3D models of the cardiac conduction system in healthy neonatal human hearts.

Iatrogenic damage to the cardiac conduction system (CCS) remains a significant risk during congenital heart surgery. Current surgical best practice involves using superficial anatomical landmarks to locate and avoid damaging the CCS. Prior work indicates inherent variability in the anatomy of the CCS and supporting tissues. This study introduces high-resolution, 3D models of the CCS in normal pediatric human hearts to evaluate variability in the nodes and surrounding structures. Human pediatric hearts were obtained with an average donor age of 2.7 days. A pipeline was developed to excise, section, stain, and image atrioventricular (AVN) and sinus nodal (SN) tissue regions. A convolutional neural network was trained to enable precise multi-class segmentation of whole-slide images, which were subsequently used to generate high- resolution 3D tissue models. Nodal tissue region models were created. All models (10 AVN, 8 SN) contain tissue composition of neural tissue, vasculature, and nodal tissues at micrometer resolution. We describe novel nodal anatomical variations. We found that the depth of the His bundle in females was on average 304 µm shallower than those of male patients. These models provide surgeons with insight into the heterogeneity of the nodal regions and the intricate relationships between the CCS and surrounding structures.

The anatomy of the atrioventricular nodal artery: A meta-analysis with implications for cardiothoracic surgery and ablation procedures.

The objective of the present meta-analysis was to evaluate recent and applicable data regarding the location and variation of the atrioventricular nodal artery (AVNA) in relation to adjacent structures. In order to minimize postoperative risks and maintain physiological anastomosis for proper cardiac function, understanding such possible variations of vascularization of the AV node is of immense importance prior to cardiothoracic surgery as well as ablations. In order to perform this meta-analysis, a systematic search was conducted in which all articles regarding, or at least mentioning, the anatomy of the AVNA was searched. In general, the results were based on 3919 patients. AVNA was found to originate only from the RCA in 82.41% (95% CI: 79.46%-85.18%). The pooled prevalence of AVNA originating only from LCA was found to be 15.25% (95% CI: 12.71%-17.97%). The mean length of AVNA was found to be 22.64 mm (SE = 1.60). The mean maximal diameter of AVNA at its origin was found to be 1.40 mm (SE = 0.14). In conclusion, we believe that this is the most accurate and up-to-date study regarding the highly variable anatomy of the AVNA. The AVNA was found to originate most commonly from the RCA (82.41%). Furthermore, the AVNA was found to most commonly have no (52.46%) or only one branch (33.74%). It is hoped that the results of the present meta-analysis will be helpful for physicians performing cardiothoracic or ablation procedures.

Anatomical study of the cardiac conduction system in swine hearts.

The cardiac conduction system (CCS) is crucial for regulating heartbeats; therefore, clinicians and comedicals involved in cardiovascular medicine treatment must have a thorough understanding of the CCS structure and function. However, anatomical education of the CCS based on actual dissection and observation is uncommon, although such educational methodology promotes three-dimensional structural understanding of the observed object. Based on previous studies, we examined the CCS structure in the heart of a swine (pig, Sus scrofa domestica) which has been used in the biological, medical and anatomical curricula as science teaching materials, by using macroscopic dissection procedures. Most CCS structures in a young pig heart were successfully identified and illustrated on a macroscopic scale. The atrioventricular bundle (His bundle) was located on the lower edge of the membranous interventricular septum and was clearly distinguished from the general myocardial fibres by its colour and fibre arrangement direction. Following the atrioventricular bundle towards the atrium or ventricle with properly removing the endocardium and myocardium, the atrioventricular node or the right and left bundles appeared respectively. In contrast, the sinoatrial node was not identified. The anatomy of the CCS in young pig hearts was essentially similar to that previously reported in humans and several domestic animals. Our findings of the CCS in young pig hearts are expected to be useful for medical and anatomical education for medical and comedical students, young clinicians and comedical workers.

Similarities and differences in the arrangement of the atrioventricular conduction axis in the canine compared to the human heart.

