Electric Field-Based Spatial Analysis of Noncontact Unipolar Electrograms to Map Regional Activation-Repolarization Intervals.

BACKGROUND: Spatial heterogeneity in repolarization plays an important role in generating and sustaining cardiac arrhythmias. Reliable determination of repolarization times remains challenging. OBJECTIVES: The goal of this study was to improve processing of densely sampled noncontact unipolar electrograms to yield reliable high-resolution activation and repolarization maps. METHODS: Endocardial noncontact unipolar electrograms were both simulated and recorded in pig left ventricle. Electrical activity on the endocardial surface was processed in terms of a pseudo-electric field. Activation and repolarization times were calculated by using an amplitude-weighted average on QRS and T waves (ie, the E-field method). This was compared vs the conventional Wyatt method on unipolar electrograms. Timing maps were validated against timing on endocardial action potentials in a simulation study. In vivo, activation and repolarization times determined by using this alternative E-field method were validated against simultaneously recorded endocardial monophasic action potentials (MAPs). RESULTS: Simulation showed that the E-field method provides viable measurements of local endocardial action potential activation and repolarization times. In vivo, correlation of E-field activation times with MAP activation times (rE = 0.76; P < 0.001) was similar to those of Wyatt (rWyatt = 0.80, P < 0.001; P[h1:rE > rWyatt] = 0.82); for repolarization times, correlation improved significantly (rE = 0.96, P < 0.001; rWyatt = 0.82, P < 0.001; P[h1:rE > rWyatt] < 0.00001). This resulted in improved correlations of activation-repolarization intervals to endocardial action potential duration on MAP (rE = 0.96, P < 0.001; rWyatt = 0.86, P < 0.001; P[h1:rE > rWyatt] < 0.00001). Spatial beat-to-beat variation of repolarization could only be calculated by using the E-field methodology and correlated well with the MAP beat-to-beat variation of repolarization (rE = 0.76; P = 0.001). CONCLUSIONS: The E-field method substantially enhances information from endocardial noncontact electrogram data, allowing for dense maps of activation and repolarization times and derived parameters.

A computational model of rabbit geometry and ECG: Optimizing ventricular activation sequence and APD distribution.

Computational modeling of electrophysiological properties of the rabbit heart is a commonly used way to enhance and/or complement findings from classic lab work on single cell or tissue levels. Yet, thus far, there was no possibility to extend the scope to include the resulting body surface potentials as a way of validation or to investigate the effect of certain pathologies. Based on CT imaging, we developed the first openly available computational geometrical model not only of the whole heart but also the complete torso of the rabbit. Additionally, we fabricated a 32-lead ECG-vest to record body surface potential signals of the aforementioned rabbit. Based on the developed geometrical model and the measured signals, we then optimized the activation sequence of the ventricles, recreating the functionality of the Purkinje network, and we investigated different apico-basal and transmural gradients in action potential duration. Optimization of the activation sequence resulted in an average root mean square error between measured and simulated signal of 0.074 mV/ms for all leads. The best-fit T-Wave, compared to measured data (0.038 mV/ms), resulted from incorporating an action potential duration gradient from base to apex with a respective shortening of 20 ms and a transmural gradient with a shortening of 15 ms from endocardium to epicardium. By making our model and measured data openly available, we hope to give other researchers the opportunity to verify their research, as well as to create the possibility to investigate the impact of electrophysiological alterations on body surface signals for translational research.

Single cardiomyocytes from papillary muscles show lower preload-dependent activation of force compared to cardiomyocytes from the left ventricular free wall.

