Redundancy reduction for improved display and analysis of body surface potential maps. I. Spatial compression

Personalized ECG monitoring and adaptive machine learning

This non-technical review introduces key concepts in personalized ECG monitoring (pECG), which aims to optimize the detection of clinical events and their warning signs as well as the selection of alarm thresholds. We review several pECG methods, including anomaly detection and adaptive machine learning (ML), in which learning is performed sequentially as new data are collected. We describe a distributed-network multiscale pECG system to show how the computational load and time associated with adaptive ML could be optimized. In this architecture, the limited analysis of ECG waveforms is performed locally (e.g., on a smart phone) to determine a small number of clinically important ECG elements, and an adaptive ML engine is located on a remote server (Internet cloud) to determine an individual's "fingerprint" basis patterns and to detect anomalies in those patterns.

Body Surface Potential Mapping: Contemporary Applications and Future Perspectives

Body surface potential mapping (BSPM) is a noninvasive modality to assess cardiac bioelectric activity with a rich history of practical applications for both research and clinical investigation. BSPM provides comprehensive acquisition of bioelectric signals across the entire thorax, allowing for more complex and extensive analysis than the standard electrocardiogram (ECG). Despite its advantages, BSPM is not a common clinical tool. BSPM does, however, serve as a valuable research tool and as an input for other modes of analysis such as electrocardiographic imaging and, more recently, machine learning and artificial intelligence. In this report, we examine contemporary uses of BSPM, and provide an assessment of its future prospects in both clinical and research environments. We assess the state of the art of BSPM implementations and explore modern applications of advanced modeling and statistical analysis of BSPM data. We predict that BSPM will continue to be a valuable research tool, and will find clinical utility at the intersection of computational modeling approaches and artificial intelligence.

The effect of interpolating low amplitude leads on the inverse reconstruction of cardiac electrical activity

Electrocardiographic imaging is an imaging modality that has been introduced recently to help in visualizing the electrical activity of the heart and consequently guide the ablation therapy for ventricular arrhythmias. One of the main challenges of this modality is that the electrocardiographic signals recorded at the torso surface are contaminated with noise from different sources. Low amplitude leads are more affected by noise due to their low peak-to-peak amplitude. In this paper, we have studied 6 datasets from two torso tank experiments (Bordeaux and Utah experiments) to investigate the impact of removing or interpolating these low amplitude leads on the inverse reconstruction of cardiac electrical activity. Body surface potential maps used were calculated by using the full set of recorded leads, removing 1, 6, 11, 16, or 21 low amplitude leads, or interpolating 1, 6, 11, 16, or 21 low amplitude leads using one of the three interpolation methods - Laplacian interpolation, hybrid interpolation, or the inverse-forward interpolation. The epicardial potential maps and activation time maps were computed from these body surface potential maps and compared with those recorded directly from the heart surface in the torso tank experiments. There was no significant change in the potential maps and activation time maps after the removal of up to 11 low amplitude leads. Laplacian interpolation and hybrid interpolation improved the inverse reconstruction in some datasets and worsened it in the rest. The inverse forward interpolation of low amplitude leads improved it in two out of 6 datasets and at least remained the same in the other datasets. It was noticed that after doing the inverse-forward interpolation, the selected lambda value was closer to the optimum lambda value that gives the inverse solution best correlated with the recorded one.

Measuring defibrillator surface potentials: The validation of a predictive defibrillation computer model

Implantable cardioverter defibrillators (ICDs) are commonly used to reduce the risk in patients with life-threatening arrhythmias, however, clinicians have little systematic guidance to place the device, especially in cases of unusual anatomy. We have previously developed a computational model that evaluates the efficacy of a delivered shock as a clinical and research aid to guide ICD placement on a patient specific basis. We report here on progress to validate this model with measured ICD surface potential maps from patients undergoing ICD implantation and testing for defibrillation threshold (DFT). We obtained body surface potential maps of the defibrillation pulses by adapting a limited lead selection and potential estimation algorithm to deal with the limited space for recording electrodes. Comparison of the simulated and measured potential maps of the defibrillation shock yielded similar patterns, a typical correlation greater than 0.9, and a relative error less than 15%. Comparison of defibrillation thresholds also showed accurate prediction of the simulations. The high agreement of the potential maps and DFTs suggests that the predictive simulation generates realistic potential values and can accurately predict DFTs in patients. These validation results pave the way for use of this model in optimization studies prior to device implantation.

Noninvasive Assessment of Atrial Fibrillation Complexity in Relation to Ablation Characteristics and Outcome

Background: The use of surface recordings to assess atrial fibrillation (AF) complexity is still limited in clinical practice. We propose a noninvasive tool to quantify AF complexity from body surface potential maps (BSPMs) that could be used to choose patients who are eligible for AF ablation and assess therapy impact.

Methods: BSPMs (mean duration: 7 ± 4 s) were recorded with a 252-lead vest in 97 persistent AF patients (80 male, 64 ± 11 years, duration 9.6 ± 10.4 months) before undergoing catheter ablation. Baseline cycle length (CL) was measured in the left atrial appendage. The procedural endpoint was AF termination. The ablation strategy impact was defined in terms of number of regions ablated, radiofrequency delivery time to achieve AF termination, and acute outcome. The atrial fibrillatory wave signal extracted from BSPMs was divided in 0.5-s consecutive segments, each projected on a 3D subspace determined through principal component analysis (PCA) in the current frame. We introduced the nondipolar component index (NDI) that quantifies the fraction of energy retained after subtracting an equivalent PCA dipolar approximation of heart electrical activity. AF complexity was assessed by the NDI averaged over the entire recording and compared to ablation strategy.

Results: AF terminated in 77 patients (79%), whose baseline AF CL was 177 ± 40 ms, whereas it was 157 ± 26 ms in patients with unsuccessful ablation outcome (p = 0.0586). Mean radiofrequency emission duration was 35 ± 21 min; 4 ± 2 regions were targeted. Long-lasting AF patients (≥12 months) exhibited higher complexity, with higher NDI values (≥12 months: 0.12 ± 0.04 vs. <12 months: 0.09 ± 0.03, p < 0.01) and short CLs (<160 ms: 0.12 ± 0.03 vs. between 160 and 180 ms: 0.10 ± 0.03 vs. >180 ms: 0.09 ± 0.03, p < 0.01). More organized AF as measured by lower NDI was associated with successful ablation outcome (termination: 0.10 ± 0.03 vs. no termination: 0.12 ± 0.04, p < 0.01), shorter procedures (<30 min: 0.09 ± 0.04 vs. ≥30 min: 0.11 ± 0.03, p < 0.001) and fewer ablation targets (<4: 0.09 ± 0.03 vs. ≥4: 0.11 ± 0.04, p < 0.01).

Conclusions: AF complexity can be noninvasively quantified by PCA in BSPMs and correlates with ablation outcome and AF pathophysiology.