BACKGROUND: Subtle differences exist between dog and human, despite use of the dog as a model for cardiac surgical and electrophysiological research. OBJECTIVE: The purpose of this study was to investigate the differences in the atrioventricular conduction axis and adjacent structures between dogs and humans. METHODS: We prepared 33 human and 5 canine hearts for serial histologic sections of the atrioventricular conduction axis, making correlations with gross anatomic findings. We additionally examined and photographed 15 intact normal human hearts obtained from infants undergoing autopsy. Furthermore, we interrogated a computed tomographic dataset from a human adolescent and from 2 autopsied canine hearts, both with normal cardiac anatomy. RESULTS: All canine hearts lacked an inferoseptal recess, with the noncoronary leaflet of the aortic valve and the right fibrous trigone having direct attachments to the septal surface of the left ventricular outflow tract. This correlated with an extensive nonbranching component of the ventricular conduction axis, which skirted half of the noncoronary aortic sinus. This anatomic arrangement was observed in 2 of 15 of autopsied infant hearts. In the human hearts with an inferoseptal recess, the relatively shorter nonbranching bundle is embedded within the fibrous tissue forming its right wall. CONCLUSION: We found a major difference between canine and the majority of human hearts, namely, the presence or absence of an inferoseptal recess. When this recess is absent, as in the canine heart and in some human hearts, a greater proportion of the atrioventricular conduction axis is found within the circumference of the subaortic outflow tract.

Morphological variations of the conduction system in the atrioventricular zone and its clinical relationship in different species.

Atrioventricular node is responsible for delaying the passage of the electrical impulse to ventricles in order to protect them from fast depolarizations coming from the atria. The importance of this study is to identify the morphological variations of the components of atrioventricular zone that affect the conduction system and its clinical relationship in different species of mammals. We analyzed ten human hearts, nine from horses, eight from pigs, and five from dogs without a clinical history of cardiac pathologies. Histological section thickness of 5 µm were obtained with a microtome and stained with hematoxylin-eosin and Masson's trichrome. We observed both an increase in collagen fibers and a decrease in the size of P cells (nodal pacemaker cells) within the atrioventricular node in dogs, horses and pigs in cases that presented cartilage in fibrous body. The percentage of fundamental substance in atrioventricular node was significantly higher in dogs and the percentage of collagen fibers was higher in pigs, both than in humans. The presence of cartilaginous metaplasia in cardiac fibrous skeleton from different species decreases the size of atrioventricular node and its cells and increases the percentage of collagen fibers within the node, which can reduce the transmission of the electrical impulse to ventricles and therefore predispose to the presentation of ventricular arrhythmias. Morphometric analysis has allowed us to objectively quantify each of the components of AV node and compare them in the different species.

Anatomy of the Atrioventricular Junction, Atrioventricular Grooves, and Accessory Pathways.

Accessory pathways that bypass all or part of the normal atrioventricular conduction system traverse the atrioventricular junction. The atrioventricular junction comprises of a limited septal component and much more extensive right and left parietal components. Its composition forms a plane of insulation between atrial and ventricular myocardium, preventing direct continuity between them. Typical accessory atrioventricular pathways located anywhere along the atrioventricular junction are muscle bundles or may involve muscle around the walls of coronary sinus aneurysms or coronary veins. Increasingly, variants or unusual accessory pathways, some involving an accessory node, are reported in clinical studies.

Localization of the sinoatrial and atrioventricular nodal region in neonatal and juvenile ovine hearts.

Localization of the components of the cardiac conduction system (CCS) is essential for many therapeutic procedures in cardiac surgery and interventional cardiology. While histological studies provided fundamental insights into CCS localization, this information is incomplete and difficult to translate to aid in intraprocedural localization. To advance our understanding of CCS localization, we set out to establish a framework for quantifying nodal region morphology. Using this framework, we quantitatively analyzed the sinoatrial node (SAN) and atrioventricular node (AVN) in ovine with postmenstrual age ranging from 4.4 to 58.3 months. In particular, we studied the SAN and AVN in relation to the epicardial and endocardial surfaces, respectively. Using anatomical landmarks, we excised the nodes and adjacent tissues, sectioned those at a thickness of 4 µm at 100 µm intervals, and applied Masson's trichrome stain to the sections. These sections were then imaged, segmented to identify nodal tissue, and analyzed to quantify nodal depth and superficial tissue composition. The minimal SAN depth ranged between 20 and 926 µm. AVN minimal depth ranged between 59 and 1192 µm in the AVN extension region, 49 and 980 µm for the compact node, and 148 and 888 µm for the transition to His Bundle region. Using a logarithmic regression model, we found that minimal depth increased logarithmically with age for the AVN (R2 = 0.818, P = 0.002). Also, the myocardial overlay of the AVN was heterogeneous within different regions and decreased with increasing age. Age associated alterations of SAN minimal depth were insignificant. Our study presents examples of characteristic tissue patterns superficial to the AVN and within the SAN. We suggest that the presented framework provides quantitative information for CCS localization. Our studies indicate that procedural methods and localization approaches in regions near the AVN should account for the age of patients in cardiac surgery and interventional cardiology.

