The electrophysiological cardiac ventricular substrate in patients after myocardial infarction: noninvasive characterization with electrocardiographic imaging

Electrical and Structural Substrate of Arrhythmogenic Right Ventricular Cardiomyopathy Determined Using Noninvasive Electrocardiographic Imaging and Late Gadolinium Magnetic Resonance Imaging

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a significant cause of sudden cardiac death in the young. Improved noninvasive assessment of ARVC and better understanding of the disease substrate are important for improving patient outcomes.

METHODS AND RESULTS

We studied 20 genotyped ARVC patients with a broad spectrum of disease using electrocardiographic imaging (a method for noninvasive cardiac electrophysiology mapping) and advanced late gadolinium enhancement cardiac magnetic resonance scar imaging. Compared with 20 healthy controls, ARVC patients had longer ventricular activation duration (median, 52 versus 42 ms; P=0.007) and prolonged mean epicardial activation-recovery intervals (a surrogate for local action potential duration; median, 275 versus 241 ms; P=0.014). In these patients, we observed abnormal and varied epicardial activation breakthrough locations and regions of nonuniform conduction and fractionated electrograms. Nonuniform conduction and fractionated electrograms were present in the early concealed phase of ARVC. Electrophysiological abnormalities colocalized with late gadolinium enhancement scar, indicating a relationship with structural disease. Premature ventricular contractions were common in ARVC patients with variable initiation sites in both ventricles. Premature ventricular contraction rate increased with exercise, and within anatomic segments, it correlated with prolonged repolarization, electric markers of scar, and late gadolinium enhancement (all P<0.001).

CONCLUSIONS

Electrocardiographic imaging reveals electrophysiological substrate properties that differ in ARVC patients compared with healthy controls. A novel mechanistic finding is the presence of repolarization abnormalities in regions where ventricular ectopy originates. The results suggest a potential role for electrocardiographic imaging and late gadolinium enhancement in early diagnosis and noninvasive follow-up of ARVC patients.

Development of Time- and Voltage-Domain Mapping (V-T-Mapping) to Localize Ventricular Tachycardia Channels During Sinus Rhythm

BACKGROUND

In ventricular scar, impulse spread is slow because it traverses split and zigzag channels of surviving muscle. We aimed to evaluate scar electrograms to determine their local delay (activation time) and inequality in voltage splitting (entropy), and their relationship to channels. We reasoned that unlike innocuous channels, which are often short with multiple side branches, ventricular tachycardia (VT) supporting channels have very slow impulse spread and possess low entropy because of their longer protected length and relative lack of side-branching.

METHODS AND RESULTS

Patients with ischemic cardiomyopathy and multiple VT were studied. In initial mapping stage (16 patients and 58 VTs), left ventricular endocardial mapping was performed in sinus rhythm. Detailed pace mapping was used to identify VT channels and confirmed, when feasible, by entrainment. Scar electrograms were analyzed in time and voltage domains to determine mean activation time, dispersion in activation time, and entropy. Predictive performances of these properties to detect VT channels were tested. In the application stage (7 patients and 20 VTs), these properties were prospectively tested to guide catheter ablation. A mean number of 763±203 sampling points were taken. From 1770 pace maps, 47 channels corresponded to VTs. A combination of scar electrograms with the latest mean activation time and minimum entropy, in a high activation dispersion region, accurately recognized regions containing VT channels (κ=0.89, sensitivity=86%, specificity=100%, positive predictive value=93%, and negative predictive value=100%). Finally, focused ablation within 5-mm rim of the prospective channel regions eliminated 18 of 20 inducible VTs.

CONCLUSIONS

Activation time and entropy mapping in the scar accurately identify VT channels during sinus rhythm. The method integrates principles of reentry formation to recognize VT channels without pace mapping or mapping during VT.

Noninvasive ECG imaging (ECGI): Mapping the arrhythmic substrate of the human heart

This short communication accompanies my presentation at the International Congress on Sudden Cardiac Death held in Prague, March 30-April 1, 2017. It summarizes briefly studies of the cardiac electrophysiological substrate in patients with hereditary arrhythmogenic syndromes - the Long QT and Brugada syndromes - conducted noninvasively, in situ, using Electrocardiographic Imaging (ECGI). The same noninvasive approach was used to map the electrophysiological substrate of a post-infarction myocardial scar and to relate this substrate to the pattern of activation during reentrant ventricular tachycardia. My thoughts about a potential role for ECGI in cardiac research and clinical care are also expressed briefly, with examples from on-going work in my laboratory.

