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.

Spatially resolved multiomics of human cardiac niches

The function of a cell is defined by its intrinsic characteristics and its niche: the tissue microenvironment in which it dwells. Here we combine single-cell and spatial transcriptomics data to discover cellular niches within eight regions of the human heart. We map cells to microanatomical locations and integrate knowledge-based and unsupervised structural annotations. We also profile the cells of the human cardiac conduction system1. The results revealed their distinctive repertoire of ion channels, G-protein-coupled receptors (GPCRs) and regulatory networks, and implicated FOXP2 in the pacemaker phenotype. We show that the sinoatrial node is compartmentalized, with a core of pacemaker cells, fibroblasts and glial cells supporting glutamatergic signalling. Using a custom CellPhoneDB.org module, we identify trans-synaptic pacemaker cell interactions with glia. We introduce a druggable target prediction tool, drug2cell, which leverages single-cell profiles and drug-target interactions to provide mechanistic insights into the chronotropic effects of drugs, including GLP-1 analogues. In the epicardium, we show enrichment of both IgG+ and IgA+ plasma cells forming immune niches that may contribute to infection defence. Overall, we provide new clarity to cardiac electro-anatomy and immunology, and our suite of computational approaches can be applied to other tissues and organs.

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.

Dynamic Cellular Integration Drives Functional Assembly of the Heart's Pacemaker Complex

Impulses generated by a multicellular, bioelectric signaling center termed the sinoatrial node (SAN) stimulate the rhythmic contraction of the heart. The SAN consists of a network of electrochemically oscillating pacemaker cells encased in a heterogeneous connective tissue microenvironment. Although the cellular composition of the SAN has been a point of interest for more than a century, the biological processes that drive the tissue-level assembly of the cells within the SAN are unknown. Here, we demonstrate that the SAN’s structural features result from a developmental process during which mesenchymal cells derived from a multipotent progenitor structure, the proepicardium, integrate with and surround pacemaker myocardium. This process actively remodels the forming SAN and is necessary for sustained electrogenic signal generation and propagation. Collectively, these findings provide experimental evidence for how the microenvironmental architecture of the SAN is patterned and demonstrate that proper cellular arrangement is critical for cardiac pacemaker biorhythmicity.

Computational assessment of the functional role of sinoatrial node exit pathways in the human heart

AIM

The human right atrium and sinoatrial node (SAN) anatomy is complex. Optical mapping experiments suggest that the SAN is functionally insulated from atrial tissue except at discrete SAN-atrial electrical junctions called SAN exit pathways, SEPs. Additionally, histological imaging suggests the presence of a secondary pacemaker close to the SAN. We hypothesise that a) an insulating border-SEP anatomical configuration is related to SAN arrhythmia; and b) a secondary pacemaker, the paranodal area, is an alternate pacemaker but accentuates tachycardia. A 3D electro-anatomical computational model was used to test these hypotheses.

METHODS

A detailed 3D human SAN electro-anatomical mathematical model was developed based on our previous anatomical reconstruction. Electrical activity was simulated using tissue specific variants of the Fenton-Karma action potential equations. Simulation experiments were designed to deploy this complex electro-anatomical system to assess the roles of border-SEPs and paranodal area by mimicking experimentally observed SAN arrhythmia. Robust and accurate numerical algorithms were implemented for solving the mono domain reaction-diffusion equation implicitly, calculating 3D filament traces, and computing dominant frequency among other quantitative measurements.

RESULTS

A centre to periphery gradient of increasing diffusion was sufficient to permit initiation of pacemaking at the centre of the 3D SAN. Re-entry within the SAN, micro re-entry, was possible by imposing significant SAN fibrosis in the presence of the insulating border. SEPs promoted the micro re-entry to generate more complex SAN-atrial tachycardia. Simulation of macro re-entry, i.e. re-entry around the SAN, was possible by inclusion of atrial fibrosis in the presence of the insulating border. The border shielded the SAN from atrial tachycardia. However, SAN micro-structure intercellular gap junctional coupling and the paranodal area contributed to prolonged atrial fibrillation. Finally, the micro-structure was found to be sufficient to explain shifts of leading pacemaker site location.

