Showing papers in "IEEE Transactions on Biomedical Engineering in 1986"

Quantitative Investigation of QRS Detection Rules Using the MIT/BIH Arrhythmia Database

Abstract: We have investigated the quantitative effects of a number of common elements of QRS detection rules using the MIT/BIH arrhythmia database. A previously developed linear and nonlinear filtering scheme was used to provide input to the QRS detector decision section. We used the filtering to preprocess the database. This yielded a set of event vectors produced from QRS complexes and noise. After this preprocessing, we tested different decision rules on the event vectors. This step was carried out at processing speeds up to 100 times faster than real time. The role of the decision rule section is to discriminate the QRS events from the noise events. We started by optimizing a simple decision rule. Then we developed a progressively more complex decision process for QRS detection by adding new detection rules. We implemented and tested a final real-time QRS detection algorithm, using the optimized decision rule process. The resulting QRS detection algorithm has a sensitivity of 99.69 percent and positive predictivity of 99.77 percent when evaluated with the MIT/BIH arrhythmia database.

An Efficient Algorithm for Spectral Analysis of Heart Rate Variability

Abstract: We present a simple efficient algorithm for the derivation of a heart rate signal from the electrocardiogram. We demonstrate that the amplitude spectrum of this heart rate signal more closely matches that of the input signal to an integral pulse frequency modulation (IPFM) model of the heart's pacemaker than do the spectra of other ECG-derived heart rate signals. The applicability of this algorithm in cross-spectral analysis between heart rate and other physiologic signals is also discussed.

Analysis of Models for External Stimulation of Axons

Abstract: Extracellular electrodes produce electrical fields at the outside of nerve fibers. Discretization of the axon's length coordinate allows simulation of the excitation by a system of differential equations in time, and difference equations in space. For myelinated fibers this segmentation is naturally given by the nodes of Ranvier, whereas unmyelinated axons can be segmented arbitrarily. In both cases the equations are similar and can be treated in parallel. The activity of the axon depends on the second space derivative of the extracellular medium. The activating function is discussed for monopolar electrodes but the principle can be extended to arbitrary configurations of electrodes.

Power Spectral Analysis of Heart Rate Varability in Sudden Cardiac Death: Comparison to Other Methods

Abstract: Power spectrum analysis of heart rate variability is described and compared to four other reported methods, with respect to their efficacy as predictors of risk of sudden cardiac death (SCD) Approximate frequency domain representations were obtained for each The underlying physiologic processes which may give rise to spectral components are considered These methods were employed to analyze 24-h ambulatory ECG's of patient populations at different degrees of risk of SCD Heart rate variability was found to be reduced in cardiac patients known to be at increased risk of SCD, when compared to those not at increased risk These differences were greatest in power spectral methods Thus, power spectrum analysis appears to be more effective than the other methods in segregating these populations, suggesting that this method may be useful in categorizing cardiac patients according to risk of sudden cardiac death

Optical Fluorescence and Its Application to an Intravascular Blood Gas Monitoring System

Abstract: Optical fluorescence has an extensive history of application in the laboratory to the measurement of ionic concentrations and the partial pressures of oxygen and carbon dioxide. The use of optical fluorescence based sensors to fulfill a recognized need for continuous invasive monitoring of arterial blood gases offers a number of inherent advantages. However, the requirements placed upon a blood gas probe and supporting instrumentation appropriate for use in the clinical environment result in significant design challenges in selection of suitable fluorescent dyes, maintenance of mechanical integrity while obtaining required miniaturization of sensors, and in the transmission, acquisition, and processing of low level light signals. An optical fluorescence based intravascular blood gas monitoring system has been developed which is particularly suited for the critical care and surgical settings and which has a sensor probe that can be introduced into the patient via a radial artery catheter. This system has shown an excellent agreement of measured with true values of pH, pCO2, and P02 in both in vitro and animal studies. Linear regression analysis of typical in vitro data, where true levels were established via tonometry and standardization to a high accuracy laboratory pH measuring instrument, shows slope/intercept values very close to 1.0/0.0 and correlation coefficients of greater than 0.99 for all three parameters.

An X-Band Microwave Life-Detection System

Abstract: An X-band microwave life-detection system has been developed for detecting the heartbeat and breathing of human subjects lying on the ground at a distance of about 30 m or located behind a cinder block wall. The basic principle of the system is to illuminate the subject with a low-intensity microwave beam, and then from the back-scattered microwave signal, extract the heart and breathing signals that modulate it. The circuit description of the system and some experimental results are presented. Potential applications of the system are noted.