Limitations and Challenges in Mapping Ventricular Tachycardia: New Technologies and Future Directions

Recurrent episodes of ventricular tachycardia in patients with structural heart disease are associated with increased mortality and morbidity, despite the life-saving benefits of implantable cardiac defibrillators. Reducing implantable cardiac defibrillator therapies is important, as recurrent shocks can cause increased myocardial damage and stunning, despite the conversion of ventricular tachycardia/ventricular fibrillation. Catheter ablation has emerged as a potential therapeutic option either for primary or secondary prevention of these arrhythmias, particularly in post-myocardial infarction cases where the substrate is well defined. However, the outcomes of catheter ablation of ventricular tachycardia in structural heart disease remain unsatisfactory in comparison with other electrophysiological procedures. The disappointing efficacy of ventricular tachycardia ablation in structural heart disease is multifactorial. In this review, we discuss the issues surrounding this and examine the limitations of current mapping approaches, as well as newer technologies that might help address them.

Regularization Techniques for ECG Imaging during Atrial Fibrillation: A Computational Study

The inverse problem of electrocardiography is usually analyzed during stationary rhythms. However, the performance of the regularization methods under fibrillatory conditions has not been fully studied. In this work, we assessed different regularization techniques during atrial fibrillation (AF) for estimating four target parameters, namely, epicardial potentials, dominant frequency (DF), phase maps, and singularity point (SP) location. We use a realistic mathematical model of atria and torso anatomy with three different electrical activity patterns (i.e., sinus rhythm, simple AF, and complex AF). Body surface potentials (BSP) were simulated using Boundary Element Method and corrupted with white Gaussian noise of different powers. Noisy BSPs were used to obtain the epicardial potentials on the atrial surface, using 14 different regularization techniques. DF, phase maps, and SP location were computed from estimated epicardial potentials. Inverse solutions were evaluated using a set of performance metrics adapted to each clinical target. For the case of SP location, an assessment methodology based on the spatial mass function of the SP location, and four spatial error metrics was proposed. The role of the regularization parameter for Tikhonov-based methods, and the effect of noise level and imperfections in the knowledge of the transfer matrix were also addressed. Results showed that the Bayes maximum-a-posteriori method clearly outperforms the rest of the techniques but requires a priori information about the epicardial potentials. Among the purely non-invasive techniques, Tikhonov-based methods performed as well as more complex techniques in realistic fibrillatory conditions, with a slight gain between 0.02 and 0.2 in terms of the correlation coefficient. Also, the use of a constant regularization parameter may be advisable since the performance was similar to that obtained with a variable parameter (indeed there was no difference for the zero-order Tikhonov method in complex fibrillatory conditions). Regarding the different targets, DF and SP location estimation were more robust with respect to pattern complexity and noise, and most algorithms provided a reasonable estimation of these parameters, even when the epicardial potentials estimation was inaccurate. Finally, the proposed evaluation procedure and metrics represent a suitable framework for techniques benchmarking and provide useful insights for the clinical practice.

Insights from Novel Noninvasive CT and ECG Imaging Modalities on Electromechanical Myocardial Activation in a Canine Model of Ischemic Dyssynchronous Heart Failure

INTRODUCTION

The interplay between electrical activation and mechanical contraction patterns is hypothesized to be central to reduced effectiveness of cardiac resynchronization therapy (CRT). Furthermore, complex scar substrates render CRT less effective. We used novel cardiac computed tomography (CT) and noninvasive electrocardiographic imaging (ECGI) techniques in an ischemic dyssynchronous heart failure (DHF) animal model to evaluate electrical and mechanical coupling of cardiac function, tissue viability, and venous accessibility of target pacing regions.

