Development of the Cardiac Conduction System.

The electrical impulses that coordinate the sequential, rhythmic contractions of the atria and ventricles are initiated and tightly regulated by the specialized tissues of the cardiac conduction system. In the mature heart, these impulses are generated by the pacemaker cardiomyocytes of the sinoatrial node, propagated through the atria to the atrioventricular node where they are delayed and then rapidly propagated to the atrioventricular bundle, right and left bundle branches, and finally, the peripheral ventricular conduction system. Each of these specialized components arise by complex patterning events during embryonic development. This chapter addresses the origins and transcriptional networks and signaling pathways that drive the development and maintain the function of the cardiac conduction system.

Remembering the canonical discoverers of the core components of the mammalian cardiac conduction system: Keith and Flack, Aschoff and Tawara, His, and Purkinje.

The mammalian cardiac conduction system (CCS) is a multifaceted continuum of electrically distinct, interconnected constructs within the myocardial mass. The key components are the sinus node (SAN), the atrioventricular node (AVN), the His bundle (HB), and the Purkinje fiber network (PF), the latter serendipitously discovered by Jan Evangelista Purkinje in 1839 in the sheep ventricle. In 1893, Wilhelm His, Jr. described a ventricular muscular tract conveying SAN-generated action potentials from the AVN (discovered by Sunao Tawara and Karl Albert Aschoff in 1906) to the PF. In 1906, Keith and Flack completed these explorations by localizing the SAN, the primum movens of CCS, which functions as a biologic oscillator emitting cadenced impulses that travel via the Bachmann bundle to the atrial myocytes and, via internodal pathways, to the AVN. Here these impulses are briefly delayed, enabling atrial systole before continuing via the AVN and the high-speed His-Purkinje conduction axis to signal ventricular contraction. The CCS canonical discoverers (Keith and Flack, Aschoff and Tawara, His, and Purkinje), historical controversies, fundamental notions of anatomy, physiology, and pathology, and therapeutic interventions pertaining to the CCS are the main themes of this review. Any scientist mentioned or unmentioned in this report who contributed directly or indirectly, with correct or inaccurate hypotheses, to the characterization of the CCS deserves our deepest gratitude for the long and painstaking hours spent microscopically scrutinizing heart specimens from multiple mammalian species, including humans.NEW & NOTEWORTHY This report presents the first comprehensive summary of the factors enabling the discovery of the cardiac conduction system (CCS). Biographical highlights and achievements of the CCS canonical discoverers, hypotheses concerning mechanisms underlying sinus node (SAN) automaticity, use of eponyms to denominate a discovery, famous controversies, possible reproachable behavior (e.g., intellectual support for eugenics in post-World War I Germany) by two of the discoverers, and examples of historical mentor-pupil relationships are discussed.

Three-dimensional visualization of the bovine cardiac conduction system and surrounding structures compared to the arrangements in the human heart.

In the human heart, the atrioventricular node is located toward the apex of the triangle of Koch, which is also at the apex of the inferior pyramidal space. It is adjacent to the atrioventricular portion of the membranous septum, through which it penetrates to become the atrioventricular bundle. Subsequent to its penetration, the conduction axis is located on the crest of the ventricular septum, sandwiched between the muscular septum and ventricular component of the membranous septum, where it gives rise to the ramifications of the left bundle branch. In contrast, the bovine conduction axis has a long non-branching component, which penetrates into a thick muscular atrioventricular septum having skirted the main cardiac bone and the rightward half of the non-coronary sinus of the aortic root. It commonly gives rise to both right and left bundle branches within the muscular ventricular septum. Unlike the situation in man, the left bundle branch is long and thin before it branches into its fascicles. These differences from the human heart, however, have yet to be shown in three-dimensions relative to the surrounding structures. We have now achieved this goal by injecting contrast material into the insulating sheaths that surround the conduction network, evaluating the results by subsequent computed tomography. The fibrous atrioventricular membranous septum of the human heart is replaced in the ox by the main cardiac bone and the muscular atrioventricular septum. The apex of the inferior pyramidal space, which in the bovine, as in the human, is related to the atrioventricular node, is placed inferiorly relative to the left ventricular outflow tract. The bovine atrioventricular conduction axis, therefore, originates from a node itself located inferiorly compared to the human arrangement. The axis must then skirt the non-coronary sinus of the aortic root prior to penetrating the thicker muscular ventricular septum, thus accounting for its long non-branching course. We envisage that our findings will further enhance comparative anatomical research.