Spatial indices of repolarization correlate with non-ST elevation myocardial ischemia in patients with chest pain

Mild-to-moderate ischemia does not result in ST segment elevation on the electrocardiogram (ECG), but rather non-specific changes in the T wave, which are frequently labeled as non-diagnostic for ischemia. Robust methods to quantify such T wave heterogeneity can have immediate clinical applications. We sought to evaluate the effects of spontaneous ischemia on the evolution of spatial T wave changes, based on the eigenvalues of the spatial correlation matrix of the ECG, in patients undergoing nuclear cardiac imaging for evaluating intermittent chest pain. We computed T wave complexity (TWC), the ratio of the second to the first eigenvalue of repolarization, from 5-min baseline and 5-min peak-stress Holter ECG recordings. Our sample included 30 males and 20 females aged 63 ± 11 years. Compared to baseline, significant changes in TWC were only seen in patients with ischemia (n = 10) during stress testing, but not among others. The absolute changes in TWC were significantly larger in the ischemia group compared to others, with a pattern that seemed to depend on the severity or anatomic distribution of ischemia. Our results demonstrate that ischemia-induced changes in T wave morphology can be meaningfully quantified from the surface 12-lead ECG, suggesting an important opportunity for improving diagnostics in patients with chest pain.

Optimal Magnetic Sensor Vests for Cardiac Source Imaging

Magnetocardiography (MCG) non-invasively provides functional information about the heart. New room-temperature magnetic field sensors, specifically magnetoresistive and optically pumped magnetometers, have reached sensitivities in the ultra-low range of cardiac fields while allowing for free placement around the human torso. Our aim is to optimize positions and orientations of such magnetic sensors in a vest-like arrangement for robust reconstruction of the electric current distributions in the heart. We optimized a set of 32 sensors on the surface of a torso model with respect to a 13-dipole cardiac source model under noise-free conditions. The reconstruction robustness was estimated by the condition of the lead field matrix. Optimization improved the condition of the lead field matrix by approximately two orders of magnitude compared to a regular array at the front of the torso. Optimized setups exhibited distributions of sensors over the whole torso with denser sampling above the heart at the front and back of the torso. Sensors close to the heart were arranged predominantly tangential to the body surface. The optimized sensor setup could facilitate the definition of a standard for sensor placement in MCG and the development of a wearable MCG vest for clinical diagnostics.

Novel measure of electrical dyssynchrony predicts response in cardiac resynchronization therapy: Results from the SMART-AV Trial

BACKGROUND

Cardiac resynchronization therapy (CRT) reduces mortality and morbidity in selected heart failure patients. However, not all patients respond to CRT.

OBJECTIVE

We hypothesized that a novel measure of electrical dyssynchrony, sum absolute QRST integral (SAI QRST), predicts CRT response independent of QRS duration and morphology.

METHODS

We retrospectively analyzed baseline 12-lead electrocardiograms of SmartDelay Determined AV Optimization: A comparison to other AV delay methods used in cardiac resynchronization therapy (SMART-AV) trial study participants (N = 234; mean age 67 years; 163 (70%) men; 140 (60%) ischemic cardiomyopathy; mean left ventricular ejection fraction 25%; mean QRS duration 152 ms; 179 (77%) had left bundle branch block). Baseline pre-implant electrocardiograms were digitized, transformed into orthogonal XYZ, and analyzed automatically by customized MATLAB software. SAI QRST was measured as an averaged arithmetic sum of absolute areas under the QRST curve. Patients were followed prospectively 6 months after CRT-defibrillator implantation. Patients with a decrease in left ventricular end-systolic volume ≥15 mL after 6 months of CRT were considered responders. The logistic regression model was adjusted for age, sex, bundle branch block morphology, left ventricular ejection fraction, cardiomyopathy type, and QRS duration.

RESULTS

Patients with the high mean SAI QRST (third tertile) had 2.5 times greater odds of response than those with the low mean SAI QRST (first tertile: odds ratio [OR] 2.5; 95% confidence interval [CI] 1.3-5.0; P = .010) and 1.9 times greater than the lower 2 tertiles combined (OR 1.9; 95% CI 1.1-3.5; P = .03). Adjustment for renal function (OR 2.33; 95% CI 1.32-4.11; P = .003) and left ventricular lead position in right anterior oblique and left anterior oblique views (OR 1.7; 95% CI 0.9-3.2; P = .087) did not attenuate association of SAI QRST with outcome.

CONCLUSION

High SAI QRST independently predicts CRT response in the SMART-AV study.

A priori model independent inverse potential mapping: the impact of electrode positioning

INTRODUCTION

In inverse potential mapping, local epicardial potentials are computed from recorded body surface potentials (BSP). When BSP are recorded with only a limited number of electrodes, in general biophysical a priori models are applied to facilitate the inverse computation. This study investigated the possibility of deriving epicardial potential information using only 62 torso electrodes in the absence of an a priori model.

METHODS

Computer simulations were used to determine the optimal in vivo positioning of 62 torso electrodes. Subsequently, three different electrode configurations, i.e., surrounding the thorax, concentrated precordial (30 mm inter-electrode distance) and super-concentrated precordial (20 mm inter-electrode distance) were used to record BSP from three healthy volunteers. Magnetic resonance imaging (MRI) was performed to register the electrode positions with respect to the anatomy of the patient. Epicardial potentials were inversely computed from the recorded BSP. In order to determine the reconstruction quality, the super-concentrated electrode configuration was applied in four patients with an implanted MRI-conditional pacemaker system. The distance between the position of the ventricular lead tip on MRI and the inversely reconstructed pacing site was determined.

RESULTS

The epicardial potential distribution reconstructed using the super-concentrated electrode configuration demonstrated the highest correlation (R = 0.98; p < 0.01) with the original epicardial source model. A mean localization error of 5.3 mm was found in the pacemaker patients.

CONCLUSION

This study demonstrated the feasibility of deriving detailed anterior epicardial potential information using only 62 torso electrodes without the use of an a priori model.

MANIFOLD LEARNING FOR ANALYSIS OF LOW-ORDER NONLINEAR DYNAMICS IN HIGH-DIMENSIONAL ELECTROCARDIOGRAPHIC SIGNALS

The dynamical structure of electrical recordings from the heart or torso surface is a valuable source of information about cardiac physiological behavior. In this paper, we use an existing data-driven technique for manifold identification to reveal electrophysiologically significant changes in the underlying dynamical structure of these signals. Our results suggest that this analysis tool characterizes and differentiates important parameters of cardiac bioelectric activity through their dynamic behavior, suggesting the potential to serve as an effective dynamic constraint in the context of inverse solutions.

Changes in dipolar structure of cardiac magnetic field maps after ST elevation myocardial infarction

BACKGROUND

Pathological changes in cardiac electrophysiology have been investigated in coronary artery disease using magnetocardiography. Aim of this work was to examine the structure of cardiac magnetic field maps (MFM) during ventricular depolarization and repolarization in patients with acute ST elevation myocardial infarction (STEMI).