METHODS AND RESULTS

Ischemic DHF was induced in 6 dogs using coronary occlusion, left bundle ablation and tachy RV pacing. Full body ECG was recorded during native rhythm followed by volumetric first-pass and delayed enhancement CT. Regional electrical activation were computed and overlaid with segmented venous anatomy and scar regions. Reconstructed electrical activation maps show consistency with LBBB starting on the RV and spreading in a "U-shaped" pattern to the LV. Previously reported lines of slow conduction are seen parallel to anterior or inferior interventricular grooves. Mechanical contraction showed large septal to lateral wall delay (80 ± 38 milliseconds vs. 123 ± 31 milliseconds, P = 0.0001). All animals showed electromechanical correlation except dog 5 with largest scar burden. Electromechanical decoupling was largest in basal lateral LV segments.

CONCLUSION

We demonstrated a promising application of CT in combination with ECGI to gain insight into electromechanical function in ischemic dyssynchronous heart failure that can provide useful information to study regional substrate of CRT candidates.

Electrophysiologic Scar Substrate in Relation to VT: Noninvasive High-Resolution Mapping and Risk Assessment with ECGI

BACKGROUND

Ischemic cardiomyopathy (ICM) can provide the substrate for ventricular tachycardia (VT).

OBJECTIVE

To map noninvasively with high resolution the electrophysiologic (EP) scar substrate, identify its relationship to reentry circuits during VT, and stratify VT risk in ICM patients.

METHODS

Noninvasive high-resolution epicardial mapping with electrocardiographic imaging (ECGI) was performed in 32 ICM patients (17 with clinical VT, 15 without VT). Abnormal scar EP substrate was determined based on electrogram (EGM) amplitude (as percentage of maximal peak-to-peak voltage over the entire ventricular epicardium; total scar [TS] < 30%; dense scar [DS] < 15%), fractionation, and presence of late potentials (LPs). Scar burden was defined as the ratio of the scar size to the total epicardial surface area. The VT activation pattern was mapped and correlated with the EP substrate to identify components of the reentry circuit.

RESULTS

Patients with VT had higher scar burden (TS: 51.0 ± 9.3% vs 36.5 ± 5.4%, P < 0.05; DS: 29.5 ± 7.3% vs 16.8 ± 6.8%, P < 0.05) with lower normalized unipolar EGM voltage (TS: 0.107 ± 0.027 vs 0.153 ± 0.031, P < 0.05; DS: 0.073 ± 0.023 vs 0.098 ± 0.026, P < 0.05), greater prevalence of fractionated EGMs (TS: 44.1 ± 10.6% vs 26.8 ± 6.3%, P < 0.05; DS: 50.8 ± 10.8% vs 30.9 ± 7.0%, P < 0.05), and LPs (TS: 26.8 ± 10.7% vs 15.8 ± 5.3, P < 0.05). VTs were mapped in eight patients; the reentry circuits were closely related to the EP substrate.

CONCLUSIONS

ECGI noninvasively identified scar EP substrate that underlies abnormal conduction in ICM patients. It identified regions within the scar that aligned with critical elements of the reentry circuit during VT. ECGI can potentially be used for VT risk stratification in ICM patients.

Sensitivity of Noninvasive Cardiac Electrophysiological Imaging to Variations in Personalized Anatomical Modeling

OBJECTIVE

Noninvasive cardiac electrophysiological (EP) imaging techniques rely on anatomically-detailed heart-torso models derived from high-quality tomographic images of individual subjects. However, anatomical modeling involves variations that lead to unresolved uncertainties in the outcome of EP imaging, bringing questions to the robustness of these methods in clinical practice. In this study, we design a systematic statistical approach to assess the sensitivity of EP imaging methods to the variations in personalized anatomical modeling.

METHODS

We first quantify the variations in personalized anatomical models by a novel application of statistical shape modeling. Given the statistical distribution of the variation in personalized anatomical models, we then employ unscented transform to determine the sensitivity of EP imaging outputs to the variation in input personalized anatomical modeling.

RESULTS

We test the feasibility of our proposed approach using two of the existing EP imaging methods: epicardial-based electrocardiographic imaging and transmural electrophysiological imaging. Both phantom and real-data experiments show that variations in personalized anatomical models have negligible impact on the outcome of EP imaging.