Use of Electric Fields to Monitor the Dynamical Aspect of Cell Behavior in Tissue Culture

Abstract: Mammalian cells can be cultured and therefore studied in vitro. Normally, the cells' morphology and other static properties are observed with the aid of a light microscope. A method is described here that allows observation of the dynamical aspects of cultured cells. Mammalian fibroblasts are cultured in polystyrene dishes that contain evaporated gold electrodes. As the cells attach to the electrodes, their presence and their motion is directly reflected in the measured impedance.

A Peripheral Nerve Information Transducer for Amputees: Long-Term Multichannel Recordings from Rabbit Peripheral Nerves

Abstract: A micromachined silicon technology is being developed for the purpose of sensing information from the stumps of amputated mammalian peripheral nerves. Information on long-term biocompatibility, anatomy, and physiology related to the structural design of this sensor is presented. Pertinent materials, fabrication, and surgical implantation issues are discussed. Noise, signal amplitude, and receptive field are considered as the prime determinants of the design of an appropriate electrode contact geometry for the structure. The selectivity of the device is also discussed in terms of the fine structure of the regenerated nerve. Examples of waveforms recorded from rabbit peripheral nerves using this sensor are presented and discussed in terms of electrical and physiological parameters. Possible use of the sensor as a source for a binary code suitable for communication of information to a computer is presented along with discussion of the limitations of the current technology and possible future applications.

ECG Data Compression Using Fourier Descriptors

Abstract: The method of Fourier descriptors (FD's) is presented for ECG data compression. The two-lead ECG data are segmented into QRS complexes and S-Q intervals, expressed as a complex sequence, and are Fourier transformed to obtain the FD's. A few lower order descriptors symmetrically situated with respect to the dc coefficient represent the data in the Fourier (compressed) domain. While compression ratios of 10:1 are feasible for the S-Q interval, the clinical information requirements limit this ratio to 3:1 for the QRS complex. With an overall compression ratio greater than 7, the quality of the reconstructed signal is well suited for morphological studies. The method is resistant to noisy signals and is simple, requiring implementation of forward and inverse FFT. The results of compression of ECG data obtained from more than 50 subjects with rhythm and morphological abnormalities are presented.

Solid-State Electrodes for Multichannel Multiplexed Intracortical Neuronal Recording

Abstract: Thin-film arrays of extracellular recording electrodes have been developed for use in studies of information processing in neural structures and eventual use in closed-loop control of neural prostheses. These probes consist of a silicon substrate which supports an array of thin-film conductors. The conductors are insulated above and below with deposited dielectrics. The electrode sites are defined by openings in the upper dielectric layer and are inlaid with gold to form low-impedance recording surfaces. The probes are typically 15 pim in thickness with shank widths as narrow as 20 ?m. The probe fabrication process is compatible with the inclusion of signal processing circuitry directly on the probe substrate. A 12 channel on-chip signal processor design with per-channel gain of 100, bandwidth of 100 Hz-6 kHz, multiplexed output, and recording-site impedance check capability is described. The probes have adequate strength to penetrate the gerbil pia-arachnoid layer and have recorded single neuron activity of over 500 ?V peak-to-peak from tip, side, and mid-carrier sites. Signal-to-noise ratios as high as 10:1 have been achieved. An equivalent circuit model for the conducting leads, the recording site, and the electrode-electrolyte interface is described. Development of biocompatible insulation and encapsulation materials for long-term implantation of active probes is underway.

Motion Artifact from Spot and Band Electrodes During Impedance Cardiography

Abstract: We have modified impedance cardiography for monitoring cardiac output during stress tests. Employing an off-line microcomputer, our instrument ensemble averaged impedance signals to minimize the effect of motion artifacts. We proposed a new four-spot electrode array and replaced the usual encircling band electrode array with it. We tested ten normal subjects and compared the signal-to-noise ratio (SNR) from our spot electrode array to that from a typical band electrode array at rest and during four levels of exercise on a treadmill. The average of the sighal-to-noise ratios for ten subjects from our spot electrode array was 13.6-45.5 percent larger than that from the band electrode array at rest and during four levels of exercise. Thus, it is desirable to replace band electrodes with spot electrodes in impedance cardiography for exercising subjects.