CONCLUSIONS

The simulations establish a relationship between anatomy and SAN electrical function. Microstructure, in the form of intercellular gap junction coupling, was found to regulate SAN function and arrhythmia.

High resolution 3-Dimensional imaging of the human cardiac conduction system from microanatomy to mathematical modeling

Cardiac arrhythmias and conduction disturbances are accompanied by structural remodelling of the specialised cardiomyocytes known collectively as the cardiac conduction system. Here, using contrast enhanced micro-computed tomography, we present, in attitudinally appropriate fashion, the first 3-dimensional representations of the cardiac conduction system within the intact human heart. We show that cardiomyocyte orientation can be extracted from these datasets at spatial resolutions approaching the single cell. These data show that commonly accepted anatomical representations are oversimplified. We have incorporated the high-resolution anatomical data into mathematical simulations of cardiac electrical depolarisation. The data presented should have multidisciplinary impact. Since the rate of depolarisation is dictated by cardiac microstructure, and the precise orientation of the cardiomyocytes, our data should improve the fidelity of mathematical models. By showing the precise 3-dimensional relationships between the cardiac conduction system and surrounding structures, we provide new insights relevant to valvar replacement surgery and ablation therapies. We also offer a practical method for investigation of remodelling in disease, and thus, virtual pathology and archiving. Such data presented as 3D images or 3D printed models, will inform discussions between medical teams and their patients, and aid the education of medical and surgical trainees.

Computer simulations of reentrant activity in the rabbit sinoatrial node

With the aid of detailed computer simulations, we have estimated distributions of membrane potential and ionic currents in the core region of a sinoatrial node reentry. We observe reduced amplitudes of the measured quantities in the core; the core sizes for potential and currents did not always coincide. Simulations revealed that acetylcholine, when applied in the vicinity of unstable reentry, attracted the reentry to become the core and to stabilize its rotation. Anatomically detailed simulations of sinoatrial node and surrounding atrial tissue revealed that reentry always rotated around small strips of connective tissue. Acetylcholine superfusion over superior part of the sinoatrial node resulted in a drift of reentry in the cranial direction. Under the latter conditions, reentry may coexist with the pacemaker in the caudal part of the sinoatrial node. Copyright © 2016 John Wiley & Sons, Ltd.

Anatomical Variations in the Sinoatrial Nodal Artery: A Meta-Analysis and Clinical Considerations

BACKGROUND AND OBJECTIVE

The sinoatrial nodal artery (SANa) is a highly variable vessel which supplies blood to the sinoatrial node (SAN). Due to its variability and susceptibility to iatrogenic injury, our study aimed to assess the anatomy of the SANa and determine the prevalence of its anatomical variations.

STUDY DESIGN

An extensive search of major electronic databases was performed to identify all articles reporting anatomical data on the SANa. No lower date limit or language restrictions were applied. Anatomical data regarding the artery were extracted and pooled into a meta-analysis.

RESULTS

Sixty-six studies (n = 21455 hearts) were included in the meta-analysis. The SANa usually arose as a single vessel with a pooled prevalence of 95.5% (95%CI:93.6-96.9). Duplication and triplication of the artery were also observed with pooled prevalence of 4.3% (95%CI:2.8-6.0) and 0.3% (95%CI:0-0.7), respectively. The most common origin of the SANa was from the right coronary artery (RCA), found in 68.0% (95%CI:55.6-68.9) of cases, followed by origin from the left circumflex artery, and origin from the left coronary artery with pooled prevalence of 22.1% (95%CI:15.0-26.2) and 2.7 (95%CI:0.7-5.2), respectively. A retrocaval course of the SANa was the most common course of the artery with a pooled prevalence of 47.1% (95%CI:36.0-55.5). The pooled prevalence of an S-shaped SANa was 7.6% (95%CI:2.9-14.1).