Fosphenytoin for control of electrical storm in acute myocardial infarction and Purkinje fiber-mediated arrhythmias.

BACKGROUND: Purkinje fiber-mediated arrhythmias in the setting of acute myocardial infarction are poorly responsive to conventional antiarrhythmic therapy, increases overall mortality and often requires radiofrequency ablation (RFA) for control. In this study, we report the use of intravenous Fosphenytoin for the control of arrhythmic storm in patients with acute myocardial infarction. METHODS AND RESULTS: Six patients with acute myocardial infarction (5 AW/1 LW) and Purkinje-triggered ventricular arrhythmias refractory to conventional antiarrhythmics were treated with intravenous Fosphenytoin before considering RFA. Arrhythmia control was obtained in all patients after the initial bolus dose. Breakthrough episodes were seen in 5/6 within 24-36 hours of the initial bolus, necessitating a second bolus. Complete arrhythmia control was obtained in all patients within 72 hours and 5/6 patients were successfully discharged from the hospital. One patient succumbed to sepsis in hospital while another patient succumbed to Sub Dural Hematoma after 3 months. CONCLUSIONS: Intravenous Fosphenytoin should be considered before RFA for control of Purkinje fiber-mediated refractory arrhythmias in acute myocardial infarction patients.

Optogenetic determination of the myocardial requirements for extrasystoles by cell type-specific targeting of ChannelRhodopsin-2.

Extrasystoles lead to several consequences, ranging from uneventful palpitations to lethal ventricular arrhythmias, in the presence of pathologies, such as myocardial ischemia. The role of working versus conducting cardiomyocytes, as well as the tissue requirements (minimal cell number) for the generation of extrasystoles, and the properties leading ectopies to become arrhythmia triggers (topology), in the normal and diseased heart, have not been determined directly in vivo. Here, we used optogenetics in transgenic mice expressing ChannelRhodopsin-2 selectively in either cardiomyocytes or the conduction system to achieve cell type-specific, noninvasive control of heart activity with high spatial and temporal resolution. By combining measurement of optogenetic tissue activation in vivo and epicardial voltage mapping in Langendorff-perfused hearts, we demonstrated that focal ectopies require, in the normal mouse heart, the simultaneous depolarization of at least 1,300-1,800 working cardiomyocytes or 90-160 Purkinje fibers. The optogenetic assay identified specific areas in the heart that were highly susceptible to forming extrasystolic foci, and such properties were correlated to the local organization of the Purkinje fiber network, which was imaged in three dimensions using optical projection tomography. Interestingly, during the acute phase of myocardial ischemia, focal ectopies arising from this location, and including both Purkinje fibers and the surrounding working cardiomyocytes, have the highest propensity to trigger sustained arrhythmias. In conclusion, we used cell-specific optogenetics to determine with high spatial resolution and cell type specificity the requirements for the generation of extrasystoles and the factors causing ectopies to be arrhythmia triggers during myocardial ischemia.

Proarrhythmic risk assessment using conventional and new in vitro assays.

