Electrocardiographic T wave and its relation with ventricular repolarization along major anatomical axes

Tpeak-Tend ECG Marker in Obesity and Cardiovascular Diseases: A Comprehensive Review

Globally, cardiovascular diseases are still the leading cause of death. Numerous methods are used to diagnose cardiovascular pathologies; there is still a place for straightforward and noninvasive techniques, such as electrocardiogram (ECG). Depolarization and repolarization parameters, including QT interval and its derivatives, are well studied. However, the Tpeak-Tend interval is a novel and promising ECG marker with growing evidence for its potential role in predicting malignant arrhythmias. In this review, we discuss the association between the Tpeak-Tend interval and several cardiovascular diseases, including long QT syndrome, cardiomyopathies, heart failure, myocardial infarction, and obesity, which constitutes one of the risk factors for cardiovascular diseases.

Cardiovascular and Vector-Cardiographic Effects of Articaine Anesthesia with Epinephrine

The aim was to investigate the vector-cardiographic effects in patients submitted to dental extraction under local anesthesia. Twenty-one patients aged 36.6 ± 12.4 years with a clinical and radiographic indication of mandibular or maxillary tooth extraction were enrolled. The intervention was a local or mandibular nerve block anesthesia with 4% articaine hydrochloride containing epinephrine (1 : 100,000; 40 mg/ml + 10 μg/ml). Blood pressure (BP), heart rate (HR), pulse wave transit time, and vector-cardiography data were recorded throughout 3 min before and 5 min after injection. QRS- and T-wave area under the curve (QRS AUC/T AUC) were calculated from the X/Y/Z QRS-vector or T-vector. T-wave amplitude (T AM), T AUC values, and diastolic BP decreased, and HR significantly increased 4 min after injection. A transient moderate HR drop and a corresponding small increase in T AM and T AUC immediately after the injection procedure may be explained by a decreased sympathetic tone due to psychological relief. In dental anesthesia, the systemic epinephrine effects are represented by a decrease in T AUC. These effects are most pronounced in the X- and Y-leads. The 3D determination of vector planes or amplitudes is a simple method to register the sympathetic tone in local anesthesia independently of possible effects on T-wave characteristics in single leads. In conclusion, T-wave determination may help to detect even small increases in systemic adrenaline concentration in case of accidental intravascular injection. At the same time, full rhythm and spatial ischemia control is provided.

Synchronous recording of magnetocardiographic and electrocardiographic signals

We present a system for simultaneous recording of the electrocardiogram and the magnetocardiogram. The measurement system contained of printed carbon electrodes and SERF magnetometer. The use of this system confirms that the position of the end of the magnetic T wave extends further than the electric T wave, which is an important indicator for the diagnosis of cardiological patients and for drug arrhythmogenicity. We analyze this phenomenon in depth, and demonstrate, that it originates from the fundamental difference between electric and magnetic measurements. The measured value is always bipolar since the electric measurements require two electrodes. We demonstrate how the dual electric and magnetic measuring system adds a new information to the commonly used electrocardiographic diagnosis. The ECG should be interpreted as the spatial asymmetry of the electric cardiac potential, and not as the potential itself. The results seem to prove, that the relation between the magnetic and the electric imaging of neural activities may be broadly applied for the benefit of medical diagnosis in cardiology and many other fields, where the neural activity is measured. This is a pilot study which requires further confirmation at the clinical level.

Modeling ventricular repolarization gradients in normal cases using the equivalent dipole layer

Background Electrical activity underlying the T-wave is less well understood than the QRS-complex. This study investigated the relationship between normal T-wave morphology and the underlying ventricular repolarization gradients using the equivalent dipole layer (EDL). Methods Body-surface-potential-maps (BSPM, 67‑leads) were obtained in nine normal cases. Subject specific MRI-based anatomical heart/torso-models with electrode positions were created. The boundary element method was used to account for the volume conductor effects. To simulate the measured T-waves, the EDL was used to apply different ventricular repolarization gradients: a) transmural, b) interventricular c) apico-basal and d) all three gradients (a-c) combined. The combined gradient (d) was optimized using an inverse procedure (Levenberg-Marquardt). Correspondence between simulated and measured T-waves was assessed using correlation coefficient (CC) and relative difference (RD). Results Realistic T-waves were simulated if repolarization times of: (a) the epicardium were smaller than the endocardium; (b) the left ventricle were smaller than the right ventricle and (c) the apex increased towards the base. The apico-basal gradient resulted in the highest correspondence between measured and simulated T-waves (CC = 0.84(0.81-0.91);RD = 0.68(0.60-0.71)) compared to a transmural gradient (CC = 0.77(0.71-0.80);RD = 1.46(0.82-1.75)) and an interventricular gradient (CC = 0.71(0.67-0.80);RD = 0.85(0.75-0.87)). All three gradients combined further improved the correspondence between measured and simulated T-waves (CC = 0.83(0.82-0.89);RD = 0.60(0.51-0.63)), especially after optimization (CC = 0.96(0.94-0.98);RD = 0.27(0.22-0.34)). Conclusion The application of all repolarization gradients combined resulted in the largest agreement between simulated and measured T-waves, followed by the apico-basal repolarization gradient. With these findings, we will optimize our EDL-based inverse procedure to assess repolarization abnormalities.

Prognostic value of the Tpeak-Tend interval for in-hospital subacute ventricular arrhythmias in tako-tsubo syndrome

INTRODUCTION AND OBJECTIVES

The clinical value of electrocardiogram (ECG) repolarization parameters associated with ventricular arrhythmias (VAs) in tako-tsubo syndrome is still under debate. We aimed to evaluate ECG predictors of subacute VAs, defined as those occurring after the first 48hours from admission.

METHODS

This single-center observational study enrolled patients admitted to the cardiology department between 2012 and 2018 with a confirmed diagnosis of tako-tsubo syndrome. Data collection included a 12-lead ECG on admission and at 48hours, continuous telemetry monitoring, blood testing, transthoracic echocardiography, and coronary angiography during hospitalization. VAs events were defined as: premature ventricular contractions ≥ 2000 within a 24-hour window of telemetry monitoring, ventricular fibrillation, sustained ventricular tachycardia (VT), polymorphic VT, and non-sustained VT.

RESULTS

A total of 87 patients (age 72±12 years) were enrolled. During a median of 8 days of hospitalization, subacute VAs were documented in 22 patients (25%) after a median of 91hours from admission. Subacute VAs were associated with an increase in mortality during hospitalization (P=.030). The corrected global (mean of the 12-lead ECG values) Tpeak-Tend interval at 48hours from admission was an independent predictor of subacute VAs and was statistically superior to the standard corrected QT interval (Z test, P=.040). A cut-off of 108 msec for the corrected global Tpeak-Tend yielded a 71% sensitivity and 72% specificity for subacute VAs.

CONCLUSIONS

In patients with tako-tsubo syndrome, subacute VAs are associated with repolarization alterations that can be identified on conventional ECG using the Tpeak-Tend interval.