CONCLUSIONS

The SANa is most commonly reported as a single vessel, originating from the RCA, and taking a retrocaval course to reach the SAN. Knowledge of high risk anatomical variants of the SANa, such as an S-shaped artery, must be taken into account by surgeons to prevent iatrogenic injuries. Specifically, interventional or cardiosurgical procedures, such as the Cox maze procedure for atrial fibrillation, open heart surgeries through the right atrium or intraoperative cross-clamping or dissection procedures during mitral valve surgery using the septal approach can all potentiate the risk for injury in the setting of high-risk morphological variants of the SANa.

Human sinoatrial node structure: 3D microanatomy of sinoatrial conduction pathways

INTRODUCTION

Despite a century of extensive study on the human sinoatrial node (SAN), the structure-to-function features of specialized SAN conduction pathways (SACP) are still unknown and debated. We report a new method for direct analysis of the SAN microstructure in optically-mapped human hearts with and without clinical history of SAN dysfunction.

METHODS

Two explanted donor human hearts were coronary-perfused and optically-mapped. Structural analyses of histological sections parallel to epicardium (∼13-21 μm intervals) were integrated with optical maps to create 3D computational reconstructions of the SAN complex. High-resolution fiber fields were obtained using 3D Eigen-analysis of the structure tensor, and used to analyze SACP microstructure with a fiber-tracking approach.

RESULTS

Optical mapping revealed normal SAN activation of the atria through a lateral SACP proximal to the crista terminalis in Heart #1 but persistent SAN exit block in diseased Heart #2. 3D structural analysis displayed a functionally-observed SAN border composed of fibrosis, fat, and/or discontinuous fibers between SAN and atria, which was only crossed by several branching myofiber tracts in SACP regions. Computational 3D fiber-tracking revealed that myofiber tracts of SACPs created continuous connections between SAN #1 and atria, but in SAN #2, SACP region myofiber tracts were discontinuous due to fibrosis and fat.

CONCLUSIONS

We developed a new integrative functional, structural and computational approach that allowed for the resolution of the specialized 3D microstructure of human SACPs for the first time. Application of this integrated approach will shed new light on the role of the specialized SAN microanatomy in maintaining sinus rhythm.

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, Ca_v3.1, Ca_v3.2, K_v1.5, SK1, K_ir3.1, and K_ir3.4) and lower expression of other key ion channels (Na_v1.5 and K_ir2.1).

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.

Contrast enhanced micro-computed tomography resolves the 3-dimensional morphology of the cardiac conduction system in mammalian hearts

The general anatomy of the cardiac conduction system (CCS) has been known for 100 years, but its complex and irregular three-dimensional (3D) geometry is not so well understood. This is largely because the conducting tissue is not distinct from the surrounding tissue by dissection. The best descriptions of its anatomy come from studies based on serial sectioning of samples taken from the appropriate areas of the heart. Low X-ray attenuation has formerly ruled out micro-computed tomography (micro-CT) as a modality to resolve internal structures of soft tissue, but incorporation of iodine, which has a high molecular weight, into those tissues enhances the differential attenuation of X-rays and allows visualisation of fine detail in embryos and skeletal muscle. Here, with the use of a iodine based contrast agent (I₂KI), we present contrast enhanced micro-CT images of cardiac tissue from rat and rabbit in which the three major subdivisions of the CCS can be differentiated from the surrounding contractile myocardium and visualised in 3D. Structures identified include the sinoatrial node (SAN) and the atrioventricular conduction axis: the penetrating bundle, His bundle, the bundle branches and the Purkinje network. Although the current findings are consistent with existing anatomical representations, the representations shown here offer superior resolution and are the first 3D representations of the CCS within a single intact mammalian heart.