Efficient pumping of the healthy left ventricle (LV) requires heterogeneities in mechanical function of individual cardiomyocytes (CM). Deformation of sub-endocardial (Endo) tissue is greater than that of sub-epicardial (Epi) regions. Papillary muscles (PM), often considered to be part of Endo tissue, show lower beat-by-beat length variation than Epi (or Endo) regions, even though they contribute to the shift in atrio-ventricular valve plane, which is essential for LV pump function. Thus far, no comparative assessment of CM mechanics for PM and LV free wall has been published. Here, we investigate contractility and cytosolic calcium concentration ([Ca2+]c) transients in rabbit single CM, freshly isolated from PM, Endo and Epi regions of the LV (free wall tissue was further subdivided into near-basal [Base], equatorial [Centre], and near-apical [Apex] parts). Functional parameters were measured in the absence of external mechanical loads (non-loaded), or during afterloaded (auxotonic) CM contractions, initiated from different levels of preload (diastolic axial stretch), using the carbon fibre technique. We note significant differences in time-course and amplitudes of sarcomere shortening between PM, Endo and Epi CM. In non-loaded CM, sarcomere shortening between regions compares as follows: Endo > Epi and Endo > PM. During afterloaded contractions, the slope of auxotonic tension-length relation and the Frank-Starling gain index (preload-dependent increase in tension and shortening) follow the sequence of Endo > Epi > PM. In terms of apico-basal gradients, time-to-peak sarcomere shortening was greater in Apex compared to Centre and Base in non-loaded CM only. Thus, CM from PM show the least pronounced preload-dependent activation of force across the LV regions assessed, while CM from Endo regions show the strongest response. This is in keeping with prior in situ observations on the smaller extent of PM shortening and their thus lower functional requirement for sensitivity to preload, compared to LV free wall. The here identified regional differences in cellular Frank-Starling responses illustrate the extent to which CM mechanical responses appear to be in keeping with in situ differences in mechanical demand.

Phosphoproteomic response of cardiac endothelial cells to ischemia and ultrasound.

Myocardial infarction and subsequent therapeutic interventions activate numerous intracellular cascades in every constituent cell type of the heart. Endothelial cells produce several protective compounds in response to therapeutic ultrasound, under both normoxic and ischemic conditions. How endothelial cells sense ultrasound and convert it to a beneficial biological response is not known. We adopted a global, unbiased phosphoproteomics approach aimed at understanding how endothelial cells respond to ultrasound. Here, we use primary cardiac endothelial cells to explore the cellular signaling events underlying the response to ischemia-like cellular injury and ultrasound exposure in vitro. Enriched phosphopeptides were analyzed with a high mass accuracy liquid chromatrography (LC) - tandem mass spectrometry (MS/MS) proteomic platform, yielding multiple alterations in both total protein levels and phosphorylation events in response to ischemic injury and ultrasound. Application of pathway algorithms reveals numerous protein networks recruited in response to ultrasound including those regulating RNA splicing, cell-cell interactions and cytoskeletal organization. Our dataset also permits the informatic prediction of potential kinases responsible for the modifications detected. Taken together, our findings begin to reveal the endothelial proteomic response to ultrasound and suggest potential targets for future studies of the protective effects of ultrasound in the ischemic heart.

Right Ventricular Longitudinal Conduction Delay in Patients with Brugada Syndrome.

BACKGROUND: The mechanism of Brugada syndrome (BrS) is still unclear, with different researchers favoring either the repolarization or depolarization hypothesis. Prolonged longitudinal activation time has been verified in only a small number of human right ventricles (RVs). The purpose of the present study was to demonstrate RV conduction delays in BrS. METHODS: The RV outflow tract (RVOT)-to-RV apex (RVA) and RVA-to-RVOT conduction times were measured by endocardial stimulation and mapping in 7 patients with BrS and 14 controls. RESULTS: Patients with BrS had a longer PR interval (180 ± 12.6 vs. 142 ± 6.7 ms, P = 0.016). The RVA-to-RVOT conduction time was longer in the patients with BrS than in controls (stimulation at 600 ms, 107 ± 9.9 vs. 73 ± 3.4 ms, P = 0.001; stimulation at 500 ms, 104 ± 12.3 vs. 74 ± 4.2 ms, P = 0.037; stimulation at 400 ms, 107 ±12.2 vs. 73 ± 5.1 ms, P = 0.014). The RVOT-to-RVA conduction time was longer in the patients with BrS than in controls (stimulation at 500 ms, 95 ± 10.3 vs. 62 ± 4.1 ms, P = 0.007; stimulation at 400 ms, 94 ±11.2 vs. 64 ± 4.6 ms, P = 0.027). The difference in longitudinal conduction time was not significant when isoproterenol was administered. CONCLUSION: The patients with BrS showed an RV longitudinal conduction delay obviously. These findings suggest that RV conduction delay might contribute to generate the BrS phenotype.