Drug-induced QT prolongation is a major safety issue in the drug discovery process. This study was conducted to assess the electrophysiological responses of four substances using established preclinical assays usually used in regulatory studies (hERG channel or Purkinje fiber action potential) and a new assay (human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs)-field potential). After acute exposure, moxifloxacin and dofetilide concentration-dependently decreased IKr amplitude (IC50 values: 102 µM and 40 nM, respectively) and lengthened action potential (100 µM moxifloxacin: +23% and 10 nM dofetilide: +18%) and field potential (300 µM moxifloxacin: +76% and 10 nM dofetilide: +38%) durations. Dofetilide starting from 30 nM induced arrhythmia in hiPSC-CMs. Overnight application of pentamidine (10 and 100 µM) and arsenic (1 and 10 µM) decreased IKr, whereas they were devoid of effects after acute application. Long-term pentamidine incubation showed a time- and concentration-dependent effect on field potential duration. In conclusion, our data suggest that hiPSC-CMs represent a fully functional cellular electrophysiology model which may significantly improve the predictive validity of in vitro safety studies. Thereafter, lead candidates may be further investigated in patch-clamp assays for mechanistic studies on individual ionic channels or in a multicellular Purkinje fiber preparation for confirmatory studies on cardiac conduction.

Endocavitary structures in the outflow tract: anatomy and electrophysiology of the conus papillary muscles.

Catheter ablation is an increasingly used and successful treatment choice for right ventricular outflow tract (RVOT) arrhythmias. While the role of endocavitary structures and the regional morphology of the ventricular inflow tract and the right atrium as a cause for difficulty with successful ablation are well described, similar issues within the RVOT are not well understood. It is also not commonly appreciated that one of the papillary muscles is located within the proximal RVOT. We report 3 patients in which ventricular arrhythmia was targeted and ablated in the conus papillary muscle. The anatomic features, potential role of the fascicular conduction system, and unique challenges with mapping arrhythmia arising from this structure are discussed.

Tissue-specific effects of acetylcholine in the canine heart.

Acetylcholine (ACh) release from the vagus nerve slows heart rate and atrioventricular conduction. ACh stimulates a variety of receptors and channels, including an inward rectifying current [ACh-dependent K⁺ current (IK,ACh)]. The effect of ACh in the ventricle is still debated. We compared the effect of ACh on action potentials in canine atria, Purkinje, and ventricular tissue as well as on ionic currents in isolated cells. Action potentials were recorded from ventricular slices, Purkinje fibers, and arterially perfused atrial preparations. Whole cell currents were recorded under voltage-clamp conditions, and unloaded cell shortening was determined on isolated cells. The effect of ACh (1-10 µM) as well as ACh plus tertiapin, an IK,ACh-specific toxin, was tested. In atrial tissue, ACh hyperpolarized the membrane potential and shortened the action potential duration (APD). In Purkinje and ventricular tissues, no significant effect of ACh was observed. Addition of ACh to atrial cells activated a large inward rectifying current (from -3.5 ± 0.7 to -23.7 ± 4.7 pA/pF) that was abolished by tertiapin. This current was not observed in other cell types. A small inhibition of Ca²âº current (ICa) was observed in the atria, endocardium, and epicardium after ACh. ICa inhibition increased at faster pacing rates. At a basic cycle length of 400 ms, ACh (1 µM) reduced ICa to 68% of control. In conclusion, IK,ACh is highly expressed in atria and is negligible/absent in Purkinje, endocardial, and epicardial cells. In all cardiac tissues, ACh caused rate-dependent inhibition of ICa.

Cortisol stimulates proliferation and apoptosis in the late gestation fetal heart: differential effects of mineralocorticoid and glucocorticoid receptors.

We have previously found that modest chronic increases in maternal cortisol result in an enlarged fetal heart. To explore the mechanisms of this effect, we used intrapericardial infusions of a mineralocorticoid receptor (MR) antagonist (canrenoate) or of a glucocorticoid receptor (GR) antagonist (mifepristone) in the fetus during maternal infusion of cortisol (1 mg·kg⁻¹·day⁻¹). We have shown that the MR antagonist blocked the increase in fetal heart weight and in wall thickness resulting from maternal cortisol infusion. In the current study we extended those studies and found that cortisol increased Ki67 staining in both ventricles, indicating cell proliferation, but also increased active caspase-3 staining in cells of the conduction pathway in the septum and subendocardial layers of the left ventricle, suggesting increased apoptosis in Purkinje fibers. The MR antagonist blocked the increase in cell proliferation, whereas the GR antagonist blocked the increased apoptosis in Purkinje fibers. We also found evidence of activation of caspase-3 in c-kit-positive cells, suggesting apoptosis in stem cell populations in the ventricle. These studies suggest a potentially important role of corticosteroids in the terminal remodeling of the late gestation fetal heart and suggest a mechanism for the cardiac enlargement with excess corticosteroid exposure.