Computer three-dimensional anatomical reconstruction of the human sinus node and a novel paranodal area

We have previously shown in rabbit that the pacemaker of the heart (the sinus node) is widespread and matches the wide distribution of the leading pacemaker site within the right atrium. There is, however, uncertainty about the precise location of the pacemaker in human heart, and its spatial relationships with the surrounding right atrial muscle. Therefore, the aim of the current study was to investigate the distribution of the sinus node tissue in a series of healthy human hearts and, for one of the hearts to construct a computer three-dimensional anatomical model of the sinus node, including the likely orientation of myocytes. A combination of experimental techniques--diffusion tensor magnetic resonance imaging (DT-MRI), histology, immunohistochemistry, image processing and computer modelling--was used. Our data show that the sinus node was larger than in previous studies and, most importantly, we identified a previously unknown area running alongside the sinus node (the "paranodal area"), which is even more extensive than the sinus node. This area possesses properties of both nodal and atrial tissues and may have a role in pacemaking. For example, it could explain the wide spread distribution of the leading pacemaker site in human right atrium, a phenomenon known as the wandering pacemaker observed in clinics. In summary, a novel 3D anatomical reconstruction presents a new picture of the distribution of nodal cells within the human right atrium.

Variation in the blood supply of the sinus node

PURPOSE

The purpose of the study was to examine the anatomical variations in the blood supply to the sinus node.

METHODS

Gross anatomical examination and angiographic evaluation were performed in 400 human hearts derived from victims of various accidents.

RESULTS

The sinus node artery was a branch of the right coronary artery in 245 cases, the left circumflex in 147 cases, and both coronary arteries in 8 cases. In one subject, two sinus node arteries were found to arise from the left circumflex artery, a finding never reported before.

CONCLUSIONS

Anatomic and postmortem angiographic findings of a previously unreported case where the sinus node is perfused by two sinus node arteries originating from the left circumflex coronary artery are demonstrated. Knowledge of this anatomical variation is useful for anatomists and of clinical significance for the interventional cardiologists and mainly for the cardiac surgeons in planning the surgical procedures.

[Mechanisms of functioning and regulation of mammalian sinoatrial node]

Actual data concerning mechanisms of automaticity in sinoatrial node, which acts as a primary pacemaker in mammalian heart, is reviewed. Studies dealing with ionic currents, maintaining automatic generation of excitation in the sinoatrial cells, and possible role of intracellular calcium turnover are discussed. Special attention is given to the differences between the central and peripheral parts of sinoatrial node, phenomenon of intranodal pacemaker shift resulting from that differences and possible role of pacemaker shift in the modulation of the sinus rhythm. Mechanisms of sinus rhythm regulation under the action of acetylcholine and noradrenalin are also discussed in detail.

A simple dissection method for the conduction system of the human heart

A simple dissection guide for the conduction system of the human heart is shown. The atrioventricular (AV) node, AV bundle, and right bundle branch were identified in a formaldehyde-fixed human heart. The sinu-atrial (SA) node could not be found, but the region in which SA node was contained was identified using the SA nodal artery. Gross anatomical observation of the conduction system is useful for understanding the structure and function of the heart.

The valve of the superior vena cava--the supernumerary structure of the precaval segment of the crista terminalis

The primitive right sinuatrial valve persists in humans as the crista terminalis, the valve of the inferior vena cava and the valve of the coronary sinus, while according to the known data the primitive left sinuatrial valve is supposed to have no derivatives. Ten human right atria were opened with intercaval incisions and the precaval segment of each crista terminalis was studied macroscopically. Three specimens did not present any peculiarities at this level, but the other 7 had sagittal muscle bundles and supernumerary valves in individual arrangements. Supernumerary valves were present in 2 specimens, one complete and the second fenestrated; these valves were located immediately below the superior vena cava orifice and covered the medial end of the crista terminalis. The supernumerary valves at the superior vena cava orifice may be termed, mirroring that of the inferior vena cava, "valves of the superior vena cava". Their exact frequency of occurrence and their embryonic precursors must be further established. The presence of such valves in the right atrium may interfere with the flow to the right side of the heart, may represent conditions for thrombotic changes and may disturb a central venous catheter placement. If present, the valve of the superior vena cava will also interfere with the catheter ablation procedures used for supraventricular tachycardia.

