Eleftheria Pervolaraki

Virtual tissue engineering of the human atrium: modelling pharmacological actions on atrial arrhythmogenesis.

Computational models of human atrial cells, tissues and atria have been developed. Cell models, for atrial wall, crista terminalis, appendage, Bachmanns bundle and pectinate myocytes are characterised by action potentials, ionic currents and action potential duration (APD) restitution. The principal effect of the ion channel remodelling of persistent atrial fibrillation (AF), and a mutation producing familial AF, was APD shortening at all rates. Electrical alternans was abolished by the modelled action of Dronedarone. AF induced gap junctional remodelling slows propagation velocity at all rates. Re-entrant spiral waves in 2-D models are characterised by their frequency, wavelength, meander and stability. For homogenous models of normal tissue, spiral waves self-terminate, due to meander to inexcitable boundaries, and by dissipation of excitation. AF electrical remodelling in these homogenous models led to persistence of spiral waves, and AF fibrotic remodelling to their breakdown into fibrillatory activity. An anatomical model of the atria was partially validated by the activation times of normal sinus rhythm. The use of tissue geometry from clinical MRI, and tissue anisotropy from ex vivo diffusion tensor magnetic resonance imaging is outlined. In the homogenous model of normal atria, a single scroll breaks down onto spatio-temporal irregularity (electrical fibrillation) that is self-terminating; while in the AF remodelled atria the fibrillatory activity is persistent. The persistence of electrical AF can be dissected in the model in terms of ion channel and intercellular coupling processes, that can be modified pharmacologically; the effects of anatomy, that can be modified by ablation; and the permanent effects of fibrosis, that need to be prevented.

Antenatal architecture and activity of the human heart

We construct the components for a family of computational models of the electrophysiology of the human foetal heart from 60 days gestational age (DGA) to full term. This requires both cell excitation models that reconstruct the myocyte action potentials, and datasets of cardiac geometry and architecture. Fast low-angle shot and diffusion tensor magnetic resonance imaging (DT-MRI) of foetal hearts provides cardiac geometry with voxel resolution of approximately 100 µm. DT-MRI measures the relative diffusion of protons and provides a measure of the average intravoxel myocyte orientation, and the orientation of any higher order orthotropic organization of the tissue. Such orthotropic organization in the adult mammalian heart has been identified with myocardial sheets and cleavage planes between them. During gestation, the architecture of the human ventricular wall changes from being irregular and isotropic at 100 DGA to an anisotropic and orthotropic architecture by 140 DGA, when it has the smooth, approximately 120° transmural change in myocyte orientation that is characteristic of the adult mammalian ventricle. The DT obtained from DT-MRI provides the conductivity tensor that determines the spread of potential within computational models of cardiac tissue electrophysiology. The foetal electrocardiogram (fECG) can be recorded from approximately 60 DGA, and RR, PR and QT intervals between the P, R, Q and T waves of the fECG can be extracted by averaging from approximately 90 DGA. The RR intervals provide a measure of the pacemaker rate, the QT intervals an index of ventricular action potential duration, and its rate-dependence, and so these intervals constrain and inform models of cell electrophysiology. The parameters of models of adult human sinostrial node and ventricular cells that are based on adult cell electrophysiology and tissue molecular mapping have been modified to construct preliminary models of foetal cell electrophysiology, which reproduce these intervals from fECG recordings. The PR and QR intervals provide an index of conduction times, and hence propagation velocities (approx. 1–10 cm s−1, increasing during gestation) and so inform models of tissue electrophysiology. Although the developing foetal heart is small and the cells are weakly coupled, it can support potentially lethal re-entrant arrhythmia.

Spatiotemporal patterning of uterine excitation patterns in human labour

The mechanisms leading to the initiation of normal, premature or dysfunctional human labour are poorly understood, as animal models are inappropriate, and experimental studies are limited. Computational modelling provides a means of linking non-invasive clinical data with the results of in vitro cell and tissue physiology. Nonlinear wave processes - propagation in an excitable medium - provides a quantitatively testable description of mechanisms of premature and full term labour, and a view of changes in uterine electrophysiology during gestation as a trajectory in excitation and intercellular coupling parameter space. Propagation phenomena can account for both premature and full term labour.

