Effects of cardiac microstructure on propagating electrical waveforms

Simulation of Ectopic Pacemakers in the Heart: Multiple Ectopic Beats Generated by Reentry inside Fibrotic Regions

The inclusion of nonconducting media, mimicking cardiac fibrosis, in two models of cardiac tissue produces the formation of ectopic beats. The fraction of nonconducting media in comparison with the fraction of healthy myocytes and the topological distribution of cells determines the probability of ectopic beat generation. First, a detailed subcellular microscopic model that accounts for the microstructure of the cardiac tissue is constructed and employed for the numerical simulation of action potential propagation. Next, an equivalent discrete model is implemented, which permits a faster integration of the equations. This discrete model is a simplified version of the microscopic model that maintains the distribution of connections between cells. Both models produce similar results when describing action potential propagation in homogeneous tissue; however, they slightly differ in the generation of ectopic beats in heterogeneous tissue. Nevertheless, both models present the generation of reentry inside fibrotic tissues. This kind of reentry restricted to microfibrosis regions can result in the formation of ectopic pacemakers, that is, regions that will generate a series of ectopic stimulus at a fast pacing rate. In turn, such activity has been related to trigger fibrillation in the atria and in the ventricles in clinical and animal studies.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

Organ explant culture of neonatal rat ventricles: a new model to study gene and cell therapy

Testing cardiac gene and cell therapies in vitro requires a tissue substrate that survives for several days in culture while maintaining its physiological properties. The purpose of this study was to test whether culture of intact cardiac tissue of neonatal rat ventricles (organ explant culture) may be used as a model to study gene and cell therapy. We compared (immuno) histology and electrophysiology of organ explant cultures to both freshly isolated neonatal rat ventricular tissue and monolayers. (Immuno) histologic studies showed that organ explant cultures retained their fiber orientation, and that expression patterns of α-actinin, connexin-43, and α-smooth muscle actin did not change during culture. Intracellular voltage recordings showed that spontaneous beating was rare in organ explant cultures (20%) and freshly isolated tissue (17%), but common (82%) in monolayers. Accordingly, resting membrane potential was -83.9±4.4 mV in organ explant cultures, -80.5±3.5 mV in freshly isolated tissue, and -60.9±4.3 mV in monolayers. Conduction velocity, measured by optical mapping, was 18.2±1.0 cm/s in organ explant cultures, 18.0±1.2 cm/s in freshly isolated tissue, and 24.3±0.7 cm/s in monolayers. We found no differences in action potential duration (APD) between organ explant cultures and freshly isolated tissue, while APD of monolayers was prolonged (APD at 70% repolarization 88.8±7.8, 79.1±2.9, and 134.0±4.5 ms, respectively). Organ explant cultures and freshly isolated tissue could be paced up to frequencies within the normal range for neonatal rat (CL 150 ms), while monolayers could not. Successful lentiviral (LV) transduction was shown via Egfp gene transfer. Co-culture of organ explant cultures with spontaneously beating cardiomyocytes increased the occurrence of spontaneous beating activity of organ explant cultures to 86%. We conclude that organ explant cultures of neonatal rat ventricle are structurally and electrophysiologically similar to freshly isolated tissue and a suitable new model to study the effects of gene and cell therapy.

Simulations of complex and microscopic models of cardiac electrophysiology powered by multi-GPU platforms

Key aspects of cardiac electrophysiology, such as slow conduction, conduction block, and saltatory effects have been the research topic of many studies since they are strongly related to cardiac arrhythmia, reentry, fibrillation, or defibrillation. However, to reproduce these phenomena the numerical models need to use subcellular discretization for the solution of the PDEs and nonuniform, heterogeneous tissue electric conductivity. Due to the high computational costs of simulations that reproduce the fine microstructure of cardiac tissue, previous studies have considered tissue experiments of small or moderate sizes and used simple cardiac cell models. In this paper, we develop a cardiac electrophysiology model that captures the microstructure of cardiac tissue by using a very fine spatial discretization (8 μm) and uses a very modern and complex cell model based on Markov chains for the characterization of ion channel's structure and dynamics. To cope with the computational challenges, the model was parallelized using a hybrid approach: cluster computing and GPGPUs (general-purpose computing on graphics processing units). Our parallel implementation of this model using a multi-GPU platform was able to reduce the execution times of the simulations from more than 6 days (on a single processor) to 21 minutes (on a small 8-node cluster equipped with 16 GPUs, i.e., 2 GPUs per node).