The Inverse Problem in Electrocardiography: A Model Study of the Effects of Geometry and Conductivity Parameters on the Reconstruction of Epicardial Potentials

Abstract: An idealized, analytic model using spherical harmonics was developed to analyze the effects of variations in torso geometry and volume conductivity parameters on the recovery of epicardial potentials from torso potentials. The model was also used to analyze the effects of these variations on individual terms in the orthogonal series expansion. The ability to reconstruct separate, local electrical events on the epicardium was examined under the following simulated situations: 1) all conductivity and geometry parameters were known accurately, 2) the conductivity of individual torso tissue layers was varied, 3) the torso-air boundary was eliminated (the "infinite medium" assumption), 4) the heart position was not accurately known, and 5) the heart size was not accurately known. Variation in conductivity and geometry parameters was found to exert a quantitative and qualitative effect on the amplitude, resolution, and position of the reconstructed epicardial maxima and minima. Significant differences were found in the ability of the inverse procedure to recover epicardial potentials resulting from posterior as opposed to anterior myocardial sources. Important conclusions regarding the narrow allowance for error in heart size and position, and the relative contributions of the torso tissue layer conductivities can provide guidelines for inverse reconstruction of epicardial potentials with a realistic model utilizing the true geometry.

A Dual-Mode Dynamic Model of the Vergence Eye Movement System

Abstract: The disparity vergence eye movement system, long thought to be a simple continuous feedback system, has recently been shown to exhibit more complex dual-mode slow and fast responses. A model has been developed which simulates this behavior. The slow component accounts for the smooth following of slowly moving stimuli. The fast component has samnpling and prediction mechanisms which account for the staircase-like step responses to fast ramp stimuli. Model simulation responses to pulse, step, step-pulse, ramp, and sinusoidal stimuli show good fit to these diverse experimental results. This robust and comprehensive model forms the basis for further understanding of the dynamic controlling mechanisms of the vergence system.

Sensors for Use with Functional Neuromuscular Stimulation

Abstract: Functional neuromuscular stimulation (FNS) designates artificially applied electrical activation of muscles to restore function lost as a result of neurological lesions. FNS prostheses are currently being designed to restore urinary bladder control, standing, walking, and hand function. All of these prostheses need sensors for interaction with the human users and the environment. This paper discusses each of these prostheses with special regard to the use of sensors and the design specifications that the sensors must meet.

The Packaging of Implantable Integrated Sensors

Abstract: Integrated sensors are being designed for long-term implantation for physiological research using animal models and for permanent implantation for various types of prostheses in clinical applications. The problem of packaging these implantable integrated sensors outside of hermetically sealed hybrid packages is described. The electrolytic corrosion of cable conductors and integrated circuit metallizations is the principal danger. The published performance of the materials which have been used to date to encapsulate nonintegrated sensors and conductors is evaluated, and found to be unreliable for these applications. So, too, are the metals which have been used as electrical conductors. New techniques being developed in the attempt to achieve "hermeticity on a chip" are described. Finally, a critical, residual aspect of the overall problem is identified: how to make a mechanically rugged and inherently noncorrodible connection between a cable and a thin film on a silicon chip. No solution to this problem has been demonstrated.

Multiple Model Adaptive Control Procedure for Blood Pressure Control

Abstract: Multiple model adaptive control procedures have been considered for a computer-based feedback system which regulates the infusion rate of a drug (nitroprusside) in order to maintain desired blood pressure. Because the transfer function parameters are different for each patient, and furthermore are time variant, such an algorithm is desirable for maintaining both steady-state and transient specifications. To this effect, computer simulation has shown that multiple model adaptive control procedures might be successfully applied to the control of blood pressure despite the uncertainty in the delays, time constant, and gains. Additional efforts concerned with the actual demonstration of these concepts on dogs have further supported the role of adaptive control for blood pressure regulation.

An Asymmetric Two Electrode Cuf for Generation of Unidirectionally Propagated Action Potentials

Abstract: An asymmetric two electrode cuff (ATEC) for generation of unidirectionally propagated action potentials (UPAP's) has been tested in animals. Results indicate that the design is well suited for applications of "collision block" of peripheral nerve transmission. This electrode cuff differs from a standard bipolar electrode cuff in that the anode is enclosed by an insulating sheath of larger diameter than the cuff's cathode and the electrodes are asymmetrically placed within the cuff. In all 13 animals studied, ATEC's with anodes of 1.6 or 3.4 mm diameter and with cathodes of 1.2 mm diameter generated UPAP's when used on a nerve trunk of approximately 1 mm diameter. The cuff length used was 16 mm and the cuff length asymmetry (i. e., distance from cathode to proximal end over distance from cathode to distal end) was 1.7:1. Stimuli were regulated-current rectangular pulses with exponential trailing phases. For pulse widths of 100-500 ?s, exponential (90-10 percent) fall-times of 100-500 f4s minimized total charge injection. The virtual cathode excitation typically seen in standard bipolar electrode cuffs was always adequately suppressed with the ATEC configuration. ATEC's generated UPAP's over a larger window of current amplitudes than monopolar electrodes of similar dimensions.