METHODS

Magnetocardiograms were recorded in 39 healthy subjects and 97 patients who had been successfully revascularized after STEMI. Using the Karhunen-Loeve transform, 12 eigenmaps were constructed for six intervals within the QT interval of each subject's signal-averaged data. The relative information content of the eigenmaps was compared between STEMI patients and healthy subjects.

RESULTS

Relative nondipolar content was between 0.03% and 0.52% higher in the STEMI group, (P < 0.001 for the repolarization intervals). Information content of the first dipolar eigenmap in the STEMI group was reduced by 2.6%-11.7% (P < 0.001 for the repolarization intervals). STT interval was best able to discriminate between groups: area-under-the-curve for nondipolar content was 85.8% (P < 0.001), for the first eigenmap 91.7% (P < 0.001). Severity of infarction was reflected in lower STT interval map 1 content for patients with anterior versus posterior infarction (83%± 11% vs. 87%± 10%, P < 0.05), with wall motion disturbances (84%± 11% vs. 92%± 7%, P < 0.001) and with microvascular obstruction (81%± 12% vs. 87%± 10%, P < 0.05). Regression analysis showed that patients with lower ejection fraction tended to have less information content (P < 0.001).

CONCLUSION

STEMI is associated with a loss of spatial coherence during repolarization, as quantified by principal component analysis of cardiac MFM.

Increased T wave complexity can indicate subclinical myocardial ischemia in asymptomatic adults

BACKGROUND

Altered ventricular repolarization and cardiovascular mortality are closely correlated, and recent novel findings show that a distorted T wave loop morphology is also strongly correlated with subsequent onset of myocardial infarction among patients with stable angina. Therefore, we hypothesized that an abnormal T wave complexity ratio (CR) can indicate vulnerability to myocardial ischemia in asymptomatic, apparently healthy adults.

METHODS

Healthy firefighters were enrolled in the current investigation where they completed symptom-limited, graded exercise treadmill testing (ETT) and 24-hour Holter electrocardiogram recording. The CR was automatically calculated using principal component analysis of the high-resolution Holter electrocardiogram signal then averaged over 24 hours (CR(24h)). End points were manually analyzed from the ETT; recordings revealing horizontal ST-segment depression (≥ 1 mm) in 2 or more leads for at least 1 minute during the peak of exercise were considered indicators of myocardial ischemia.

RESULTS

One hundred four firefighters (age, 44 ± 8 years; 96% men) completed both ETT and Holter recording. Firefighters with positive end points (n = 34, or 33%) had higher CR(24h) compared with those with negative end points (0.14 ± 0.06 vs 0.09 ± 0.04, P < .01); there were no demographic differences between the 2 groups. After controlling for age, smoking status, hypertension, and obesity, an abnormal CR(24h) (≥ 20%) significantly predicted exercise-induced myocardial ischemia (odds ratio, 4.6; P = .01).

CONCLUSIONS

Increased T wave CR(24h) can predict myocardial ischemia in asymptomatic middle-age adults; this suggests that the distorted T wave loop morphology can reflect an altered ventricular repolarization caused by prolonged subclinical myocardial ischemia possibly caused by early coronary artery disease.

Beat-to-beat interplay of heart rate, ventricular depolarization, and repolarization

To improve malignant arrhythmia risk stratification, the causal and random components of spatiotemporal dynamics of heart rate (RR distances), ventricular depolarization sequence, and repolarization disparity were studied based on body surface potential map records taken for 5 minutes, in resting, supine position on 14 healthy subjects (age range, 20-65 years) and on 6 arrhythmia patients (age range, 59-70 years). Beat-to-beat QRS and QRST integral maps, Karhunen-Loève (KL) coefficients, RR, and nondipolarity index time series were computed. Tight relationship was found between RR and QRS integrals in healthy subjects with less association in arrhythmia patients. Tight KL-domain multiple linear association (r(2) > 0.72) was found between the QRS and QRST integral dynamics (ie, depolarization sequence and repolarization disparity). Beat-to-beat probability of the generation of significant nondipolarity index spikes was proportional to the QRST KL-component standard deviations (SD(i)) and inversely proportional with the mean dipolar KL components (M(i)) of the average QRST integral map.

Applicability of body surface potential map in computerized optimization of biventricular pacing

Biventricular pacing (BVP) could be improved by identifying the patient-specific optimal electrode positions. Body surface potential map (BSPM) is a non-invasive technique for obtaining the electrophysiology and pathology of a patient. The study proposes the use of BSPM as input for an automated non-invasive strategy based on a personalized computer model of the heart, to identify the patient pathology and specific optimal treatment with BVP devices. The anatomy of a patient suffering from left bundle branch block and myocardial infarction is extracted from a series of MR data sets. The clinical measurements of BSPM are used to parameterize the computer model of the heart to represent the individual pathology. Cardiac electrophysiology is simulated with ten Tusscher cell model and excitation propagation is calculated with adaptive cellular automaton, at physiological and pathological conduction levels. The optimal electrode configurations are identified by evaluating the QRS error between healthy and pathology case with/without pacing. Afterwards, the simulated ECGs for optimal pacing are compared to the post-implantation clinically measured ECGs. Both simulation and clinical optimization methods identified the right ventricular (RV) apex and the LV posterolateral regions as being the optimal electrode configuration for the patient. The QRS duration is reduced both in measured and simulated ECG after implantation with 20 and 14%, respectively. The optimized electrode positions found by simulation are comparable to the ones used in hospital. The similarity in QRS duration reduction between measured and simulated ECG signals indicates the success of the method. The computer model presented in this work is a suitable tool to investigate individual pathologies. The personalized model could assist therapy planning of BVP in patients with congestive heart failure. The proposed method could be used as prototype for further clinically oriented investigations of computerized optimization of biventricular pacing.

Electrocardiographic body surface mapping: potential tool for the detection of transient myocardial ischemia in the 21st century?

Coronary artery disease (CAD) is one of the leading causes of cardiovascular mortality and morbidity worldwide. CAD presents as a wide spectrum of clinical disease from stable angina to ST segment elevation myocardial infarction. The 12-lead electrocardiogram (ECG) has been the main tool for the diagnosis of these events for almost a century but is limited in its diagnostic ability. For patients with suspected angina, the exercise tolerance test is often used to provoke and detect stress-induced ischemia but does not provide a definitive answer in a substantial proportion of patients. Body surface mapping (BSM) is a technique that samples multiple points around the thorax to provide a more comprehensive electrocardiographic data set than the conventional 12-lead ECG. Moreover, recent preliminary data demonstrate that BSM can detect and display transient regional myocardial ischemia in an intuitive fashion, employing subtraction color mapping, making it potentially valuable for diagnosing CAD causing transient regional ischemia. Research is ongoing to determine the full extent of its utility.