A miniaturized endocardial electromagnetic energy harvester for leadless cardiac pacemakers.

Life expectancy of contemporary cardiac pacemakers is limited due to the use of an internal primary battery. Repeated device replacement interventions are necessary, which leads to an elevated risk for patients and an increase of health care costs. The aim of our study is to investigate the feasibility of powering an endocardial pacemaker by converting a minimal amount of the heart's kinetic energy into electric energy. The intrinsic cardiac muscle activity makes it an ideal candidate as continuous source of energy for endocardial pacemakers. For this reason, we developed a prototype able to generate enough power to supply a pacing circuit at different heart rates. The prototype consists of a mass imbalance that drives an electromagnetic generator while oscillating. We developed a mathematical model to estimate the amount of energy harvested from the right ventricle. Finally, the implemented prototype was successfully tested during in-vitro and in-vivo experiments.

Epicardial Ablation Biophysics and Novel Radiofrequency Energy Delivery Techniques.

Important physiologic and anatomic differences exist between the epicardium and endocardium, particularly of the ventricles, and these differences affect ablation biophysics. Absence of passive convective effects conferred by circulating blood as well as the presence of epicardial fat and vessels and absence of intracavitary ridges and structures affect ablation lesion size when performing epicardial catheter-based ablation, whether using radiofrequency or cryothermal energy. Understanding differential effects in each environment is important in informing strategies to increase ablation lesion depth. When using actively cooled radiofrequency ablation, local impedance can be altered to selectively augment energy delivery.

Endocardial electro-anatomic mapping in healthy horses: Normal sinus impulse propagation in the left and right atrium and the ventricles.

Understanding the depolarisation pattern of the equine heart under normal physiologic conditions, and its relationship to the surface electrocardiogram (ECG), is of uppermost importance before any further research can be done about the pathophysiology of complex arrhythmias. In the present study, a 3D electro-anatomical mapping system was used to evaluate the qualitative and quantitative depolarisation patterns and correlation to the surface ECG of both the atrial and ventricular endocardium in seven healthy horses in sinus rhythm under general anaesthesia. Bipolar activation maps of the endocardium were analysed. The first atrial activation was located at the height of the terminal crest. Only one interatrial conduction pathway was recognised. The first and second P wave deflections represent the right and left atrial depolarisation, respectively. Bundle of His electrograms could be recorded in 5/7 horses. Left ventricular activation started at the mid septum and right ventricular activation started apically from the supraventricular crest. This was followed by separate depolarisations at the height of the mid free wall. Further ventricular depolarisation occurred in an explosive pattern. Electrically active tissue could be found in all pulmonary veins. In contrast to findings of previous studies, all parts of the ventricular depolarisation contributed to the surface ECG QRS complex. This study provides a reference for the normal sinus impulse endocardial propagation pattern and for conduction velocities in equine atria and ventricles.

The Formation of Coronary Vessels in Cardiac Development and Disease.

Understanding how coronary blood vessels form and regenerate during development and progression of cardiac diseases will shed light on the development of new treatment options targeting coronary artery diseases. Recent studies with the state-of-the-art technologies have identified novel origins of, as well as new, cellular and molecular mechanisms underlying the formation of coronary vessels in the postnatal heart, including collateral artery formation, endocardial-to-endothelial differentiation and mesenchymal-to-endothelial transition. These new mechanisms of coronary vessel formation and regeneration open up new possibilities targeting neovascularization for promoting cardiac repair and regeneration. Here, we highlight some recent studies on cellular mechanisms of coronary vessel formation, and discuss the potential impact and significance of the findings on basic research and clinical application for treating ischemic heart disease.

Coronary Revascularization During Heart Regeneration Is Regulated by Epicardial and Endocardial Cues and Forms a Scaffold for Cardiomyocyte Repopulation.