ECG T-Wave Morphologic Variations Predict Ventricular Arrhythmic Risk in Low- and Moderate-Risk Populations

Background Early identification of individuals at risk of sudden cardiac death (SCD) remains a major challenge. The ECG is a simple, common test, with potential for large-scale application. We developed and tested the predictive value of a novel index quantifying T-wave morphologic variations with respect to a normal reference (TMV), which only requires one beat and a single-lead ECG. Methods and Results We obtained reference T-wave morphologies from 23 962 participants in the UK Biobank study. With Cox models, we determined the association between TMV and life-threatening ventricular arrhythmia in an independent data set from UK Biobank study without a history of cardiovascular events (N=51 794; median follow-up of 122 months) and SCD in patients with coronary artery disease from ARTEMIS (N=1872; median follow-up of 60 months). In UK Biobank study, 220 (0.4%) individuals developed life-threatening ventricular arrhythmias. TMV was significantly associated with life-threatening ventricular arrhythmias (hazard ratio [HR] of 1.13 per SD increase [95% CI, 1.03-1.24]; P=0.009). In ARTEMIS, 34 (1.8%) individuals reached the primary end point. Patients with TMV ≥5 had an HR for SCD of 2.86 (95% CI, 1.40-5.84; P=0.004) with respect to those with TMV <5, independently from QRS duration, corrected QT interval, and left ventricular ejection fraction. TMV was not significantly associated with death from a cause other than SCD. Conclusions TMV identifies individuals at life-threatening ventricular arrhythmia and SCD risk using a single-beat single-lead ECG, enabling inexpensive, quick, and safe risk assessment in large populations.

Electrocardiographic marker of the cardiac action potential triangulation induced by antiarrhythmic drugs in perfused guinea-pig heart

NEW FINDINGS

What is the central question of this study? Can triangular appearance of ventricular action potential, indicating proarrhythmic profile of antiarrhythmic agent, be approximated by specific changes on ECG? What is the main finding and its importance? The triangulation of the ventricular action potential seen when antiarrhythmic drugs induce a greater lengthening of the late repolarization compared to the initial repolarization in epicardium, is closely approximated by a greater prolongation of the T wave upslope relative to the interval between the J point and the start of the T wave (the JT_start interval) on ECG. These findings may improve the power of ECG assessments in predicting the drug-induced arrhythmia resulting from slowed phase 3 repolarization.

ABSTRACT

Antiarrhythmic drugs prescribed to treat atrial fibrillation can occasionally precipitate ventricular tachyarrhythmia through a prominent slowing of the phase 3 repolarization. The latter results in the triangular shape of ventricular action potential, indicating high arrhythmic risks. However, clinically, the utilility of triangulation assessments for predicting arrhythmia is limited owing to the invasive nature of the ventricular action potential recordings. This study examined whether the triangulation effect can be detected indirectly from ECG analysis. Epicardial monophasic action potentials and ECG were simultaneously recorded in perfused guinea-pig hearts. With antiarrhythmics (dofetilide, quinidine, procainamide and flecainide), a prolongation of the initial repolarization seen in the action potential recordings was closely approximated by lengthening of the interval bewteen the J point and the start of the T wave (the JT_start interval) on ECG, whereas a prolongation of the late repolarization was paralleled by widening of the T wave upslope. Dofetilide, quinidine and procainamide induced a prominent slowing of the phase 3 repolarization in epicardium, leading to triangulation of the action potential. These effects were accompanied by a greater prolongation of the T wave upslope compared to the JT_start interval. Flecainide elicited a proportional prolongation of the initial and the late ventricular repolarization, and therefore failed to induce triangulation, based on analysis of both epicardial action potential and ECG profiles. Collectively, these findings suggest that the ratio between the durations of the T wave upslope and the JT_start interval may represent ECG metric of the ventricular action potential triangulation induced by antiarrhythmic drugs. This article is protected by copyright. All rights reserved.

Why are you talking with snakes? To get new evolutionary insights in cardiac electrophysiology!

Brette, Le Guennec, and Thireau discuss recent findings on evolutionary cardiac electrophysiology.

Repolarization Dispersion Is Associated With Diastolic Electromechanical Discoordination in Children With Pulmonary Arterial Hypertension

Background

Electromechanical dyssynchrony is a well described comorbidity in pulmonary arterial hypertension (PAH). ECG‐derived measurements reflective of diastolic dysfunction and electromechanical imaging markers are yet to be investigated. In this study we investigated the ECG‐ derived marker of repolarization dispersion, interval between the peak and end of T wave (TpTe), in pediatric patients with PAH and left ventricular (LV) diastolic dysfunction.

Methods and Results

We measured TpTe from a standard 12‐lead ECG and in 30 children with PAH and matched control subjects. All participants underwent same‐day echocardiography and myocardial strain analysis to calculate the diastolic electromechanical discoordination marker diastolic relaxation fraction. When compared with control subjects, patients with PAH had increased TpTe (93±15 versus 81±12 ms, P=0.001) and elevated diastolic relaxation fraction (0.33±0.10 versus 0.27±0.03, P=0.001). Patients with PAH with LV diastolic dysfunction had significantly increased TpTe when compared with patients with PAH without diastolic dysfunction (P=0.012) and when compared with control group (P<0.001). Similarly, patients with PAH with LV diastolic dysfunction had increased diastolic relaxation fraction when compared with PAH patients without diastolic dysfunction (P=0.007) and when compared with control group (P<0.001). A 10‐ms increase in TpTe was significantly associated with 0.023 increase in diastolic relaxation fraction (P=0.008) adjusting for body surface area, heart rate, right ventricular volumes, and function.

Conclusions

Prolonged myocardial repolarization and abnormal LV diastolic electromechanical discoordination exist in parallel in children with PAH and are associated with worse LV diastolic function and functional class.

Seasonal changes of electrophysiological heterogeneities in the rainbow trout ventricular myocardium

Introduction

Thermal adaptation in fish is accompanied by morphological and electrophysiological changes in the myocardium. Little is known regarding seasonal changes of spatiotemporal organization of ventricular excitation and repolarization processes. We aimed to evaluate transmural and apicobasal heterogeneity of depolarization and repolarization characteristics in the rainbow trout in-situ ventricular myocardium in summer and winter conditions.

Methods

The experiments were done in summer-acclimatized (SA, 18°C, n = 8) and winter-acclimatized (WA, 3°C, n = 8) rainbow trout (Oncorhynchus mykiss). 24 unipolar electrograms were recorded with 3 plunge needle electrodes (eight lead terminals each) impaled into the ventricular wall. Activation time (AT), end of repolarization time (RT), and activation-repolarization interval (ARI, a surrogate for action potential duration) were determined as dV/dt min during QRS-complex, dV/dt max during T-wave, and RT-AT difference, respectively.