Histological and morphometric study of the components of the sinus and atrioventricular nodes in horses and dogs.

The cardiac nodes are the source of the electrical impulse that is transmitted to the heart, the aim of this work is study the histological and morphometric characteristics of the different components of the sinus and atrioventricular nodes in horses and dogs that help to know the physiopathology of these nodes. A group of ten horse hearts and five dog hearts were used. The region of the sinus and atrioventricular nodes was sectioned serially, and the block of tissue removed for study. The samples were assessed using a morphometric analysis with the Image-Pro Plus 7.1 software and the acquisition of the images using a Leica DMD108 optic microscope. The shape of the horse's sinus node is oblong and its P cells are large. The shape of the dog's sinus is rounded or oblong. The P cells are large and pale. The area of P cells in horses was 976 (SD 223.7) µm2 and in dogs the area for P cells was 106 (SD 30.4) µm2, which indicates that the value for P cells in horses are significantly higher than in dogs (p = .001). The horse atrioventricular node presented an oblong shape and in dogs, presents a spindle shape. The lower cell density in any of the cardiac nodes, especially in P cells of sinus node, can decrease electrical conduction within the nodes and in the internodal tracts, which would reflect the presence of cardiac arrhythmias derived from poor conduction, even in morphologically normal hearts.

Clarifying the anatomy of the atrioventricular node artery.

BACKGROUND: Reports of conduction abnormalities necessitating permanent pacemaker implantation due to atrioventricular node artery injury and increasing evidence of stenosis of the atrioventricular node artery in cases of sudden death are of unsolved clinical importance. Unfortunately, technical issues associated with physical and virtual dissections of the atrioventricular conduction axis make it difficult to accurately assess its arterial supply. METHODS: We used a specialized dissection technique to gather anatomical information on the atrioventricular node artery and described them using attitudinally appropriate terminology. RESULTS: The mean number of atrioventricular node artery branches was 1.6 in 103 submacroscopic examinations and 2.3 in 17 histological reconstructions. The artery had 5 origins in the modified AHA anatomy guidelines: distal RCA (#3), 10.4%; right posterior interventricular artery (#4PI), 7.3%; proximal RCA posterolateral branch (proximal #4PL), 76.8%; distal RCA posterolateral branch detouring the coronary sinus (distal #4PL), 1.8%; distal LCX (#13), 3.7%. Histological examination revealed that most atrioventricular node arteries immediately left the distal compact node (71.8%), suggesting that they supply mainly the proximal part of the AV conduction axis. The artery to the atrioventricular node tended to originate from the medial and atrial aspect of RCA posterolateral branch, and supplied adjacent structures within the inferior pyramidal space before entering the compact atrioventricular node. CONCLUSIONS: Based on the visualisation of the atrioventricular conduction axis and its arterial supply, we herein provide the 'gold standard' for understanding the origin, course and distribution of the artery to the atrioventricular node.

A contemporary view of atrioventricular nodal physiology.