Ventricular myocardium development and the role of connexins in the human fetal heart

The developmental timeline of the human heart remains elusive. The heart takes on its characteristic four chambered appearance by ~56 days gestational age (DGA). However, owing to the complexities (both technical and logistical) of exploring development in utero, we understand little of how the ventricular walls develop. To address this, we employed diffusion tensor magnetic resonance imaging to explore the architecture and tissue organization of the developing heart aged 95–143 DGA. We show that fractional anisotropy increases (from ~0.1 to ~0.5), diffusion coefficients decrease (from ~1 × 10−3mm2/sec to ~0.4 × 10−3mm2/sec), and fiber paths, extracted by tractography, increase linearly with gestation, indicative of the increasing organization of the ventricular myocytes. By 143 DGA, the developing heart has the classical helical organization observed in mature mammalian tissue. This was accompanied by an increase in connexin 43 and connexin 40 expression levels, suggesting their role in the development of the ventricular conduction system and that electrical propagation across the heart is facilitated in later gestation. Our findings highlight a key developmental window for the structural organization of the fetal heart.

Atrioventricular Node Dysfunction and Ion Channel Transcriptome in Pulmonary Hypertension

Background—Heart block is associated with pulmonary hypertension, and the aim of the study was to test the hypothesis that the heart block is the result of a change in the ion channel transcriptome of the atrioventricular (AV) node. Methods and Results—The most commonly used animal model of pulmonary hypertension, the monocrotaline-injected rat, was used. The functional consequences of monocrotaline injection were determined by echocardiography, ECG recording, and electrophysiological experiments on the Langendorff-perfused heart and isolated AV node. The ion channel transcriptome was measured by quantitative PCR, and biophysically detailed computer modeling was used to explore the changes observed. After monocrotaline injection, echocardiography revealed the pattern of pulmonary artery blood flow characteristic of pulmonary hypertension and right-sided hypertrophy and failure; the Langendorff-perfused heart and isolated AV node revealed dysfunction of the AV node (eg, 50% incidence of heart block in isolated AV node); and quantitative PCR revealed a widespread downregulation of ion channel and related genes in the AV node (eg, >50% downregulation of Cav1.2/3 and HCN1/2/4 channels). Computer modeling predicted that the changes in the transcriptome if translated into protein and function would result in heart block. Conclusions—Pulmonary hypertension results in a derangement of the ion channel transcriptome in the AV node, and this is the likely cause of AV node dysfunction in this disease.

Dynamic Action Potential Restitution Contributes To Mechanical Restitution In Right Ventricular Myocytes From Pulmonary Hypertensive Rats

We investigated the steepened dynamic action potential duration (APD) restitution of rats with pulmonary artery hypertension (PAH) and right ventricular (RV) failure and tested whether the observed APD restitution properties were responsible for negative mechanical restitution in these myocytes. PAH and RV failure were provoked in male Wistar rats by a single injection of monocrotaline (MCT) and compared with saline-injected animals (CON). Action potentials were recorded from isolated RV myocytes at stimulation frequencies between 1 and 9 Hz. Action potential waveforms recorded at 1 Hz were used as voltage clamp profiles (action potential clamp) at stimulation frequencies between 1 and 7 Hz to evoke rate-dependent currents. Voltage clamp profiles mimicking typical CON and MCT APD restitution were applied and cell shortening simultaneously monitored. Compared with CON myocytes, MCT myocytes were hypertrophied; had less polarized diastolic membrane potentials; had action potentials that were triggered by decreased positive current density and shortened by decreased negative current density; APD was longer and APD restitution steeper. APD90 restitution was unchanged by exposure to the late Na+-channel blocker (5 μM) ranolazine or the intracellular Ca2+ buffer BAPTA. Under AP clamp, stimulation frequency-dependent inward currents were smaller in MCT myocytes and were abolished by BAPTA. In MCT myocytes, increasing stimulation frequency decreased contraction amplitude when depolarization duration was shortened, to mimic APD restitution, but not when depolarization duration was maintained. We present new evidence that the membrane potential of PAH myocytes is less stable than normal myocytes, being more easily perturbed by external currents. These observations can explain increased susceptibility to arrhythmias. We also present novel evidence that negative APD restitution is at least in part responsible for the negative mechanical restitution in PAH myocytes. Thus, our study links electrical restitution remodeling to a defining mechanical characteristic of heart failure, the reduced ability to respond to an increase in demand.

