Application of computer modelling and lead field theory in developing multiple aimed impedance cardiography measurements

Conventional impedance cardiography (ICG) methods estimate parameters related to the function of the heart from a single waveform that reflects an integrated combination of complex sources. We have previously developed methods and tools for calculating measurement sensitivity distributions of ICG electrode configurations. In this study, the methods were applied to investigate the prospects of recording multiple aimed ICG waveforms utilizing the 12-lead electrocardiography (ECG) electrode locations. Three anatomically realistic volume conductor models were used: one based on Visible Human Man cryosection data and two on magnetic resonance (MR) images representing end diastolic and end systolic phases of the cardiac cycle. Based on the sensitivity distributions obtained, 236 electrode configurations were selected for preliminary clinical examination on 12 healthy volunteers and 9 valvular patients. The model study suggested that a variety of configurations had clearly enhanced sensitivity to the cardiovascular structures as compared to conventional ICGs. Simulation data and clinical experiments showed logical correspondence supporting the theoretically predicted differences between the configurations. Recorded 12-lead ICG signals had characteristic waveforms and landmarks not coinciding with those of conventional ICG. Furthermore, configurations showing resemblance to invasive data and morphological variations in disease are of interest. The results indicate the applicability of the modelling approach in developing ICG measurement configurations. However, the level of clinical relevance and potential of the 12-lead method remains to be explored in studies employing dynamic modelling and acquisition of invasive reference data.     

Sensitivity Distributions of Impedance Cardiography Using Band and Spot Electrodes Analyzed by a Three-Dimensional Computer Model

Impedance cardiography (ICG) offers a safe, noninvasive, and inexpensive method to track stroke volume estimates over long periods of time. Several modified ICG measurement configurations have been suggested where for convenience or improved performance the standard band electrodes are replaced with electrocardiogram electrodes. This report assesses the sensitivity of the conventional and three modified ICG methods in detecting regional conductivity changes in the simulated human thorax. The theoretical analyses of the measurement sensitivity employ the reciprocity theorem and the lead field theory with a highly detailed, anatomically accurate, three-dimensional computer thorax model. This model is based on the finite-difference element method and the U.S. National Library of Medicine's Visible Human Man anatomy data. The results obtained indicate that the conventional four-band ICG is not specifically sensitive to detect conductivity changes in the region of the heart, aortas, and lungs. Analyzed modified electrode configurations do not reproduce exactly the measurement sensitivity distribution of the conventional four-band ICG. Thus, although the signals measured with modified spot arrangements may appear similar to the four-band configuration, the distribution of the signal origin may not be the same. Changing from band to spot electrodes does not overcome the methodological problems associated with ICG. © 1998 Biomedical Engineering Society. PAC98: 8790+y, 8710+e, 8437+q     

A Simulation Study on the Effect of Thoracic Conductivity Inhomogeneities on Sensitivity Distributions

A high-resolution 3D finite difference model of the electrical conductivity distribution in a human thorax based on a 43-slice MRI data set along with lead field theory was used to examine the effect of thoracic conductivity inhomogeneities on sensitivity distributions. The electrode configurations used in the present study were based on an eight-electrode array positioned evenly around the thoracic model at a level close the nipple line. Sensitivity distributions of each possible adjacent pair current excitation pattern for both the homogeneous thoracic model and the heterogeneous thoracic model were evaluated. The results show that thoracic inhomogeneities significantly perturb sensitivity distribution patterns. Although for a given thoracic geometry the electrode configuration gives the overall sensitivity distribution features, sharp large local changes occur near the boundaries between different tissues in the heterogeneous model. The results of sensitivity distributions of the heterogeneous thoracic model demonstrate the feasibility of impedance source localization. Selectivity can be used to as a guide to finding favorable electrode configuration for regional impedance monitoring.     