Eigenleads: ECG leads for maximizing information capture and improving SNR

There is currently much interest in exploring new ways to optimize ECG acquisition. In the current study, we have investigated optimal configurations of ECG leads with respect to: 1) best signal magnitude (maximal signal variance) and 2) best reconstruction of the total body surface potential distribution and the 12-lead ECG. Principal component analysis was applied to a set of 117-lead body surface potential maps (BSPMs) recorded from 559 subjects. Three bipolar leads, referred to as "eigenleads," were identified from the extrema on the resulting eigenvectors. Recording sites for the three leads were largely located in the precordial region. The magnitude of the signals recorded from the eigenleads was calculated on a set of 185 unseen subjects. The accuracy of the eigenleads in the reconstruction of BSPMs and the 12-lead ECG was also assessed for each subject. These results were compared to existing limited lead systems. It was found that, when compared to conventional leads, eigenleads could be used to increase signal strength (rms voltage) by 27.9%, 39.0%, and 20.3% for P-waves, QRS segments, and STT segments, respectively. Although the eigenleads were not able to reconstruct total body surface information as well as the 12-lead ECG (24.4 mu V versus 20.2 mu V), the eigenleads did perform comparably with other limited lead systems in the estimation of the 12-lead ECG. In particular, the eigenleads performed well in the reconstruction of precordial leads in comparison to the EASI lead system and a limited lead system made up of a subset of precordial leads. The proposed leads are a suitable alternative limited leads system, and can be used to improve SNR. More work is needed to test the practicality of such leads.

Complex T-wave morphology in body surface potential mapping in prediction of arrhythmic events in patients with acute myocardial infarction and cardiac dysfunction

AIMS

Heterogeneous ventricular repolarization is associated with sudden cardiac death after myocardial infarction (MI). This prospective study investigated repolarization disparity with parameters based on T-wave morphology in body surface potential mapping (BSPM) in the assessment of arrhythmia risk in patients with a recent MI and cardiac dysfunction.

METHODS AND RESULTS

Patients (n = 158) had 120-lead BSPM and 12-lead electrocardiogram (ECG) registered soon after acute MI. Principal component analysis (PCA) of the T-wave and T-wave vector loop descriptors were applied to compute parameters describing T-wave morphology and its variation. The study endpoints were arrhythmic events and all-cause mortality. During a mean follow-up of 50 months, 30 patients (19%) died and 16 (10%) had an arrhythmic event. Most of the parameters differed significantly between patients with and without arrhythmic events. In univariate analysis, T-wave vector loop length (TLL) and PCA parameter PCA(3) in BSPM and TLL in ECG were significant predictors of arrhythmic events. In multivariate analysis including several clinical variables, these parameters also showed an independent prediction, with parameters in BSPM performing somewhat better. None of the parameters predicted all-cause mortality.

CONCLUSION

Complex T-wave morphology in BSPM is a marker of arrhythmia propensity in patients with a recent MI and cardiac dysfunction.

T-wave morphology parameters based on principal component analysis reproducibility and dependence on T-offset position

BACKGROUND

T-wave morphology parameters based on principal component analysis (PCA) are candidate to better understand the relation between QT prolongation and torsades de pointes. We aimed to assess the repeatability and to determine the influence of T-end position on PCA parameters.

METHODS

Digital ECGs recorded from 30 subjects were used to assess short term (5 minutes), circadian and long-term (28 days) repeatability of PCA parameters. The T-end cursor position was moved backward and forward (+/- 8 ms) from its optimal position. We calculated QRS-T angle, PCA ratio, and T-wave residuum (TWR).

RESULTS

At long-term evaluation, coefficients of variation were 11.3 +/- 9.9%, 11.7 +/- 7.1%, and 23.0 +/- 22.0% for the QRS-T angle, PCA ratio, TWR, respectively. After moving the T-end cursor, repeatability was 0.42 +/- 0.2%, 1.00 +/- 1.04%, 4.0 +/- 4.2% for the same PCA parameters.

CONCLUSIONS

T-wave morphology parameters based on PCA are reproducible with the exception of TWR and QRS-T angle. In addition, PCA is robust, showing only little dependence on T-end cursor position. These data should be taken into account for safety pharmacology trials.

Karhunen-Loève representation distinguishes ST-T wave morphology differences in emergency department chest pain patients with non-ST-elevation myocardial infarction versus nonacute coronary syndrome

Patients presenting to the emergency department with chest pain are triaged to early reperfusion therapies based on their initial 12-lead electrocardiogram (ECG). The standard 12-lead ECG lacks sensitivity to detect acute myocardial infarction (AMI). Electrocardiographic diagnosis of non-ST-elevation myocardial infarction (non-STEMI) is especially difficult and is delayed until cardiac biomarkers turn positive, indicating onset of myocardial necrosis.

STUDY AIMS

The purpose of this analysis was to extract global ST-T waveform features from patients with chest pain, compare these features in patients with and without AMI, and then identify features that distinguish diagnostic categories.

METHODS

This is a secondary analysis of data from the Ischemia Monitoring and Mapping in the Emergency Department in Appropriate Triage and Evaluation of Acute Ischemic Myocardium study, a prospective clinical trial in which patients were attached to Holter monitor devices to obtain 24 hours of continuous ECG data. Digital recordings from 176 patients were analyzed: 88 with AMI (STEMI and non-STEMI) and 88 without AMI or unstable angina. The non-acute coronary syndrome (ACS) group was further subdivided into those with non-ACS cardiac conditions such as heart failure and those without cardiac disease who had noncardiac chest pain. For each patient, 10 consecutive waveforms were obtained within the first 120 minutes of emergency department presentation. The waveforms were time-aligned to the QRS, signal-averaged, baseline-adjusted. ST-T waveforms were complied according to diagnostic category and pooled for further analysis. Eigenvector-lead feature coefficients (Karhunen-Loève [K-L] coefficients) were obtained for each patient by taking the dot product of the ST-T wave (ST segment or entire waveform) and the first 3 common eigenvectors, producing 24 K-L coefficients. Cumulative probability distribution function curves were plotted for each diagnostic category. Statistical significance of category coefficient distribution differences was determined. Multinomial regression was used to assess accuracy of feature coefficients to predict diagnostic category.

RESULTS

Non-STEMI and non-ACS cardiac category K-L coefficient curves were statistically different in 11 of 24 feature curves (P < .001-.047). ST-segment (50 samples) coefficients predicted non-ACS cardiac patients 11.5% more often (P = .02) than those derived from the entire ST-T wave.

CONCLUSION

Patients diagnosed with non-STEMI have distinct distribution of K-L coefficients compared with non-ACS cardiac patients. Coefficients from the first 50 samples of the ST-T wave (ST segment) better predict diagnostic category than do coefficients derived from the entire ST-T wave. Karhunen-Loève coefficient feature analysis may provide early diagnostic information to distinguish patients with non-STEMI vs non-ACS cardiac patients.