CONCLUSION

This study verifies the robustness of EP imaging methods to the errors in personalized anatomical modeling and suggests the possibility to simplify the process of anatomical modeling in future clinical practice.

SIGNIFICANCE

This study proposes a systematic statistical approach to quantify anatomical modeling variations and assess their impact on EP imaging, which can be extended to find a balance between the quality of personalized anatomical models and the accuracy of EP imaging that may improve the clinical feasibility of EP imaging.

Noninvasive reconstruction of cardiac electrical activity: update on current methods, applications and challenges

Electrical activity at the level of the heart muscle can be noninvasively reconstructed from body-surface electrocardiograms (ECGs) and patient-specific torso-heart geometry. This modality, coined electrocardiographic imaging, could fill the gap between the noninvasive (low-resolution) 12-lead ECG and invasive (high-resolution) electrophysiology studies. Much progress has been made to establish electrocardiographic imaging, and clinical studies appear with increasing frequency. However, many assumptions and model choices are involved in its execution, and only limited validation has been performed. In this article, we will discuss the technical details, clinical applications and current limitations of commonly used methods in electrocardiographic imaging. It is important for clinicians to realise the influence of certain assumptions and model choices for correct and careful interpretation of the results. This, in combination with more extensive validation, will allow for exploitation of the full potential of noninvasive electrocardiographic imaging as a powerful clinical tool to expedite diagnosis, guide therapy and improve risk stratification.

Cardiac electrophysiological substrate underlying the ECG phenotype and electrogram abnormalities in Brugada syndrome patients

BACKGROUND

Brugada syndrome (BrS) is a highly arrhythmogenic cardiac disorder, associated with an increased incidence of sudden death. Its arrhythmogenic substrate in the intact human heart remains ill-defined.

METHODS AND RESULTS

Using noninvasive ECG imaging, we studied 25 BrS patients to characterize the electrophysiological substrate and 6 patients with right bundle-branch block for comparison. Seven healthy subjects provided control data. Abnormal substrate was observed exclusively in the right ventricular outflow tract with the following properties (in comparison with healthy controls; P<0.005): (1) ST-segment elevation and inverted T wave of unipolar electrograms (2.21±0.67 versus 0 mV); (2) delayed right ventricular outflow tract activation (82±18 versus 37±11 ms); (3) low-amplitude (0.47±0.16 versus 3.74±1.60 mV) and fractionated electrograms, suggesting slow discontinuous conduction; (4) prolonged recovery time (381±30 versus 311±34 ms) and activation-recovery intervals (318±32 versus 241±27 ms), indicating delayed repolarization; (5) steep repolarization gradients (Δrecovery time/Δx=96±28 versus 7±6 ms/cm, Δactivation-recovery interval/Δx=105±24 versus 7±5 ms/cm) at right ventricular outflow tract borders. With increased heart rate in 6 BrS patients, reduced ST-segment elevation and increased fractionation were observed. Unlike BrS, right bundle-branch block had delayed activation in the entire right ventricle, without ST-segment elevation, fractionation, or repolarization abnormalities on electrograms.

CONCLUSIONS

The results indicate that both slow discontinuous conduction and steep dispersion of repolarization are present in the right ventricular outflow tract of BrS patients. ECG imaging could differentiate between BrS and right bundle-branch block.

Electrocardiographic imaging of heart rhythm disorders: from bench to bedside

Noninvasive electrocardiographic imaging (ECGI; also called ECG mapping) can reconstruct potentials, electrograms, activation sequences, and repolarization patterns on the epicardial surface of the heart with high resolution. ECGI can possibly be used to quantify synchrony, identify potential responders/nonresponders to cardiac resynchronization therapy, and guide electrode placement for effective resynchronization therapy. This article provides a brief description of the ECGI procedure and selected previously published examples of its application in important clinical conditions, including heart failure, cardiac resynchronization therapy, atrial arrhythmias, and ventricular tachycardia.

