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Objective. This study aims to assess the sensitivity of epicardial potential-based electrocardiographic imaging (ECGI) to the removal or interpolation of bad leads. Approach. We utilized experimental data from two distinct centers. Langendorff-perfused pig ( n = 2) and dog ( n = 2) hearts were suspended in a human torso-shaped tank and paced from the ventricles. Six different bad lead configurations were designed based on clinical experience. Five interpolation methods were applied to estimate the missing data. Zero-order Tikhonov regularization was used to solve the inverse problem for complete data, data with removed bad leads, and interpolated data. We assessed the quality of interpolated ECG signals and ECGI reconstructions using several metrics, comparing the performance of interpolation methods and the impact of bad lead removal versus interpolation on ECGI. Main results. The performance of ECG interpolation strongly correlated with ECGI reconstruction. The hybrid method exhibited the best performance among interpolation techniques, followed closely by the inverse-forward and Kriging methods. Bad leads located over high amplitude/high gradient areas on the torso significantly impacted ECGI reconstructions, even with minor interpolation errors. The choice between removing or interpolating bad leads depends on the location of missing leads and confidence in interpolation performance. If uncertainty exists, removing bad leads is the safer option, particularly when they are positioned in high amplitude/high gradient regions. In instances where interpolation is necessary, the inverse-forward and Kriging methods, which do not require training, are recommended. Significance. This study represents the first comprehensive evaluation of the advantages and drawbacks of interpolating versus removing bad leads in the context of ECGI, providing valuable insights into ECGI performance., (Creative Commons Attribution license.)

Objective: Despite a long history of ECG-based monitoring of acute ischemia quantified by several widely used clinical markers, the diagnostic performance of these metrics is not yet satisfactory, motivating a data-driven approach to leverage underutilized information in the electrograms. This study introduces a novel metric for acute ischemia, created using a machine learning technique known as Laplacian eigenmaps (LE), and compares the diagnostic and temporal performance of the LE metric against traditional metrics., Methods: The LE technique uses dimensionality reduction of simultaneously recorded time signals to map them into an abstract space in a manner that highlights the underlying signal behavior. To evaluate the performance of an electrogram-based LE metric compared to current standard approaches, we induced episodes of transient, acute ischemia in large animals and captured the electrocardiographic response using up to 600 electrodes within the intramural and epicardial domains., Results: The LE metric generally detected ischemia earlier than all other approaches and with greater accuracy. Unlike other metrics derived from specific features of parts of the signals, the LE approach uses the entire signal and provides a data-driven strategy to identify features that reflect ischemia., Conclusion: The superior performance of the LE metric suggests there are underutilized features of electrograms that can be leveraged to detect the presence of acute myocardial ischemia earlier and more robustly than current methods., Significance: The earlier detection capabilities of the LE metric on the epicardial surface provide compelling motivation to apply the same approach to ECGs recorded from the body surface., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Background: Computational models of myocardial ischemia often use oversimplified ischemic source representations to simulate epicardial potentials. The purpose of this study was to explore the influence of biophysically justified, subject-specific ischemic zone representations on epicardial potentials., Methods: We developed and implemented an image-based simulation pipeline, using intramural recordings from a canine experimental model to define subject-specific ischemic regions within the heart. Static epicardial potential distributions, reflective of ST segment deviations, were simulated and validated against measured epicardial recordings., Results: Simulated epicardial potential distributions showed strong statistical correlation and visual agreement with measured epicardial potentials. Additionally, we identified and described in what way border zone parameters influence epicardial potential distributions during the ST segment., Conclusion: From image-based simulations of myocardial ischemia, we generated subject-specific ischemic sources that accurately replicated epicardial potential distributions. Such models are essential in understanding the underlying mechanisms of the bioelectric fields that arise during ischemia and are the basis for more sophisticated simulations of body surface ECGs., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Background: Near-peer teaching and electronic learning (e-learning) are two effective modern teaching styles. Near-peer sessions provide a supportive learning environment that benefits both the students and the tutor. E-learning resources are flexible and easily distributed. Careful construction and regular editing can ensure that students receive all of the essential material. The aim of this study is to compare the efficacy of e-learning and near-peer teaching during the pre-clinical medical curriculum., Methods: Thirty-nine second-year medical students were consented and randomised into two groups. Each group received teaching on electrocardiogram (ECG) interpretation from a predefined syllabus. Eighteen students completed an e-learning module and 21 students attended a near-peer tutorial. Students were asked to complete a multiple-choice exam, scored out of 50. Each student rated their confidence in ECG interpretation before and after their allocated teaching session., Results: The near-peer group (84%) demonstrated a significantly higher performance than the e-learning group (74.5%) on the final assessment (p = 0.002). Prior to the teaching, the students' mean confidence scores were 3/10 in both the near-peer and e-learning groups (0, poor; 10, excellent). These increased to 6/10 in both cases following the teaching session., Discussion: Both teaching styles were well received by students and improved their confidence in ECG interpretation. Near-peer teaching led to superior scores in our final assessment. Given the congested nature of the modern medical curriculum, direct comparison of the efficacy of these methods may aid course design. The aim of this study is to compare the efficacy of e-learning and near-peer teaching., (© 2015 John Wiley & Sons Ltd.)