Computer simulation of ECG manifestations of left ventricular electrical remodeling

An increased QRS voltage is considered to be specific for the electrocardiogram (ECG) diagnosis of left ventricular hypertrophy (LVH). However, the QRS-complex patterns in patients with LVH cover a broader spectrum: increased QRS voltage, prolonged QRS duration, left axis deviation, and left anterior fascicular block- and left bundle branch block-like patterns, as well as pseudo-normal QRS patterns. The classical interpretation of the QRS patterns in LVH relates these changes to increased left ventricular mass (LVM) per se, while tending to neglect the modified active and passive electrical properties of the myocardium. However, it has been well documented that both active and passive electrical properties in LVH are altered. Using computer simulations, we have shown that an increased LVM is not the only determinant of QRS complex changes in LVH, as these changes could also be produced without changing the left ventricular mass, implying that these QRS patterns can be present in patients before their LVM exceeds the arbitrary upper normal limits. Our results link the experimental evidence on electrical remodeling with clinical interpretation of ECG changes in patients with LVH and stress the necessity of a complex interpretation of the QRS patterns considering both spatial and nonspatial determinants in terms of the spatial angle theory. We assume that hypertrophic electrical remodeling in combination with changes in left ventricular size and shape explains the variety of ECG patterns as well as the discrepancies between ECG and left ventricular mass.

MR diffusion tensor imaging study of postinfarct myocardium structural remodeling in a porcine model

This study aimed to investigate postinfarct left ventricular (LV) fiber structural alterations by ex vivo diffusion tensor imaging (DTI) in a porcine heart model. In vivo cardiac MR imaging was first performed to measure ventricular function in six adult pigs with septal infarction near apex induced by the LAD ligation 13 weeks earlier. Hearts were then excised from the infarct pigs (n = 6) and six intact controls (n = 6) and fixed in formalin. High-resolution DTI was employed to examine changes in fractional anisotropy (FA), apparent diffusion coefficient (ADC), and transmural helix angle distribution in the infarct, adjacent and remote regions as compared to the sham regions in the controls. FA values were found to decrease in the infarct and differ between the adjacent and remote regions. ADC increase in the infarct region was substantial, while changes in the adjacent and remote regions were insignificant. Structurally, the double-helix myocardial structure shifted toward more left-handed around the infarcted myocardium. Accordingly, the histological analysis revealed clear fiber structural degradation in the adjacent region. These findings confirmed the subtle alterations in the myocardial fiber quality and structure not only in the infarcted but also in the surrounding noninfarcted myocardium or borderzone.

Electrical and structural remodeling in left ventricular hypertrophy-a substrate for a decrease in QRS voltage?

Electrical remodeling in advanced stages of cardiovascular diseases creates a substrate for triggering and maintenance of arrhythmias. The electrical remodeling is a continuous process initiated already in the early stages of cardiological pathology. The aim of this opinion article was to discuss the changes in electrical properties of myocardium in left ventricular hypertrophy (LVH), with special focus on its early stage, as well as their possible reflection in the QRS amplitude of the electrocardiogram. It critically appraises the classical hypothesis related to the QRS voltage changes in LVH. The hypothesis of the relative voltage deficit is discussed in the context of supporting evidence from clinical studies, animal experiments, and simulation studies. The underlying determinants of electrical impulse propagation which may explain discrepancies between "normal" ECG findings and increased left ventricular size/mass in LVH are reviewed.