Intramural Purkinje fibers facilitate rapid ventricular activation in the equine heart.

BACKGROUND: The Purkinje fibers convey the electrical impulses at much higher speed than the working myocardial cells. Thus, the distribution of the Purkinje network is of paramount importance for the timing and coordination of ventricular activation. The Purkinje fibers are found in the subendocardium of all species of mammals, but some mammals also possess an intramural Purkinje fiber network that provides for relatively instantaneous, burst-like activation of the entire ventricular wall, and gives rise to an rS configuration in lead II of the ECG. AIM: To relate the topography of the horse heart and the distribution and histology of the conduction system to the pattern of ventricular activation as a mechanism for the unique electrical axis of the equine heart. METHODS: The morphology and distribution of the cardiac conduction system was determined by histochemistry. The electrical activity was measured using ECG in the Einthoven and orthogonal configuration. RESULTS: The long axis of the equine heart is close to vertical. Outside the nodal regions the conduction system consisted of Purkinje fibers connected by connexin 43 and long, slender parallel running transitional cells. The Purkinje fiber network extended deep into the ventricular walls. ECGs recorded in an orthogonal configuration revealed a mean electrical axis pointing in a cranial-to-left direction indicating ventricular activation in an apex-to-base direction. CONCLUSION: The direction of the mean electrical axis in the equine heart is determined by the architecture of the intramural Purkinje network, rather than being a reflection of ventricular mass.

Pro-Arrhythmic Effects of Discontinuous Conduction at the Purkinje Fiber-Ventricle Junction Arising From Heart Failure-Induced Ionic Remodeling - Insights From Computational Modelling.

Heart failure is associated with electrical remodeling of the electrical properties and kinetics of the ion channels and transporters that are responsible for cardiac action potentials. However, it is still unclear whether heart failure-induced ionic remodeling can affect the conduction of excitation waves at the Purkinje fiber-ventricle junction contributing to pro-arrhythmic effects of heart failure, as the complexity of the heart impedes a detailed experimental analysis. The aim of this study was to employ computational models to investigate the pro-arrhythmic effects of heart failure-induced ionic remodeling on the cardiac action potentials and excitation wave conduction at the Purkinje fiber-ventricle junction. Single cell models of canine Purkinje fiber and ventricular myocytes were developed for control and heart failure. These single cell models were then incorporated into one-dimensional strand and three-dimensional wedge models to investigate the effects of heart failure-induced remodeling on propagation of action potentials in Purkinje fiber and ventricular tissue and at the Purkinje fiber-ventricle junction. This revealed that heart failure-induced ionic remodeling of Purkinje fiber and ventricular tissue reduced conduction safety and increased tissue vulnerability to the genesis of the unidirectional conduction block. This was marked at the Purkinje fiber-ventricle junction, forming a potential substrate for the genesis of conduction failure that led to re-entry. This study provides new insights into proarrhythmic consequences of heart failure-induced ionic remodeling.

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue.

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca2+ current (ICaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 µM ACh caused an ≈15% decrease in ICaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K+ currents, 5 µM ACh resulted in an ≈23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in ICaL and ACh-induced changes in K+ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward ICaL and outward K+ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization.

New Insights into the Development and Morphogenesis of the Cardiac Purkinje Fiber Network: Linking Architecture and Function.