Introduction: The "Experimental Data and Geometric Analysis Repository", or EDGAR is an Internet-based archive of curated data that are freely distributed to the international research community for the application and validation of electrocardiographic imaging (ECGI) techniques. The EDGAR project is a collaborative effort by the Consortium for ECG Imaging (CEI, ecg-imaging.org), and focused on two specific aims. One aim is to host an online repository that provides access to a wide spectrum of data, and the second aim is to provide a standard information format for the exchange of these diverse datasets., Methods: The EDGAR system is composed of two interrelated components: 1) a metadata model, which includes a set of descriptive parameters and information, time signals from both the cardiac source and body-surface, and extensive geometric information, including images, geometric models, and measure locations used during the data acquisition/generation; and 2) a web interface. This web interface provides efficient, search, browsing, and retrieval of data from the repository., Results: An aggregation of experimental, clinical and simulation data from various centers is being made available through the EDGAR project including experimental data from animal studies provided by the University of Utah (USA), clinical data from multiple human subjects provided by the Charles University Hospital (Czech Republic), and computer simulation data provided by the Karlsruhe Institute of Technology (Germany)., Conclusions: It is our hope that EDGAR will serve as a communal forum for sharing and distribution of cardiac electrophysiology data and geometric models for use in ECGI research., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Introduction: We hypothesize that electrocardiographic measurements from the intramyocardial space contain more sensitive markers of ischemia than those detectable on the epicardium. The goal of this study was to evaluate different electrical markers for their potential to detect the earliest phases of acute myocardial ischemia., Methods: We conducted acute ischemia studies in open chest animal, by creating finely controlled demand or supply ischemic episodes and recording intramyocardial and epicardial potentials., Results: Under the conditions of mild perfusion deficit, acute ischemia induced changes in the T wave that were larger and could be detected earlier on the epicardial surface than ST-segment changes., Conclusions: Our findings indicate that in the setting of very acute ischemia, epicardial T waves have higher sensitivity to mild degrees of acute ischemia than epicardial ST potentials. These results suggest that changes in the T wave shape may augment shifts in ST segments to improve ECG based localization of ischemia., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Inverse electrocardiography in recent years has generally been approached using one of two quite distinct source models, either a potential-based approach or an activation-based approach. Each approach has advantages and disadvantages relative to the other, which are inherited by all specific methods based on a given approach. Recently our group has been working to develop models which can bridge between these two approaches, hoping to capture some of the most important advantages of both. In this work we present one such effort, which we term wavefront-based potential reconstruction (WBPR). It is a modification of standard regularization methods for potential-based inverse electrocardiography, into which we incorporate a constraint based on a wavefront-like approximation to the potential-based solution. Initial results indicate significant improvement with respect to localization and characterization of the wavefront in simulations using both epicardially and supra-ventricularly paced heartbeats.