In delaying transmission of the cardiac impulse from the atria to the ventricles, the atrioventricular (AV) node serves a critical function in augmenting ventricular filling during diastole and limiting the ventricular response during atrial tachyarrhythmias. The complex structure of the nodal region, however, also provides the substrate for reentrant rhythms. Recent discoveries have elucidated the cellular basis and anatomical determinants of slow conduction in the node. Based on analysis of gap junction proteins, distinct structural components of the AV node have been defined, including the compact node, right and left inferior nodal extensions, the lower nodal bundle, and transitional tissue. Emerging evidence supports the role of the inferior nodal extensions in mediating slow pathway conduction. The most common form of reentry involving the node, slow-fast AV nodal reentrant tachycardia (AVNRT), utilizes the inferior nodal extensions for anterograde slow pathway conduction; the structures responsible for retrograde fast pathway activation in the superior septum are less well defined and likely heterogeneous. Atypical forms of AVNRT arise from circuits that activate at least one of the inferior extensions in the retrograde direction.

Histological topography of the atrioventricular node and its extensions in relation to the cardiothoracic surgical landmarks in normal human hearts.

BACKGROUND: Atrioventricular (AV) nodal injury which results in cardiac conduction disorders is one of the potential complications of heart valve surgeries and radiofrequency catheter ablations. Understanding the topography of the AV conduction system in relation to the tricuspid and mitral valves will help in reducing these complications. METHODS: A tissue block of 3cmx4cm, which contain the AV node, bundle of His and the AV nodal extensions, was excised at the AV septal junction in 20 apparently normal human hearts. The block was divided into three equal segments through vertical incisions perpendicular to the insertion of the septal leaflet of the tricuspid valve. Each segment was processed and stained with H&E and Gomori to study the different parts of the AV conduction system. RESULTS: The lower pole of the AV node was located vertically above the tricuspid septal leaflet (TSL) in 100% (20/20) of cases and at the level of the muscular interventricular septum in 65% (13/20) of cases. The upper pole of the compact AV node was located at the level of the mitral valve leaflet (MVL) in 50% (10/20) of cases. The penetrating bundle of His was seen at the level of the TSL, while the branching bundle of His was situated 1.9±1.5 mm inferior to the TSL. The right and left posterior extensions of the AV node spanned from the MVL to 2.9±1.3 mm above the TSL. CONCLUSIONS: A rectangular area (2.5 mm × 12 mm) in the Koch's triangle was devoid of AV nodal tissue and could be labeled as a safe area with no risk of conduction defects during valve surgeries. Information on the separation of AV nodal extensions from the TSL, MVL and muscular interventricular septum may play a crucial role in guiding and improving the safety of radiofrequency ablations.

Morphological study of the atrioventricular conduction system and Purkinje fibers in yak.

We studied the morphology of the atrioventricular conduction system (AVCS) and Purkinje fibers of the yak. Light and transmission electron microscopy were used to study the histological features of AVCS. The distributional characteristics of the His-bundle, the left bundle branch (LBB), right bundle branch (RBB), and Purkinje fiber network of yak hearts were examined using gross dissection, ink injection, and ABS casting. The results showed that the atrioventricular node (AVN) of yak located in the right side of interatrial septum and had a flattened ovoid shape. The AVN of yak is composed of the slender, interweaving cells formed almost entirely of the transitional cells (T-cells). The His-bundle extended from the AVN, and split into left LBB and RBB at the crest of the interventricular septum. The LBB descended along the left side of interventricular septum. At approximately the upper 1/3 of the interventricular septum, the LBB typically divided into three branches. The RBB ran under the endocardium of the right side of interventricular septum, and extended to the base of septal papillary muscle, passed into the moderator band, crossed the right ventricular cavity to reach the base of anterior papillary muscle, and divided into four fascicles under the subendocardial layer. The Purkinje fibers in the ventricle formed a complex spatial network. The distributional and cellular component characteristics of the AVCS and Purkinje fibers ensured normal cardiac function.

The Current State and Future Potential of Pediatric and Congenital Electrophysiology.

Pediatric electrophysiologists specialize in the diagnosis and treatment of rhythm abnormalities in pediatric, congenital heart disease, and inherited arrhythmia syndrome patients. The field originated out of the unique knowledge base that rhythm management in young patients required. In the 1970s, pediatric electrophysiology was recognized as a distinct cardiac subspecialty and it has evolved rapidly since that time. Despite the considerable growth in personnel, technology, and complexity that the field has undergone, further opportunities to progress pediatric electrophysiology exist. In this review, we highlight some of the clinical focus of pediatric and adult congenital electrophysiologists to date and identify areas within this specialty where the pediatric and congenital electrophysiology community could come together in order to drive improvements in rhythm management for patients.