Results

The SA fish demonstrated relatively flat apicobasal and transmural AT and ARI profiles. In the WA animals, ATs and ARIs were longer as compared to SA animals (p≤0.001), ARIs were shorter in the compact layer than in the spongy layer (p≤0.050), and within the compact layer, the apical region had shorter ATs and longer ARIs as compared to the basal region (p≤0.050). In multiple linear regression analysis, ARI duration was associated with RR-interval and AT in SA and WA animals. The WA animals additionally demonstrated an independent association of ARIs with spatial localization across the ventricle.

Conclusion

Cold conditions led to the spatial redistribution of repolarization durations in the rainbow trout ventricle and the formation of repolarization gradients typically observed in mammalian myocardium.

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Spatiotemporal electrophysiological pattern is essential for cardiac function.

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A role of this pattern is unclear, specifically in seasonal changes in fish.

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Transmural repolarization gradients develop in cold conditions in rainbow trout.

The relation of 12 lead ECG to the cardiac anatomy: The normal CineECG

BACKGROUND

The interpretation of the 12‑lead ECG is notoriously difficult and requires experts to distinguish normal from abnormal ECG waveforms. ECG waveforms depend on body build and electrode positions, both often different in males and females. To relate the ECG waveforms to cardiac anatomical structures is even more difficult. The novel CineECG algorithm enables a direct projection of the 12‑lead ECG to the cardiac anatomy by computing the mean location of cardiac activity over time. The aim of this study is to investigate the cardiac locations of the CineECG derived from standard 12‑lead ECGs of normal subjects.

METHODS

In this study we used 6525 12‑lead ECG tracings labelled as normal obtained from the certified Physionet PTB XL Diagnostic ECG Database to construct the CineECG. All 12 lead ECGs were analyzed, and then divided by age groups (18-29,30-39,40-49,50-59,60-69,70-100 years) and by gender (male/female). For each ECG, we computed the CineECG within a generic 3D heart/torso model. Based on these CineECG's, the average normal cardiac location and direction for QRS, STpeak, and TpeakTend segments were determined.

RESULTS

The CineECG direction for the QRS segment showed large variation towards the left free wall, whereas the STT segments were homogeneously directed towards the septal/apical region. The differences in the CineECG location for the QRS, STpeak, and TpeakTend between the age and gender groups were relatively small (maximally 10 mm at end T-wave), although between the gender groups minor differences were found in the 4 chamber direction angles (QRS 4°, STpeak 5°, and TpeakTend 8°) and LAO (QRS 1°, STpeak 13°, and TpeakTend 30°).

CONCLUSION

CineECG demonstrated to be a feasible and pragmatic solution for ECG waveform interpretation, relating the ECG directly to the cardiac anatomy. The variations in depolarization and repolarization CineECG were small within this group of normal healthy controls, both in cardiac location as well as in direction. CineECG may enable an easier discrimination between normal and abnormal QRS and T-wave morphologies, reducing the amount of expert training. Further studies are needed to prove whether novel CineECG can significantly contribute to the discrimination of normal versus abnormal ECG tracings.

Identification to cardiac conduction cells in humans and pigs according to their zonal distribution, using histological, immunohistochemical and morphometric study

Histologically, the cardiac conduction network is formed of electrically isolated subendocardial fibers that comprise specialized cells with fewer myofibrils and mitochondria than cardiomyocytes. Our aim is to uncover regional variations of cardiac conduction fibers through histological and morphometric study in a porcine and human model. We analyzed five male adult human hearts and five male pig hearts. The left ventricles were dissected and sectioned in the axial plane into three parts: basal, middle third and apex regions. Cardiac conduction fibers study was carried out using hematoxylin-eosin and Masson's trichrome staining, and cardiac conduction cells and their junctions were identified using desmin, and a PAS method. Cardiac conduction fibers were difficult to pinpoint in humans, mostly showing a darker color or equal to cardiomyocytes. Cardiac conduction fibers in humans were in the subendocardium and in pigs in the myocardium and subendocardium. Cardiac conduction fibers were located mainly in the septal region in both humans and pigs. In our morphometric analysis, we were able to determine that cardiac conduction cells in humans (18.52 +/- 5.41 μm) and pigs (21.32 +/- 6.45 μm) were large, compared to cardiomyocytes. Conduction fiber-myocardial junctions were present in 10% in humans and 24.2% in pigs. The performance of immunohistochemical methods made it possible to improve the identification of cardiac conduction cells in the species studied. Study of cardiac conduction fibers and cells and their myocardial junctions is vital to gain insight into their normal distribution in the species analyzed, and thus advance the use of pigs in experimental models of the cardiac conduction system in humans.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Dispersion of ventricular repolarization: Temporal and spatial

Repolarization heterogeneity (RH) is an intrinsic property of ventricular myocardium and the reason for T-wave formation on electrocardiogram (ECG). Exceeding the physiologically based RH level is associated with appearance of life-threatening ventricular arrhythmias and sudden cardiac death. In this regard, an accurate and comprehensive evaluation of the degree of RH parameters is of importance for assessment of heart state and arrhythmic risk. This review is devoted to comprehensive consideration of RH phenomena in terms of electrophysiological processes underlying RH, cardiac electric field formation during ventricular repolarization, as well as clinical significance of RH and its reflection on ECG parameters. The formation of transmural, apicobasal, left-to-right and anterior-posterior gradients of action potential durations and end of repolarization times resulting from the heterogenous distribution of repolarizing ion currents and action potential morphology throughout the heart ventricles, and the different sensitivity of myocardial cells in different ventricular regions to the action of pharmacological agents, temperature, frequency of stimulation, etc., are being discussed. The review is focused on the fact that RH has different aspects – temporal and spatial, global and local; ECG reflection of various RH aspects and their clinical significance are being discussed. Strategies for comprehensive assessment of ventricular RH using different ECG indices reflecting various RH aspects are presented.

Transgenic Rabbit Models in Proarrhythmia Research

Drug-induced proarrhythmia constitutes a potentially lethal side effect of various drugs. Most often, this proarrhythmia is mechanistically linked to the drug's potential to interact with repolarizing cardiac ion channels causing a prolongation of the QT interval in the ECG. Despite sophisticated screening approaches during drug development, reliable prediction of proarrhythmia remains very challenging. Although drug-induced long-QT-related proarrhythmia is often favored by conditions or diseases that impair the individual's repolarization reserve, most cellular, tissue, and whole animal model systems used for drug safety screening are based on normal, healthy models. In recent years, several transgenic rabbit models for different types of long QT syndromes (LQTS) with differences in the extent of impairment in repolarization reserve have been generated. These might be useful for screening/prediction of a drug's potential for long-QT-related proarrhythmia, particularly as different repolarizing cardiac ion channels are impaired in the different models. In this review, we summarize the electrophysiological characteristics of the available transgenic LQTS rabbit models, and the pharmacological proof-of-principle studies that have been performed with these models—highlighting the advantages and disadvantages of LQTS models for proarrhythmia research. In the end, we give an outlook on potential future directions and novel models.