Noninvasive mapping of ventricular arrhythmias

Several decades of research has led to the development of a 252-lead electrocardiogram-based three-dimensional imaging modality to refine noninvasive diagnosis and improve the management of heart rhythm disorders. This article reviews the clinical potential of this noninvasive mapping technique in identifying the sources of electrical disorders and guiding the catheter ablation of ventricular arrhythmias (premature ventricular beats and ventricular tachycardia). The article also briefly refers to the noninvasive electrical imaging of the arrhythmogenic ventricular substrate based on the electrophysiologic characteristics of postinfarction ventricular myocardium.

Non Invasive ECG Mapping To Guide Catheter Ablation

Since more than 100 years, 12-lead electrocardiography (ECG) is the standard-of-care tool, which involves measuring electrical potentials from limited sites on the body surface to diagnose cardiac disorder, its possible mechanism and the likely site of origin. Several decades of research has led to the development of a 252-lead-ECG and CT-scan based, three dimensional, electro-imaging modality to non-invasively map abnormal cardiac rhythms including fibrillation. These maps provide guidance towards ablative therapy and thereby help advance the management of complex heart rhythm disorders. Here, we describe the clinical experience obtained using non-invasive technique in mapping the electrical disorder and guide the catheter ablation of atrial arrhythmias (premature atrial beat, atrial tachycardia, atrial fibrillation), ventricular arrhythmias (premature ventricular beats) and ventricular pre-excitation (Wolff-Parkinson-White syndrome).

Noninvasive imaging of 3-dimensional myocardial infarction from the inverse solution of equivalent current density in pathological hearts

We propose a new approach to noninvasively image the 3-D myocardial infarction (MI) substrates based on equivalent current density (ECD) distribution that is estimated from the body surface potential maps (BSPMs) during S-T segment. The MI substrates were identified using a predefined threshold of ECD. Computer simulations were performed to assess the performance with respect to: 1) MI locations; 2) MI sizes; 3) measurement noise; 4) numbers of BSPM electrodes; and 5) volume conductor modeling errors. A total of 114 sites of transmural infarctions, 91 sites of epicardial infarctions, and 36 sites of endocardial infarctions were simulated. The simulation results show that: 1) Under 205 electrodes and 10-μV noise, the averaged accuracies of imaging transmural MI are 83.4% for sensitivity, 82.2% for specificity, 65.0% for Dice's coefficient, and 6.5 mm for distances between the centers of gravity (DCG). 2) For epicardial infarction, the averaged imaging accuracies are 81.6% for sensitivity, 75.8% for specificity, 45.3% for Dice's coefficient, and 7.5 mm for DCG; while for endocardial infarction, the imaging accuracies are 80.0% for sensitivity, 77.0% for specificity, 39.2% for Dice's coefficient, and 10.4 mm for DCG. 3) A reasonably good imaging performance was obtained under higher noise levels, fewer BSPM electrodes, and mild volume conductor modeling errors. The present results suggest that this method has the potential to aid in the clinical identification of the MI substrates.

Noninvasive electrocardiographic mapping to guide ablation of outflow tract ventricular arrhythmias

Background

Localizing the origin of outflow tract ventricular tachycardias (OTVT) is hindered by lack of accuracy of electrocardiographic (ECG) algorithms and infrequent spontaneous premature ventricular complexes (PVCs) during electrophysiological studies.

Objectives

To prospectively assess the performance of noninvasive electrocardiographic mapping (ECM) in the pre-/periprocedural localization of OTVT origin to guide ablation and to compare the accuracy of ECM with that of published ECG algorithms.

Methods

Patients with symptomatic OTVT/PVCs undergoing clinically indicated ablation were recruited. The OTVT/PVC origin was mapped preprocedurally by using ECM, and 3 published ECG algorithms were applied to the 12-lead ECG by 3 blinded electrophysiologists. Ablation was guided by using ECM. The OTVT/PVC origin was defined as the site where ablation caused arrhythmia suppression. Acute success was defined as abolition of ectopy after ablation. Medium-term success was defined as the abolition of symptoms and reduction of PVC to less than 1000 per day documented on Holter monitoring within 6 months.