Study of myocardial fiber pathway using magnetic resonance diffusion tensor imaging

The purpose of this study was to investigate myocardial fiber pathway distribution in order to provide supplemental information on myocardial fiber architecture and cardiac mechanics. Diffusion tensor imaging (DTI) with medium diffusion resolution (15 directions) was performed on normal canine heart samples (N=6) fixed in formalin. With the use of diffusion tensor fiber tracking, left ventricle (LV) myocardial fiber pathways and helix angles were computed pixel by pixel at short-axis slices from base to apex. Distribution of DTI-tracked fiber pathway length and number was analyzed quantitatively as a function of fiber helix angle in step of 9 degrees . The long fiber pathways were found to have small helix angles. They are mostly distributed in the middle myocardium and run circumferentially. Fiber pathways tracked at the middle and upper LV are generally longer than those near the apex. Majority of fiber pathways have small helix angles between -20 degrees and 20 degrees , dominating the fiber architecture in myocardium. Likely, such myocardial fiber pathway measurement by DTI may reflect the spatial connectiveness or connectivity of elastic myofiber bundles along their preferential pathway of electromechanical activation. The dominance of the long and circumferentially running fiber pathways found in the study may explain the circumferential predominance in left ventricular contraction.

Fragmented, Long-Duration, Low-Amplitude Electrograms Characterize the Origin of Focal Atrial Tachycardia

BACKGROUND

Focal atrial tachycardias (FAT) originate from areas with poor cell-to-cell coupling. Due to cellular uncoupling extracellular potentials become fractionated. The degree of fragmentation may be used to identify the site of origin of FAT prior to catheter ablation. We studied electrical fragmentation in relation to the distance to the site of earliest activity during FAT.

METHODS

Three-dimensional (3-D) electroanatomical activation/voltage maps obtained from patients (n = 15: 6 male, age 40 +/- 14 (19-72) years) referred for catheter ablation of FAT were analyzed. Bipolar atrial potentials (BP) were categorized according to the number of deflections. The peak-to-peak amplitude and the time interval between the first and last deflection (fractionation duration) of each BP were measured.

RESULTS

Eighteen different atrial tachycardias (AT) (CL 346 +/- 109 [190-550]) msec were analyzed. The incidence of single potentials ranged from 30 to 81 (59 +/- 16)%. The occurrence of double and fractionated potentials < or = 2 cm of the site of earliest activity ("focal area") was higher compared to the remainder of the atria (67 +/- 22% vs 34 +/- 14%, P < 0.001). Focal area potentials were characterized by a longer duration (49 +/- 22 msec vs 34 +/- 17 msec, P < 0.001) and a lower peak-to-peak amplitude (0.51 +/- 0.43 [0.12-1.7] mV vs 0.94 +/- 0.69 [0.22-2.58] mV, P = 0.03). Fractionation was not associated with FAT cycle length (r =-0.26, P = 0.29) or left atrial diameter (r = 0.47, P = 0.30).

CONCLUSION

Significant differences in fractionation, fractionation duration, and peak-to-peak-amplitude of atrial potentials between the focal area and the remainder of the atria exist. Fractionation and voltage mapping can be used, besides activation mapping, to identify the site of origin of FAT during catheter ablation.

A new approach to the determination of cardiac potential distributions: Application to the analysis of electrode configurations

This paper presents a mathematical model and new solution technique for studying the electric potential in a slab of cardiac tissue. The model is based on the bidomain representation of cardiac tissue and also allows for the effects of fibre rotation between the epicardium and the endocardium. A detailed solution method, based on Fourier Series and a simple one-dimensional finite difference scheme, for the governing equations for electric potential in the tissue and the blood, is also presented. This method has the advantage that the potential can be calculated only at points where it is required, such as the measuring electrodes. The model is then used to study various electrode configurations which have been proposed to determine cardiac tissue conductivity parameters. Three electrode configurations are analysed in terms of electrode spacing, placement position and the effect of including fibre rotation: the usual surface four-electrode configuration; a single vertical analogue of this and a two probe configuration, which has the current electrodes on one probe and the measuring electrodes on the other, a fixed distance away. It is found that including fibre rotation has no effect on the potentials measured in the first two cases; however, in the two probe case, non-zero fibre rotation causes a significant drop in the voltage measured. This leads to the conclusion that it is necessary to include the effects of fibre rotation in any model which involves the use of multiple plunge electrodes.