Defective coronary network function and insufficient blood supply are both cause and consequence of myocardial infarction. Efficient revascularization after infarction is essential to support tissue repair and function. Zebrafish hearts exhibit a remarkable ability to regenerate, and coronary revascularization initiates within hours of injury, but how this process is regulated remains unknown. Here, we show that revascularization requires a coordinated multi-tissue response culminating with the formation of a complex vascular network available as a scaffold for cardiomyocyte repopulation. During a process we term "coronary-endocardial anchoring," new coronaries respond by sprouting (1) superficially within the regenerating epicardium and (2) intra-ventricularly toward the activated endocardium. Mechanistically, superficial revascularization is guided by epicardial Cxcl12-Cxcr4 signaling and intra-ventricular sprouting by endocardial Vegfa signaling. Our findings indicate that the injury-activated epicardium and endocardium support cardiomyocyte replenishment initially through the guidance of coronary sprouting. Simulating this process in the injured mammalian heart should help its healing.

New Adjusted Cutoffs for "Normal" Endocardial Voltages in Patients With Post-Infarct LV Remodeling.

OBJECTIVES: This study sought to determine new reference cutoffs for normal unipolar voltage (UV) and bipolar voltage (BV) that would be adjusted for the LV remodeling. BACKGROUND: The definition of "normal" left ventricular (LV) endocardial voltage in patients with post-infarct scar is still lacking. The reference voltage of the noninfarcted myocardium (NIM) may differ between patients depending on LV structural remodeling and the ensuing interstitial fibrosis. METHODS: Electroanatomic voltage mapping was integrated with isotropic late gadolinium-enhanced cardiac magnetic resonance in 15 patients with nonremodeled LV and 12 patients with remodeled LV (end-systolic volume index >50 ml/m2 with ejection fraction <47% assessed by cardiac magnetic resonance). Reference voltages (fifth percentile values) were determined from pooled NIM segments without late gadolinium enhancement. RESULTS: The cutoffs for normal BV and UV were ≥3.0 and ≥6.7 mV for nonremodeled LV and ≥2.1 and ≥6.4 mV for remodeled LV. Endocardial low-voltage area (LVA) defined by the adjusted cutoffs corresponded better to late gadolinium enhancement-detected scar than did LVA defined by uniform cutoffs. In 15 patients who underwent successful ablation of ventricular tachycardia, the LVA contained >97% of targeted evoked delayed potentials. Insights from whole-heart T1 mapping revealed more fibrotic NIM in patients with remodeled LV compared with nonremodeled LV. CONCLUSIONS: This study found substantial differences in endocardial voltage of NIM in post-infarct patients with remodeled versus nonremodeled LV. The new adjusted cutoffs for "normal" BV and UV enable a patient-tailored approach to electroanatomic voltage mapping of LV.

Hemodynamic-mediated endocardial signaling controls in vivo myocardial reprogramming.

Lower vertebrate and neonatal mammalian hearts exhibit the remarkable capacity to regenerate through the reprogramming of pre-existing cardiomyocytes. However, how cardiac injury initiates signaling pathways controlling this regenerative reprogramming remains to be defined. Here, we utilize in vivo biophysical and genetic fate mapping zebrafish studies to reveal that altered hemodynamic forces due to cardiac injury activate a sequential endocardial-myocardial signaling cascade to direct cardiomyocyte reprogramming and heart regeneration. Specifically, these altered forces are sensed by the endocardium through the mechanosensitive channel Trpv4 to control Klf2a transcription factor expression. Consequently, Klf2a then activates endocardial Notch signaling which results in the non-cell autonomous initiation of myocardial Erbb2 and BMP signaling to promote cardiomyocyte reprogramming and heart regeneration. Overall, these findings not only reveal how the heart senses and adaptively responds to environmental changes due to cardiac injury, but also provide insight into how flow-mediated mechanisms may regulate cardiomyocyte reprogramming and heart regeneration.

Quantitative analysis of regional endocardial geometry dynamics from 4D cardiac CT images: endocardial tracking based on the iterative closest point with an integrated scale estimation.