Uterine EHG Processing for Obstetrical Monitorng

Abstract: The temporal and spectral properties of the human uterine electromyogram are first described, related to two different situations: pregnancy and parturition. Thus, a parameter set is selected, and a discriminant analysis is performed, in order to obtain the best discriminant vector for these two situations. A dynamic control of the efficiency of the contractions during labor is described. The good results of this dynamic control permit us to propose a monitoring device providing information on contraction rate and efficiency.

A Discrete-Time Model of Electrcally Stimulated Muscle

Abstract: A model describing the input/output properties of electrically stimulated isometric muscle is developed and experimentally tested. A discrete-time model gives the force output at the times of stimulation during pulse width modulation of recruitment at fixed stimulus amplitudes and periods. Two elements are necessary in the model: a static nonlinear element followed by a linear dynamic element. The static nonlinearity describes the relationship between pulse width and steady-state force. The dynamic properties are described with less than 10 percent error by a second-order discrete-time deterministic autoregressive moving average (DARMA) model. Exponentially weighted recursive least squares methods allow efficient parameter estimation. Model parameters are found to vary systematically with muscle length and stimulus frequency. Tests comparing actual and simulated feedback control of electrically stimulated muscle indicate that the model is adequate for digital controller design for applications in functional electrical stimulation.

An Integrated Sensor for Electrochemical Measurements

Abstract: A method for the fabrication of a completely integrated solid-state electrochemical sensor which combines a minature liquid junction reference electrode with a CMOS ISFET is presented. The reference electrode is fabricated by preferentially etching silicon to form a porous silicon frit. The CMOS process provides electrical encapsulation of the ISFET. The performance of the reference electrode and CMOS ISFET as an integrated sensor is demonstrated.

Reduction of Heart Sounds from Lung Sounds by Adaptive Filterng

Abstract: Auscultation of the chest is an attractive diagnostic method used by physicians, owing to its simplicity and noninvasiveness. Hence, there is interest in lung sound analysis using time and frequency domain techniques to increase its usefulness in diagnosis. The sounds recorded or heard are, however, contaminated by incessant heart sounds which interfere in the diagnosis based on, and analysis of, lung sounds. A common method to minimize the effect of heart sounds is to filter the sound with linear high-pass filters which, however, also eliminates the overlapping spectrum of breath sounds. In this work we show how adaptive filtering can be used to reduce heart sounds without significantly affecting breath sounds. The technique is found to reduce the heart sounds by 50?80 percent.

Thin-Film Multiple Electrode Probes: Possibilities and Limitations

Abstract: Thin-film multiple electrode probes are produced by means of thin-film techniques. They are successfully employed for potential measurements in brain research. The most advantageous feature of these probes is that the electrodes can be designed and arranged accurately and close together. The geometric size of the electrode areas is usually in the range of between 50 and 10 000 ?m2. The size limitations of these thin-film probes are mainly determined by the electrode-electrolyte interface and insulation layer qualities. Since medical research problems, as well as surgical requests, are stressing these limitations, some estimates of maximum resolution and probe dimensions are presented.

Dielectric Properties of VX-2 Carcinoma Versus Normal Liver Tissue

Abstract: The bulk electrical properties of an implanted VX-2 carcinoma in rabbit liver tissue were measured from 1 kHz to 13 MHz, together with those of normal rabbit liver tissue. At the lower end of the frequency range, the conductivity of the tumor tissue was 6-7.5 times higher and its permittivity was 2-5 times lower than that of the normal tissue. The increased conductivity of the tumor tissue is believed to arise from the presence of widespread necrosis in the tumor nodules. Such differences, if generally present between tumor and surrounding normal tissues, could be used to advantage in clinical applications of electromagnetic fields.

A Stochastic Model of the Backscattered Doppler Ultrasound from Blood

Abstract: A stochastic model of the continuous wave (CW) Doppler ultrasound signal backscattered from blood is developed. The model incorporates the effects due to the flow-dependent packing and aggregation of red blood cells (RBC's) at a given hematocrit. The Doppler signal is shown to be a zero-mean, stationary, Gaussian random process which can be completely specified by its autocorrelation function. From the autocorrelation function, explicit closed-form expressions are derived for the power spectral density and the backscattering coefficient. Both expressions are found to be more general than those derived from previous blood models. Further, the values of the statistical parameters in the model are discussed in relation to recently published data on the backscattering coefficient of bovine RBC's suspended in saline and of bovine whole blood. It is shown that the change in backscatter with flow conditions can be explained by RBC orientation in saline and by RBC aggregation in whole blood.