Cardiac electrophysiology and tissue structure: bridging the scale gap with a joint measurement and modelling paradigm

Significant tissue structures exist in cardiac ventricular tissue that are of supracellular dimension. It is hypothesized that these tissue structures contribute to the discontinuous spread of electrical activation, may contribute to arrhymogenesis and also provide a substrate for effective cardioversion. However, the influences of these mesoscale tissue structures in intact ventricular tissue are difficult to understand solely on the basis of experimental measurement. Current measurement technology is able to record at both the macroscale tissue level and the microscale cellular or subcellular level, but to date it has not been possible to obtain large volume, direct measurements at the mesoscales. To bridge this scale gap in experimental measurements, we use tissue-specific structure and mathematical modelling. Our models have enabled us to consider key hypotheses regarding discontinuous activation. We also consider the future developments of our intact tissue experimental programme.

Wave Similarity Mapping Shows the Spatiotemporal Distribution of Fibrillatory Wave Complexity in the Human Right Atrium During Paroxysmal and Chronic Atrial Fibrillation

INTRODUCTION

The complexity of waveforms during atrial fibrillation may reflect critical activation patterns for the arrhythmia perpetuation. In this study, we introduce a novel concept of map, based on the analysis of the wave morphology, which gives a direct evidence in the human right atrium on the spatiotemporal distribution of fibrillatory wave complexity in paroxysmal (PAF) and chronic (CAF) atrial fibrillation.

METHODS AND RESULTS

Electrograms were recorded from a 64-electrode catheter in the right atrium of 15 patients during PAF (n = 8) and CAF (n = 7). Wave similarity maps were constructed by calculating the degree of morphological similarity of activation waves (S) at each atrial site and by following its temporal evolution. During PAF the spatiotemporal distribution of the waveforms was highly consistent across the subjects and was determined by the anatomic location. Wave similarity maps showed the existence of an extended area with low similarity index, which covered the low posteroseptal atrium (S = 0.28 +/- 0.09) and the septal region (S = 0.22 +/- 0.04), and the presence of a large tongue with high similarity index, which penetrated the lateral wall (S = 0.55 +/- 0.08) starting from the high anterolateral atrium (S = 0.54 +/- 0.06). A completely different spatiotemporal pattern was seen during CAF. No distinct regions with different similarity indexes were recognized, but a uniformly distributed low similarity index (S = 0.27 +/- 0.07) was found. The spatial pattern was highly stable in time with fluctuations of S < 0.04.

CONCLUSION

Quantification of the spatiotemporal distribution of fibrillatory wave complexity is feasible in humans by wave similarity mapping. Anatomic anchoring of waveforms during PAF and pattern destruction during CAF was determined.

Numerical Simulation of Paced Electrogram Fractionation: Relating Clinical Observations to Changes in Fibrosis and Action Potential Duration

Simulating paced electrogram fractionation.

INTRODUCTION

Paced electrogram fractionation analysis (PEFA) may identify a re-entrant substrate in patients at risk of ventricular fibrillation (VF) by detecting prolonged, fractionated ventricular electrograms ("fractionation") in response to premature extrastimuli. Numerical simulations of action potential (AP) propagation through human myocardium following such premature stimulation were performed to study the relationship between electrogram fractionation, fibrosis, and changes in AP currents.

METHODS AND RESULTS

Activation in a resistive monodomain 2 cm2 sheet of myocardium containing nonconducting fibrous tissue was modeled using standard numerical methods for solutions of partial differential equations using the Priebe-Beukelmann (PB) AP equations. Myocardial fibrosis significantly influenced electrogram morphology. High densities of closely spaced fibrous septa caused functional block and altered propagation paths at short coupling intervals, and produced large increases in electrogram duration similar to those associated with increased risk of VF in clinical studies. Prolongation of the cardiac AP using the heart failure variant of the PB model further increased the amount of fractionation and thereby replicated clinical recordings more closely than did fibrosis alone. Increasing AP dispersion by a variable reduction in the potassium current I(Kr) simulated results seen in patients with the long QT syndrome with an abrupt increase in electrogram duration, while a uniform reduction in I(Kr) alone did not result in fractionated electrograms. In contrast, increases in cytosolic Ca2+ and Ca2+ buffering by troponin to simulate HCM had little effect on fractionation.

CONCLUSIONS

These results relate the effects of fibrosis, AP abnormalities, and dispersion of AP duration to the characteristic electrograms recorded in patients at risk of sudden death.