This study identifies the most sensitive electrocardiographic leads for monitoring ST-segment changes caused by acute coronary ischemia. The data set consisted of 120-lead electrocardiograms (ECGs) digitally recorded during balloon-inflation angioplasty in 3 groups of patients with single-vessel disease (left anterior descending [LAD], 32; right coronary artery [RCA], 36; left circumflex [LCx], 23). The ST deviation was measured in all recorded leads during baseline and ischemic states, and its difference between these 2 states (DeltaST) was calculated at 352 sites and plotted as DeltaST maps. The patients in each group were divided, by means of DeltaST criteria, into subgroups of "responders" and "nonresponders." Mean DeltaSTs for each group/subgroup were calculated and standardized by the corresponding standard deviation (SD); these values were plotted as mean DeltaST and t maps. Sites where extrema of DeltaST occurred most frequently were sought in bootstrap trials, performed in each group/subgroup. The results suggest that the optimal sites for the ischemia-sensitive leads are: V(3) (+) and just below V(8) (-) for LAD-related ischemia; the left iliac crest (+) and above V(3) at the third intercostal space (-) for RCA-related ischemia; and just below V(8) (+) and above V(2) at the third intercostal space (-) for LCx-related ischemia. Three "optimal" bipolar leads using these sites registered, in the responders from the LAD, RCA, and LCx groups, mean DeltaST (+/-SD) of 232 +/- 59, 245 +/- 96 and 158 +/- 91 microV, respectively; the corresponding t values were 15.14, 9.90, and 6.75. In the 12-lead ECG, only lead V(3) approached optimal DeltaST and t values for the LAD responders (187 +/- 61 microV; t = 11.75) and lead III for the RCA responders (191 +/- 76 microV; t = 9.73), but even these values were significantly suboptimal (P = 0.0011 and P = 0.0120, respectively). We found that the "optimal" bipolar leads can be derived, to an excellent approximation, from the 12 standard leads or from 3 EASI leads (with 3 electrodes at Frank's transverse level and 1 on the manubrium), by using precalculated regression coefficients. By means of bootstrap trials, we estimated the mean sensitivity (SE) and the mean positive predictive value (PPV) with which 3 "optimal" vessel-specific leads could identify ischemia related to the LAD, RCA, and LCx arteries, in the test set, as (SE/PPV) 94.7/92.8%, 78.7/80.9%, and 81.5/80.9%. A similar diagnostic performance can be achieved by vessel-specific leads derived from the 12-lead ECG (93.0/93.4%, 76.6/82.0%, and 82.7/77.1%) and, interestingly, from the EASI lead system (97.8/88.4%, 78.0/80.2%, and 76.8/83.2%). Thus, although the "optimal" bipolar leads for detecting ischemia related to each of the 3 coronary arteries were found to require sampling outside the 12-lead ECG, these leads can be derived from the full set of 12 standard leads or--for clinical monitoring applications--from the EASI lead system by using fewer electrodes at convenient locations.

Despite many years of study, certain aspects of myocardial ischemia remain incompletely understood. One observation that motivated this study is that acute, complete occlusion produces elevations but never depression of the ST-segment potentials in electrocardiographic leads over the ischemic zone. Limited flow, on the other hand, leads to ST-segment depression, both in in situ experiments and during clinical stress tests. The prevailing biophysical theory of ischemia suggests that complete occlusion should produce at least transient ST-segment depression, a finding we have neither observed in our own studies nor uncovered in the literature. Our goal with these experiments was to understand the difference between complete occlusion and reduced coronary flow, specifically the behavior at the transition between the two. We have performed experiments by using isolated dog hearts with a cannulated left anterior descending artery suspended in a human shaped electrolytic tank. To create a range of ischemic conditions, we changed coronary flow rates both suddenly and in controlled sequences and varied the heart rate of the isolated heart. The main finding was that in the isolated heart preparation, epicardial ST-segment depression over the ischemic zone arose only under conditions of combined restricted flow and elevated heart rate. Reduced coronary flow alone never produced ST-segment depression. These findings suggest that heart rate and probably metabolic work create the conditions necessary for subendocardial ischemia that reduced flow alone cannot provoke. They furthermore suggest that the degree of ST-segment depression for a given restriction in coronary flow may depend on heart rate, which supports the notion of rate correction for clinical stress electrocardiogram testing.

Background: Repolarization dispersion (Rd) is frequently mentioned as a predictor of cardiac abnormalities. We present a new measure of Rd based on the root-mean-square (RMS) curve of an ECG lead set and compare its performance with that of the commonly used QT dispersion (QTd) measure with the use of recovery times measured from directly recorded canine electrograms., Methods and Results: Using isolated, perfused canine hearts suspended in a torso-shaped electrolytic tank, we simultaneously recorded electrograms from 64 epicardial sites and ECGs from 192 "body surface" sites. RMS curves were derived from 4 lead sets: epicardial, body surface, precordial, and a 6-lead optimal set. Repolarization was altered by changing cycle length, temperature, and activation sequence. Rd, calculated directly from recovery times of the 64 epicardial potentials, was then compared with the width of the T wave of the RMS curve and with QTd for each of these 4 lead sets. The correlation between T-wave width and Rd for each lead set, respectively, was epicardium, 0.91; body surface, 0.84; precordial, 0.72; and optimal leads, 0.81. The correlation between QTd and Rd for each lead set was epicardium, 0.46; body surface, 0.47; precordial, 0.17; and optimal leads, 0.11., Conclusions: RMS curve analysis provides an accurate method of estimating Rd from the body surface. In contrast, QTd analysis provides a poor estimate of Rd.