[The discovery of the sinus node 100 years ago and the part of K. F. Wenckebach]

Hundred years ago, in 1907, A. Keith and M. Flack described histologically in the right atrium of different mammalian including human hearts a structure which they called sinu-auricular node and which they interpreted as the place where the heart beat originates. One year earlier K.F. Wenckebach had reported on an arrhythmia which he explained by polygraphic technique as "vein-atrial block" (today: sinoatrial block). To such an assumption he had to suppose that the heart action begins in the vena cava superior where he described a "small but interesting musculature" above the atrium as the morphological basis of the origin. Recently, with regard to the publication dates of these findings it was claimed (W. Ehrlich) that we should owe the honour of having discovered the sinus node morphologically to Wenckebach and not to Keith and Flack. Referring to the original publications it is shown that Wenckebach as well as Keith and Flack refered to different ideas. Wenckebach supposed the location of the origin of the heart beat in a macroscopically discernible muscle placed at the vena cava superior just above but separated from the right atrium. As the only connection capable of conduction between both he described a special bundle. On the contrary, Keith and Flack depicted as the place where the heart action begins a microscopically defined structur in the atrium at the junction of the vena cava superior with the sorrounding venous and atrial musculature being not separated from each other. Scientific progress corroborated the interpretation of Keith and Flack, while Wenckebach desisted only slowly from his position which, after all, proved incorrect.

Anatomical variations in the human sinuatrial nodal artery

OBJECTIVE

To analyze the anatomical variations of sinuatrial nodal branch(es) of the coronary artery mainly regarding their number; a recent report from Japan claims the presence of 2 branches in up to 50% of cases, an occurrence that would permit adequate flow compensation in case of occlusion or section of 1 of these branches.

METHODS

The sinuatrial nodal branch(es) of 50 human hearts fixed in formol solution were dissected with the aid of a Normo Health 3.0 degree visor magnifying lens, measured, and classified as to the origin, route, and number of branches.

RESULTS

In 94% (n = 47) of cases, a single sinuatrial nodal branch was found. classified: (A) two right side types, R1 (in 46% of cases, n = 23), situated medial to the right auricle and R2 (in 4% of cases, n = 2), situated on the posterior surface of the right atrium; (B) three left side types, L1 (in 24% of cases, n = 12), situated medial to the left auricle, L2 (in 16% of cases, n = 8), situated posterior to the left auricle, and L3 (in 4% of cases, n = 2), situated on the posterior surface of the left atrium. Except for R2, each type was subdivided into 'a' or 'b' types, according to whether the sinuatrial nodal branch(es) occurred in a clockwise or counterclockwise orientation around the base of the superior cava vena. In 4% of cases (n = 2), 2 sinuatrial nodal branch(es) were observed with 1 branch originating from each of the coronary arteries. In 1 case (2%), 3 sinuatrial nodal branch(es) were found, 2 from the right coronary artery and the third probably from the bronchial branch of the thoracic aorta. In 30% of the cases, the sinuatrial nodal branch(es) formed a ring around the base of the superior cava vena. In all cases, the sinuatrial nodal branch(es) supplied collateral branches to the atrium and/or the auricle of the same side as its origin and/or to the opposite side.

CONCLUSION

The low frequency of 2 sinuatrial nodal branch(es) in Brazilian individuals, compared to the higher frequency found among the Japanese, is probably due to a variation associated with ethnic group origin.

Organisation of the mouse sinoatrial node: structure and expression of HCN channels

OBJECTIVE

To reveal the structural characteristics of the sinoatrial node (SAN) and the distribution of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN) in the SAN in the mouse.

METHODS

The structure of the SAN and the distribution of HCN channels in the SAN in the mouse were studied by histology and immunolabelling of ANP, Cx43 and HCN channels.