Regional cardiac function analysis is important for the diagnosis and treatment planning of ischemic heart disease, but has not been sufficiently developed in the field of computed tomography (CT). Therefore, we propose a 3D endocardial tracking framework for cardiac CT using local point cloud registration based on the iterative closest point with an integrated scale estimation algorithm. We also introduce regional function descriptors that express the curvature and stretching of the endocardium: Surface distortion (E) and Scaling rate (S). For a region-to-region comparison, we propose endocardial segmentation according to coronary perfusion territories defined by the Voronoi partition based on coronary distribution. Our study of 65 endocardial segments in ten subjects showed that global endocardial deformation has a positive relationship with the stroke volume index (r = 0.896 and 0.829 in [Formula: see text] and [Formula: see text], respectively) and ejection fraction (r = 0.804 and 0.835), and a positive relationship with the brain natriuretic peptide level (r = 0.690 and 0.776). A positive relationship between segmental E and S (r = 0.845), a higher value of E in ischemic segments (p  = 0.021) that are determined by fractional flow reserve estimated from coronary CT data, and a higher value of S in the left circumflex artery territory (p  < 0.05) were also observed. The required radiation dose was 5.0 ± 0.7 mSv and the computation time was 7.2 ± 1.1 min. The result suggests that proposed endocardial deformation analysis using CT can be conducted on site and in time for the acute setting, and may be useful for the diagnosis of cardiac dysfunction or myocardial ischemia.

Noninvasive Imaging of Epicardial and Endocardial Potentials With Low Rank and Sparsity Constraints.

In this study, we explore the use of low rank and sparse constraints for the noninvasive estimation of epicardial and endocardial extracellular potentials from body-surface electrocardiographic data to locate the focus of premature ventricular contractions (PVCs). The proposed strategy formulates the dynamic spatiotemporal distribution of cardiac potentials by means of low rank and sparse decomposition, where the low rank term represents the smooth background and the anomalous potentials are extracted in the sparse matrix. Compared to the most previous potential-based approaches, the proposed low rank and sparse constraints are batch spatiotemporal constraints that capture the underlying relationship of dynamic potentials. The resulting optimization problem is solved using alternating direction method of multipliers. Three sets of simulation experiments with eight different ventricular pacing sites demonstrate that the proposed model outperforms the existing Tikhonov regularization (zero-order, second-order) and L1-norm based method at accurately reconstructing the potentials and locating the ventricular pacing sites. Experiments on a total of 39 cases of real PVC data also validate the ability of the proposed method to correctly locate ectopic pacing sites.

Influence of fiber connectivity in simulations of cardiac biomechanics.

PURPOSE: Personalized computational simulations of the heart could open up new improved approaches to diagnosis and surgery assistance systems. While it is fully recognized that myocardial fiber orientation is central for the construction of realistic computational models of cardiac electromechanics, the role of its overall architecture and connectivity remains unclear. Morphological studies show that the distribution of cardiac muscular fibers at the basal ring connects epicardium and endocardium. However, computational models simplify their distribution and disregard the basal loop. This work explores the influence in computational simulations of fiber distribution at different short-axis cuts. METHODS: We have used a highly parallelized computational solver to test different fiber models of ventricular muscular connectivity. We have considered two rule-based mathematical models and an own-designed method preserving basal connectivity as observed in experimental data. Simulated cardiac functional scores (rotation, torsion and longitudinal shortening) were compared to experimental healthy ranges using generalized models (rotation) and Mahalanobis distances (shortening, torsion). RESULTS: The probability of rotation was significantly lower for ruled-based models [95% CI (0.13, 0.20)] in comparison with experimental data [95% CI (0.23, 0.31)]. The Mahalanobis distance for experimental data was in the edge of the region enclosing 99% of the healthy population. CONCLUSIONS: Cardiac electromechanical simulations of the heart with fibers extracted from experimental data produce functional scores closer to healthy ranges than rule-based models disregarding architecture connectivity.

Ventricular Endocardial Tissue Geometry Affects Stimulus Threshold and Effective Refractory Period.