Common Genetic Variants Modulate the Electrocardiographic Tpeak-to-Tend Interval

Sudden cardiac death is responsible for half of all deaths from cardiovascular disease. The analysis of the electrophysiological substrate for arrhythmias is crucial for optimal risk stratification. A prolonged T-peak-to-Tend (Tpe) interval on the electrocardiogram is an independent predictor of increased arrhythmic risk, and Tpe changes with heart rate are even stronger predictors. However, our understanding of the electrophysiological mechanisms supporting these risk factors is limited. We conducted genome-wide association studies (GWASs) for resting Tpe and Tpe response to exercise and recovery in ∼30,000 individuals, followed by replication in independent samples (∼42,000 for resting Tpe and ∼22,000 for Tpe response to exercise and recovery), all from UK Biobank. Fifteen and one single-nucleotide variants for resting Tpe and Tpe response to exercise, respectively, were formally replicated. In a full dataset GWAS, 13 further loci for resting Tpe, 1 for Tpe response to exercise and 1 for Tpe response to exercise were genome-wide significant (p ≤ 5 × 10-8). Sex-specific analyses indicated seven additional loci. In total, we identify 32 loci for resting Tpe, 3 for Tpe response to exercise and 3 for Tpe response to recovery modulating ventricular repolarization, as well as cardiac conduction and contraction. Our findings shed light on the genetic basis of resting Tpe and Tpe response to exercise and recovery, unveiling plausible candidate genes and biological mechanisms underlying ventricular excitability.

Low-Frequency Oscillations in Cardiac Sympathetic Neuronal Activity

Sudden cardiac death caused by ventricular arrhythmias is among the leading causes of mortality, with approximately half of all deaths attributed to heart disease worldwide. Periodic repolarization dynamics (PRD) is a novel marker of repolarization instability and strong predictor of death in patients post-myocardial infarction that is believed to occur in association with low-frequency oscillations in sympathetic nerve activity. However, this hypothesis is based on associations of PRD with indices of sympathetic activity that are not directly linked to cardiac function, such as muscle vasoconstrictor activity and the variability of cardiovascular autospectra. In this review article, we critically evaluate existing scientific evidence obtained primarily in experimental animal models, with the aim of identifying the neuronal networks responsible for the generation of low-frequency sympathetic rhythms along the neurocardiac axis. We discuss the functional significance of rhythmic sympathetic activity on neurotransmission efficacy and explore its role in the pathogenesis of ventricular repolarization instability. Most importantly, we discuss important gaps in our knowledge that require further investigation in order to confirm the hypothesis that low frequency cardiac sympathetic oscillations play a causative role in the generation of PRD.

Left ventricular endocardial pacing is less arrhythmogenic than conventional epicardial pacing when pacing in proximity to scar

Background

Epicardial pacing increases risk of ventricular tachycardia (VT) in patients with ischemic cardiomyopathy (ICM) when pacing in proximity to scar. Endocardial pacing may be less arrhythmogenic as it preserves the physiological sequences of activation and repolarization.

Objective

The purpose of this study was to determine the relative arrhythmogenic risk of endocardial compared to epicardial pacing, and the role of the transmural gradient of action potential duration (APD) and pacing location relative to scar on arrhythmogenic risk during endocardial pacing.

Methods

Computational models of ICM patients (n = 24) were used to simulate left ventricular (LV) epicardial and endocardial pacing 0.2–3.5 cm from a scar. Mechanisms were investigated in idealized models of the ventricular wall and scar. Simulations were run with/without a 20-ms transmural APD gradient in the physiological direction and with the gradient inverted. Dispersion of repolarization was computed as a surrogate of VT risk.

Results

Patient-specific models with a physiological APD gradient predict that endocardial pacing decreases VT risk (34%; P <.05) compared to epicardial pacing when pacing in proximity to scar (0.2 cm). Endocardial pacing location does not significantly affect VT risk, but epicardial pacing at 0.2 cm compared to 3.5 cm from scar increases it (P <.05). Inverting the transmural APD gradient reverses this trend. Idealized models predict that propagation in the direction opposite to APD gradient decreases VT risk.

Conclusion

Endocardial pacing is less arrhythmogenic than epicardial pacing when pacing proximal to scar and is less susceptible to pacing location relative to scar. The physiological repolarization sequence during endocardial pacing mechanistically explains reduced VT risk compared to epicardial pacing.

ECG markers of local but not global increase in dispersion of ventricular repolarization (simulation study)

BACKGROUND

An increase in local dispersion of repolarization (DOR) may contribute more to arrhythmogenesis as compared to changes of global DOR. The aim of this simulation study was to find ECG markers of local increase in DOR in conditions where global DOR remains normal.

METHODS

In the framework of van Oosterom and Oostendorp ECGSIM model, the local DOR was increased in 10 different ventricular locations by (1) action potential duration (APD) shortening/lengthening both on epi- and endocardium, (2) epicardial APD shortening, and (3) endocardial APD shortening. The simulation cases where the increase in local DOR was accompanied by increase in global DOR were excluded from consideration. T-wave parameters were analyzed in the simulated precordial and anatomically ordered limb leads.

RESULTS

The increase in local DOR resulted in increased lead-to‑lead differences in Tpeak and Tend instants in 28 out of 32 simulated scenarios, and in an increased dispersion of Tpeak-Tend interval throughout 12 standard leads in 8 out of 32 simulated scenarios. In all simulations, the global DOR measured as a difference between earliest and latest repolarization times and standard APD deviation was the same.

CONCLUSIONS

The local increase in DOR was expressed in increased lead-to‑lead differences in Tpeak and Tend instants between adjacent anatomically ordered standard leads (aVL, I, aVR(-), II, aVF, III, and V1-V6), even if global DOR, Tpeak-Tend interval and Tpeak-Tend dispersion were within a normal range.

Inhalation of printer-emitted particles impairs cardiac conduction, hemodynamics, and autonomic regulation and induces arrhythmia and electrical remodeling in rats

BACKGROUND

Using engineered nanomaterial-based toners, laser printers generate aerosols with alarming levels of nanoparticles that bear high bioactivity and potential health risks. Yet, the cardiac impacts of printer-emitted particles (PEPs) are unknown. Inhalation of particulate matter (PM) promotes cardiovascular morbidity and mortality, and ultra-fine particulates (< 0.1 μm aerodynamic diameter) may bear toxicity unique from larger particles. Toxicological studies suggest that PM impairs left ventricular (LV) performance; however, such investigations have heretofore required animal restraint, anesthesia, or ex vivo preparations that can confound physiologic endpoints and/or prohibit LV mechanical assessments during exposure. To assess the acute and chronic effects of PEPs on cardiac physiology, male Sprague Dawley rats were exposed to PEPs (21 days, 5 h/day) while monitoring LV pressure (LVP) and electrocardiogram (ECG) via conscious telemetry, analyzing LVP and heart rate variability (HRV) in four-day increments from exposure days 1 to 21, as well as ECG and baroreflex sensitivity. At 2, 35, and 70 days after PEPs exposure ceased, rats received stress tests.