RESULTS

The mouse SAN is a comma-shaped structure with a length of approximately 1.5 mm parallel to the crista terminalis and is separated from atrial muscle by connective tissue at the border both with the crista terminalis and the atrial septum. A unique compact nodal structure with densely-packed nodal cells was identified at the head of the comma-shaped SAN. Cell size and fibre orientation vary regionally in the SAN: the cells in the compact node are small and are orientated perpendicular to the crista terminalis, whereas the cells in the more inferior part are larger and more loosely-packed and are orientated parallel to the crista terminalis. All SAN cells exhibited labelling of HCN4, but no cell exhibited detectable labelling of HCN1, HCN2, ANP and Cx43, while surrounding atrial cells exhibited labelling of ANP and Cx43, but not HCN1, HCN2 and HCN4. A specialised interface between the SAN and surrounding atrial muscle was also identified: strands of HCN4-positive nodal cells protrude into the atrial muscle and strands of Cx43-positive atrial cells protrude into the SAN; thus, there are interdigitations between the SAN and atrial muscle.

CONCLUSIONS

In the mouse, (i) the SAN is structurally complex with a densely-packed head and loosely-packed tail; (ii) HCN4 is the only HCN isoform detectable and is present throughout the SAN; and (iii) there is a specialised interface between the SAN and surrounding atrium that may be necessary for the SAN to drive the more hyperpolarized atrial muscle.

Percutaneous Mitral Annuloplasty

Background— To allow performance of “stand-alone” mitral annuloplasty with minimal invasiveness, percutaneous techniques consisting of delivery into the coronary sinus (CS) of devices intended to shrink the mitral valve annulus have recently been tested in animal models. These techniques exploit the anatomic proximity of the CS and mitral valve annulus in ovine or dogs. Knowledge of a detailed anatomic relationship between the CS, coronary arteries, and mitral valve annulus in humans is essential to define the safety and efficacy of percutaneous techniques in clinical practice. We sought to determine the qualitative and quantitative anatomic relationships between CS and surrounding structures in human hearts.

Methods and Results— The distance from the CS to the mitral valve annulus and the relationship between the CS and surrounding structures were studied in 61 excised cadaveric human hearts. Maximal distance from the CS to the mitral valve annulus was found to be up to 19 mm (mean, 9.7±3.2 mm). A diagonal or ramus branch, main circumflex artery, or its branches were located between anterior interventricular vein/CS and the mitral valve annulus in 16.4% and 63.9% of cases, respectively.

Conclusions— Surgical anatomy suggests that in humans the CS is located behind the left atrial wall at a significant distance from the mitral valve annulus. Percutaneous mitral annuloplasty devices probably shrink the mitral valve annulus only by an indirect traction mediated by the left atrial wall; a theoretical risk of compressing coronary artery branches exists. Chronic studies are needed to address this problem and to determine long-term efficacy of such methods.

Relative distribution densities of cholinergic and adrenoceptor structures in the central part of the sinoatrial node in rat heart

Characteristics of distribution of cholinergic and adrenoceptor structures along the sinoatrial node artery in rat heart were evaluated by autoradiography on semithin sections by determining the density of (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate binding sites. The relative density of binding sites for (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate was minimum in the functional nucleus of the sinoatrial node and asymmetrically increased to maximum values to cranial (sharply) and caudal (smoothly) directions. The relative level of binding for (3)H-dihydroalprenolol in the perinodal atrial myocardium tissue was markedly lower than in the periarterial zone of the central part of the sinoatrial node and comparable to that for (3)H-quinuclidinyl benzilate.

Considerations on the sinus node microangioarchitecture

The importance of the sinus node as the cardiac pacemaker is well known. The aim of the present study was to investigate the microangioarchitecture at the level of the sinus node. Ten human adult hearts were injected with India ink in the initial segments of the coronary arteries. Pieces were drawn and diaphanized. The results of the study can be summarized: (1) the sinus node is rather an irregularly shaped structure, with peripheral strands intermingling with strands of the atrial myocardium; at this level two vascular patterns can be recognized: (a) the myocardial capillary networks that parallels the muscular bundles, and (b) the peripheral nodal networks built upon dichotomizing arterioles; (2) it seems that while the thick and large sinus node artery does not branch in the nodal tissue, the blood supply of this tissue is ensured by the peripheral nodal networks; (3) characteristically, in the periphery of the nodal tissue are largely present glomeruli made by capillaries with pericellular dispositions. The results strongly suggest that the nodal tissue is mainly supplied from its periphery and the sinus node artery is rather a scaffold than a supplier of that tissue.