New method for analysing sensitivity distributions of electroencephalography measurements

In this paper, we introduce a new modelling related parameter called region of interest sensitivity ratio (ROISR), which describes how well the sensitivity of an electroencephalography (EEG) measurement is concentrated within the region of interest (ROI), i.e. how specific the measurement is to the sources in ROI. We demonstrate the use of the concept by analysing the sensitivity distributions of bipolar EEG measurement. We studied the effects of interelectrode distance of a bipolar EEG lead on the ROISR with cortical and non-cortical ROIs. The sensitivity distributions of EEG leads were calculated analytically by applying a three-layer spherical head model. We suggest that the developed parameter has correlation to the signal-to-noise ratio (SNR) of a measurement, and thus we studied the correlation between ROISR and SNR with 254-channel visual evoked potential (VEP) measurements of two testees. Theoretical simulations indicate that source orientation and location have major impact on the specificity and therefore they should be taken into account when the optimal bipolar electrode configuration is selected. The results also imply that the new ROISR method bears a strong correlation to the SNR of measurement and can thus be applied in the future studies to efficiently evaluate and optimize EEG measurement setups.     

Development of a Tri-polar Concentric Ring Electrode for Acquiring Accurate Laplacian Body Surface Potentials

Potentials recorded on the body surface from the heart are of a spatial and temporal function. The 12-lead electrocardiogram (ECG) provides a useful means of global temporal assessment; however, it yields limited spatial information due to the smoothing effect caused by the volume conductor. In an attempt to circumvent the smoothing problem, researchers have used the five-point method (FPM) to numerically estimate the analytical solution of the Laplacian with an array of monopolar electrodes. Researchers have also developed a bipolar concentric ring electrode system to estimate the analytical Laplacian, and others have used a quasi-bipolar electrode configuration. In a search to find an electrode configuration with a close approximation to the analytical Laplacian, development of a tri-polar concentric ring electrode based on the nine-point method (NPM) was conducted. A comparison of the NPM, FPM, and discrete form of the quasi-bipolar configuration was performed over a 400 × 400 mesh with 1/400 spacing by computer modeling. Different properties of bipolar, quasi-bipolar and tri-polar concentric ring electrodes were evaluated and compared, and verified with tank experiments. One-way analysis of variance (ANOVA) with post hoc t-test and Bonferroni corrections were performed to compare the performance of the various methods and electrode configurations. It was found that the tri-polar electrode has significantly improved accuracy and local sensitivity. This paper also discusses the development of an active sensor using the tri-polar electrode configuration. A 1-cm active Laplacian tri-polar sensor based on the NPM was tested and deemed feasible for acquiring Laplacian cardiac surface potentials.     

Multiple lead recordings improve accuracy of bio-impedance plethysmographic technique

We have developed the theory and instrumentation of multiple multi-electrode bio-impedance (BI) measurements based on lead field theoretical approach. To derive reliable information based on BI data, a quantity of measurements should be taken with electrode configurations possessing regional measurement sensitivity. An apparatus has been developed with an eye to the requirements imposed by the theoretical aspects of achieving multiple multi-electrode BI measurements. It has features compensating electrode-contact related errors and errors due to imbalance between the conductive pathways when multiple electrodes are utilised for BI measurement. The proposed design allows simultaneous multi-electrode BI and bioelectric recording with the same electrode system. Initial operation experiences in clinical environment indicate that the device functions as intended, and allows user-friendly utilisation of multiple BI measurements. Contributions presented to BI methodology and instrumentation improve the reliability of BI measurements.     