Towards computational modelling of the human foetal electrocardiogram: normal sinus rhythm and congenital heart block

AIMS We aim to engineer a computational model of propagation during normal sinus rhythm in the foetal human heart, by modifying models for adult cardiac tissue to match foetal electrocardiogram (fECG) characteristics. The model will be partially validated by fECG data, and applied to explore possible mechanisms of arrhythmogenesis in the foetal heart. METHODS AND RESULTS Foetal electrocardiograms have been recorded during pregnancy, with P- and T-waves, and the QRS complex, identified by averaging and signal processing. Intervals of the fECG are extracted and used to modify currently available human adult cardiomyocyte models. RR intervals inform models of the pacemaking cells by constraining their rate, the QT interval and its rate dependence constrain models of ventricular cells, and the width of the P-wave, the QR and PR intervals constrain propagation times, conduction velocities, and intercellular coupling. These cell models are coupled into a one-dimensional (1D) model of propagation during normal sinus rhythm in the human foetal heart. We constructed a modular, heterogeneous 1D model for propagation in the foetal heart, and predicted the effects of reduction in L-type Ca(++) current. These include bradycardia and atrioventricular conduction blocks. These may account quantitatively for congenital heart block produced by positive IgG antibodies. CONCLUSION The fECG can be interpreted mechanistically and quantitatively by using a simple computational model for propagation. After further validation, by clinical recordings of the fECG and the electrophysiological experiments on foetal cardiac cells and tissues, the model may be used to predict the effects of maternally administered pharmaceuticals on the fECG.

Human uterine excitation patterns leading to labour: synchronization or propagation?

The mechanisms leading to the initiation of normal, premature or dysfunctional human labour are poorly understood, as animal models are inappropriate, and experimental studies are limited. Computational modelling provides a means of linking non-invasive clinical data with the results of in vitro cell and tissue physiology. Nonlinear wave processes --- propagation in an excitable medium --- provides a quantitatively testable description of mechanisms of premature and full term labour, and a view of changes in uterine electrophysiology during gestation as a trajectory in excitation and intercellular coupling parameter space. Propagation phenomena can account for both premature and full term labour.

Multichannel electrocardiogram diagnostics for the diagnosis of arrhythmogenic right ventricular dysplasia

Aims The identification of arrhythmogenic right ventricular dysplasia (ARVD) from 12-channel standard electrocardiogram (ECG) is challenging. High density ECG data may identify lead locations and criteria with a higher sensitivity. Methods and results Eighty-channel ECG recording from patients diagnosed with ARVD and controls were quantified by magnitude and integral measures of QRS and T waves and by a measure (the average silhouette width) of differences in the shapes of the normalized ECG cycles. The channels with the best separability between ARVD patients and controls were near the right ventricular wall, at the third intercostal space. These channels showed pronounced differences in P waves compared to controls as well as the expected differences in QRS and T waves. Conclusion Multichannel recordings, as in body surface mapping, add little to the reliability of diagnosing ARVD from ECGs. However, repositioning ECG electrodes to a high anterior position can improve the identification of ECG variations in ARVD. Additionally, increased P wave amplitude appears to be associated with ARVD.

Self-terminating re-entrant cardiac arrhythmias: quantitative characterization

Atrial and ventricular tachyarrhythmia are often sustained by re-entrant propagation, and explained by deterministic models. A quantitative, stochastic description of self-termination provides an alternative to the current paradigm for re-entrant tachyarrhythmia - that of triggers and a substrate, modelled by parametrically heterogeneous deterministic partial differential equations Atrial and ventricular data was from recordings obtained during routine clinical monitoring and treatment, either noninvasively or invasively. Atrial and ventricular tachycardia are characterised by their initiation times and durations, re-presented as instantaneous rates, whose means estimate transition probabilities/s for onset and termination. These estimated probabilities range from 10-9 to 10-1/s.