PubMed Health⌕ Search

Biomedical subjects

R A Stratbucker

Publications and source records attributed to R A Stratbucker.

15 recordsLinked to original sources
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

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

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↗
PubMed1992

Efficacy and safety of transcutaneous low-impedance cardiac pacing in human volunteers using conventional polymeric defibrillation pads.

STUDY OBJECTIVES: To assess the safety and efficacy of transcutaneous cardiac pacing using low-impedance defibrillation-type, self-adhesive polymer electrode pads positioned in the same anatomic sites typical of such pad placement in emergency defibrillation attempts. DESIGN: Prospective, randomized, single-blinded normal subject investigation. METHODS: Thirty healthy unmedicated adult volunteers of both sexes were paced transcutaneously to the threshold of capture and beyond by an intensity factor of 125%. A commercial defibrillator-pacer operating in demand mode had its rate set-point higher than the subjects' base-line rate by 125%. Threshold was established as the pulse current required to effect 75% captured beats. Pacing was continued at the 125% level for two minutes, during which subjective discomfort levels were recorded on a 1-to-5 pain scale. RESULTS: All subjects but one were able to complete the study as designed. The single exception complained of intolerable pain early in the two-minute trial. Capture current had a mean value of 80 mA while the pain assessment averaged 3.2 on our five-point scale. No subject showed any untoward cardiovascular effects either during or after the study. CONCLUSION: In normal human volunteers, transcutaneous cardiac pacing with low-impedance polymer pad electrodes is safe. By extrapolation from this study population, it is sufficiently effective to accommodate successfully the vast majority of clinical circumstances likely to be encountered in emergency medicine.

Adult↗
PubMed1992

Quantitation of metastatic tumor burden from human colon tumor xenografts using radiolabelled monoclonal antibody 17-A fragments.

In realistic models of human tumor xenograft metastasis, the metastatic foci arise in perivascular sites and rarely grow to sizes which are easily quantifiable by visual inspection. As an alternative approach, we have used monoclonal antibody (MAb) 17-1A F(ab')2 fragments labelled with radioiodine (125I) to study the differential accumulation of label in xenografts and metastatic tumor sites as well as in noninvolved tissues of NIH Swiss nude mice receiving HT-29 human colon tumor cells. Images of the whole-body distribution and sites of localization were determined using a pinhole-collimated Angergamma camera. Radioactivity was determined in tissue samples using a well scintillation system, and pharmacokinetics were assessed during the initial 72 h after injection of antibody fragments. The half-life of 125I-F(ab')2 fragments in the blood, 8.6 h, was similar in nontumor-bearing control and tumor-bearing mice. The half-life in subcutaneous tumor xenografts was 30.1 h. The tumor xenograft to tissue activity ratios per unit weight (radiolocalization indices) at 72 h were: blood 90, lung 65, pancreas 50, muscle 35, spleen 20, liver and mesenteric lymph node 10. All subcutaneous xenografts were successfully imaged, and images of 5 of 9 mice (55%) appeared to demonstrate the presence of metastatic tumor by differential and focal accumulation of MAb fragments after 48 or 72 h in the lung (2 cases) or abdominal cavity (3 cases). Necropsy and subsequent histological and biodistribution studies confirmed the presence of metastatic tumor in these sites and identified tumor in several additional sites. The smallest volume of metastatic tissue in liver or lung determined at necropsy which appeared to have been detected by imaging was about 20 mm3. Generally, for mice with metastatic tumors, the radioactivity per unit weight of metastatic tumor-bearing organs compared to tumor-free organs was 2- to 7-fold greater. The results indicate that a radiolocalization index of > or = 2 is generally necessary for metastatic tumor detection by imaging although this is influenced by the extent of anatomical location of the tumor. It was possible to predict the tissue distribution of the fragments from the planar image for the amounts of radioactivity (approximately 1 mCi/kg body weight) employed in this study. These results demonstrate the utility of this approach to quantitate the metastatic burden arising from human colon tumor xenografts in this experimental model.

Animals↗
PubMed1990

The effect of baseline electrocardiographic abnormalities on the diagnostic accuracy of exercise-induced ST segment changes.

Although exercise-induced ST segment depression is thought to be unreliable marker of myocardial ischemia in the presence of resting electrocardiographic changes, this conclusion is based on limited and disparate data from studies often lacking acceptable measures of ischemia. To determine the diagnostic accuracy of the ST segment response in a blinded prospective protocol, we compared ST deviation to thallium201 SPECT scintigraphy in 95 patients during exercise. Diagnostic accuracy was poor in the 95 patients with resting abnormalities: left bundle branch block (LBBB) = 70%, complete right bundle branch block (cRBBB) = 75%, incomplete right bundle branch block (incRBBB) = 79%, intraventricular conduction delay (IVCD) = 44%, left ventricular hypertrophy (LVH) = 59%, digitalis = 53%, compared with a diagnostic accuracy of 90% in 29 patients without resting changes. There were 20 false negative and 17 false positive ST segment responses. The extent and direction of resting ST deviation varied substantially and had no influence on diagnostic accuracy. The extent of change in ST deviation with exercise required for a positive response did not alter diagnostic accuracy: -1.0 mm = 61%, -1.5 mm = 63%, and -2.0 = 61%. While the location of regional ischemia did not influence the accuracy of ST segment analysis, a QRS duration less than 120 msec did improve diagnostic accuracy. Our data confirm that ST segment analysis with exercise testing is not reliable in patients with resting electrocardiographic abnormalities and demonstrates that accuracy is not improved by adjusting for either resting or exercise-induced ST segment changes or for location of the ischemic region.

Aged↗
PubMed1988

Brain structure and function in sexual molesters of children and adolescents.

Sixteen men incarcerated in a state psychiatric facility pursuant to sexual molestation of children and/or adolescents were evaluated by computed tomography (CT) head scans and regional cerebral blood flow (rCBF) estimation. Compared with controls, child molesters were found to have thinner and less dense skulls and lower rCBF values. Variations from control values in rCBF and characteristics of CT head scans for the sex offenders suggest a role for cerebral dysfunction in the etiology of their aberrant behavior.

Adult↗