Reduction of motion artefacts during paediatric/infant apnoea monitoring.
Explore the source record for details and available documents.
Apnea↗
Biomedical subjects
J G Webster
Publications and source records attributed to J G Webster.
The origin of skin-stretch-caused motion artifacts under electrodes.
The outside of the skin of the forearm is typically 15 mV more negative than the inside. Stretching the skin causes a reduction in the magnitude of this skin potential V, which we observe as a motion artifact delta V. We seek to determine the origin of this motion artifact by successively stripping 12 layers of the skin using Scotch Tape. Between each stripping we measure artifact delta V, 13 Hz impedance Z, and change in impedance delta Z. On the interior surface of the forearm, Z decreases with number of strippings. delta Z can be first either positive or negative, then is always negative and decreases linearly with Z. delta V first remains constant and then decreases with Z and delta Z. delta V and delta Z increase with stretch force following a logarithmic relationship. delta Z has a rectangular shape waveform, whereas the rising edge of delta V shows a fast followed by a slow component and its falling edge decays exponentially with a large time constant. We have expanded the model of Thakor and Webster to best fit the waveform of delta V and delta Z caused by stretch.
Adult↗
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↗
An ultrasonic time-of-flight system for hand movement measurement.
We have designed and built an ultrasonic time-of-flight system to measure the three-dimensional coordinates of up to three moving emitters. These emitters are fixed on the hand and arm of a human subject or on a hand-held stylus, to provide information concerning motor control. Three stationary receivers are suspended above the experimental space and provide three diagonal distances which are converted to x, y and z coordinates. The sampling rate for one emitter is 200-400 Hz, depending on the nature of the experiment. Multiple emitters are time-multiplexed. The piezoelectric resonator emitter is pulsed with 200 V for 12 microseconds of each cycle and rings at 40 kHz with a Q factor of four. The piezoelectric resonator receiver drives circuitry with a gain of 500 and a bandwidth of 60 kHz. The system provided highly linear results with a repeatability of +/- 1.64 mm and a resolution of 0.7 mm.
Ergometry↗
A system for non-invasively measuring blood pressure on a treadmill.
Measurement of signal to artifact ratio yielded optimal frequencies for a Korotkoff-based automatic system for measuring blood pressure on a treadmill. Maximal Korotkoff sounds occurred just above the crease of the elbow over the brachial artery. Output of a piezoelectric microphone and charge amplifier was measured during treadmill exercise for frequency bands from 8 to 57 Hz. An automatic system filtered output from 40 to 45 Hz, rectified it, then compared it to a threshold that had fixed and exponentially decaying components. For five subjects, the system decisions of systolic and diastolic pressures compared well with those of two observers using stethoscopes. The system shows promise for improved measurements of blood pressure during treadmill exercise.
Blood Pressure Determination↗
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↗
Signal-to-motion artifact ratio versus frequency for impedance pneumography.
We measured transthoracic impedance between 12.5 and 185 kHz in nine adults. We used a system with two impedance channels, both simultaneously detecting the real part of impedance at two different frequencies. We used only two electrodes in the midaxillary line, connecting both channels in parallel. The amplitude relation between the two channels was measured for different maneuvers and frequencies. Results show for normal breathing an increase of the signal of 20% and a decrease in motion artifacts from 12.5 to 185 kHz. We conclude that, for the maneuvers studied, it is better to work at higher frequencies than the ones commonly used. Also, we suggest a method to further increase the signal-to-motion artifact ratio based on measurement at two frequencies.
Adult↗
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↗
Reduction of motion artifacts using a two-frequency impedance plethysmograph and adaptive filtering.
We measured transthoracic impedance in nine presumed healthy adult subjects with a two-frequency plethysmograph at 57 kHz and 185 kHz. The measurement protocol included periods of normal breathing without motion and periods of motion without breathing. We analyzed the cross-correlation and the ratio between the signals at both frequencies for all the different maneuvers. The correlation coefficient was between 0.97 and 1 for breathing, the minimal cross-correlation (0.81) was for simulated obstructive apnea. We found that the amplitude ratio between the two-frequency signals was different for normal breathing and for motion. Based on these results, we designed and tested an adaptive filter to increase the signal-to-artifact ratio (SAR). The increase in SAR (mean +/- standard deviation) compared with the signal at 57 kHz was: 183% +/- 117% for arm movement, 133% +/- 93% for leg movement, and 34% +/- 62% for simulated obstructive apnea.
Adult↗
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↗
Finite-element method in electrical impedance tomography.
In electrical impedance tomography (EIT), current patterns are injected into a subject and boundary voltages are measured to reconstruct a cross-sectional image of resistivity distribution. Static EIT image reconstruction requires a computer model of a subject, an efficient data-collection method and robust and fast reconstruction algorithms. The finite-element method is used as the computer model. The paper describes the finite-element analysis software package developed, including an interactive graphical mesh generator and fast algorithms for solving linear systems of equations using sparse-matrix and vector techniques. Various models of irregularly shaped subjects are developed using mesh-design tools, including automatic mesh generation and optimisation using the Delaunay algorithm. Even though the software package is customised for use in electrical impedance tomography, it can be used for other biomedical research areas, such as impedance cardiography, cardiac defibrillation and impedance pneumography.
Algorithms↗
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↗
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↗
A comparison of electrodes for potential use in paediatric/infant apnoea monitoring.
We measured the signal-to-motion-artifact ratio for ten paediatric/infant electrodes for infant apnoea monitoring. Electrodes that have good stability, strong adhesion, low face-to-face impedance, low transthoracic-plus-electrode impedance, large effective area, and large total area are optimal for impedance pneumography. The gel make-up may also affect optimality.
Adhesiveness↗
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↗
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↗
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↗
Using compound electrodes in electrical impedance tomography.
In electrical impedance tomography, we inject currents and measure voltages to estimate an object's resistivity distribution. The electrode configuration affects measured voltage data because the electrode-skin contact impedance is high and varies with electrode location. We developed a compound electrode which is composed of two electrodes: a large outer electrode to inject current and a small inner electrode to sense voltage. We used these compound electrodes to measure voltages from a physical phantom. We showed that the measured voltages from the compound electrodes are smaller in amplitude than those from conventional electrodes. This demonstrates that the compound electrode can minimize contact impedance voltage drop from the measured data. We used a finite element model for the compound electrode and incorporated the model into the regularized Newton-Raphson reconstruction algorithm. We performed a sensitivity study and showed that the reconstructed resistivity distributions are less dependent on the unknown contact resistance values for a compound electrode than a conventional electrode and that the use of a compound electrode results in improved images for the reconstruction algorithm.
Algorithms↗