This study investigates a technique to estimate dispersion based on the root mean square (RMS) signal of multiple electrocardiographic leads. Activation and recovery times were measured from 64 sites on the epicardium of canine hearts using acute in situ or Langendorff perfused isolated heart preparations. Repolarization and its dispersion were altered by varying cycle length, myocardial temperature, or ventricular pacing site. Mean and dispersion of activation and recovery times, and activation-recovery interval (ARI) were calculated for each beat. The waveform was then calculated from all leads. Estimates of mean and dispersion of activation and recovery times and mean ARI were derived using only inflection points from the RMS waveform. QT intervals were also measured and QT dispersion was determined. Estimates determined from the RMS waveform provided accurate estimates of repolarization and were, in particular, a better measure of repolarization dispersion than QT dispersion.

Background: Myocardial infarction and subsequent remodeling create substrates with altered electrophysiological (EP) properties that are highly arrhythmogenic. Existing ECG methods cannot always detect the existence of such substrates nor provide any detailed information about their EP characteristics. A noninvasive method with such capabilities is greatly needed for identifying patients at risk of arrhythmias and for guidance and evaluation of therapy. Recently, we developed a noninvasive ECG imaging modality that can reconstruct epicardial EP information from body surface potentials. We extended its application to hearts with structural disease and examined its ability to detect and characterize abnormal EP substrates., Methods and Results: Epicardial potentials were recorded with a 490-electrode sock from an open-chest dog. Recordings were obtained from a normal heart and from the same heart 2 hours after left anterior descending coronary artery occlusion and ethanol injection to create an infarct. Body surface potentials were generated from these epicardial potentials in a human torso model. Realistic geometry errors and measurement noise were added to the torso data, which were then used to noninvasively reconstruct epicardial potentials and electrograms (EGMs), with excellent accuracy. EP characteristics associated with the infarct substrate were reconstructed, including (1) a negative region over the infarct, (2) EGMs with large predominant negative deflections (eg, Q-wave EGMs), (3) Q-wave EGMs with superimposed RS deflections reflecting local activation of surviving myocardium within the infarct border zone, (4) reduced magnitudes of EGM negative derivatives, and (5) negative QRS integrals of EGMs over the infarct., Conclusions: ECG imaging can noninvasively detect and map abnormal EP substrates associated with infarction and structural heart disease.

The long-term goal of electrocardiography is to relate electric potentials on the body surface with activities in the heart. Many previously reported studies have focused on direct links between heart and body surface potentials. The goals of this study were first to validate computational methods of determining volume potentials and currents with high-resolution experimental measurements and then to use interactive visualization of thoracic currents to understand features of the electrocardiographic fields from measured cardiac sources. We developed both simulation and experimental studies based on a realistic shaped torso phantom containing an isolated, perfused dog heart. Interventions included atrial pacing, single pacing and simultaneously pacing at multiple locations on the ventricles. Simulated torso volume potentials closely matched measured potentials in the torso-tank preparation (mean correlation coefficients of 0.95). Simulation further provided a means of estimating the current field in the torso from the computed torso volume potentials and the local geometric and conductive properties of the medium. Applying these techniques to the torso electric fields under a variety of pacing conditions, we have further demonstrated that thoracic current can provide many insights into the relationship between heart surface potential and body surface potentials. Specifically, we have shown that geometric factors including cardiac source configuration and location play an important role in determining to what extent electric activity in the heart is directly visible on the body surface electrocardiogram. The computation and visualization toolkit we developed in this study to explore current fields associated with cardiac events may provide new insights into electrocardiology.

Multielectrode catheters provide a percutaneous means of recording activation near the epicardium but only for a relatively small number of sites that are restricted to the major coronary vessels. We have applied a statistical signal processing technique to estimate the value of activation time over the entire epicardium (490 sites) from leadsets consisting of 4 to 40 sites aligned with major branches of the coronary veins. We tested this method using data from high-resolution epicardial mapping from six dog hearts and 153 activation sequences. A study including data from both normal and infarcted dog hearts yielded estimates of activation time, with mean correlation coefficients ranging from 0.97 to 0.84 and achieved localization of earliest site of activation to within 3 to 15 mm, depending on training parameters and leadset. These results suggest that with 10 to 15 catheter-mounted electrodes, it may be possible to reconstruct epicardial activation maps from percutaneous recordings.

We describe two new methods for the inverse problem of electrocardiography. Both employ regularization with multiple constraints, rather than the standard single-constraint regularization. In one method, multiple constraints on the spatial behavior of the solution are used simultaneously. In the other, spatial constraints are used simultaneously with constraints on the temporal behavior of the solution. The specific cases of two spatial constraints and one spatial and one temporal constraint are considered in detail. A new method, the L-Surface, is presented to guide the choice of the required pairs of regularization parameters. In the case when both spatial and temporal regularization are used simultaneously, there is an increased computational burden, and two methods are presented to compute solutions efficiently. The methods are verified by simulations using both dipole sources and measured canine epicardial data.