The rapid propagation of electrical activity through the ventricular conduction system (VCS) controls spatiotemporal contraction of the ventricles. Cardiac conduction defects or arrhythmias in humans are often associated with mutations in key cardiac transcription factors that have been shown to play important roles in VCS morphogenesis in mice. Understanding of the mechanisms of VCS development is thus crucial to decipher the etiology of conduction disturbances in adults. During embryogenesis, the VCS, consisting of the His bundle, bundle branches, and the distal Purkinje network, originates from two independent progenitor populations in the primary ring and the ventricular trabeculae. Differentiation into fast-conducting cardiomyocytes occurs progressively as ventricles develop to form a unique electrical pathway at late fetal stages. The objectives of this review are to highlight the structure-function relationship between VCS morphogenesis and conduction defects and to discuss recent data on the origin and development of the VCS with a focus on the distal Purkinje fiber network.

Histological study on the heart ventricle of Egyptian bovines (Bos aegyptiacus).

Background: The heart ventricles have thicker walls than atrium as they pump blood through blood vessels into all body organs. Aim: This study aimed to describe the histological changes of the heart ventricles in Egyptian bovine (Bos aegyptiacus) with special reference to Purkinje fibers. Methods: A total of 10 male Egyptian bovines of 1-10 years old were divided into three groups according to age; immature, mature, and adult animals. Results: The histological sections from all examined animals' groups revealed three different layers of the wall of both right and left ventricles; endocardium, myocardium, and epicardium. The endocardium was lined with endothelium and filled with fibrous connective tissue. The endocardium of adult bovine was the thickest. Purkinje fibers appeared of pale cytoplasm with few myofibrils. They were present in the deep layer of the endocardium and in the myocardium. The size of Purkinje fibers and the amount of their myofibrils appeared to be increased with advanced age. Bundles of cardiac muscles were the main constituent of the myocardium. The myocardial bundles were separated by fine connective tissue in immature animals that showed an increased amount in the adult animals. The hypereosinophilic cardiac muscle cells were observed in the ventricles of both mature and adult animals suggesting hypercontraction during rigor mortis. An external layer of the ventricles was the epicardium which consisted of connective tissue and covered with mesothelium. Conclusion: Overall, this study revealed histological changes in the wall of the ventricle and Purkinje fibers of Egyptian bovines (B. aegyptiacus) in relation to age. Additionally, the hypereosinophilia of the cardiac muscle cells was recorded in the ventricles of mature and adult bovines.

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.

Hemodynamic effects of Purkinje potential pacing in the left ventricular endocardium in patients with advanced heart failure.

BACKGROUND: Various difficulties can occur in patients who undergo cardiac resynchronization therapy for drug-refractory heart failure with respect to placement of the left ventricular (LV) lead, because of anatomical features, pacing thresholds, twitching, or pacing lead anchoring, possibly requiring other pacing sites. The goal of this study was to determine whether Purkinje potential (PP) pacing could provide better hemodynamics in patients with left bundle branch block and heart failure than biventricular (BiV) pacing. METHODS: Eleven patients with New York Heart Association functional class II or III heart failure despite optimal medical therapy were selected for this study. All patients underwent left- and right-sided cardiac catheterization for measurement of LV functional parameters in the control state during BiV and PP pacing. RESULTS: Maximum dP/dt increased during BiV and PP pacing when compared with control measurements. This study compared parameters measured during BiV pacing with PP pacing and non-paced beats as the control state in each patient (717±171 mmHg/s vs. 917±191 mmHg/s, p<0.05; and 921±199 mmHg/s, p<0.005); however, the difference between PP pacing and BiV pacing was not significant. There was no difference in heart rate, electrocardiographic wave complex duration, minimum dP/dt, left ventricular end-diastolic pressure, left ventricular end-systolic pressure, pulmonary capillary wedge pressure, or cardiac index when comparing BiV pacing and PP pacing to control measurements. CONCLUSIONS: The hemodynamic outcome of PP pacing was comparable to that of BiV pacing in patients with advanced heart failure.