RESULTS

On day 21 of exposure, PEPs significantly (P < 0.05 vs. Air) increased LV end systolic pressure (LVESP, + 18 mmHg) and rate-pressure-product (+ 19%), and decreased HRV indicating sympathetic dominance (root means squared of successive differences [RMSSD], - 21%). Overall, PEPs decreased LV ejection time (- 9%), relaxation time (- 3%), tau (- 5%), RMSSD (- 21%), and P-wave duration (- 9%). PEPs increased QTc interval (+ 5%) and low:high frequency HRV (+ 24%; all P < 0.05 vs. Air), while tending to decrease baroreflex sensitivity and contractility index (- 15% and - 3%, P < 0.10 vs. Air). Relative to Air, at both 2 and 35 days after PEPs, ventricular arrhythmias increased, and at 70 days post-exposure LVESP increased. PEPs impaired ventricular repolarization at 2 and 35 days post-exposure, but only during stress tests. At 72 days post-exposure, PEPs increased urinary dopamine 5-fold and protein expression of ventricular repolarizing channels, Kv1.5, Kv4.2, and Kv7.1, by 50%.

CONCLUSIONS

Our findings suggest exposure to PEPs increases cardiovascular risk by augmenting sympathetic influence, impairing ventricular performance and repolarization, and inducing hypertension and arrhythmia. PEPs may present significant health risks through adverse cardiovascular effects, especially in occupational settings, among susceptible individuals, and with long-term exposure.

IK1 channel agonist zacopride suppresses ventricular arrhythmias in conscious rats with healing myocardial infarction

AIMS

Arrhythmogenesis of chronic myocardial infarction (MI) is associated with the prolongation of action potential, reduction of inward rectifier potassium (I_K1, Kir) channels and hyper-activity of Calcium/calmodulin-dependent kinase II (CaMKII) in cardiomyocytes. Zacopride, a selective I_K1 agonist, was applied to clarify the cardioprotection of I_K1 agonism via a CaMKII signaling on arrhythmias post-MI.

METHODS

Male SD rats were implanted wireless transmitter in the abdominal cavity and subjected to left main coronary artery ligation or sham operation. The telemetric ECGs were monitored per day throughout 4 weeks. At the endpoint, isoproterenol (1.28 mg/kg, i.v.) was administered for provocation test. The expressions of Kir2.1 (dominant subunit of I_K1 in ventricle) and CaMKII were detected by Western-blotting.

KEY FINDINGS

In the telemetric rats post-MI, zacopride significantly reduced the episodes of atrioventricular conduction block (AVB), premature ventricular contraction (PVC), ventricular tachycardia (VT) and ventricular fibrillation (VF), without significant effect on superventricular premature contraction (SPVC). In provocation test, zacopride suppressed the onset of ventricular arrhythmias in conscious PMI or sham rats. The expression of Kir2.1 was significantly downregulated and p-CaMKII was upregulated post-MI, whereas both were restored by zacopride treatment.

SIGNIFICANCE

I_K1/Kir2.1 might be an attractive target for pharmacological controlling of lethal arrhythmias post MI.

Prolonged Tpeak -Tend interval is a risk factor for sudden cardiac death in adults with congenital heart disease

Objective

Adult congenital heart disease (ACHD) patients are at risk of sudden cardiac death (SCD). However, methods for risk stratification are not yet well‐defined. The T_peak‐T_end (TpTe) interval, a measure of dispersion of ventricular repolarization, is a risk factor for SCD in non‐ACHD patients. We aim to evaluate whether TpTe can be used in risk stratification for SCD in ACHD patients.

Design

From an international multicenter cohort of 25 790 ACHD patients, we identified all SCD cases. Cases were matched to controls by age, gender, congenital defect, and (surgical) intervention.

Outcome Measures

TpTe was measured on a standard 12‐lead ECG. The maximum TpTe of all ECG leads (TpTe‐max), mean (TpTe‐mean), and TpTe dispersion (maximum minus minimum) were obtained. Odds ratios (OR) for SCD cases vs controls were calculated using conditional logistic regression analysis.

Results

ECGs were available for 147 cases (median age at death 33.5 years (quartiles 26.2, 48.7), 66% male) and 267 controls. The mean TpTe‐max was 97 ± 24 ms in cases vs 84 ± 17 ms in controls (P < .001); TpTe‐mean was 70 ± 16 vs 63 ± 10 ms (P < .001); and dispersion was 51 ± 22 ms vs 41 ± 16 ms (P = .02), respectively. Assessing each ECG lead separately, TpTe in lead aVR predicted SCD most accurately. TpTe in lead aVR was 71 ± 23 ms in cases vs 61 ± 13 ms in controls (P < .001). After adjusting for impaired ventricular function, heart failure symptoms, and prolonged QRS duration, the OR of SCD of TpTe in lead aVR at an optimal cutoff of 80 ms was 5.8 (95% CI 2.7‐12.4, P < .001).

Conclusions

The TpTe interval is associated with SCD in ACHD patients. Particularly, TpTe in lead aVR can be used as an independent risk factor for SCD in ACHD patients and may, therefore, add precision to current risk prediction models.

Differences in the upslope of the precordial body surface ECG T wave reflect right to left dispersion of repolarization in the intact human heart

BACKGROUND

The relationship between the surface electrocardiogram (ECG) T wave to intracardiac repolarization is poorly understood.

OBJECTIVE

The purpose of this study was to examine the association between intracardiac ventricular repolarization and the T wave on the body surface ECG (SECGTW).

METHODS

Ten patients with a normal heart (age 35 ± 15 years; 6 men) were studied. Decapolar electrophysiological catheters were placed in the right ventricle (RV) and lateral left ventricle (LV) to record in an apicobasal orientation and in the lateral LV branch of the coronary sinus (CS) for transmural recording. Each catheter (CS, LV, RV) was sequentially paced using an S1-S2 restitution protocol. Intracardiac repolarization time and apicobasal, RV-LV, and transmural repolarization dispersion were correlated with the SECGTW, and a total of 23,946 T waves analyzed.

RESULTS

RV endocardial repolarization occurred on the upslope of lead V1, V2, and V3 SECGTW, with sensitivity of 0.89, 0.91, and 0.84 and specificity of 0.67, 0.68, and 0.65, respectively. LV basal endocardial, epicardial, and mid-endocardial repolarization occurred on the upslope of leads V6 and I, with sensitivity of 0.79 and 0.8 and specificity of 0.66 and 0.67, respectively. Differences between the end of the upslope in V1, V2, and V3 vs V6 strongly correlated with right to left dispersion of repolarization (intraclass correlation coefficient 0.81, 0.83, and 0.85, respectively; P <.001). Poor association between the T wave and apicobasal and transmural dispersion of repolarization was seen.