Results

In 24 patients (mean age 50 ± 18 years) recruited ECM successfully identified OTVT/PVC origin in 23/24 (96%) (right ventricular outflow tract, 18; left ventricular outflow tract, 6), sublocalizing correctly in 100% of this cohort. Acute ablation success was achieved in 100% of the cases with medium-term success in 22 of 24 patients. PVC burden reduced from 21,837 ± 23,241 to 1143 ± 4039 (P < .0001). ECG algorithms identified the correct chamber of origin in 50%–88% of the patients and sublocalized within the right ventricular outflow tract (septum vs free-wall) in 37%–58%.

Conclusions

ECM can accurately identify OTVT/PVC origin in the left and the right ventricle pre- and periprocedurally to guide catheter ablation with an accuracy superior to that of published ECG algorithms.

Noninvasive electrocardiographic imaging of arrhythmogenic substrates in humans

Cardiac excitation is determined by interactions between the source of electric activation (membrane depolarization) and the load that cardiac tissue presents. This relationship is altered in pathology by remodeling processes that often create a substrate favoring the development of cardiac arrhythmias. Most studies of arrhythmia mechanisms and arrhythmogenic substrates have been conducted in animal models, which may differ in important ways from the human pathologies they are designed to represent. Electrocardiographic imaging is a noninvasive method for mapping the electric activity of the heart in humans in real-world conditions. This review summarizes results from electrocardiographic imaging studies of arrhythmogenic substrates associated with human clinical arrhythmias. Examples include heart failure, myocardial infarction scar, atrial fibrillation, and abnormal ventricular repolarization.

Computing ischemic regions in the heart with the bidomain model--first steps towards validation

We investigate whether it is possible to use the bidomain model and body surface potential maps (BSPMs) to compute the size and position of ischemic regions in the human heart. This leads to a severely ill posed inverse problem for a potential equation. We do not use the classical inverse problems of electrocardiography, in which the unknown sources are the epicardial potential distribution or the activation sequence. Instead we employ the bidomain theory to obtain a model that also enables identification of ischemic regions transmurally. This approach makes it possible to distinguish between subendocardial and transmural cases, only using the BSPM data. The main focus is on testing a previously published algorithm on clinical data, and the results are compared with images taken with perfusion scintigraphy. For the four patients involved in this study, the two modalities produce results that are rather similar: The relative differences between the center of mass and the size of the ischemic regions, suggested by the two modalities, are 10.8% ± 4.4% and 7.1% ± 4.6%, respectively. We also present some simulations which indicate that the methodology is robust with respect to uncertainties in important model parameters. However, in contrast to what has been observed in investigations only involving synthetic data, inequality constraints are needed to obtain sound results.

Computational cardiology: how computer simulations could be used to develop new therapies and advance existing ones

This article reviews the latest developments in computational cardiology. It focuses on the contribution of cardiac modelling to the development of new therapies as well as the advancement of existing ones for cardiac arrhythmias and pump dysfunction. Reviewed are cardiac modelling efforts aimed at advancing and optimizing existent therapies for cardiac disease (defibrillation, ablation of ventricular tachycardia, and cardiac resynchronization therapy) and at suggesting novel treatments, including novel molecular targets, as well as efforts to use cardiac models in stratification of patients likely to benefit from a given therapy, and the use of models in diagnostic procedures.

Confirmation of Novel Noninvasive High-Density Electrocardiographic Mapping With Electrophysiology Study

Background—

Twelve lead ECGs have limited value in precisely identifying atrial and ventricular activation during arrhythmias, including accessory atrioventricular conduction activation. The aim of this study was to report a single center’s clinical experience validating a novel, noninvasive, whole heart, beat-by-beat, 3-dimensional mapping technology with invasive electrophysiological studies, including ablation, where applicable.

Methods and Results—

Using an electrocardiographic mapping (ECM) system in 27 patients, 3-dimensional epicardial activation maps were generated from >250 body surface ECGs using heart–torso geometry obtained from computed tomographic images. ECM activation maps were compared with clinical diagnoses, and confirmed with standard invasive electrophysiological studies mapping. (1) In 6 cases of Wolff–Parkinson–White syndrome, ECM accurately identified the ventricular insertion site of an accessory atrioventricular connection. (2) In 10 patients with premature ventricular complexes, ECM accurately identified their ventricular site of origin in 8 patients. In 2 of 10 patients transient premature ventricular complex suppression was observed during ablation at the site predicted by ECM as the earliest. (3) In 10 cases of atrial tachycardia/atrial flutter, ECM accurately identified the chamber of origin in all 10, and distinguished isthmus from nonisthmus dependent atrial flutter. (4) In 1 patient with sustained exercise induced ventricular tachycardia, ECM accurately identified the focal origin in the left ventricular outflow tract.