Imaging the heart: computer 3-dimensional anatomic models of the heart

Since the 1960s, models of the action potential in various cardiac cell types have been developed, and since the 1990s, 3-dimensional anatomic (or geometric) models of various cardiac structures have been developed. We are approaching the time when, for one species, we should have a complete set of action potential and anatomic models for the various cardiac tissues and then we will have realized the aim of constructing a "virtual heart" with accurate anatomy and electrophysiology. However, already the two types of model are beginning to be used in tandem to reconstruct the activation sequence of the heart both during sinus rhythm and arrhythmias.

Sinus node revisited in the era of electroanatomical mapping and catheter ablation

OBJECTIVE

To study the architecture of the human sinus node to facilitate understanding of mapping and ablative procedures in its vicinity.

METHODS

The sinoatrial region was examined in 47 randomly selected adult human hearts by histological analysis and scanning electron microscopy.

RESULTS

The sinus node, crescent-like in shape, and 13.5 (2.5) mm long, was not insulated by a sheath of fibrous tissue. Its margins were irregular, with multiple radiations interdigitating with ordinary atrial myocardium. The distances from the node to endocardium and epicardium were variable. In 72% of the hearts, the whole nodal body was subepicardial and in 13 specimens (28%) the inner aspect of the nodal body was subendocardial. The nodal body cranial to the sinus nodal artery was more subendocardial than the remaining nodal portion, which was separated from the endocardium by the terminal crest. In 50% of hearts, the most caudal boundaries of the body of the node were at least 3.5 mm from the endocardium. When the terminal crest was > 7 mm thick (13 hearts, 28%), the tail was subepicardial or intramyocardial and at least 3 mm from the endocardium.

CONCLUSIONS

The length of the node, the absence of an insulating sheath, the presence of nodal radiations, and caudal fragments offer a potential for multiple breakthroughs of the nodal wavefront. The very extensive location of the nodal tissue, the cooling effect of the nodal artery, and the interposing thick terminal crest caudal to this artery have implications for nodal ablation or modification with endocardial catheter techniques.

Computer three-dimensional reconstruction of the sinoatrial node

BACKGROUND

There is an effort to build an anatomically and biophysically detailed virtual heart, and, although there are models for the atria and ventricles, there is no model for the sinoatrial node (SAN). For the SAN to show pacemaking and drive atrial muscle, theoretically, there should be a gradient in electrical coupling from the center to the periphery of the SAN and an interdigitation of SAN and atrial cells at the periphery. Any model should include such features.

METHODS AND RESULTS

Staining of rabbit SAN preparations for histology, middle neurofilament, atrial natriuretic peptide, and connexin (Cx) 43 revealed multiple cell types within and around the SAN (SAN and atrial cells, fibroblasts, and adipocytes). In contrast to atrial cells, all SAN cells expressed middle neurofilament (but not atrial natriuretic peptide) mRNA and protein. However, 2 distinct SAN cell types were observed: cells in the center (leading pacemaker site) were small, were organized in a mesh, and did not express Cx43. In contrast, cells in the periphery (exit pathway from the SAN) were large, were arranged predominantly in parallel, often expressed Cx43, and were mixed with atrial cells. An approximately 2.5-million-element array model of the SAN and surrounding atrium, incorporating all cell types, was constructed.

CONCLUSIONS

For the first time, a 3D anatomically detailed mathematical model of the SAN has been constructed, and this shows the presence of a specialized interface between the SAN and atrial muscle.