CONCLUSION

The precordial SECGTW reflects regional repolarization differences between right and left heart. These findings have important implications for accurately identifying biomarkers of arrhythmogenic risk in disease.

I feel what I do: Relating interoceptive processes and reward-related behavior

Interoceptive signalling has been shown to contribute to action regulation and action experience. Here, we assess whether motor behaviour can be influenced by anticipated homeostatic feeling states induced through different predictable contexts. Participants performed a reward incentive paradigm in which accurate responses increased (gain) or avoided the depletion (averted loss) of a credit score. Across two types of blocks, we varied the predictability of the outcome state. In predictable blocks, a cue signaled a gain, loss or control trial (motor response did not affect the credit score). This allowed participants to anticipate the interoceptive feeling state associated with the outcome. In unpredictable blocks, the cue had no relation to the type of outcome. Thus, participants were unable to anticipate the feeling state it produced. Via EEG, we measured the Heartbeat Evoked Potential (HEP) and the Contingent Negative Variation (CNV) as indices of interoceptive and motor processing respectively. In addition, we measured feedback P3 amplitude following outcome presentation and accuracy and reaction times of the required motor response. We observed higher HEP and CNV amplitudes as well as faster and more accurate motor responses in predictable compared to unpredictable outcome blocks. Similarly, feedback-related P3 amplitudes were significantly lower for predictable relative to unpredictable outcomes. Crucially, HEP amplitudes measured prior to feedback predicted feedback-related P3 amplitudes for anticipated outcome events. Results suggest that accurate anticipation of homeostatic feeling states associated with gain, loss or control outcomes facilitates motor execution and outcome evaluation. Findings are hereby the first to empirically assess the link between interoceptive and motor domains and provide primary evidence for a joint processing structure.

Arrhythmogenic drugs can amplify spatial heterogeneities in the electrical restitution in perfused guinea-pig heart: An evidence from assessments of monophasic action potential durations and JT intervals

Non-uniform shortening of the action potential duration (APD90) in different myocardial regions upon heart rate acceleration can set abnormal repolarization gradients and promote arrhythmia. This study examined whether spatial heterogeneities in APD90 restitution can be amplified by drugs with clinically proved proarrhythmic potential (dofetilide, quinidine, procainamide, and flecainide) and, if so, whether these effects can translate to the appropriate changes of the ECG metrics of ventricular repolarization, such as JT intervals. In isolated, perfused guinea-pig heart preparations, monophasic action potentials and volume-conducted ECG were recorded at progressively increased pacing rates. The APD90 measured at distinct ventricular sites, as well as the JTpeak and JTend values were plotted as a function of preceding diastolic interval, and the maximum slopes of the restitution curves were determined at baseline and upon drug administration. Dofetilide, quinidine, and procainamide reverse rate-dependently prolonged APD90 and steepened the restitution curve, with effects being greater at the endocardium than epicardium, and in the right ventricular (RV) vs. the left ventricular (LV) chamber. The restitution slope was increased to a greater extent for the JTend vs. the JTpeak interval. In contrast, flecainide reduced the APD90 restitution slope at LV epicardium without producing effect at LV endocardium and RV epicardium, and reduced the JTpeak restitution slope without changing the JTend restitution. Nevertheless, with all agents, these effects translated to the amplified epicardial-to-endocardial and the LV-to-RV non-uniformities in APD90 restitution, paralleled by the increased JTend vs. JTpeak difference in the restitution slope. In summary, these findings suggest that arrhythmic drug profiles are partly attributable to the accentuated regional heterogeneities in APD90 restitution, which can be indirectly determined through ECG assessments of the JTend vs. JTpeak dynamics at variable pacing rates.

Mechanism of electrocardiographic T-wave flattening in diabetes mellitus: experimental and simulation study

In the present study we investigated the contribution of ventricular repolarization time (RT) dispersion (the maximal difference in RT) and RT gradients (the differences in RT in apicobasal, anteroposterior and interventricular directions) to T-wave flattening in a setting of experimental diabetes mellitus. In 9 healthy and 11 diabetic (alloxan model) open-chest rabbits, we measured RT in ventricular epicardial electrograms. To specify the contributions of apicobasal, interventricular and anteroposterior RT gradients and RT dispersion to the body surface potentials we determined T-wave voltage differences between modified upper- and lower-chest precordial leads (T-wave amplitude dispersions, TWAD). Expression of RT gradients and RT dispersion in the correspondent TWAD parameters was studied by computer simulations. Diabetic rabbits demonstrated flattened T-waves in precordial leads associated with increased anteroposterior and decreased apicobasal RT gradients (P<0.05) due to RT prolongation at the apex. For diabetics, simulations predicted the preserved T-vector length and altered sagittal and longitudinal TWAD proven by experimental measurements. T-wave flattening in the diabetic rabbits was not due to changes in RT dispersion, but reflected the redistributed ventricular repolarization pattern with prolonged apical repolarization resulting in increased anteroposterior and decreased apicobasal RT gradients.

Experimental Validation of Noninvasive Epicardial and Endocardial Activation Imaging

BACKGROUND

Noninvasive imaging of cardiac activation before ablation of the arrhythmogenic substrate can reduce electrophysiological procedure duration and help choosing between an endocardial or epicardial approach. A noninvasive imaging technique was evaluated that estimates both endocardial and epicardial activation from body surface potential maps. We performed a study in isolated and in situ pig hearts, estimating activation from body surface potential maps during sinus rhythm and localizing endocardial and epicardial stimulation sites.

METHODS AND RESULTS

From 3 Langendorff-perfused pig hearts, 180 intramural unipolar electrograms were recorded during sinus rhythm and ectopic activation, together with pseudo-body surface potential map ECGs in 2 of them. From 4 other anesthetized pigs, 64-lead body surface potential maps were recorded during sinus rhythm and ventricular stimulation from 27 endocardial and epicardial sites. The ventricular activation pattern was computed from the recorded QRS complexes. For both Langendorff-perfused hearts, the calculated epicardial and endocardial activation patterns showed good qualitative correspondence to the patterns obtained with needle electrodes. Absolute timing difference for sinus rhythm was 10±5 and 11±8 ms respectively, and for ectopic activation 6±5 and 7±6 ms, respectively. Calculated activation for the in situ hearts in sinus rhythm was similar to patterns recorded in Langendorff-perfused hearts. During stimulation, the distance between the stimulation site and calculated site of earliest activation was 18 (15-27) mm, and 23 of 27 stimulation sites were correctly mapped to either endocardium or epicardium.

CONCLUSIONS

Noninvasive activation imaging is able to determine earliest ventricular activation and discriminate endocardial from epicardial origin of activation with clinically relevant accuracy.