A Bidomain Model for the Extracellular Potential and Magnetic Field of Cardiac Tissue

Abstract: In this brief communication, a bidomain volume conductor model is developed to represent cardiac muscle strands, enabling the magnetic field and extracellular potential to be calculated from the cardiac transmembrane potential. The model accounts for all action currents, including the interstitial current between the cardiac cells, and thereby allows quantitative interpretation of magnetic measurements of cardiac muscle.

Human Body Impedance and Threshold Currents for Perception and Pain for Contact Hazard Analysis in the VLF-MF Band

Abstract: The body impedance and threshold currents needed to produce sensations of perception and pain have been measured for 367 human subjects for the frequency range 10 kHz to 3 MHz. A sufficient number of subjects (197 male and 170 female subjects of ages between 18 and 70 years) were utilized in the study to make valid statistical predictions for the general adult population. Various types of contact with metallic electrodes were used to simulate the situation where a human being would be in contact with a large metallic object (car, van, school bus, etc.) in an electromagnetic field in the VLF to MF band. Based on these measurements, it is speculated that the body impedance of a human being is inversely proportional to the body dimensions and the threshold current for perception is directly proportional to the square of the body dimensions. Predictions are made, based on scaling, for the corresponding threshold values for ten-year-old children. The average measured impedance and threshold current values are used to calculate threshold electric fields required to produce sensations of perception and pain in humans in contact with these vehicles. It is concluded from these calculations that many situations can exist in which the present ANSI (American National Standards Institute) recommended standard of 632 V/m for the frequency band 0.3-3 MHz is too high.

Time Domain Resolution of Forward and Reflected Waves in the Aorta

Abstract: A simple, fast time domain method for the resolution of aortic pressure and flow pulses into their forward and reflected components is presented. Accuracy of the method depends on the estimation of characteristic impedance and the frequency responses of the transducer systems.

Role of Light Scatterng in Whole Blood Oximetry

Abstract: We investigated the role of light scattering in whole blood oximetry by transmission spectrophotometry. To delineate the role of scattering and absorbance in the measurement of oxyhemoglobin saturation, we applied Twersky's theory of radiation scattering and measured the apparent optical density of whole blood and hemoglobin solutions. The optical density versus hematocrit relationship predicted by Twersky's theory was found to give a good fit to the data obtained at 660, 813, 880, and 940 nm. A semi-empirical variation of Twersky's equation and photon diffusion equations were also compared to the data, and Twersky's original equation was found to give the best fit. Therefore, Twersky's equation was employed throughout the rest of the data analysis. Total scattering effects were shown to be wavelength and oxygenation dependent. Moreover, the relationship between total scattering effects and percent O2 saturation was approximately linear, and it had a greater slope (at 660 nm) than absorbance versus O2 saturation. Thus, scattering effects in the red-infrared range do not detract from the linearity of whole blood oximeters. By contrast, scattering effects increase the sensitivity of oximeters by contributing linearly to the total O. D. change that occurs with altered oxygenation.

Recording from the Aplysia Abdominal Ganglion with a Planar Microelectrode Array

Abstract: A passive multimicroelectrode array has been fabricated and used to record neural events from the abdominal ganglion of the marine mollusk, Aplysia californica. The array consists of a pattern of gold conductor lines on a glass substrate which is insulated with a polyimide. The 32 electrodes are 25, in diameter and are arranged in a 4 x 8 matrix on 200 um centers. The array is durable and reusable, and can be safely autoclaved. The recording environment surrounding each electrode is sufficiently uniform so as to permit spatial localization of identified cells in the ganglion. The array can record large numbers of unique and often interrelated extracellular neural potentials in relatively simple experiments.

Classification of EEG Spatial Patterns with a Tree-Structured Methodology: CART

Abstract: The increasing use of computers in statistics has spawned a new generation of multivariate statistical techniques. Chief among these is a tree-structured approach to classification and regression analysis. The CART, or Classification and Regression Trees, program implements a recursive partitioning procedure based on an iterative search for best binary "splits" of data. Resultant classifiers consist of binary trees whose leaves determine class labeling. Extensive use of data resampling techniques replaces biased classifier performance measures.