In experimental studies using Langendorff perfused, isolated canine hearts immersed in a torso-shaped electrolytic tank we studied repolarization and its dispersion using direct epicardial measurements and newly derived, noninvasive body surface indices. Activation recovery intervals (ARIs) measured from 64 epicardial sites based on differences between activation times (ATs) and recovery times (RTs) provided direct measures of repolarization. The indirect, torso surface indices were derived from inflections of the root-mean-square (RMS) voltage of the torso tank surface electrocardiograms recorded simultaneously with the epicardial data. For cycle lengths ranging from 300 to 900 ms, and electrolyte temperatures ranging from 32 degrees C to 40 degrees C we calculated mean, variance, and range of ATs, RTs, and ARIs from the epicardium. From epicardial and torso surface RMS waveforms, we used times of R and T peaks and their differences to estimate mean ATs, RTs, and ARIs, respectively. The RMS T wave width as determined from the second derivative inflections on either side of the T peak served as an estimate of the dispersion of RTs. In parallel studies, we showed that the direct measures of repolarization and its dispersion were reflected in RMS waveforms generated from the epicardial electrograms themselves. In this study, we confirm that the torso and epicardial RMS waveforms reflect comparable information for estimating repolarization and its dispersion. Furthermore, the derived measures provide a method to assess mean ARIs and dispersion of RTs on a beat-to-beat basis and during abnormal (ectopic ventricular) activation sequences.

In mapping the electrical activity of the heart, interpolation of electric potentials plays two important roles. First, it permits the estimation of potentials in regions that could not be sampled or where signal quality was poor, and second, it supports the construction of isopotential lines and surfaces for visualization. The difficulty in developing robust interpolation techniques for cardiac applications lies in the abrupt change in potential in the vicinity of the activation wave front. Despite the resulting nonlinearities in spatial potential distributions, simple linear interpolation methods are the current standard and the resulting errors due to aliasing can be large if electrode spacing does not lie on the order of 0.5-2 mm--the thickness of the activation wave front. We have developed a novel interpolation method that is based on two observations specific to the spread of excitation in the heart: (1) that propagation velocity changes smoothly within a region large enough to contain several measurement electrodes and (2) that electrogram morphology varies very little in the neighborhood of each sample point except for a time shift in the potential wave forms. The resulting interpolation scheme breaks the interpolation of one highly nonlinear variable--extracellular potential--into two separate interpolations of variables with much less drastic spatial variation--activation time and electrogram morphology. We have applied this method to potentials originally recorded at 1.5 mm spacing and then subsampled at a range of densities for testing of the interpolation. The results based both on reconstruction of isopotential contour maps and statistical comparison showed significant improvement of this novel approach over standard linear techniques. The applications of the new method include improved determination of electrophysiological parameters such as spatial gradients of potential and the path of cardiac activation and recovery, estimation of electrograms at desired locations, and visualization of electric potential distributions.

The goal of the inverse problem of electrocardiography is noninvasive discrimination and characterization of normal/abnormal cardiac electrical activity from measurements of body surface potentials. Smoothing and attenuation in the torso volume conductor cause the problem to be ill posed. Standard regularized solutions employ an a priori constraint to achieve reliability and may be biased by the constraint chosen as well as the regularization parameter used to weight it. In this paper, we describe an approach that reformulates this inverse problem as the search for a solution that is a member of an admissible solution set; admissibility is defined in terms of the available constraints. In principle, this approach can utilize as many constraints as may be available, unlike standard techniques which do not easily permit the use of multiple constraints. No regularization parameter is required; instead, we need to choose the nature and size of the constraint sets. Constraints described include several spatial constraints, weighted constraints, and temporal constraints. We describe a solution approach based on iterative convex optimization, and the algorithm--the ellipsoid algorithm--which we have used. Accuracy and feasibility of the method are illustrated with simulation results using dipole sources and measured epicardial potentials.

The QT interval (QTI) has long been useful as a clinical index of the duration of ventricular repolarization, particularly as a marker of prolonged repolarization and its well-established association with arrhythmogenic cardiac states. Likewise, inhomogeneity (dispersion) of repolarization has been linked definitively to increased susceptibility to reentrant arrhythmias. Recent studies have reported the use of QTI dispersion as a meaningful clinical index to identify patients at risk, but the interpretation of the measurement has been controversial. A Langendorff-perfused, isolated canine heart suspended in a torso-shaped, electrolytic tank filled with NaCl-sucrose solution was used to investigate the relationship between body surface QTIs and ventricular repolarization measured directly from the cardiac surface by using activation-recovery intervals, which have been documented to reflect the duration of local action potentials as well as local refractory periods. The data showed poor correlation between cardiac surface activation-recovery intervals and QTIs, as well as the insensitivity of QTIs to regional repolarization shortening in the presence of prolonged repolarization elsewhere. Furthermore, the data confirmed that torso tank QTI dispersion does not reflect directly the full range of measured ventricular repolarization inhomogeneity. It is concluded that body surface QTI dispersion is not a reliable index of repolarization dispersion.

