Cardiac Output Monitoring by Impedance Cardiography During Treadmill Exercise

We have modified impedance cardiography for monitoring cardiac output during stress tests. We adapted an ensemble-averaging technique for eliminating motion artifacts. We applied an array consisting of four spot ECG electrodes for impedance cardiogram (ICG) monitoring and compared it to conventional encircling band electrodes. We tested ten normal adults, compared the cardiac output obtained by our ICG monitoring system to that simultaneously obtained by the carbon dioxide (CO2) rebreathing method at rest and during three levels of treadmill exercise. The results show that the correlation coefficient between the spot electrode ICG and the CO2 rebreathing method is r = 0.90, and between the band electrode ICG and the CO2 rebreathing method is r = 0.96. If we use the peak-to-valley height of dz/dt instead of the peak height of dZ/dt in computing cardiac output, the correlation coefficient between the spot electrode ICG and the CO2 rebreathing method can be improved to r = 0.95.     

A finite-element study of the effects of electrode position on themeasured impedance change in impedance cardiography

Traditional impedance cardiography (ICG) technique uses band electrodes both for delivering current to and measuring impedance change in the thorax. The use of spot electrodes increases the ease of electrode placement and comfort level for patients. Research has shown that changes in thoracic impedance can have multiple causes. In this study, we used finite element modeling to investigate the sources of impedance change for both band-electrode and spot-electrode ICG, and focused on how differences in electrode location affect the contribution of different sources to changes in impedance. The ultimate purpose is to identify the optimal electrode type and placement for the sensing of stroke volume (SV). Our models were built on sets of end-diastolic and end-systolic magnetic resonance images of a healthy human subject. The results showed that the effect of ventricular contraction is opposite to that of the other changes in systole: the expansion of major vessels, decrease in blood resistivity due to increased blood flow velocity, and decrease in lung resistivity due to increased blood perfusion. Ventricular contraction, the only factor that tends to increase systolic impedance, has a larger effect than any of the other factors. When spot electrodes are placed on the anterior chest wall near the heart, ventricular contraction is so dominant that the measured impedance increases from end-diastole to end-systole, and the change represents 82% of the contribution from ventricular contraction. When using the common band-electrode configuration, the change in measured impedance is a more balanced combination of the four effects, and ventricular contraction is overcome by the other three factors so that the impedance decreases. These results suggest that the belief that ICG can be used to directly measure SV based on the change in the whole thoracic impedance may be invalid, and that spot electrodes may be more useful for understanding local physiological events such as ventricular volume change. These findings are supported by previously reported experimental observations.     

Impedance cardiography using band and regional electrodes insupine, sitting, and during exercise

The electrical impedance and its first derivative (dZ/dt) were measured at 100 kHz on 10 normal males in supine, sitting, and during upright bicycle exercise in order to compare the contribution of regional electrodes to the standard band electrode signal and to evaluate the possible use of spot electrodes for stroke volume (SV) measurements. Simultaneous measurements were made from band electrodes placed around the neck and lower thorax and from spot electrodes which recorded signals from the neck, upper thorax, and lower thorax. The results showed that approximately equal parts of the dZ/dt waveform came from the neck and upper thorax with the lower thorax contribution small but providing important features of the band signal. Changing from supine to sitting showed percentage decreases of 35% and 46% for the band and neck signals, respectively, with an increase of 19% for the upper thorax signal. The percentage increases in SV with upright exercise were 34%, 52%, and 24% for the bands, neck, and upper thorax signals, respectively. Band signal is made up of different signals from various regions of the thorax. Its ability to predict correct changes in SV may result from some "lucky" coincidences. The use of regional electrodes will probably not give the same SV information but may be important in measuring regional activities of the central circulation.     

Electrocardiographic motion artifact versus electrode impedance

We degraded electrocardiographic electrodes by exposing them to air for four days and evaluated them on 12 subjects. After application, we recorded the electrocardiogram (including motion artifact), missed QRS detections and electrode impedance during 5 min of arm and body movements. Missed QRS detections increased with electrode impedance but correlation was poor. Increased electrode impedance was not a reliable predictor of a poor electrode and the need to replace it.     