Cochlear-implant high pulse rate and narrow electrode configuration impair transmission of temporal information to the auditory cortex

In the most commonly used cochlear prosthesis systems, temporal features of sound are signaled by amplitude modulation of constant-rate pulse trains. Several convincing arguments predict that speech reception should be optimized by use of pulse rates > or approximately 2,000 pulses per second (pps) and by use of intracochlear electrode configurations that produce restricted current spread (e.g., bipolar rather than monopolar configurations). Neither of those predictions has been borne out in consistent improvements in speech reception. Neurons in the auditory cortex of anesthetized guinea pigs phase lock to the envelope of sine-modulated electric pulse trains presented through a cochlear implant. The present study used that animal model to quantify the effects of carrier pulse rate, electrode configuration, current level, and modulator wave shape on transmission of temporal information from a cochlear implant to the auditory cortex. Modulation sensitivity was computed using a signal-detection analysis of cortical phase-locking vector strengths. Increasing carrier pulse rate in 1-octave steps from 254 to 4,069 pps resulted in systematic decreases in sensitivity. Comparison of sine- versus square-wave modulator waveforms demonstrated that some, but not all, of the loss of modulation sensitivity at high pulse rates was a result of the decreasing size of pulse-to-pulse current steps at the higher rates. Use of a narrow bipolar electrode configuration, compared with the monopolar configuration, produced a marked decrease in modulation sensitivity. Results from this animal model suggest explanations for the failure of high pulse rates and/or bipolar electrode configurations to produce hoped-for improvements in speech reception.     

Simultaneous acquisition of functional magnetic resonance images and impedance cardiography

While simultaneous acquisition of electrocardiography (ECG) data during MRI is a widely used clinical technique, the effects of the MRI environment on impedance cardiography (ICG) data have not been characterized. We collected echo planar MRI scans while simultaneously recording ECG and thoracic impedance using carbon fiber electrodes and customized amplifiers. Here, we show that the key changes in impedance (dZ/dt) and features of the ECG waveforms are not obstructed during MRI. We present a method for ensemble averaging ICG/ECG signals collected during MRI and show that it performs comparably with signals collected outside the MRI environment. These results indicate that ICG can be used during MRI to measure stroke volume, cardiac output, preejection period, and left ventricular ejection time.     

Pulse pressure waveform estimation using distension profiling with contactless optical probe

The pulse pressure waveform has, for long, been known as a fundamental biomedical signal and its analysis is recognized as a non-invasive, simple, and resourceful technique for the assessment of arterial vessels condition observed in several diseases. In the current paper, waveforms from non-invasive optical probe that measures carotid artery distension profiles are compared with the waveforms of the pulse pressure acquired by intra-arterial catheter invasive measurement in the ascending aorta. Measurements were performed in a study population of 16 patients who had undergone cardiac catheterization. The hemodynamic parameters: area under the curve (AUC), the area during systole (AS) and the area during diastole (AD), their ratio (AD/AS) and the ejection time index (ETI), from invasive and non-invasive measurements were compared. The results show that the pressure waveforms obtained by the two methods are similar, with 13% of mean value of the root mean square error (RMSE). Moreover, the correlation coefficient demonstrates the strong correlation. The comparison between the AUCs allows the assessment of the differences between the phases of the cardiac cycle. In the systolic period the waveforms are almost equal, evidencing greatest clinical relevance during this period. Slight differences are found in diastole, probably due to the structural arterial differences. The optical probe has lower variability than the invasive system (13% vs 16%). This study validates the capability of acquiring the arterial pulse waveform with a non-invasive method, using a non-contact optical probe at the carotid site with residual differences from the aortic invasive measurements.     

The clinical application and limitation of electrocardiography

Electrocardiography (EGG) is one of the most useful methods for diagnosis of the heart diseases, including ischemic heart disease, cardiomegaly and arrhythmias. However, image methods such as echocardiography, CT-scan and MRI for diagnosis of heart diseases developed in the recent several years gave rise to some conflictions between the findings obtained by ECG and the imaging methods. Therefore, the diagnostic criteria of ECG had to be re-examined. The body Surface Map (MAP) with 87 leads on the body surface revealed more circumstantial electric phenomena of the heart than the standard 12-lead ECG and vectorcardiography. However, the MAP needs a lot of time for recording and diagnosis because of its many electrodes. Computer diagnosis adequately developed, and simplification and abbreviation of recording are required. Morphologic disorders of the heart can be diagnosed more adequately by imaging methods such as echocardiography and CT-scan than ECGs. However for the study of arrhythmias, ECGs including standard 12-lead, vector and Holter electrocardiographies, and MAP are almost the only methods now available. Each method of ECG has different characteristics and is widely accepted not only for clinical diagnosis but also research of arrhythmias. In the near future, noninvasive examination from the body surface to reveal a minute electric change of the heart will be expected instead of invasive examinations as His-bundle ECGs.     