Conclusions—

ECM successfully provided valid activation sequence maps obtained noninvasively in a variety of rhythm disorders that correlated well with invasive electrophysiological studies.

Computational cardiology: the heart of the matter

This paper reviews the newest developments in computational cardiology. It focuses on the contribution of cardiac modeling to the development of new therapies as well as the advancement of existing ones for cardiac arrhythmias and pump dysfunction. Reviewed are cardiac modeling efforts aimed at advancing and optimizing existent therapies for cardiac disease (defibrillation, ablation of ventricular tachycardia, and cardiac resynchronization therapy) and at suggesting novel treatments, including novel molecular targets, as well as efforts to use cardiac models in stratification of patients likely to benefit from a given therapy, and the use of models in diagnostic procedures.

Repolarization changes underlying long-term cardiac memory due to right ventricular pacing: noninvasive mapping with electrocardiographic imaging

BACKGROUND

Cardiac memory refers to the observation that altered cardiac electrical activation results in repolarization changes that persist after the restoration of a normal activation pattern. Animal studies, however, have yielded disparate conclusions, both regarding the spatial pattern of repolarization changes in cardiac memory and the underlying mechanisms. The present study was undertaken to produce 3-dimensional images of the repolarization changes underlying long-term cardiac memory in humans.

METHODS AND RESULTS

Nine adult subjects with structurally normal hearts and dual-chamber pacemakers were enrolled in the study. Noninvasive electrocardiographic imaging was used before and after 1 month of ventricular pacing to reconstruct epicardial activation and repolarization patterns. Eight subjects exhibited cardiac memory in response to ventricular pacing. In all subjects, ventricular pacing resulted in a prolongation of the activation recovery interval (a surrogate for action potential duration) in the region close to the site of pacemaker-induced activation from 228.4±7.6 ms during sinus rhythm to 328.3±6.2 ms during cardiac memory. As a consequence, increases are observed in both apical-basal and right-left ventricular gradients of repolarization, resulting in a significant increase in the dispersion of repolarization.

CONCLUSIONS

These results demonstrate that electrical remodeling in response to ventricular pacing in human subjects results in action potential prolongation near the site of abnormal activation and a marked dispersion of repolarization. This dispersion of repolarization is potentially arrhythmogenic and, intriguingly, was less evident during continuous right ventricular pacing, suggesting the novel possibility that continuous right ventricular pacing at least partially suppresses pacemaker-induced cardiac memory.

Bridging experiments, models and simulations: an integrative approach to validation in computational cardiac electrophysiology

Computational models in physiology often integrate functional and structural information from a large range of spatiotemporal scales from the ionic to the whole organ level. Their sophistication raises both expectations and skepticism concerning how computational methods can improve our understanding of living organisms and also how they can reduce, replace, and refine animal experiments. A fundamental requirement to fulfill these expectations and achieve the full potential of computational physiology is a clear understanding of what models represent and how they can be validated. The present study aims at informing strategies for validation by elucidating the complex interrelations among experiments, models, and simulations in cardiac electrophysiology. We describe the processes, data, and knowledge involved in the construction of whole ventricular multiscale models of cardiac electrophysiology. Our analysis reveals that models, simulations, and experiments are intertwined, in an assemblage that is a system itself, namely the model-simulation-experiment (MSE) system. We argue that validation is part of the whole MSE system and is contingent upon 1) understanding and coping with sources of biovariability; 2) testing and developing robust techniques and tools as a prerequisite to conducting physiological investigations; 3) defining and adopting standards to facilitate the interoperability of experiments, models, and simulations; 4) and understanding physiological validation as an iterative process that contributes to defining the specific aspects of cardiac electrophysiology the MSE system targets, rather than being only an external test, and that this is driven by advances in experimental and computational methods and the combination of both.