The electrode system in impedance-based ventilation measurement

In this paper, we determined which electrode types, sizes, and locations were best suited for impedance-based ventilation measurement. Optimal electrodes provide high signal-to-(motion) artifact ratio (SAR) and reliability by meeting the following criteria: 1) low baseline impedance, 2) high adhesion, 3) good physical stability, 4) large effective area, 5) thin with high flexibility. We compared 14 electrodes from two main groups: adhesive-gel and conductive rubber electrodes. Adhesive-gel electrodes are easy to apply, make good body contact, and do not slip during the course of an experiment. We found that higher SAR's are obtained when electrode area is increased by connecting several small electrodes together rather than by using a single electrode with a larger area. The peak SAR is achieved when two electrode arrays (area = 70 cm2) are centered at the 8th intercostal spaces on opposite midaxillary lines. To determine the optimal electrode locations, we placed 32 electrodes on the trunk and recorded impedance between 171 electrode combinations on ten normal adult subjects. Based on these data, we conclude that the SAR's are highest when one electrode is placed on the midpoint between the left and right second intercostal spaces on the sternum and the other electrode is placed in the opposite position on the back.     

Human Body Impedance for Electromagnetic Hazard Analysis in the VLF to MF Band

A knowledge of the average electrical impedance of the human body is essential for the analysis of electromagnetic hazards in the VLF to MF band. The purpose of our measurements was to determine the average body impedance of several human subjects as a function of frequency. Measurements were carried out with the subjects standing barefoot on a ground plane and touching various metal electrodes with the hand or index finger. The measured impedance includes the electrode polarization and skin impedances, spread impedance near the electrode, body impedance, stray capacitance between the body surface and ground, and inductance due to the body and grounding strap. These components are separated and simplifed equivalent circuits are presented for body impedance of humans exposed to free-space electromagnetic waves as well as in contact with large ungrounded metaltic objects therein.     

Filtering noncorrelated noise in impedance cardiography

Impedance cardiography (ICG) may be altered by noises as respiration and movement artifacts, mainly during exercise. In this work, a scaled Fourier linear combiner (SFLC) event-related to the R-R interval of ECG is proposed. It estimates the deterministic component of the impedance cardiographic signal and removes the noises uncorrelated to this interval. The impedance cardiographic signal is modeled as Fourier series with the coefficients estimated by the least mean square (LMS) algorithm. Simulations have been carried out to evaluate the filter performance for different noise conditions. Moreover, the method capability to remove uncorrelated noises was also examined in physiological data obtained in rest and exercise, by synchronizing respiration and pedalling with a metronome. Analyzing the ICG power spectrum, it was concluded that the proposed filter could remove the noises that are not synchronized with heart rate     

基于铂黑的微针电极阵列表面修饰方法

由于微针电极阵列尖端直径小,空间分辨率高,可以记录单个神经元的放电活动,已成为神经信号记录的首选.但商用微针电极阵列的阻抗较高,降低电极阻抗有利于提高信噪比,改善记录信号质量.采用超声电镀铂黑的方法对微针电极表面进行修饰.测试结果表明铂黑修饰后的微针电极电化学性能优异,1 kHz处阻抗约为2.5 kΩ,相比裸金电极降低...     

Selective activation of distant nerve by surface electrode array

Neural prostheses for restoring lost functions can benefit from selective activation of nerves with limited number and density of electrodes. Here, we show by simulations and animal experiments that multipoint simultaneous stimulation with a surface electrode array can selectively activate nerves in a bundle at a desired location in between the array or at a desired depth, which are referred to as lateral or depth-wise gating stimulation, respectively. The stimulation broadly generates action potentials with cathodic source electrodes, and simultaneously blocks unnecessary propagation with downstream anodic gate electrodes. In general, stimulation with a small diameter electrode can affect a nearly hemispherical region, while a large electrode is effective at a more vertically compressed region, i.e., a surface of nerve bundle. The gating stimulation takes advantage of the size effects by utilizing an asymmetrical electrode array. The array of the lateral gating stimulation is designed to have four electrodes; a pair of large source electrodes and a pair of small gate electrodes. The depth-wise gating stimulation array consists of two electrodes; a large gate and small source electrodes. The simulation first demonstrated that appropriate combination of currents at the source and gate electrodes can change recruitment patterns of nerves with lateral or depth-wise selectivity as desired. We then applied the lateral gating stimulation on the rat spinal cords and obtained a preliminary support for the feasibility.     