Modelling transthoracic defibrillation waveforms

Recent investigations connected with implantable defibrillators yielded new data on heart electrophysiology, resulting in reassessment of existing and advancing of new types of electrical impulses. Different electrical equivalent circuits were proposed for modelling intracardiac and transthoracic defibrillation pulse waveforms, comprising generator, electrode interface and tissue resistances. We attempted modelling of the transmembrane voltage V_m time course, induced by different applied voltage V_s waveforms, taking into account only the shapes and the relative V_s and V_m amplitudes. The excitable cell membrane impedance Z was modeled with higher resistance and lower capacitance, so that a shunting effect on the generator and tissue resistances was avoided. The result was a very simple equivalent circuit. We proposed criteria for efficient defibrillation pulse waveforms yielding a straightforward approach to model existing and new pulses and to assess their efficiency.     

Modelling transthoracic defibrillation waveforms

Recent investigations connected with implantable defibrillators yielded new data on heart electrophysiology, resulting in reassessment of existing and advancing of new types of electrical impulses. Different electrical equivalent circuits were proposed for modelling intracardiac and transthoracic defibrillation pulse waveforms, comprising generator, electrode interface and tissue resistances. We attempted modelling of the transmembrane voltage Vm time course, induced by different applied voltage Vs waveforms, taking into account only the shapes and the relative Vs and Vm amplitudes. The excitable cell membrane impedance Zm was modelled with higher resistance and lower capacitance, so that a shunting effect on the generator and tissue resistances was avoided. The result was a very simple equivalent circuit. We proposed criteria for efficient defibrillation pulse waveforms yielding a straightforward approach to model existing and new pulses and to assess their efficiency.     

Conversion of polarographic electrode measurements--a computer based approach

The polarographic measurement of tissue oxygenation is one of the most widely used methods in clinical practice for the quantification of tumour hypoxia. However, due to the particular features of the electrode measuring process, the results of the measurements do not accurately reflect the tumour oxygenation. This study aimed to find a correlation between the electrode measurements and the tumour oxygenation in an attempt to improve the accuracy of the predictions regarding the response to treatment based on electrode measurements. A previously developed computer model that allows the simulation of tumour tissue and electrode measurements was used. The oxygenation of a large number of tumours with biologically relevant distributions of blood vessels was theoretically calculated. Simulations of electrode measurements allowed the comparison between the real tissue oxygenation and the results obtained with the electrode. A semi-empirical relationship between the hypoxic fraction measured by the electrode and the real hypoxic fraction in the tissue has been found. The impact of the correction of the electrode measurements in terms of predictions for tumour control probability was estimated for a few clinical examples. The range of possible true values corresponding to one measurement has also proven useful for explaining the apparently unexpected response to the treatment of some patients. The corrected hypoxic fraction which is believed to be closer to the real value of tissue hypoxia predicts much smaller control probabilities than the raw electrode measurements. This could provide an explanation for the apparently unexpected failure to respond to the treatment of some of the patients with apparently favourable tumour oxygenation. This also means that the electrode measurements cannot be used directly for the quantitative modelling of tumour response to the treatment. The conversion method proposed in this paper might however strengthen the statistical power of the correlations between the electrode measurements and the treatment outcome.     