BACKGROUND: Understanding the biophysical processes by which electrical stimuli applied to cardiac tissue may result in local activation is important in both the experimental and clinical electrophysiology laboratory environments, as well as for gaining a more in-depth knowledge of the mechanisms of focal-trigger-induced arrhythmias. Previous computational models have predicted that local myocardial tissue architecture alone may significantly modulate tissue excitability, affecting both the local stimulus current required to excite the tissue and the local effective refractory period (ERP). In this work, we present experimental validation of this structural modulation of local tissue excitability on the endocardial tissue surface, use computational models to provide mechanistic understanding of this phenomena in relation to localized changes in electrotonic loading, and demonstrate its implications for the capture of afterdepolarizations. METHODS AND RESULTS: Experiments on rabbit ventricular wedge preparations showed that endocardial ridges (surfaces of negative mean curvature) had a stimulus capture threshold that was 0.21 ± 0.03 V less than endocardial grooves (surfaces of positive mean curvature) for pairwise comparison (24% reduction, corresponding to 56.2 ± 6.4% of the energy). When stimulated at the minimal stimulus strength for capture, ridge locations showed a shorter ERP than grooves (n = 6, mean pairwise difference 7.4 ± 4.2 ms). When each site was stimulated with identical-strength stimuli, the difference in ERP was further increased (mean pairwise difference 15.8 ± 5.3 ms). Computational bidomain models of highly idealized cylindrical endocardial structures qualitatively agreed with these findings, showing that such changes in excitability are driven by structural modulation in electrotonic loading, quantifying this relationship as a function of surface curvature. Simulations further showed that capture of delayed afterdepolarizations was more likely in trabecular ridges than grooves, driven by this difference in loading. CONCLUSIONS: We have demonstrated experimentally and explained mechanistically in computer simulations that the ability to capture tissue on the endocardial surface depends upon the local tissue architecture. These findings have important implications for deepening our understanding of excitability differences related to anatomical structure during stimulus application that may have important applications in the translation of novel experimental optogenetics pacing strategies. The uncovered preferential vulnerability to capture of afterdepolarizations of endocardial ridges, compared to grooves, provides important insight for understanding the mechanisms of focal-trigger-induced arrhythmias.

Conduction in the Heart Wall: Helicoidal Fibers Minimize Diffusion Bias.

The mammalian heart must function as an efficient pump while simultaneously conducting electrical signals to drive the contraction process. In the ventricles, electrical activation begins at the insertion points of the Purkinje network in the endocardium. How does the diffusion component of the subsequent excitation wave propagate from the endocardium in a healthy heart wall without creating directional biases? We show that this is a consequence of the particular geometric organization of myocytes in the heart wall. Using a generalized helicoid to model fiber orientation, we treat the myocardium as a curved space via Riemannian geometry, and then use stochastic calculus to model local signal diffusion. Our analysis shows that the helicoidal arrangement of myocytes minimizes the directional biases that could lead to aberrant propagation, thereby explaining how electrophysiological principles are consistent with local measurements of cardiac fiber geometry. We discuss our results in the context of the need to balance electrical and mechanical requirements for heart function.

The face of epicardial and endocardial derived cells in zebrafish.

Zebrafish hearts can regenerate through activation of growth factors and trans-differentiation of fibroblasts, epicardial, myocardial and endocardial cells, all positive for GATA4 during the process. A possible model of regeneration of the whole heart and the regenerating cells in ex-vivo culture is presented here by a stimulation of cocktail of growth factors. In ex-vivo growth-factors-supplemented culture the heart regeneration was quite complete without signs of fibrosis. Epicardial- and endocardial-derived cells have been analyzed by electron microscopy evidencing two main types: 1) larger/prismatic and 2) small/rounded. Type (1) showed on the surface protein-sculptures, while type(2) was smooth with sparse globular proteins. To confirm their nature we have contemporarily analyzed their proliferative capability and markers-positivity. The cells treated by growth factors have at least two-fold more proliferation with GATA4-positivity. The type (1) cell evidenced WT1+(marker of embryonic epicardium); the type (2) showed NFTA2+(marker of embryonic endocardium); whereas cTNT-cardiotroponin was negative. Under growth factors stimulation, GATA4+/WT1+ and GATA4+/NFTA2+ could be suitable candidates to be the cells with capability to move in/out of the tissue, probably by using their integrins, and it opens the possibility to have long term selected culture to future characterization.