A simultaneous multichannel monophasic action potential electrode array for in vivo epicardial repolarization mapping

While the recording of extracellular monophasic action potentials (MAPs) from single epicardial or endocardial sites has been performed for over a century, we are unaware of any previous successful attempt to record MAPs simultaneously from a large number of sites in vivo. We report here the design and validation of an array of MAP electrodes which records both depolarization and repolarization simultaneously at up to 16 epicardial sites in a square array on the heart in vivo. The array consists of 16 sintered Ag-AgCl electrodes mounted in a common housing with individual suspensions allowing each electrode to exert a controlled pressure on the epicardial surface. The electrodes are arranged in a square array, with each quadrant of four having an additional recessed sintered Ag-AgCl reference electrode at its center. A saline-soaked sponge establishes ionic contact between the reference electrodes and the tissue. The array was tested on six anesthetized open-chested pigs. Simultaneous diagnostic-quality MAP recordings were obtained from up to 13 out of 16 ventricular sites. Ventricular MAPs had amplitudes of 10-40 mV with uniform morphologies and stable baselines for up to 30 min. MAP duration at 90% repolarization was measured and shown to vary as expected with cycle length during sustained pacing. The relationship between MAP duration and effective refractory period was also confirmed. The ability of the array to detect local differences in repolarization was tested in two ways. Placement of the array straddling the atrioventricular (AV) junction yielded simultaneous atrial or ventricular recordings at corresponding sites during 1:1 and 2:1 AV conduction. Localized ischemia via constriction of a coronary artery branch resulted in shortening of the repolarization phase at the ischemic, but not the nonischemic, sites. In conclusion, these results indicate that the simultaneous multichannel MAP electrode array is a viable method for in vivo epicardial repolarization mapping. The array has the potential to be expanded to increase the number of sites and spatial resolution.     

Antimicrobial Peptide Functionalized Conductive Nanowire Array Electrode as a Promising Candidate for Bacterial Environment Application

Conductive polymer electrodes are widely used for electrical signal detection owing to their unique mechanical, redox, and impedance characteristics. However, the performance of electrodes is compromised due to the interference of adhered bacteria and most of the scientists have not taken the microbial environment into consideration during electrode design. Here, a facile approach to construct antimicrobial peptide (AMP) functionalized polypyrrole nanowire array conductive electrodes (PNW‐AMP) is reported. Instead of compromising the electrochemical properties as the other antibacterial agents do, the PNW‐AMP electrodes exhibit excellent redox and low interfacial impedance properties. More importantly, the PNW‐AMP can eliminate bacterial adhesion and maintain electrochemical stability simultaneously in the microbial microenvironment for a long time. The antibacterial rate of the PNW‐AMP electrode reaches 95.8% after exposing the electrode to air for one month, while the charge transfer resistance ( R_ct) value only increases by 9% at a bacterium (Escherichia. Coli ) concentration of 1 × 10⁴ colony forming unit (CFU) mL^?1. This research makes it possible to construct highly stable conductive polymer electrodes for bacterial environment electrical signal detection.     

Calibrated single-plunge bipolar electrode array for mapping myocardial vector fields in three dimensions during high-voltage transthoracic defibrillation

Mapping of the myocardial scalar electric potential during defibrillation is normally performed with unipolar electrodes connected to voltage dividers and a global potential reference. Unfortunately, vector potential gradients that are calculated from these data tend to exhibit a high sensitivity to measurement errors. This paper presents a calibrated single-plunge bipolar electrode array (EA) that avoids the error sensitivity of unipolar electrodes. The EA is triaxial, uses a local potential reference, and simultaneously measures all three components of the myocardial electric field vector. An electrode spacing of approximately 500 microm allows the EA to be direct-coupled to high-input-impedance, isolated, differential amplifiers and eliminates the need for voltage dividers. Calibration is performed with an electrolytic tank in which an accurately measured, uniform electric field is produced. For each EA, unique calibration matrices are determined which transform potential difference readings from the EA to orthogonal components of the electric field vector. Elements of the matrices are evaluated by least squares multiple regression analysis of data recorded during rotation of the electric field. The design of the electrolytic tank and electrode holder allows the electric field vector to be rotated globally with respect to the electrode axes. The calibration technique corrects for both field perturbation by the plunge electrode body and deviations from orthogonality of the electrode axes. A unique feature of this technique is that it eliminates the need for mechanical measurement of the electrode spacing. During calibration, only angular settings and voltages are recorded. For this study, ten EAs were calibrated and their root-mean-square (rms) errors evaluated. The mean of the vector magnitude rms errors over the set of ten EAs was 0.40% and the standard deviation 0.07%. Calibrated EAs were also tested for multisite mapping in four dogs during high-voltage transthoracic shocks.     