The Cardiac Conduction System: Generation and Conduction of the Cardiac Impulse.

In this article, the authors outline the key components behind the automated generation of the cardiac impulses and the effect these impulses have on cardiac myocytes. Also, a description of the key components of the normal cardiac conduction system is provided, including the sinoatrial node, the atrioventricular node, the His bundle, the bundle branches, and the Purkinje network. Finally, an outline of how each stage of the cardiac conduction system is represented on the electrocardiogram is described, allowing the reader of the electrocardiogram to translate background information about the normal cardiac conduction system to everyday clinical practice.

Location and functional characterization of the right atrioventricular pacemaker ring in the adult avian heart.

Previous histological studies showed that in addition to a sinus node, an atrioventricular (AV) node, an AV bundle, left and right bundle branches, birds also possess a right AV-Purkinje ring that is located in the atrial sheet of the right muscular AV-valve along all its base length. The functionality of the AV-Purkinje ring is unknown. In this work, we studied the topology of pacemaker myocytes in the atrial side of the isolated chicken spontaneously contracting right muscular AV-valve using the method of microelectrode mapping of action potentials. We show that AV-cells having the ability to show pacemaking reside in the right muscular AV-valve. Pacemaker action potentials were exclusively recorded close to the base of the valve along its whole length from dorsal to the ventral attachment to the interventricular septum. These action potentials have much slower rate of depolarization, lower amplitude, and higher diastolic depolarization than action potentials of Purkinje (conducting) cells. We conclude the right AV-valve has a ring bundle of pacemaker cells (but not Purkinje cells) in the adult chicken heart.

The sinus venosus myocardium contributes to the atrioventricular canal: potential role during atrioventricular node development?

The presence of distinct electrophysiological pathways within the atrioventricular node (AVN) is a prerequisite for atrioventricular nodal reentrant tachycardia to occur. In this study, the different cell contributions that may account for the anatomical and functional heterogeneity of the AVN were investigated. To study the temporal development of the AVN, the expression pattern of ISL1, expressed in cardiac progenitor cells, was studied in sequential stages performing co-staining with myocardial markers (TNNI2 and NKX2-5) and HCN4 (cardiac conduction system marker). An ISL1+/TNNI2+/HCN4+ continuity between the myocardium of the sinus venosus and atrioventricular canal was identified in the region of the putative AVN, which showed a pacemaker-like phenotype based on single cell patch-clamp experiments. Furthermore, qPCR analysis showed that even during early development, different cell populations can be identified in the region of the putative AVN. Fate mapping was performed by in ovo vital dye microinjection. Embryos were harvested and analysed 24 and 48 hrs post-injection. These experiments showed incorporation of sinus venosus myocardium in the posterior region of the atrioventricular canal. The myocardium of the sinus venosus contributes to the atrioventricular canal. It is postulated that the myocardium of the sinus venosus contributes to nodal extensions or transitional cells of the AVN since these cells are located in the posterior region of the AVN. This finding may help to understand the origin of atrioventricular nodal reentrant tachycardia.

Functional, anatomical, and molecular investigation of the cardiac conduction system and arrhythmogenic atrioventricular ring tissue in the rat heart.

BACKGROUND: The cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node-like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3-dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node. METHODS AND RESULTS: Three-dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Cav3.1, Cav3.2, Kv1.5, SK1, Kir3.1, and Kir3.4) and lower expression of other key ion channels (Nav1.5 and Kir2.1). CONCLUSIONS: The AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.

Identification of nodal tissue in the living heart using rapid scanning fiber-optics confocal microscopy and extracellular fluorophores.

