood conductance and the right ventricular volume. Previous studies reported a quasi-linear dependence between these two quantities. We quantified the effects of the resistivities of the surrounding tissues (e.g. heart wall, lungs) on this relationship and performed simulations for four different right ventricular longitudinal sectional areas to assess the linearity of the relationship. The relationship was most significantly affected by the blood conductivity. However, the effects of the cardiac muscle and the lungs could not be neglected. The dependence of the intraventricular blood conductance on the ventricular volume was found to be non-linear. Although to some extent inaccurate, a linear approximation of this relationship is useful for the development of rate-responsive implantable cardiac pacemakers, where the pacing rate is adjusted based on the need for cardiac output. The cardiac output is computed from the product of the heart rate and the stroke volume. The stroke volume can be estimated by measuring the changes in the intraventricular blood conductance. The electrodes needed for the stroke volume estimation can be placed on the same catheter as those used for pacing. The use of this method for clinical monitoring or diagnosis has to be investigated further given that its errors in the estimation of the stroke volume are considerably larger than those corresponding to standard methods such as dye- or thermo-dilution.

Blood Physiological Phenomena↗

PubMed1994

Design of an inductive plethysmograph for ventilation measurement.

We have designed an inductive plethysmograph to obtain a non-invasive measure of ventilation. Two elastic bands containing insulated wires encircle the chest and abdomen--the inductance of each band depends on the enclosed cross sectional area. Each inductive band forms an element in a tank circuit, which determines the resonant frequency of a Colpitts oscillatory. By measuring the oscillatory frequency, we indirectly measure the changes in cross sectional area that occur during breathing. Independent measures of chest and abdominal cross sectional area provide a way to detect both normal breathing and airway obstruction. Magnetic coupling due to the mutual inductance between chest and abdominal bands modulates the desired oscillation frequencies. When modulation is excessive, frequency locking occurs and we cannot make independent measures of chest and abdominal area. We have performed simulations that show that, as the chest and abdominal band oscillator frequencies are sufficiently separated, we decrease modulation and avoid frequency locking. We have compared simulataneous recordings of ventilation using our inductive plethysmography and a commercial impedance pneumograph and spirometer. Recordings of normal ventilation by all methods appear similar; however, our inductive device is less prone than the impedance pneumography to artifacts caused by applied pressure and body movements. In addition, during simulated airway obstruction, signals from the chest and abdominal bands are out of phase--suggesting that the inductive technique may be useful for detecting airway obstruction.

Abdomen↗

PubMed1994

Modeling current density distributions during transcutaneous cardiac pacing.

We 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 1842 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 we 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, we 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 were nearly as good. The efficiency of pacing increased as electrode size increased up to 70 cm2 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.

Anisotropy↗

PubMed1994

A nonlinear electrical-thermal model of the skin.

This work 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 we 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. We 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.

Electric Conductivity↗

PubMed1994

A nonlinear finite element model of the electrode-electrolyte-skin system.

This study presents a two-dimensional finite element model of the electrode-electrolyte-skin system which takes into account the nonlinear behavior of the skin with respect to the amplitude of the voltage. The nonlinear modeling approach has practical value for studies related to transcutaneous stimulation (e.g. maximizing the dynamic range of sensory substitution systems, optimization of TENS, optimization of transcutaneous cardiac pacing, etc.). The model has three main regions: 1) the electrolyte; 2) the skin; and 3) the body. The model consists of 364 nodes, 690 elements and was generated on a MacIntosh II using a version of FEHT (Finite Element for Heat Transfer) adapted for electromagnetics. The electrodes are equipotential lines and the electrolyte is modeled as a pure resistive region with constant conductivity. Although the electrode-electrolyte interface can introduce nonlinearities, we did not take them into account because the skin displays a much higher impedance. The skin is modeled as a nonlinear material with the conductivity dependent on the applied voltage. To account for the mosaic structure of the skin, we used ten different nonlinear subregions of five different values of breakdown voltage. The region designated "body" models the effects of the resistance associated with the dermis and the tissues underneath the skin, and has a constant high conductivity. We studied the effects of two different electrolytes on the comfort of stimulation and found that there was less potential pain delivered when high-resistivity electrolytes were used. This was due to the larger nonuniformities in the current density distribution which appeared for low-resistivity electrolytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Electric Conductivity↗

PubMed1993

The mosaic electrical characteristics of the skin.

The goals of this study are: 1) to characterize the structure of the skin by measuring impedance variations with a suction microelectrode; 2) to correlate the observed impedance variations with dc current pathways through the skin; 3) to characterize the breakdown phenomenon. We constructed a suction microelectrode with a 200-microns internal diameter and performed several tests on two male subjects. Skin impedance measured from different locations on the forearm and palm varied considerably. We 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 is larger for the forearm than for the palm. For both the forearm and the palm the magnitude and variance of skin impedance decrease with increasing stimulus frequency. The density of low impedance points observed on the forearm and palm are consistent with the density of dc current pathways through the skin as indicated by traces left on 1-cm2 Ag electrodes when we passed dc current through the skin. The ratio between the highest and lowest impedances decreased as temperature decreased--at low temperatures the skin displayed mostly high impedances. We were not able to break down the skin using the suction microelectrode. The tests with dry and wet electrodes 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. In conjunction with the larger impedance values, this would tend to increase the breakdown voltage.

Body Temperature↗