Circuit Models and Simulation Analysis of Electromyographic Signal Sources?I: The Impedance of EMG Electrodes

The impedance of surface and intramuscular biopotential electrodes was measured during rest and muscle contraction in humans. A frequency-dependent parallel RC circuit model of the skin-electrode interface that captures the dependence of this impedance on the size and geometry of electrode placement was developed and its components were estimated. The model could explain between 86-97 percent of the variations in impedance (Z), 72-92 percent of the variations in resistance (R), and 34-93 percent of the variations in capacitance (C). The impedance and resistance of these electrodes decrease by about tenfold for a 20-fold increase in frequency, while the capacitance decreases by about twofold for the same change in frequency (f). Thus, the overall FRC factor of this parallel RC circuit model remains nearly unchanged over the range of frequencies studied (50-2000 Hz). A significant difference was found between the impedances of the electrodes comprising the differential electrode pair. This imbalance in impedance was between 8-14 percent for surface electrodes and 6-19 percent for wire electrodes. These data provide essential design criteria for the development and simulation of a system for the measurement of electromyographic activity.     

Test Strategies for Electrode Degradation in Bio-Fluidic Microsystems

Electrode technology is fundamental to numerous actuation and sensing functions in bio-fluidic microsystems that target portable bio-analytical instruments. Within these systems high levels of reliability and robustness are crucial and normally complemented by requirements for extremely low probabilities of false positives or negatives being generated. New methods of validating functionality and integrity of the reading are hence required. Embedded test and condition monitoring are crucial technologies for delivering these capabilities. This paper presents two solutions for detecting degradation in electrodes that interface to fluidic or biological systems. In the first solution, a low frequency, impedance based method for identifying degraded structures within an array is proposed. This method depends on measuring and comparing the impedance of each sensing electrode. This research is backed up by physical measurements from an electrode array for drug testing on cardiac and neuron tissue. In the second solution, a mid-frequency oscillation test technique is proposed that is sensitive to degradation in the bio-fluidic interface capacitance, to contamination and to fouling.     

Electrode and Electrolyte Impedance in the Detection of Bacterial Growth

The growth of bacteria in culture was found to produce impedance changes between two metal electrodes which result from changes both in conductivity and electrode impedance. Predominant changes were found in the imaginary impedance component at low frequency. Linear ac impedance characteristics were found in the relation between real and imaginary impedance components before and during bacterial growth. This linear relation is consistent with characteristics of diffusion and electron exchange (oxidation-reduction) at the electrode. The measurement of impedance in this system was limited below 100 Hz by nonlinearity in the voltage-current relation. Bacterial growth was best detected with stainless steel electrodes at low frequency (100 Hz) and including the imaginary component of impedance.     

How Many Leads Are Necessary for a Reliable Reconstruction of Surface Potentials During Atrial Fibrillation?

In this study, we aimed at determining how many leads are necessary for accurately reconstructing ECG potentials during atrial fibrillation (AF) on the body surface. Although the standard ECG is appropriate for the detection of this arrhythmia, its accuracy for extracting other diagnostic features or constructing surface potential maps may not be optimal. We evaluated the suitability of the standard ECG in AF and proposed a new lead system for improving the information content of AF signals in limited lead systems. We made use of 64-lead body surface potential mapping recordings of 17 patients during AF and 18 healthy subjects. Lead selection was performed by making use of a lead selection algorithm proposed by Lux, and error curves were calculated for increasing number of selected leads for QRS complexes and P waves from healthy subjects and AF signals. From our results, at least 23 leads are needed in order to have the same degree of accuracy in the derivation of AF waves as the 12-lead ECG for a normal QRS complex (25% error). The 12-lead ECG allows a reconstruction of surface potentials with 53% error. If a limited lead set is to be chosen, a repositioning of only four electrodes from the standard ECG reduces reconstruction error in 11%. This repositioning of electrodes may include more right anterior electrodes and one posterior electrode.     