Predicting the QRS complex and detecting small changes using principal component analysis

In this paper, a new method for QRS complex analysis and estimation based on principal component analysis (PCA) and polynomial fitting techniques is presented. Multi-channel ECG signals were recorded and QRS complexes were obtained from every channel and aligned perfectly in matrices. For every channel, the covariance matrix was calculated from the QRS complex data matrix of many heartbeats. Then the corresponding eigenvectors and eigenvalues were calculated and reconstruction parameter vectors were computed by expansion of every beat in terms of the principal eigenvectors. These parameter vectors show short-term fluctuations that have to be discriminated from abrupt changes or long-term trends that might indicate diseases. For this purpose, first-order poly-fit methods were applied to the elements of the reconstruction parameter vectors. In healthy volunteers, subsequent QRS complexes were estimated by calculating the corresponding reconstruction parameter vectors derived from these functions. The similarity, absolute error and RMS error between the original and predicted QRS complexes were measured. Based on this work, thresholds can be defined for changes in the parameter vectors that indicate diseases.

Body surface ECG signal shape dispersion

The spatial distribution of the shape of the electrocardiography (ECG) waves obtained by body surface potential mapping (BSPM) is studied, using a 64-channel high-resolution ECG system. The index associated to each lead is the shape difference between its ECG wave and a reference computed taking into account all the leads on the same column. The reference is either a selected real wave or a synthetic signal computed by integral shape averaging (ISA). Better results are obtained with the ISA signal using the distribution function method (DFM) for computing the shape difference. The spatial dispersion of ECG waves is showed to allow the separation of patients after myocardial infarction (MI) from healthy subjects. In addition, the reference signal position for each column is computed. The path linking these positions appears as an invariant, i.e., it is independent of the subject and the ECG wave.

Diagnosing Old MI by Searching for a Linear Boundary in the Space of Principal Components

Body surface potential mapping (BSPM) is a technique employing multiple electrodes to capture, via noninvasive means, an indication of the heart's condition. An inherent problem with this technique is the resulting high-dimensional recordings and the subsequent problems for diagnostic classifiers. A data set, recorded from a 192-lead BSPM system, containing 74 records is investigated. QRS isointegral maps, offering a summary of the information obtained during ventricular depolarization, were derived from 30 old inferior myocardial infarction and 44 normal recordings. Principal component analysis was applied to reduce the dimensionality of the recordings and a linear classifier was employed for classification. This perceptron-based classifier has been adapted so that the final weight and bias values are estimated prior to the learning process. This estimation process, referred to as the linear hyperplane approach (LHA), derives the estimated weights from a bisector hyperplane, placed orthogonal to the means of two class distributions in an n-dimensional Euclidean space. Estimating weights encourages a network to exhibit better generalization ability. Utilizing a number of different principal components as input features, the LHA achieved an average sensitivity and specificity of 79.58% and 76.45%, respectively, across all experiments. The average accuracy of 76.73% achieved with this approach was significantly better than the other benchmark classifiers evaluated against it.

QT peak dispersion, not QT dispersion, is a more useful diagnostic marker for detecting exercise-induced myocardial ischemia

BACKGROUND

The electrocardiographic indices of QT dispersion (QTd), QT peak dispersion (QTpd), and the principal component analysis ratio (PCAr) are related to the occurrence of fatal arrhythmia and are influenced by physical exercise.

OBJECTIVE

The purpose of this study was to investigate whether or not the QT parameters can be used as markers for exercise-induced myocardial ischemia.

METHODS

We measured these QT parameters at rest and at 3 minutes after exercise using exercise-stress thallium-201 scintigraphy (SPECT), compared with conventional ST segment changes in 161 patients with suspected or known coronary artery disease. The patients were classified into four groups (normal, redistribution, fixed defect, and redistribution with fixed defect) according to SPECT.

RESULTS

At rest, QTd and PCAr were greater in the fixed defect and redistribution with fixed defect groups. PCAr, however, increased after exercise in the redistribution and redistribution with fixed defect groups. Although QTpd at rest was not significantly different among the four groups, it increased in the redistribution and redistribution with fixed defect groups after exercise (QTpd after exercise: normal, 36 +/- 16 ms vs. redistribution, 51 +/- 23 ms, redistribution with fixed defect, 53 +/- 19 ms; P<.05). For myocardial infarction reflected by fixed defect, QTd at rest was the most useful indicator, while QTpd after exercise was the most useful indicator for exercise-induced myocardial ischemia according to multiple logistic regression analysis with receiver operating characteristic curves. In addition, the change in PCAr by exercise was an independent predictor for exercise-induced ischemia.

CONCLUSIONS

QTpd and PCAr could be useful indices for exercise-induced myocardial ischemia. Determining the QTpd of a patient after exercising can improve the diagnostic accuracy of ischemia in a routine clinical setting.

Selection of optimal recording sites for limited lead body surface potential mapping: a sequential selection based approach

Background

In this study we propose the development of a new algorithm for selecting optimal recording sites for limited lead body surface potential mapping. The proposed algorithm differs from previously reported methods in that it is based upon a simple and intuitive data driven technique that does not make any presumptions about deterministic characteristics of the data. It uses a forward selection based search technique to find the best combination of electrocardiographic leads.

Methods

The study was conducted using a dataset consisting of body surface potential maps (BSPM) recorded from 116 subjects which included 59 normals and 57 subjects exhibiting evidence of old Myocardial Infarction (MI). The performance of the algorithm was evaluated using spatial RMS voltage error and correlation coefficient to compare original and reconstructed map frames.

Results

In all, three configurations of the algorithm were evaluated and it was concluded that there was little difference in the performance of the various configurations. In addition to observing the performance of the selection algorithm, several lead subsets of 32 electrodes as chosen by the various configurations of the algorithm were evaluated. The rationale for choosing this number of recording sites was to allow comparison with a previous study that used a different algorithm, where 32 leads were deemed to provide an acceptable level of reconstruction performance.

Conclusion

It was observed that although the lead configurations suggested in this study were not identical to that suggested in the previous work, the systems did bear similar characteristics in that recording sites were chosen with greatest density in the precordial region.

Drug-induced QT dispersion: does it predict the risk of torsade de pointes?

Drug-induced delay in ventricular repolarization and proarrhythmias have attracted considerable regulatory attention. The measure of delayed ventricular repolarization most frequently used clinically is the ability of the new chemical entity (NCE) to prolong the QTc interval on surface electrocardiogram. Before they can be approved, new chemical entities with systemic bioavailability require characterization for their potential to prolong the QTc interval. Inevitably, QTc interval prolongation has come to be recognized as a surrogate marker of the risk of torsade de pointes (TdP)--a unique form of potentially fatal polymorphic ventricular tachycardia. Although it is the best and the simplest clinical measure that is available at present, QTc interval is not a reliable surrogate of TdP. Intramyocardial dispersion of repolarization appears to play a more important role both in electrical stability of the ventricles and in arrhythmogenesis. The potential importance of myocardial dispersion of refractoriness in arrhythmogenesis has led to a number of attempts to assess it from the surface electrocardiogram. This review summarizes the evidence for and against the predictive value of one of these attempts-measurement of the so-called QT dispersion. Although the concept of QT dispersion is the best known and most widely investigated, it has also proved to be the least successful in predicting the risks of drug-induced TdP.