Association between electrocardiographic findings and cardiac dysfunction in adult isolated traumatic brain injury

Introduction:

Abnormal electrocardiographic (ECG) findings can be seen in traumatic brain injury (TBI) patients. ECG may be an inexpensive tool to identify patients at high risk for developing cardiac dysfunction after TBI. The aim of this study was to examine abnormal ECG findings after isolated TBI and their association with true cardiac dysfunction, based on echocardiogram.

Methods:

Data from adult patients with isolated TBI between 2003 and 2010 was retrospectively examined. Inclusion criteria included the presence of a 12-lead ECG within 24 h of admission and a formal echocardiographic examination within 72 h of admission after TBI. Patients with preexisting cardiac disease were excluded. Baseline clinical characteristics, 12-lead ECG, and echocardiogram report were abstracted. Logistic regression was used to identify the relationship of specific ECG abnormalities with cardiac dysfunction.

Results:

We examined data from 59 patients with isolated TBI who underwent 12-lead ECG and echocardiographic evaluation. In this cohort, 13 (22%) patients had tachycardia (heart rate >100 bpm), 25 (42.4%) patients had a prolonged QTc, and 6 (10.2%) patients had morphologic end-repolarization abnormalities (MERA), with each having an association with abnormal echocardiographic findings: Odds ratios (and 95% confidence intervals) were 4.14 (1.02-17.05), 9.0 (1.74-46.65), and 5.63 (1.96-32.94), respectively. Ischemic-like ECG changes were not associated with echocardiographic abnormalities.

Conclusions:

Repolarization abnormalities (prolonged QTc and MERA), but not ischemic-like ECG changes, are associated with cardiac dysfunction after isolated TBI. 12-lead ECG may be an inexpensive screening tool to evaluate isolated TBI patients for cardiac dysfunction prior to more expensive or invasive studies.     

Automatic analysis of pre‐ejection period during sleep using impedance cardiogram

The pre‐ejection period (PEP) is a valid index of myocardial contractility and beta‐adrenergic sympathetic control of the heart defined as the time between electrical systole (ECG Q wave) to the initial opening of the aortic valve, estimated as the B point on the impedance cardiogram (ICG). B‐point detection accuracy can be severely impacted if ICG cardiac cycles corrupted by motion artifact, noise, or electrode displacement are included in the analyses. Here, we developed new algorithms to detect and exclude corrupted ICG cycles by analyzing their level of activity. PEP was then estimated and analyzed on ensemble‐averaged clean ICG cycles using an automatic algorithm previously developed by the authors for the detection of B point in awake individuals. We investigated the algorithms’ performance relative to expert visual scoring on long‐duration data collected from 20 participants during overnight recordings, where the quality of ICG could be highly affected by movement artifacts and electrode displacements and the signal could also vary according to sleep stage and time of night. The artifact rejection algorithm achieved a high accuracy of 87% in detection of expert‐identified corrupted ICG cycles, including those with normal amplitude as well as out‐of‐range values, and was robust to different types and levels of artifact. Intraclass correlations for concurrent validity of the B‐point detection algorithm in different sleep stages and in‐bed wakefulness exceeded 0.98, indicating excellent agreement with the expert. The algorithms show promise toward sleep applications requiring accurate and reliable automatic measurement of cardiac hemodynamic parameters.     

Model-based noninvasive estimation of intracranial pressure from cerebral blood flow velocity and arterial pressure