Cultivation in rotating bioreactors promotes maintenance of cardiac myocyte electrophysiology and molecular properties

We tested the hypothesis that cardiomyocytes maintained their phenotype better if cultured as three-dimensional tissue constructs than if cultured as confluent monolayers. Neonatal rat cardiomyocytes were cultured on biomaterial scaffolds in rotating bioreactors for 1 week, and resulting tissue constructs were compared with confluent monolayers and slices of native ventricular tissue with respect to proteins involved in cell metabolism (creatine kinase isoform MM), contractile function (sarcomeric myosin heavy chain), and intercellular communication (connexin 43), as well as action potential characteristics (e.g., membrane resting potential, maximum depolarization slope, and action potential duration), and macroscopic electrophysiological properties (maximum capture rate). The molecular and electrophysiological properties of cardiomyocytes cultured in tissue constructs, although inferior to those of native neonatal ventricles, were superior to those of the same cells cultured as monolayers. Construct levels of creatine kinase, myosin heavy chain, and connexin 43 were 40-60% as high as ventricle levels, whereas monolayer levels of the same proteins were only 11-20% as high. Construct action potential durations were 1.8-fold higher than those in ventricles, whereas monolayer action potential durations were 2.4-fold higher. Pharmacological studies using 4-aminopyridine showed that prolonged action potential duration and reduced maximum capture rate in tissue constructs as compared with native ventricles could be explained by decreased transient outward potassium current.

Electrotonic load triggers remodeling of repolarizing current Ito in ventricle

A change in activation sequence electrically remodels ventricular myocardium, causing persistent changes in repolarizing currents (T-wave memory). However, the underlying mechanism for triggering activation sequence-dependent remodeling is unknown. Optical action potentials were mapped with high resolution from the epicardial surface of the arterially perfused canine wedge preparation (n = 23) during 30 min of baseline endocardial stimulation, followed by 40 min of epicardial stimulation, and, finally, restoration of endocardial stimulation. Immediately after the change from endocardial to epicardial stimulation, phase 1 notch amplitude of epicardial cells was attenuated by 74 +/- 8% (P < 0.001) compared with baseline and continued to diminish during the period of epicardial pacing, suggesting progressive remodeling of the transient outward current (Ito). When endocardial pacing was restored, notch amplitude did not immediately recover but remained attenuated by 23 +/- 10% (P < 0.001), also consistent with a remodeling effect. Peak Ito current measured from isolated epicardial myocytes changed by 12 +/- 4% (P < 0.025), providing direct evidence for Ito remodeling occurring on a surprisingly short time scale. The mechanism for triggering remodeling of Ito was a significant reduction (by 14 +/- 4%, P < 0.001) of upstroke amplitude in epicardial cells during epicardial stimulation. Reduction in upstroke amplitude during epicardial pacing was explained by electrotonic load on epicardial cells by fully repolarized downstream endocardial cells. These data suggest a novel mechanism for triggering electrical remodeling in the ventricle. Electrotonic load imposed by a change in activation sequence reduces upstroke amplitude, which, in turn, attenuates Ito according to its known voltage-dependent properties, triggering downregulation of current.

Electrode systems for measuring cardiac impedances using optical transmembrane potential sensors and interstitial electrodes--theoretical design

The cardiac electrical substrate is a challenge to direct measurement of its properties. Optical technology together with the capability to fabricate small electrodes at close spacings opens new possibilities. Here, those possibilities are explored from a theoretical viewpoint. It appears that with careful measurements from a well-designed set of electrodes one can obtain structural conductivities, separating intracellular from interstitial values, and longitudinal from transverse. Resting membrane resistance also can be obtained.

Electrical and structural remodeling of the failing ventricle

Heart failure (HF) is a complex disease that presents a major public health challenge to Western society. The prevalence of HF increases with age in the elderly population, and the societal disease burden will increase with prolongation of life expectancy. HF is initially characterized by an adaptive increase of neurohumoral activation to compensate for reduction of cardiac output. This leads to a combination of neurohumoral activation and mechanical stress in the failing heart that trigger a cascade of maladaptive electrical and structural events that impair both the systolic and diastolic function of the heart.