Despite the widespread use of electrocardiogram (ECG), changes in cardiac activity resulting from ischemia or altered recovery characteristics sometimes remain electrocardiographically "silent" or are first detectable by techniques that measure ventricular contractility, such as ultrasound or blood pressure. Especially local changes in repolarization can go undetected when ECG electrodes do not lie close to the area of the heart affected. Experiments were performed on an isolated, perfused canine heart suspended in a realistically shaped, instrumented, electrolytic torso tank with the goal of determining some mechanisms for these ambiguities. By recording simultaneously both epicardial and torso tank surface potentials, complete descriptions were obtained of the electrical response to interventions such as coronary occlusions and alterations in pacing site and frequency. One hypothesis was that some interventions produce highly variable ECG responses primarily because of differences in their location within the heart. To test this, the effect was measured of repeating the same intervention as the heart's location and orientation in the tank were varied. A numerical forward solution was also used to investigate variation of torso tank potentials with heart location. The resulting changes in tank surface potentials illustrate how, for example, precordial ST-segment shifts following occlusion change from elevation to depression to become almost undetectable as the heart rotates in the tank. The results suggest that some events are electrocardiographically silent because of the complex geometric relationship of the heart, torso, and site of the lesion, as well as the spatial sampling and analysis techniques used in detection.

One of the fundamental problems in theoretical electrocardiography can be characterized by an inverse problem. We present new methods for achieving better estimates of heart surface potential distributions in terms of torso potentials through an inverse procedure. First, we outline an automatic adaptive refinement algorithm that minimizes the spatial discretization error in the transfer matrix, increasing the accuracy of the inverse solution. Second, we introduce a new local regularization procedure, which works by partitioning the global transfer matrix into sub-matrices, allowing for varying amounts of smoothing. Each submatrix represents a region within the underlying geometric model in which regularization can be specifically 'tuned' using an a priori scheme based on the L-curve method. This local regularization method can provide a substantial increase in accuracy compared to global regularization schemes. Within this context of local regularization, we show that a generalized version of the singular value decomposition (GSVD) can further improve the accuracy of ECG inverse solutions compared to standard SVD and Tikhonov approaches. We conclude with specific examples of these techniques using geometric models of the human thorax derived from MRI data.

The diagnostic information contained in the standard 12-lead electrocardiogram was assessed by comparing the classification results produced by the standard leads for various clinical settings, such as normal versus myocardial infarction or versus left ventricular hypertrophy to those achieved by 120-lead data or body surface potential maps (BSPMs). Separately, optimal signal leads were extracted from the BSPM by ranking all leads in function of their capability of reconstructing the BSPM. Ranking was achieved by deriving eigenvalues from the covariance matrix calculated from all leads and corresponding measurements. Thus, while comparing the results from the standard leads (diagnostic leads) to those from the original raw map data, a comparison was also performed with respect to the best signal leads, namely the four best and the eight best. From the results observed for all bi- and multigroup classifications, it appeared that the diagnostic yield of the 12 standard leads matched those obtained with a number of signal leads lying between 4 and 8. This indicated that a large overlap still existed between the leads composing the 12-lead ECG (in fact, only 8 independent leads). Another interesting observation resulted from this investigation: although classifiers (discriminating variables) used for classification were identical, whether they originated from the raw standard leads (derived from the raw maps) or from standard leads reconstructed with four or eight signal leads, reconstructed measurements performed better than original measurements. This paradox can be explained by looking at the respective F values. Indeed, since increased F values result from higher ratios between the difference of group means and the composite variance from the pooled groups, higher differences and/or smaller variances produce larger ratios and hence, better group separations.

With the availability of low-cost, high-speed computers with (relatively) vast amounts of storage has come something of an explosion in the application of "quantitative electrocardiography." A search of the Medline medical reference database on the subject string "quantitative AND electrocardiography" reveals no less than 509 citations, suggesting that the term has gained widespread acceptance. However, while quantitative techniques are, in general, to be welcomed to clinical medicine and research, their use as a diagnostic or patient monitoring tool begs a careful examination of just what is being counted and how it is being linked to physiology. In this study, the authors focus on the use of standard electrocardiographic lead systems as the basis for quantitative patient evaluation and attempt to highlight some limitations in the ability to extract meaningful parameters with such a limited sampling of human thoracic electrical activity.