Intracranial pressure (ICP) is affected in many neurological conditions. Clinical measurement of pressure on the brain currently requires placing a probe in the cerebrospinal fluid compartment, the brain tissue, or other intracranial space. This invasiveness limits the measurement to critically ill patients. Because ICP is also clinically important in conditions ranging from brain tumors and hydrocephalus to concussions, noninvasive determination of ICP would be desirable. Our model-based approach to continuous estimation and tracking of ICP uses routinely obtainable time-synchronized, noninvasive (or minimally invasive) measurements of peripheral arterial blood pressure and blood flow velocity in the middle cerebral artery (MCA), both at intra-heartbeat resolution. A physiological model of cerebrovascular dynamics provides mathematical constraints that relate the measured waveforms to ICP. Our algorithm produces patient-specific ICP estimates with no calibration or training. Using 35 hours of data from 37 patients with traumatic brain injury, we generated ICP estimates on 2665 nonoverlapping 60-beat data windows. Referenced against concurrently recorded invasive parenchymal ICP that varied over 100 millimeters of mercury (mmHg) across all records, our estimates achieved a mean error (bias) of 1.6 mmHg and SD of error (SDE) of 7.6 mmHg. For the 1673 data windows over 22 hours in which blood flow velocity recordings were available from both the left and the right MCA, averaging the resulting bilateral ICP estimates reduced the bias to 1.5 mmHg and SDE to 5.9 mmHg. This accuracy is already comparable to that of some invasive ICP measurement methods in current clinical use.     

Correlation between signal-to-noise ratios and region of interest sensitivity ratios of bipolar EEG measurements

We have developed a parameter, which describes how well the measurement is concentrated on the region of interest source area compared to other source areas in the volume conductor. The parameter concept is called the region of interest sensitivity ratio (ROISR). We assume that ROISR is also connected to the SNR of the measurement. The objective of the present study was to investigate the assumed correlation between the ROISR and SNR of the measurement with three-layer spherical head model. We studied how the source distribution and orientation affect the correlation and thus how applicable the ROISR is in analysing the sensitivity distributions of measurements. We simulated bipolar EEG-evoked potential measurements with 16 combinations of four-source distribution and four-source orientation models. The results indicate that the ROISR correlates with the SNR of the measurement with all tested source distributions and orientations. Thus the ROISRs concept can be applied to analyse measurement setups by modelling and analysing the sensitivity distributions.     

Technical aspects of impedance plethysmography

This paper describes the basic methods for measurement of body impedance, electrodes and their configuration, and the measuring instrument with its limitations. A microcomputer assisted impedance plethysmograph system, developed at BARC and different lead configurations for impedance plethysmographic investigation are also described. Typical impedance plethysmographic waveforms recorded from a normal subject and measurement of their amplitude and various time intervals are illustrated.     

12导心电图同步采集系统USB接口设计

为了快速、方便地传输数据,采用USB接口作为12导心电图同步采集系统与计算机连接的接口.基于该心电采集系统,介绍了USB接口相关的硬件结构、固件程序、驱动程序和应用程序.最后,通过USB接口实现了采样分辨率为16位,采样频率为200Hz/导的12导心电图同步采集和动态描述.     

Effects of temporal modelling on the statistical uncertainty of spatiotemporal distributions estimated directly from dynamic SPECT projections

Artefacts can result when reconstructing a dynamic image sequence from inconsistent single photon emission computed tomography (SPECT) projection data acquired by a slowly rotating gantry. The artefacts can lead to biases in kinetic parameters estimated from time-activity curves generated by overlaying volumes of interest on the images. Insufficient sampling and truncation of projections by cone-beam collimators can cause additional artefacts. To overcome these sources of bias in conventional image based dynamic data analysis, we have been investigating the estimation of time-activity curves and kinetic model parameters directly from dynamic SPECT projection data by modelling the spatial and temporal distribution of the radiopharmaceutical throughout the projected field of view. In the present work, we perform Monte Carlo simulations to study the effects of the temporal modelling on the statistical variability of the reconstructed spatiotemporal distributions. The simulations utilize fast methods for fully four-dimensional (4D) direct estimation of spatiotemporal distributions and their statistical uncertainties, using a spatial segmentation and temporal B-splines. The simulation results suggest that there is benefit in modelling higher orders of temporal spline continuity. In addition, the accuracy of the time modelling can be increased substantially without unduly increasing the statistical uncertainty, by using relatively fine initial time sampling to capture rapidly changing activity distributions.