Methods for Compensating for Variable Electrode Contact in EIT

Electrical impedance tomography (EIT) is an imaging modality that currently shows promise for the detection and characterization of breast cancer. A very significant problem in EIT imaging is the proper modeling of the interface between the body and the electrodes. We have found empirically that it is very difficult, in a clinical setting, to assure that all electrodes make satisfactory contact with the body. In addition, we have observed a capacitive effect at the skin/electrode boundary that is spatially heterogeneous. To compensate for these problems, we have developed a hybrid nonlinear–linear reconstruction algorithm using the complete electrode model in which we first estimate electrode surface impedances, by means of a Levenberg–Marquardt iterative optimization procedure with an analytically computed Jacobian matrix. We, subsequently, use a linearized algorithm to perform a 3-D reconstruction of perturbations in both contact impedances, and in the spatial distributions of conductivity and permittivity. Results show that, with this procedure, artifacts due to electrodes making poor contact can be greatly reduced. If the experimental apparatus physically applies voltages and measures currents, we show that it is preferable to compute the reconstruction with respect to the Dirichlet-to-Neumann map rather than the Neumann-to-Dirichlet map if there is a significant possibility that electrodes will be fully disconnected. Finally, we test our electrode compensation algorithms for a set of clinical data, showing that we can significantly improve the fit of our model to the measurements by allowing the electrode surface impedances to vary.      

The Impedance Plethysmographic Sampling Field in the Human Calf

Impedance plethysmography has been widely used clinically because of its convenience and sensitivity, but its use has been questioned by some investigators because of uncertainty concerning the source of the signal. Quantitative definitions of the sensitivity to conductivity changes as a function of position have been lacking. The present study defines the sampling efficiency for impedance measurements made with four electrodes on a uniform cylinder model of the calf. Predictions based on electric field theory compare favorably with results from an in vitro model (average rms error = 8 percent). These studies demonstrate that for a given voltage electrode separation, the proximity of the current electrodes determines the relative sensitivity to conductivity changes within the sampling field. For voltage and current electrode separations of 10 and 18 cm on a 37 cm circumference calf (typical measurements for clinical testing of the lower leg), the sampling field extends about 3 cm beyond the voltage electrodes and the deep vessels are sampled with at least 80 percent of the efficiency of the superficial vessels. These studies also indicate that it is possible to predict the sampling field associated with various electrode configurations, a possibility which should aid the design of electrode configurations for specific applications of impedance plethysmography.     

Electrical impedance measurement of urinary bladder fullness

A tetrapolar 75-kHz, 0.2-mA constant-current electrical impedance measuring system was used to monitor urinary volume change over 12 four-hour sessions in 20 male and 20 female normal human subjects. Two spot voltage electrodes (E) and two spot current electrodes (I) were applied 5 cm above the symphysis pubis at 15 cm interelectrode distances, 7.5 cm bilaterally from the midline. Five measurements of impedance and skin temperature were made at 15-minute intervals over 4 hours. Specific gravity, impedance change, and volume were recorded with each voided specimen. Suprailiac and infrascapular skinfolds, and circumference at iliac crests were measured. The subjects drank 175 ml of fluids per hour during the testing session. Subjects remained in the supine position during measurements. They walked to the lavatory to void. Bladder fullness is defined as the urge to void. Results include: (1) a poor negative correlation between specific gravity of urine and impedance (r2 = 0.1240, p less than or equal to 0.01); (2) baseline impedance was dependent upon individual subject characteristics: in males skin area, skinfold thickness, and suprailiac circumference; in females skinfold thicknesses and time since last menstrual period; (3) impedance decreased with urinary bladder filling and increased upon voiding (p less than or equal to 10(-10)); (4) the cumulative sum test (CUSUM) predicted time to void in 78.9% of voids (p less than or equal to 0.05) and no void (when voiding did not occur) in 66.8% of no voids (p less than or equal to 0.05). Overall accuracy of the CUSUM test was 74.6% (p less than or equal to 0.05).     

A reconstruction algorithm for electrical impedance tomography data collected on rectangular electrode arrays

A three-dimensional reconstruction algorithm in electrical impedance imaging is presented for determining the conductivity distribution beneath the surface of a medium, given surface voltage data measured on a rectangular array of electrodes. Such an electrode configuration may be desirable for using electrical impedence tomography to detect tumors in the human breast. The algorithm is based on linearizing the conductivity about a constant value. Here, we describe a simple implementation of the algorithm on a four-electrode--by-four-electrode array and the reconstructions obtained from numerical and experimental tank data. The results demonstrate significantly better spatial resolution in the plane of the electrodes than with respect to depth.