The utility of body surface potential mapping to improve interpretation of electrocardiographic information lies in the presentation of thoracic surface distributions to characterize underlying electrophysiology less ambiguously than that afforded by conventional electrocardiography. Localized cardiac disease or abnormal electrophysiology presents itself electrocardiographically on the body surface in a manner in which pattern plays an important role for identifying or characterizing these abnormalities. Thus, in myocardial infarction, transient myocardial ischemia, Wolff-Parkinson-White syndrome, or ventricular ectopy, observation of electrocardiographic potential patterns, their extrema, and their magnitudes permits localization and quantization of the abnormal activity. Conventional electrocardiography assesses pattern information incompletely and does not use information of distribution extrema locations or magnitudes. Thus, increases or decreases in the magnitudes of electrocardiographic features (ST-segment potential displacement, amplitude, or morphology of Q, R, S, or T waves) associated with changes in cardiac sources (ischemia, infarction, conduction abnormalities, etc.) as measured from fixed leads have a high likelihood of being misinterpreted if the distribution itself is changing. In this study, the authors demonstrate the utility of estimating distributions from small numbers of optimally selected leads, including conventional leads, to reduce uncertainty in the interpretation of electrocardiographic information. This issue is highly relevant when thresholds are used to detect significance of potential levels (exercise testing, detection of myocardial infarction, and continuous monitoring to assess ST-segment changes). Significance of this work lies in improved detection and characterization of abnormal electrophysiology using conventional or enhanced leadsets and methods to estimate thoracic potential distributions.

Localization of Ischemia. This study demonstrates the utility of an electrocardiographic inverse solution, coupled with body surface potential mapping (BSPM), in localizing acute ischemia in patients undergoing percutaneous transluminal coronary angioplasty (PTCA). PTCA balloon inflations produce complete occlusion and acute transient ischemia, which can be detected electrocardiographically with BSPM. Comparisons between maps recorded both during and before the inflation of the PTCA balloon allow patient- and artery-specific characterizations of the resulting ischemia. Knowledge of the patient's coronary anatomy and the location of the occlusion site by coronary angiography permit an estimation based on cardiac hemodynamics of the region of myocardium most likely to suffer from PTCA-induced ischemia. Electrocardiographic inverse solutions provide a means of predicting cardiac potentials from body surface maps. In this study, we describe an inverse solution we have developed to localize the transient ischemia produced by PTCA. To validate the procedure, we compared the locations of predicted ischemia in seven patients with a qualitative estimate of the perfusion region based on fluoroscopic examination of each patient's coronary anatomy and PTCA balloon location. In each case, the region of ischemia predicted by the model included the perfusion zone determined fluoroscopically. These results suggest that electrical changes induced by acute ischemia can be localized with an electrocardiographic inverse solution.

This study reports preliminary results on 45 patients who underwent percutaneous transluminal coronary angioplasty (PTCA); 120-lead data (including the 12-lead standard electrocardiogram [ECG]) were recorded before, during, and after balloon inflation. Twenty-one patients underwent PTCA for left anterior descending coronary disease, 13 for right coronary artery disease, and 10 for left circumflex; 1 patient had combined left anterior descending and right coronary artery disease. In each patient, voltage data recorded during the various phases of the procedure were compared with the patient's own baseline data. In 18 patients, 120 leads were also recorded 24 hours after PTCA. In this study, the usefulness of the standard 12-lead ECG was investigated in locating the coronary artery being occluded, in elucidating the mechanisms of the QRS changes, and in identifying changes occurring 24 hours after completion of the procedure. Results indicate that the observation of ST elevation in the 12-lead ECG may lead to ambiguous interpretation. Also, limiting observation to ST-T patterns alone instead of including QRS changes further hampers correct identification of the involved vessel. QRS modifications during inflation are interpreted as conduction disturbances, although other mechanisms are evoked: study of surface maps may contribute to the understanding of these mechanisms. Changes present 24 hours later are visible in the standard leads, but again, in the absence of the thoracic potential distribution, these are difficult to interpret. These changes were different from those observed after cessation of inflation at the end of the procedure. It is hypothesized that next-day changes may reflect reperfusion injury and/or represent myocardial stunning. Presence of injury and reversibility of changes require further investigation. Also, biochemical markers such as creatine kinase-MB mass, creatine kinase-MB activity, myoglobin, and troponin-T may help elucidate the significance of these findings.