Sudden Intense Exercise Increases QT Heart Rate Slope and T Wave Complexity in Long QT Syndrome and Normal Subjects

Sudden intense physical activity is a trigger for ventricular arrhythmias in long QT syndrome (LQTS), and beta-blockers (B) reduce the risk of ventricular arrhythmias in LQTS. We compared the effect of graded (gradual intensity) versus burst (sudden intensity) exercise on QT-rate adaptation and T wave complexity in LQTS + B (n = 21), LQTS - B (n = 5), and normal controls (n = 20). Graded exercise consisted of symptom-limited bicycle ergometry (30 W, increment 20 W/min). Burst exercise involved a fixed 200 W load for 1.0 minute. ECGs were digitally recorded every 10 seconds during exercise and a 10 minute recovery period. QT-rate adaptation was quantified using the slope of the QT cycle length relationship fit to a quadratic function. Principle component analysis (PCA) was used to quantify T wave complexity. The QT-rate slope was two-fold greater with burst exercise than graded exercise for LQTS + B (-82E +/- 40E vs -36E +/- 40E, P = 0.0016), LQTS - B (-85E +/- 60E vs -30E +/- 50E, P = 0.011) and controls (-100E +/- 60E vs -48E +/- 100E, P = 0.0011) (E = x10(-5)). For each exercise protocol, there was no difference in QT-rate slope between the three groups. In contrast, the QT-rate slope during the 10 minute recovery period was similar between the burst and graded protocol for LQTS + B (25E +/- 40E vs 30E +/- 50E), LQTS - B (81E +/- 80E vs 85E +/- 70E) and controls (90E +/- 80E vs 82E +/- 80E). The coefficient of variability of PCA (T wave complexity variability) during burst exercise was greater than that during graded exercise for LQTS + B (41 +/- 15 vs 30 +/- 10, P = 0.017), LQTS - B (47 +/- 25 vs 26 +/- 4, P = 0.016), and control (46 +/- 14 vs 33 +/- 19, P = 0.012). For each exercise protocol, no difference in T wave complexity variability was seen between the three groups. In conclusion, QT heart rate slope and T wave complexity variability are greater during sudden intense exercise than graded exercise in LQTS patients (on and off beta-blockers) and normal subjects, with similar findings among the three groups of patients.

Age, gender, and autonomic tone effects on surface electrocardiographic indices of ventricular repolarization

BACKGROUND

Prolonged QT offset dispersion (QToD), an index of heterogeneity of ventricular repolarization, is thought to be an independent predictor of all-cause and cardiovascular mortality. However the influence of gender and autonomic tone in healthy adults on age-related changes in measures of ventricular repolarization are not well characterized.

METHODS

QToD and T wave complexity were measured in 56 healthy subjects with no detectable heart disease (by echo and stress test)-38 young subjects with a mean age of 28 +/- 4 years and 18 old subjects with a mean age of 71 +/- 7 years. QToD and T wave complexity were computed from 12-lead ECGs using the GE Marquette QT Guard automated analysis program with manual overreading at rest (baseline), following exercise, and double autonomic blockade with atropine and propranolol. Data was analyzed using factorial ANOVA.

RESULTS

Young males had a significantly greater QToD than young and old females at baseline (28 +/- 5 ms, 23 +/- 5 ms, and 22 +/- 5 ms, respectively, P < 0.01), an intrinsic effect independent of changes in autonomic tone. In contrast, females had significantly greater T wave complexity than males following exercise and double autonomic blockade with a definite trend at baseline. There was no correlation between T wave complexity and QToD.

CONCLUSIONS

Age and gender demonstrate a complex interaction on indices of myocardial repolarization with different measures behaving differently. These findings have implications for better understanding age and gender effects on myocardial electrophysiology.

Uncertainty of the electrocardiogram: old and new ideas for assessment and interpretation

The electrocardiogram (ECG) is a highly complex, dynamic and stochastic phenomenon. Although it provides a valuable, noninvasive and rapid means of assessing cardiac state and its change, uncertainties in its measurement and variation in the underlying electrophysiology that generates the ECG make difficult further improvement in its reliability for detecting and monitoring cardiac pathologies and conditions. This article reviews the sources of variability and uncertainty in ECG measurement and interpretation, revisits some old ideas for dealing with them, and proposes some novel directions for improving accuracy of ECG assessment and interpretation. We shall explore relative information content of lead systems, representation of ECG signals and patterns, and estimation of ECG distributions from limited lead systems. In addition, we will compare strategies for measuring ECG information and suggest new paradigms for feature extraction that reduce the sensitivity of assessment accuracy to intrinsic and extrinsic measurement errors. Finally, we review the importance of including dynamic information in ECG assessment, both for interpreting current cardiac state as well as for monitoring its change and significance.

Activation-recovery intervals of 12-lead electrocardiograms before and after catheter ablation in patients with Wolff-Parkinson-White syndrome

Preexcitation in Wolff-Parkinson-White syndrome (WPW) has been reported to induce long-lasting changes in ventricular recovery properties. However, there has not been a report concerning changes in the activation-recovery interval (ARI) in 12-lead ECGs before and after catheter ablation (CA) in patients with WPW syndrome. The present study compared changes in ARIs from 12-lead ECGs with those from body surface unipolar leads before and after CA to examine whether ARIs from limb leads of 12-lead ECGs provide useful information on changes in recovery properties in addition to the ARIs from precordial leads. The study population consisted of 27 manifest WPW patients with a left- (n=18, group A) or right-sided accessory pathway (n=9, group B). ARIs in leads I, II, and III were strongly correlated with those in unipolar leads over the left lateral, left lower, and right lower chest, respectively. ARIs in leads aVR, aVL, and aVF showed a significant correlation with those in unipolar leads over the right upper, left upper, and lower anterior chest, respectively. These correlations were maintained before and after CA. Furthermore, in group A, ARIs in lead V1 tended to increase on day 7 post CA compared with before CA and on day 1. In group B, ARIs in lead III significantly decreased on day 7 compared with before CA and on day 1. These findings suggest that ARIs from the limb leads of 12-lead ECGs may represent those from unipolar leads of a particular area over the body surface, and that ARIs from 12-lead ECGs may provide useful quantitative information on changes in recovery properties before and after CA in patients with manifest WPW syndrome.

Continuous localization of cardiac activation sites using a database of multichannel ECG recordings

Monomorphic ventricular tachycardia and ventricular extrasystoles have a specific exit site that can be localized using the multichannel surface electrocardiogram (ECG) and a database of paced ECG recordings. An algorithm is presented that improves on previous methods by providing a continuous estimate of the coordinates of the exit site instead of selecting one out of 25 predetermined segments. The accuracy improvement is greatest, and most useful, when adjacent pacing sites in individual patients are localized relative to each other. Important advantages of the new method are the objectivity and reproducibility of the localization results.

