Research areas of the laboratory

Cardiac arrhythmias remain the major cause of human mortality in the developed world. Various factors contribute to an abnormal wavefront generation and/or conduction, which may result in dangerous cardiac arrhythmias. We investigate the mechanisms of wavefront initiation in the sino-atrial node or by an artificial electronic pacemaker. We also investigate wavefront propagation is cardiac structures with heterogeneous ion channel and gap junction expression, such as atrio-ventricular and sino-atrial nodes. Using fluorescent imaging we study propagation of electrical impulses in the heart during normal and abnormal cardiac rhythm(s). Specifically, we are interested in extending our understanding of (1) impulse generation (excitation) by a groups of cells and by local electrical stimulus, (2) impulse termination and break-excitation by strong electrical defibrillating shock, (3) impulse propagation in the structures with strong degree of heterogeneity of ion channel and gap junction expression.

Specific Projects

1. Structure/function of the pacemaking and conduction system of the heart. The pacemaking and conduction system (PCS) of the heart is a profoundly complex structure, which orchestrates orderly contractions of cardiac chambers by generating and transmitting action potentials at an appropriate rate, conduction velocity, and delay between the chambers. As evident from neurofilament 160 staining in the developing and adult rabbit heart, PCS is anatomically and functionally a continuous structure. There are various types of cells within PCS, which differ morphologically. However, they all have pacemaking properties, unlike cells of the working atrial and ventricular myocardium which lack them. In addition to morphological heterogeneity, PCS possesses striking heterogeneity of functional properties. For example, there is a nearly 100-fold difference in the conduction velocity between the anatomically adjacent compact atrioventricular (AV) node (~2-3 cm/sec) and the His-Purkinje system (up to 2.5 m/sec). Such a difference is required for the proper delay of excitation between the atria and ventricles, on the one hand, and synchronized excitation of ventricles, on the other hand. We hypothesize that this functional heterogeneity has both a structural and molecular basis: heterogeneity of expression of genes encoding gap junction and ion channels, and receptors of the PCS provides the substrate for normal and abnormal pacemaking and conduction and for arrhythmias. We also hypothesize that quantification of the PCS 3D structure is required to more completely understand its function. We plan to implement such quantification, using state-of-the art optical mapping with voltage-sensitive dye, optical coherence tomography (OCT), and immunohistochemical mapping of several fundamental proteins, which define intercellular coupling and cell types. The long term goal of this project is to develop a structure/function framework of the cardiac PCS in 3D. Numerous studies examined contribution of various isoforms of connexins, ion channels, and receptors to cellular physiology in the pacemaking and conduction system of the heart. We aim to apply a systems physiology approach which will lay down foundation for future integration of molecular and cellular information into a comprehensive mathematical model of the heart. Our data will allow us to examine factors responsible for stability of normal pacemaking and conduction, for abnormal impulse generation and failed conduction, and for arrhythmogenesis mediated by the autonomic nervous system and stretch. This new knowledge will be helpful in future development of novel genetic and device therapies of PCS abnormalities. We will also explore the potential of novel OCT imaging technology for diagnosis and research of PCS abnormalities.