In patients without significant collaterals, percutaneous transluminal coronary angioplasty (PTCA) produces acute transient ischemia that is detectable in both standard electrocardiograms (ECG) and body surface potential maps (BSPMs). Control recordings made before or between inflations provide personalized baselines, which isolate the effects of ischemia from interpatient differences, such as torso shape and electrode location. In this study, two methods of evaluating PTCA-induced ischemia from BSPM recordings are presented. In the first method, an ECG inverse solution that estimates epicardial potentials from body surface signals using a realistic model of torso geometry is applied. The strength of this method lies in its potential ability to localize areas of cardiac ischemia on the epicardial surface. In the second approach, wavelet transforms were used to perform a multiresolution decomposition of the BSPM data into different frequency bands. The basis functions of the wavelet transform are time-limited and narrow band and hence can be expected to be sensitive to features of the BSPM that originate in discrete electrophysiologic events, such as intrusion of the activation front onto regions of ischemia or arrhythmias due to local conduction abnormalities. The method also offers a means of temporal and frequency localization of cardiac events related to the initiation of injury currents and abnormal conduction due to PTCA-induced ischemia. The inverse solution and the wavelet transform each offer new views of the spatial and temporal courses of acute ischemia potentially leading to new diagnostic insights in ECG patient examination.

Measurement of dynamics and spatial characteristics of ventricular repolarization is of interest in assessing patients with ischemic heart disease, particularly in relation to the detection and characterization of ischemic events, identification of patients at risk of ventricular arrhythmias, or determination of the efficacy of drugs intended to alter repolarization. The QT interval (QTI) has been the index of choice for assessing repolarization abnormalities. It is a general measure of repolarization duration but lacks the power to assess the spatial aspects of repolarization and the ability to detect localized shortening in the setting of global prolongation. For direct cardiac surface measurement, QRST integrals and activation recovery intervals (ARIs) were used to assess repolarization and its disparity. The use of similar measurements from the body surface was proposed to provide better characterization of repolarization, its disparity, and its dynamics than is possible using the QTI. In one open-chest experiment using an intact canine heart and two experiments using isolated canine hearts suspended in a torso-shaped electrolytic tank, 64 epicardial electrograms and 192 torso surface electrocardiograms were measured simultaneously. Ventricular repolarization was globally altered by varying pacing cycle lengths or tank temperature. Atrial and ventricular pacing were used to assess sensitivity of repolarization indices to activation sequence. At the cardiac surface, (1) QTI tracks global repolarization changes but is affected by activation sequence and insensitive to localized shortening of repolarization; (2) distribution of QRST integrals reflects disparity of repolarization and is largely independent of activation sequence; and (3) ARI measures local repolarization duration and is only weakly affected by activation sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiography has played an important role in the detection and characterization of heart function, both in normal and abnormal states. In this paper we present an inhomogeneous, anisotropic computer model of the human thorax for use in electrocardiography with emphasis on the calculation of transthoracic potential and current distributions. Knowledge of the current pathways in the thorax has many applications in electrocardiography and has direct utility in studies pertaining to cardiac defibrillation, forward and inverse problems, impedance tomography, and electrode placement in electrocardiography.

One hundred twenty-lead body surface potential maps (BSPMs) were recorded immediately before and 24 hours after coronary angioplasty (PTCA) in 24 patients with symptomatic coronary artery disease (single-vessel in 21 and two-vessel in 3). All PTCAs were uncomplicated and successful. The modified Gensini score decreased in every patient and the mean score fell from 43 +/- 36 to 21 +/- 28 (p less than 0.001). Resting spatial patterns of QRS, ST-segment, and T wave integral distributions over the torso surface were unchanged from before to after PTCA. Quantitative temporal subtraction maps, however, revealed a large precordial area of decreased T wave integral values after PTCA. The sum (sigma) positive T wave integrals fell from 20,501 +/- 10,544 microV.s before PTCA to 17,647 +/- 10,310 microV.s after PTCA (p less than 0.02). In contrast, the sigma positive QRS (10,115 +/- 4,848 microV.s before PTCA vs. 9,656 +/- 4,556 microV.s after PTCA) and the sigma negative ST integrals (-2,489 +/- 1,467 microV.s before PTCA vs. -2,359 +/- 1,505 microV.s after PTCA) were unchanged (NS). Thus, successful PTCA does not produce any persistent change in depolarization or early repolarization electrocardiographic variables but is associated with a decrease in late repolarization potentials that persists for at least 24 hours after the procedure. The pathophysiology of this persistent change is speculative, but myocardial ischemia during the PTCA procedure is a likely possibility. The clinical significance, including predictive value for subsequent stenosis, and the natural history of T wave effect remain incompletely defined. These data suggest that measures to decrease myocardial ischemia during PTCA are warranted.