Significance of QT dispersion in the long QT syndrome

The long QT syndrome (LQTS) has often been considered as a model to study the abnormalities of cardiac repolarization in humans because it represents a pure electrical disease with no evidence of cardiac structural abnormalities. The arrhythmogenic potential of prolonged ventricular repolarization has been extensively studied both in experimental models and at the clinical level in LQTS patients, and many studies pointed to the pathogenetic role of the dispersion of ventricular recovery times (i.e., dispersion of ventricular repolarization). In the last few years, a new critical knowledge has been achieved thanks to the molecular biology techniques that are unveiling the genetic bases of LQTS. Indeed, the understanding of the genes and mutations that may cause the LQTS opened the way to understanding the molecular determinants of the altered ventricular repolarization that can be found in LQTS patients. From the clinical standpoint, the traditional tools applied for the detection and quantification of the dispersion of ventricular repolarization (monophasic action potential, QT dispersion) showed their effectiveness but also their limitations. More recently, the availability of new algorithms and the development of powerful computerized supports allowed the evaluation of innovative techniques, which now represent possible attractive alternatives intended to quantify the degree of repolarization abnormalities in LQTS patients and possibly to noninvasively quantify the risk of cardiac events.

Relation between activation sequence fluctuation and arrhythmogenicity in sodium-channel blockades

To examine the correlation between activation sequence fluctuation and arrhythmogenicity, we investigated temporal changes in the activation sequence by measuring activation times [negative first derivative of voltage over time (-dV/dt) in QRS] from the entire heart in 18 dogs. The heart was paced by constant atrial stimulation. The character of the activation sequence fluctuation was established by a principal component analysis, in which the first principal component was defined as a stable component of the sequence and the second or third component as a fluctuated component. Steady state contained 2.2 +/- 0.6% (percent total principal component, mean +/- SD) of fluctuated components, which appeared in a beat-by-beat manner (activation sequence alternans). Activation sequence alternans was observed only during flecainide administration and not during lidocaine or disopyramide administration. Fluctuated components at a high dose of flecainide significantly increased (3.3 +/- 0.8%). Ventricular fibrillation ensued in all dogs (n = 6) exposed to flecainide after an increase in activation sequence alternans. In conclusion, flecainide evoked local activation sequence alternans. This phenomenon correlated with the occurrence of ventricular fibrillation.

Value of magnetocardiographic QRST integral maps in the identification of patients at risk of ventricular arrhythmias

It has been shown that regional ventricular repolarization properties can be reflected in body surface distributions of electrocardiographic QRST deflection areas (integrals). We hypothesize that these properties can be reflected also in the magnetocardiographic QRST areas and that this may be useful for predicting vulnerability to ventricular tachyarrhythmias. Magnetic field maps were obtained during sinus rhythm from 49 leads above the anterior chest in 22 healthy (asymptomatic) control subjects (group A) and in 29 patients with ventricular arrhythmias (group B). In each subject, the QRST deflection area was calculated for each lead and displayed as an integral map. The mean value of maximum was significantly larger in the control group A than in the patient group B (1,626+/-694 pTms vs. 582+/-547 pTms, P<0.0001). To quantitatively assess intragroup variability in the control group A and intergroup variability of the control and patient groups, we used the correlation coefficient r and covariance sigma. These indices showed significantly less intragroup than intergroup variation (e.g., in terms of sigma, 28.0x10(-6)+/-12.3x10(-6) vs. 3.4x10(-6)+/-12.5x10(-6), P<0.0001). Each QRST integral map was also represented as a weighted sum of 24 basis functions (eigenvectors) by means of Karhunen-Loeve transformation to calculate the contribution of the nondipolar eigenvectors (all eigenvectors beyond the third). This percentage nondipolar content of magnetocardiographic QRST integral maps was significantly higher in the patient group B than in the control group A (13.0%+/-9.1 % vs. 2.6%+/-2.0%, P<0.0001). Discriminations between control subjects and patients with ventricular arrhythmias based on magnitude of the maximum, covariance sigma, and nondipolar content were 90.2%, 90.2%, and 86.3% accurate, with a sensitivity of 89.7%, 93.1%, and 75.9%, and a specificity of 90.9%, 86.4%, and 100%. We have shown that magnitude of the maximum and indices of variability and nondipolarity of the magnetocardiographic QRST integral maps may predict arrhythmia vulnerability. This finding is in agreement with earlier studies that used body surface potential mapping and suggests that magneticfield mapping may also be a useful diagnostic tool for risk analysis.

Spatial, temporal and wavefront direction characteristics of 12-lead T-wave morphology

Three new approaches for the analysis of ventricular repolarisation in 12-lead electrocardiograms (ECGs) are presented: the spatial and temporal variations in T-wave morphology and the wavefront direction difference between the ventricular depolarisation and repolarisation waves. The spatial variation characterises the morphology differences between standard leads. The temporal variation measures the change in interlead relationships. A minimum dimensional space, constructed by ECG singular value decomposition, is used. All descriptors are measured using the ECG vector in the constructed space and the singular vectors that define this space. None of the descriptors requires time domain measurements (e.g. the precise detection of the T-wave offset), and so the inaccuracies associated with conventional QT interval related parameters are avoided. The new descriptors are compared with the conventional measurements provided by a commercial system for an automatic evaluation of QT interval and QT dispersion in digitally recorded 12-lead ECGs. The basic comparison uses a set of 1100 normal ECGs. The short-term intrasubject reproducibility of the new descriptors is compared with that of the conventional measurements in a set of 760 ECGs recorded in 76 normal subjects and a set of 630 ECGs recorded in 63 patients with hypertrophic cardiomyopathy (ten serial recordings in each subject of both these sets). The discriminative power of the new and conventional parameters to distinguish normal and abnormal repolarisation patterns is compared using the same set. The results show that the new parameters do not correlate with the conventional QT interval-related descriptors (i.e. they assess different ECG qualities), are generally more reproducible than the conventional parameters, and lead to a more significant separation between normal and abnormal ECGs, both univariately and in multivariate regression models.

Spatial features in body surface potential maps of patients with ventricular tachyarrhythmias with or without coronary artery disease

Body surface potential maps (BSPM) from patients with coronary artery disease or no structural heart disease were analyzed with respect to their spatial features and QT/QTc dispersion in order to determine whether BSPM allows identification of patients with ventricular fibrillation. QRST integral maps and QT/QTc dispersion were acquired from simultaneous recordings of 62 ECG leads during sinus rhythm in patients with idiopathic ventricular fibrillation (n=13), ventricular fibrillation and coronary artery disease (n=22), coronary artery disease without ventricular fibrillation (n=21) and healthy controls (n=18). The Karhunen-Loeve transformation was applied to reduce the dimensionality of the data matrix of the QRST map to eight coefficients. Linear discriminant analysis allowed discrimination between idiopathic ventricular fibrillation patients and controls with high sensitivity (85%) and specificity (89%). However, discrimination between coronary artery disease patients with or without ventricular fibrillation was poor (68% and 67%, respectively). QTc dispersion calculated from BSPM was longer in idiopathic ventricular fibrillation patients than in controls (99+/-30 ms vs 70+/-14 ms, P=0.009) in contrast to QTc dispersion taken from 12-lead ECG (53+/-21 ms vs. 47+/-12 ms, P=n.s.). No significant difference was noted for coronary artery disease patients with or without ventricular fibrillation. In conclusion, repolarization disturbances detected by BSPM allow identification of ventricular fibrillation patients without structural heart disease. However, our results do not suggest a major impact of QT/QTc dispersion or QRST integral mapping for identification of ventricular fibrillation patients with coronary artery disease.