T-Wave Morphology Restitution Predicts Sudden Cardiac Death in Patients With Chronic Heart Failure

BACKGROUND

Patients with chronic heart failure are at high risk of sudden cardiac death (SCD). Increased dispersion of repolarization restitution has been associated with SCD, and we hypothesize that this should be reflected in the morphology of the T-wave and its variations with heart rate. The aim of this study is to propose an electrocardiogram (ECG)-based index characterizing T-wave morphology restitution (TMR), and to assess its association with SCD risk in a population of chronic heart failure patients.

METHODS AND RESULTS

Holter ECGs from 651 ambulatory patients with chronic heart failure from the MUSIC (MUerte Súbita en Insuficiencia Cardiaca) study were available for the analysis. TMR was quantified by measuring the morphological variation of the T-wave per RR increment using time-warping metrics, and its predictive power was compared to that of clinical variables such as the left ventricular ejection fraction and other ECG-derived indices, such as T-wave alternans and heart rate variability. TMR was significantly higher in SCD victims than in the rest of patients (median 0.046 versus 0.039, P<0.001). When TMR was dichotomized at TMR=0.040, the SCD rate was significantly higher in the TMR≥0.040 group (P<0.001). Cox analysis revealed that TMR≥0.040 was strongly associated with SCD, with a hazard ratio of 3.27 (P<0.001), independently of clinical and ECG-derived variables. No association was found between TMR and pump failure death.

CONCLUSIONS

This study shows that TMR is specifically associated with SCD in a population of chronic heart failure patients, and it is a better predictor than clinical and ECG-derived variables.

Mechano-electric coupling, heterogeneity in repolarization and the electrocardiographic T-wave

Stretch influences repolarization by mechano-electric coupling (MEC) and contributes to arrhythmogenesis. Although there is an abundance of research on electrophysiological effects of MEC, it is still unclear how MEC translates to the ECG. We aim to provide an overview of the MEC research focused on the ECG and the underlying changes in electrophysiology. In addition, we present new data on the effect of left ventricular pressure on the electrocardiographic T-wave. We show that an increase in left ventricular pressure leads to prolonged QT-intervals with increased amplitudes of the STT-segment. This corresponds to a prolongation in repolarization and an increased interventricular dispersion of repolarization. MEC is dependent on timing, intensity and modality of stretch and these three factors should be taken into account to analyse the effects of MEC on the heart and on the ECG. In addition, the deformation of the heart itself should be considered, since it influences the amplitude of the STT-segment. Because the electrocardiographic T-wave represents heterogeneity in repolarization, left ventricular pressure increases may have significant influence on the inducibility of (re-entrant) arrhythmias.

A quantitative structural and morphometric analysis of the Purkinje network and the Purkinje-myocardial junctions in pig hearts

The morpho-functional properties of the distal section of the cardiac Purkinje network (PN) and the Purkinje-myocardial junctions (PMJs) are fundamental to understanding the sequence of electrical activation in the heart. The overall structure of the system has already been described, and several computational models have been developed to gain insight into its involvement in cardiac arrhythmias or its interaction with implantable devices, such as pacemakers. However, anatomical descriptions of the PN in the literature have not enabled enough improvements in the accuracy of anatomical-based electrophysiological simulations of the PN in 3D hearts models. In this work, we study the global distribution and morphological properties of the PN, with special emphasis on the cellular and architectural characterization of its intramural branching structure, mesh-like sub-endocardial network, and the PMJs in adult pig hearts by both histopathological and morphometric evaluation. We have defined three main patterns of PMJ: contact through cell bodies, contact through cell prolongations either thick or piliform, and contact through transitional cells. Moreover, from hundreds of micrographs, we quantified the density of PMJs and provided data for the basal/medial/apical regions, anterior/posterior/septal/lateral regions and myocardial/sub-endocardial distribution. Morphometric variables, such as Purkinje cell density and thickness of the bundles, were also analyzed. After combining the results of these parameters, a different septoanterior distribution in the Purkinje cell density was observed towards the cardiac apex, which is associated with a progressive thinning of the conduction bundles and the posterolateral ascension of intramyocardial terminal scattered fibers. The study of the PMJs revealed a decreasing trend towards the base that may anatomically explain the early apical activation. The anterolateral region contains the greatest number of contacts, followed by the anterior and septal regions. This supports the hypothesis that thin distal Purkinje bundles create a junction-rich network that may be responsible for the quick apical depolarization. The PN then ascends laterally and spreads through the anterior and medial walls up to the base. We have established the first morphometric study of the Purkinje system, and provided quantitative and objective data that facilitate its incorporation into the development of models beyond gross and variable pathological descriptions, and which, after further studies, could be useful in the characterization of pathological processes or therapeutic procedures.

Automated Algorithm for J-Tpeak and Tpeak-Tend Assessment of Drug-Induced Proarrhythmia Risk

BACKGROUND

Prolongation of the heart rate corrected QT (QTc) interval is a sensitive marker of torsade de pointes risk; however it is not specific as QTc prolonging drugs that block inward currents are often not associated with torsade. Recent work demonstrated that separate analysis of the heart rate corrected J-Tpeakc (J-Tpeakc) and Tpeak-Tend intervals can identify QTc prolonging drugs with inward current block and is being proposed as a part of a new cardiac safety paradigm for new drugs (the "CiPA" initiative).

METHODS

In this work, we describe an automated measurement methodology for assessment of the J-Tpeakc and Tpeak-Tend intervals using the vector magnitude lead. The automated measurement methodology was developed using data from one clinical trial and was evaluated using independent data from a second clinical trial.

RESULTS

Comparison between the automated and the prior semi-automated measurements shows that the automated algorithm reproduces the semi-automated measurements with a mean difference of single-deltas <1 ms and no difference in intra-time point variability (p for all > 0.39). In addition, the time-profile of the baseline and placebo-adjusted changes are within 1 ms for 63% of the time-points (86% within 2 ms). Importantly, the automated results lead to the same conclusions about the electrophysiological mechanisms of the studied drugs.

CONCLUSIONS

We have developed an automated algorithm for assessment of J-Tpeakc and Tpeak-Tend intervals that can be applied in clinical drug trials. Under the CiPA initiative this ECG assessment would determine if there are unexpected ion channel effects in humans compared to preclinical studies. The algorithm is being released as open-source software.

TRIAL REGISTRATION

NCT02308748 and NCT01873950.

Transmural, interventricular, apicobasal and anteroposterior action potential duration gradients are all essential to the genesis of the concordant and realistic T wave: A whole-heart model study

BACKGROUND

It has been reported that ventricular repolarization dispersion resulting from transmural, apicobasal and interventricular action potential duration (APD) gradients makes the T wave concordant with the QRS complex.

METHOD AND RESULTS

A whole-heart model integrating transmural, apicobasal, interventricular and anteroposterior APD gradients was used, and the corresponding electrocardiograms were simulated to study the influence of these APD gradients on the T-wave amplitudes. The simulation results showed that changing a single APD gradient (e.g., interventricular APD gradient alone) only made substantial changes to the T-wave amplitudes in a limited number of leads and was not able to generate T waves with amplitudes comparable with clinical findings in all leads. A combination of transmural, apicobasal and interventricular APD gradients could simulate T waves with amplitudes similar to clinical values in the limb leads only. Adding the anteroposterior APD gradient into the model greatly improved the consistency between the simulated T-wave amplitudes and the clinical values.