2. Low-voltage defibrillation. According to the American Heart Association, an “overwhelming number of sudden cardiac deaths (SCD) from coronary disease (estimated at about 340,000 per year) is thought to be from ventricular fibrillation” (VF). Sudden cardiac death usually starts as ventricular tachycardia (VT) and then degenerates into VF. Defibrillation by high-intensity electric shock is the only reliable treatment of VT/VF, which however often results in myocardial dysfunction and damage. We propose to develop a novel approach for defibrillation therapy, which may result in a significant reduction in the energy delivered to the heart. The current defibrillation approach requires termination of all propagating waves of excitation in the entire fibrillating heart. It is commonly assumed that in order to achieve this goal, one has to deliver an electric field of at least 5.4 V/cm to all or critical amount of myocardium in the heart. We propose to target only the leading centers that sustain arrhythmia, rather than the whole organ. The majority of patients receiving ICD therapy have a history of coronary disease; their hearts develop anatomical heterogeneities, which could provide a substrate for reentrant tachyarrhythmias. Based on our observations we hypothesize that: (a) The leading center of reentrant tachyarrhythmia tends to anchor at anatomical heterogeneities, such as infarction border zones, scars, areas of fibrosis; (b) Attachment to nonconductive anatomical heterogeneities stabilizes the leading center and thus reduces the efficacy of the current low-energy therapy, anti-tachycardia pacing (ATP); (c) An externally applied electric shock induces maximum tissue polarization, known as virtual electrode polarization, at anatomical heterogeneities including the same anatomical substrate which anchors the leading center of arrhythmia; (d) A low-energy properly timed (applied during a termination window) electric shock can destabilize the leading center of arrhythmia via virtual electrode polarization and unpin it from the substrate; (e) Following unpinning, arrhythmia can be terminated by properly timed anti-repining pacing (ARP), which will terminate VT and prevent its degeneration into VF.

3. The mechanisms of electroporation and its role in defibrillation.  Defibrillation by electric shock is the only known effective therapy of sudden cardiac death. However, defibrillation has a significant side effect, associated with electric field induced rupture of sarcolemmal membrane known as electroporation. Electroporation is believed to be responsible for clinical post-shock depression of cardiac electrical and mechanical dysfunction, metabolic inhibition, bradicardia, complete heart block and increased pacing thresholds. Despite evident importance of electroporation, little is known about spatiotemporal mechanisms of development of electroporation and its consequences in complex three-dimensional tissue structure of the heart. The most striking puzzle is the existence of experimental evidence for both pro- and anti-arrhythmic effects of electroporation. The overall objective of the project is to investigate the role of electroporation in defibrillation.

 Research of laboratory members

• Vadim Fedorov, PhD: Cardiac protection in hibernation, mechanisms of impulse generation and conduction in the SA node, mechanisms of low-voltage defibrillation in the atria, mechanisms of VT/VF low voltage termination in canine 4-day  infarction model, mechanisms of electroporation and arrhythmia in the heart, optical imaging of the human heart;
• Christina Ambrosi: Optical coherence tomography and optical imaging of the human heart, imaging of the AV junction of the human heart, transcriptional regulation of gene expression in the human AV junction;
• Qing Luo: Panoramic imaging of cardiac defibrillation, effect of blebbistatin on genesis and maintenance of ventricular arrhythmia, calcium imaging of ventricular arrhythmia;
• Wenwen Li: Low-voltage termination of reentrant arrhythmias in hearts with infarction and heart failure;
• Kelley Foyil: Low voltage atrial defibrllation, optical imaging of atrial arrhythmia, regional heterogeneity of gene expression in the atria.
• Alexey Glukhov, PhD, Mechanisms of electroporation, optical imaging of the mouse heart, mechanisms of impulse geenration in the mouse SA node.

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Selected publications

Mechanisms of arrhythmogenesis in the heart

• Flourescent imaging of stimulus-induced arrhythmogenesis [reprint]
• Computer simulations of cardiac excitation and arrhythmias [reprint]
• Topology of 3D scroll-waves induced by electric stimulation in the heart [reprint]
• Dynamics of scroll-waves in the heart during ventricular tachycardia
• 3D fluorescent imaging of the AV nodal reentrant tachycardia [slides] [reprint]

• Mapping of cardiac stimulation [reprint]
• Epicardial Stimulation [reprint]
• Internal Defibrillation [reprint]
• Mechanisms of upper limit of vulnerability and defibrillation [reprint]
• External Defibrillation [reprint]
• Electroporation [reprint]

• Optical mapping of the AV node [reprint]
• Effects of voltage-sensitive dyes on cardiac cells [reprint]
• Data acquisition and processing: development of multi-channel data acquisition & analysis system [reprint]
• Application of voltage-sensitive dye recording techniques to human heart