SVD-based on-line exercise ECG signal orthogonalization

An orthogonalization method to eliminate unwanted signal components in standard 12-lead exercise electrocardiograms (ECG's) is presented in this work. A singular-value-decomposition-based algorithm is proposed to decompose the signal into two time-orthogonal subspaces; one containing the ECG and the other containing artifacts like baseline wander and electromyogram. The method makes use of redundancy in 12-lead ECG. The same method is also tested for reconstruction of a completely lost channel. The online implementation of the method is given. It is observed that the first two decomposed channels with highest energy are sufficient to reconstruct the ST-segment and J-point. The dimension of the signal space, on the other hand, does not exceed three. Data from 23 patients, with duration ranging from 9 to 21 min, are used.

Comparative study of local and Karhunen-Loève-based ST-T indexes in recordings from human subjects with induced myocardial ischemia

In this work we studied ST-T complex changes in the ECG as result of induced ischemia. The principal aim was to determine whether global changes in the ST-T complex were more sensitive markers of ischemic alterations than those based on measurements of changes at specific locations on ST segment or T wave. High-resolution ECGs from patients undergoing percutaneous transluminal coronary angioplasty in one of the major coronary arteries were analyzed to give a description of the period from the end of active depolarization (QRS complex) to the end of active repolarization (T wave). During artery occlusion traditional local measurements of the ST-T complex were compared to global measurements based on the Karhunen-Loève transform. An ischemic change sensor parameter was estimated for each of the studied indexes showing that global measurements detected changes better in the repolarization period in a larger number of leads and with higher sensitivity (more than 85%) than was done using local measurements (sensitivity of 64% with ST level, 33% with T-wave maximum position, and 37% with T-wave maximum amplitude). Using these global indexes it was found that most cases of ST-segment changes were accompanied by T-wave changes (72% of patients). With the use of traditional indexes 23% of patients showed no changes in the repolarization period, whereas with global indexes this percentage decreased to 8%. Thus a global representation of the entire ST-T complex appears to be more suitable than local measurements when studying the initial stages of myocardial ischemia.

Useful lessons from body surface mapping

Useful Lessons from Body Surface Mapping. Body surface potential maps (BSMs) depict the time varying distribution of cardiac potentials on the entire surface of the torso. Hundreds of studies have shown that BSMs contain more diagnostic and prognostic information than can be elicited from the 12-lead ECG. Despite these advantages, body surface mapping has not become a routinely used clinical method. One reason is that visual examination and sophisticated analysis of BSMs do not permit inferring the sequence of excitation and repolarization in the heart with a sufficient degree of certainty and detail. These limitations can be partially overcome by implementing inverse procedures that reconstruct epicardial potentials, isochrones, and ECGs from body surface measurements. Furthermore, ongoing experimental work and simulation studies show that a great deal of information about intramural events can be elicited from measured or reconstructed epicardial potential distributions. Interpreting epicardial data in terms of deep activity requires extensive knowledge of the architecture of myocardial fibers, their anisotropic properties, and the role of rotational anisotropy in affecting propagation and the associated potential fields.

Electrocardiographic identification of abnormal ventricular depolarization and repolarization in patients with idiopathic ventricular fibrillation

OBJECTIVES

We sought to gain more insight into the arrhythmogenic etiology of idiopathic ventricular fibrillation (VF) by assessing ventricular depolarization and repolarization properties by means of various electrocardiographic (ECG) techniques.

BACKGROUND

Idiopathic VF occurs in the absence of demonstrable structural heart disease. Abnormalities in ventricular depolarization or repolarization have been related to increased vulnerability to VF in various cardiac disorders and are possibly also present in patients with idiopathic VF.

METHODS

In 17 patients with a first episode of idiopathic VF, 62-lead body surface QRST integral maps, QT dispersion on the 12-lead ECG and XYZ-lead signal-averaged ECGs were computed.

RESULTS

All subjects of a healthy control group had a normal dipolar QRST integral map. In patients with idiopathic VF, either a normal dipolar map (29%,), a dipolar map with an abnormally large negative area on the right side of the thorax (24%) or a nondipolar map (47%) were recorded. Only four patients (24%) had increased QT dispersion on the 12-lead ECG and late potentials could be recorded in 6 (38%) of 16 patients. During a median follow-up duration of 56 months (range 9 to 136), a recurrent arrhythmic event occurred in 7 patients (41%), all of whom had an abnormal QRST integral map. Five of these patients had late potentials, and three showed increased QT dispersion on the 12-lead ECG.

CONCLUSIONS

In patients with idiopathic VF, ventricular areas of slow conduction, regionally delayed repolarization or dispersion in repolarization can be identified. Therefore, various electrophysiologic conditions, alone or in combination, may be responsible for the occurrence of idiopathic VF. Body surface QRST integral mapping may be a promising method to identify those patients who do not show a recurrent episode of VF.

Lead system transformation of body surface map data

Multicenter application of body surface map data (multilead electrocardiographic [ECG] data) is hampered by the fact that the centers involved in body surface mapping use lead systems differing in lead placement as well as in the number of leads. In this study, the performance of two methods for converting multilead ECGs from one lead system to another is evaluated in their application to the major lead systems presently in use throughout the world. The first method is based on Laplacian interpolation, and the second method is derived from the correlations between the signals in an extensive lead system. Through analyzing the representation errors, it was found that, for lead systems incorporating over 60 leads, both methods work well, yielding errors comparable to interbeat differences in individuals. For lead systems incorporating fewer leads, the correlation method is to be preferred.

Spatial organization, predictability, and determinism in ventricular fibrillation

The degree of spatial organization of ventricular fibrillation (VF) is a fundamental dynamical property of the arrhythmia and may determine the success of proposed therapeutic approaches. Spatial organization is closely related to the dimension of VF, and hence to its predictability and controllability. We have explored several techniques to quantify spatial organization during VF, to predict patterns of activity, and to see how spatial organization and predictability change as the arrhythmia progresses. Epicardial electrograms recorded from pig hearts using rectangular arrays of unipolar extracellular electrodes (1 mm spacing) were analyzed. The correlation length of VF, the number of Karhunen-Loeve modes required to approximate data during VF, the number, size and recurrence of wavefronts, and the mean square error of epicardial potential fields predicted 0.256 seconds into the future were all estimated. The ability of regularly-timed pacing stimuli to capture areas of fibrillating myocardium during VF was confirmed by a significant increase in local spatial organization. Results indicate that VF is neither "low-dimensional chaos" (dimension <5) nor "random" behavior (dimension= infinity ), but is a high-dimensional response with a degree of spatial coherence that changes as the arrhythmia progresses. (c) 1998 American Institute of Physics.