CONCLUSION

The simulation results support that the transmural, apicobasal, interventricular and the anteroposterior APD gradient are all essential to the genesis of the clinical T wave.

Dispersion in ventricular repolarization in the human, canine and porcine heart

Dispersion in repolarization is important for the genesis of the T wave, and for the induction of reentrant arrhtyhmias. Because the T wave differs across species our intent here is to review the epicardial, endocardial and transmural repolarization patterns contributing to repolarization in whole hearts from man, dog and pig. The major points we emphasize are: transmural repolarization time gradients are small and are directed from endocardium (early) to epicardium (late) in dog and human and from epicardium to endocardium in pig; the right ventricle tends to repolarize before the left ventricle and this difference is larger in dog than in pig; a negative relation between the activation times and the repolarization times is rare in man, and absent in dog and pig. Given the above, a large dispersion in repolarization between two myocardial areas does not lead to arrhythmias without a premature beat. Moreover, an arrhythmic substrate can be identified by a metric composed of activation times and repolarization times, the reentry vulnerability index, RVI.

Modeling our understanding of the His-Purkinje system

The His-Purkinje System (HPS) is responsible for the rapid electric conduction in the ventricles. It relays electrical impulses from the atrioventricular node to the muscle cells and, thus, coordinates the contraction of ventricles in order to ensure proper cardiac pump function. The HPS has been implicated in the genesis of ventricular tachycardia and fibrillation as a source of ectopic beats, as well as forming distinct portions of reentry circuitry. Despite its importance, it remains much less well characterized, structurally and functionally, than the myocardium. Notably, important differences exist with regard to cell structure and electrophysiology, including ion channels, intracellular calcium handling, and gap junctions. Very few computational models address the HPS, and the majority of organ level modeling studies omit it. This review will provide an overview of our current knowledge of structure and function (including electrophysiology) of the HPS. We will review the most recent advances in modeling of the system from the single cell to the organ level, with considerations for relevant interspecies distinctions.

Transmural APD gradient synchronizes repolarization in the human left ventricular wall

AIMS

The duration and morphology of the T wave predict risk for ventricular fibrillation. A transmural gradient in action potential duration (APD) in the ventricular wall has been suggested to underlie the T wave in humans. We hypothesize that the transmural gradient in APD compensates for the normal endocardium-to-epicardium activation sequence and synchronizes repolarization in the human ventricular wall.

METHODS AND RESULTS

We made left ventricular wedge preparations from 10 human donor hearts and measured transmural activation and repolarization patterns by optical mapping, while simultaneously recording a pseudo-ECG. We also studied the relation between local timings of repolarization with the T wave in silico. During endocardial pacing (1 Hz), APD was longer at the subendocardium than at the subepicardium (360 ± 17 vs. 317 ± 20 ms, P < 0.05). The transmural activation time was 32 ± 4 ms and resulted in final repolarization of the subepicardium at 349 ± 18 ms. The overall transmural dispersion in repolarization time was smaller than that of APD. During epicardial pacing, the dispersion in repolarization time increased, whereas that of APD remained similar. The morphology of the T wave did not differ between endocardial and epicardial stimulation. Simulations explained the constant T wave morphology without transmural APD gradients.

CONCLUSION

The intrinsic transmural difference in APD compensates for the normal cardiac activation sequence, resulting in more homogeneous repolarization of the left ventricular wall. Our data suggest that the transmural repolarization differences do not fully explain the genesis of the T wave.

Synchronization of repolarization by mechano-electrical coupling in the porcine heart

AIMS

The aim of this study was to evaluate the effect of increase in left ventricular (LV) pressure on repolarization and activation-recovery intervals.

METHODS AND RESULTS

Six pig hearts were Langendorff-perfused. A compliant liquid-filled balloon, connected with a pressure transducer, inserted through the mitral orifice, could be filled until the required LV systolic pressure was obtained. A grid of 121 electrodes (11 × 11; 5 mm interelectrode distance) was sutured on the LV free wall. Ventricular pacing at 600 ms and at 400 or 450 ms was either performed from the LV wall or from the ventricular septum. Under all these four conditions, the pressure wave occurred at the same moment relative to the onset of the QRS complex. Consequently, the time relation between local repolarization and the pressure wave differed between the various pacing sites. Repolarization times (RTs) at a cycle length (CL) of 600 ms were prolonged by increased pressure. With stimulation from the LV, when the pressure wave coincides with the action potentials (APs) late in their phase (sites with relatively early repolarization), an increase in pressure from 0 to 100 mmHg delayed repolarization more than with stimulation from the septum, when the pressure wave occurs at a relatively earlier phase of the AP (sites with relatively late repolarization). At pacing at CL 400/450 ms, an increase in pressure caused RT prolongation at the LV free wall during LV stimulation, but less RT prolongation or even shortening during septal stimulation.

CONCLUSION

The effect of increased LV pressure is synchronization of repolarization.

Action potential duration gradients in the heart ventricles and the cardiac electric field during ventricular repolarization (a model study)

BACKGROUND

We simulated contributions of transmural, apicobasal, anteroposterior and interventricular action potential duration (APD) gradients to the body surface potential distribution (BSPD) with constant or varied magnitudes of the transmural and apicobasal gradients.

METHODS

Simulations were done in the framework of the discrete computer model of the rabbit heart ventricles on the basis of realistic activation sequence and APDs. The APD gradients were set constant at 20 ms or varied in the range of ±80 ms.

RESULTS

The apicobasal, transmural and interventricular APD gradients of 20 ms produced similar BSPDs, whereas the BSPD inversion was caused by the inverted apicobasal or transmural 80 ms gradients. The transmural APD gradient produced transversal and mainly apicobasal T-wave vectors due to wall curvature and cancellation effects. The "normal" transversal and apicobasal repolarization gradients were decreased and increased by activation sequence, respectively.

CONCLUSION

The different APD gradients contributed consistently to the development of BSPD.

Misinterpretation of the mouse ECG: 'musing the waves of Mus musculus'

The ECG is a primary diagnostic tool in patients suffering from heart disease, underscoring the importance of understanding factors contributing to normal and abnormal electrical patterns. Over the past few decades, transgenic mouse models have been increasingly used to study pathophysiological mechanisms of human heart diseases. In order to allow extrapolation of insights gained from murine models to the human condition, knowledge of the similarities and differences between the mouse and human ECG is of crucial importance. In this review, we briefly discuss the physiological mechanisms underlying differences between the baseline ECG of humans and mice, and provide a framework for understanding how these inherent differences are relevant to the interpretation of the mouse ECG during pathology and to the translation of the results from the mouse to man.