BACKGROUND: Risks associated with pediatric reconstructive heart surgery include injury of the sinoatrial node (SAN) and atrioventricular node (AVN), requiring cardiac rhythm management using implantable pacemakers. These injuries are the result of difficulties in identifying nodal tissues intraoperatively. Here we describe an approach based on confocal microscopy and extracellular fluorophores to quantify tissue microstructure and identify nodal tissue. METHODS AND RESULTS: Using conventional 3-dimensional confocal microscopy we investigated the microstructural arrangement of SAN, AVN, and atrial working myocardium (AWM) in fixed rat heart. AWM exhibited a regular striated arrangement of the extracellular space. In contrast, SAN and AVN had an irregular, reticulated arrangement. AWM, SAN, and AVN tissues were beneath a thin surface layer of tissue that did not obstruct confocal microscopic imaging. Subsequently, we imaged tissues in living rat hearts with real-time fiber-optics confocal microscopy. Fiber-optics confocal microscopy images resembled images acquired with conventional confocal microscopy. We investigated spatial regularity of tissue microstructure from Fourier analysis and second-order image moments. Fourier analysis of fiber-optics confocal microscopy images showed that the spatial regularity of AWM was greater than that of nodal tissues (37.5 ± 5.0% versus 24.3 ± 3.9% for SAN and 23.8 ± 3.7% for AVN; P<0.05). Similar differences of spatial regularities were revealed from second-order image moments (50.0 ± 7.3% for AWM versus 29.3 ± 6.7% for SAN and 27.3 ± 5.5% for AVN; P<0.05). CONCLUSIONS: The study demonstrates feasibility of identifying nodal tissue in living heart using extracellular fluorophores and fiber-optics confocal microscopy. Application of the approach in pediatric reconstructive heart surgery may reduce risks of injuring nodal tissues.

A simple model of the right atrium of the human heart with the sinoatrial and atrioventricular nodes included.

Existing atrial models with detailed anatomical structure and multi-variable cardiac transmembrane current models are too complex to allow to combine an investigation of long time dycal properties of the heart rhythm with the ability to effectively simulate cardiac electrical activity during arrhythmia. Other ways of modeling need to be investigated. Moreover, many state-of-the-art models of the right atrium do not include an atrioventricular node (AVN) and only rarely--the sinoatrial node (SAN). A model of the heart tissue within the right atrium including the SAN and AVN nodes was developed. Looking for a minimal model, currently we are testing our approach on chosen well-known arrhythmias, which were until now obtained only using much more complicated models, or were only observed in a clinical setting. Ultimately, the goal is to obtain a model able to generate sequences of RR intervals specific for the arrhythmias involving the AV junction as well as for other phenomena occurring within the atrium. The model should be fast enough to allow the study of heart rate variability and arrhythmias at a time scale of thousands of heart beats in real-time. In the model of the right atrium proposed here, different kinds of cardiac tissues are described by sets of different equations, with most of them belonging to the class of Liénard nonlinear dynamical systems. We have developed a series of models of the right atrium with differing anatomical simplifications, in the form of a 2D mapping of the atrium or of an idealized cylindrical geometry, including only those anatomical details required to reproduce a given physiological phenomenon. The simulations allowed to reconstruct the phase relations between the sinus rhythm and the location and properties of a parasystolic source together with the effect of this source on the resultant heart rhythm. We model the action potential conduction time alternans through the atrioventricular AVN junction observed in cardiac tissue in electrophysiological studies during the ventricular-triggered atrial tachycardia. A simulation of the atrio-ventricular nodal reentry tachycardia was performed together with an entrainment procedure in which the arrhythmia circuit was located by measuring the post-pacing interval (PPI) at simulated mapping catheters. The generation and interpretation of RR times series is the ultimate goal of our research. However, to reach that goal we need first to (1) somehow verify the validity of the model of the atrium with the nodes included and (2) include in the model the effect of the sympathetic and vagal ANS. The current paper serves as a partial solution of the 1). In particular we show, that measuring the PPI-TCL entrainment response in proximal (possibly-the slow-conducting pathway), the distal and at a mid-distance from CS could help in rapid distinction of AVNRT from other atrial tachycardias. Our simulations support the hypothesis that the alternans of the conduction time between the atria and the ventricles in the AV orthodromic reciprocating tachycardia can occur within a single pathway. In the atrial parasystole simulation, we found a mathematical condition which allows for a rough estimation of the location of the parasystolic source within the atrium, both for simplified (planar) and the cylindrical geometry of the atrium. The planar and the cylindrical geometry yielded practically the same results of simulations.