Dimensionality in cardiac modelling

The development of mathematical models of the heart has been an ongoing concern for many decades. The initial focus of this work was on single cell models that incorporate varyingly detailed descriptions of the mechanisms that give rise to experimentally observed action potential shapes. Clinically relevant heart rhythm disturbances, however, are multicellular phenomena, and there have been many initiatives to develop multidimensional representations of cardiac electromechanical activity. Here, we discuss the merits of dimensionality, from 0D single cell models, to 1D cell strands, 2D planes and 3D volumes, for the simulation of normal and disturbed rhythmicity. We specifically look at models of: (i) the origin and spread of cardiac excitation from the sino-atrial node into atrial tissue, and (ii) stretch-activated channel effects on ventricular cell and tissue activity. Simulation of the spread of normal and disturbed cardiac excitation requires multicellular models. 1D architectures suffer from limitations in neighbouring tissue effects on individual cells, but they can (with some modification) be applied to the simulation of normal spread of excitation or, in ring-like structures, re-entry simulation (colliding wave fronts, tachycardia). 2D models overcome many of the limitations imposed by models of lower dimensionality, and can be applied to the study of complex co-existing re-entry patterns or even fibrillation. 3D implementations are closest to reality, as they allow investigation of scroll waves. Our results suggest that 2D models offer a good compromise between computational resources, complexity of electrophysiological models, and applicability to basic research, and that they should be considered as an important stepping-stone towards anatomically detailed simulations. This highlights the need to identify and use the most appropriate model for any given task. The notion of a single and ultimate model is as useful as the idea of a universal mechanical tool for all possible repairs and servicing requirements in daily life. The ideal model will be as simple as possible and as complex as necessary for the particular question raised.

[Sinus-like tachycardias]

Various tachycardias presenting with positive P waves in the standard leads are described in this article. Sinus tachycardia may occur as a normal adaptation reaction to the environment or in the setting of autonomic dysregulation. It may also be mimicked by various arrhythmias which share the earliest depolarisation in the sinus node area. The authors expose a review of these mechanisms.

[Anatomy of cardiac nodes and atrioventricular specialized conduction system]

Concomitant with the development of catheter ablation techniques for the treatment of atrial arrhythmias, there has been renewed interest in the morphologic arrangement of the cardiac conduction system. The first descriptions of the anatomy of the nodes and atrioventricular conduction system appeared nearly 100 years ago. Since then the subject has been controversial, possibly because of the early researchers' imprecise knowledge of histology. The components and structure of the specific conduction system in humans are similar to those found in commonly used laboratory animals. The conduction system is composed of specialized myocytes. Its atrial components, the sinus node and the atrioventricular node, are in contact with atrial myocardium. The His bundle penetrates the right fibrous trigone, then divides into two specialized ventricular bundle branches (right and left), which also are surrounded by a fibrous sheath that separates the specialized myocytes from the ordinary myocardium. Only at the distal ramifications of the bundle branches do the fibrous sheaths disappear, allowing continuity with the ventricular myocardium. Knowledge of the specialized myocardium can help in the development of potentially useful therapies for some forms of cardiac arrhythmia.

The morphological significance of the human sinuatrial nodal branch (artery)

The sinuatrial nodal branch/artery (SANB), the sinuatrial nodal branch of the coronary artery, is anatomically regarded as a significant artery since it is used as a landmark to identify the sinuatrial node, in addition to its clinical significance. In previous reports, the SANB has been shown to have 2-5 routes and it had only one branch in 91%-100% of hearts. These results indicate that compensation for the SANB is not possible in the case of its being cut or occluded. Therefore, we macroscopically reinvestigated the SANB using 106 human adult hearts to obtain a detailed understanding of its morphology. The following results and discussions were obtained from our study. (1) The SANB was observed to take six routes and two or more branches were found in 57 out of 106 cases (53.7%). (2) In those cases in which the SANB had only one branch (46.3%), it was observed to be the result of forming the proximal arterial loop in 25 cases (51.0%). The total of these cases with one branch with the proximal arterial loop between the right and left coronary arteries and those with two or more branches were 82 cases (77.4%). These results strongly suggest that compensation for the SANB could occur in the majority of cases. (3) We speculated that the SANB was generated by the disappearance of and the anastomosis between the lateral arterial loop, lateral to both auricles, and the medial arterial loop, medial to both auricles.

Examination of the cardiac conduction system: forensic application in cases of sudden cardiac death

Forensic pathologists may occasionally encounter cases of apparent sudden cardiac death without gross cardiac abnormality. In some of these cases, evaluation of the cardiac conduction system may reveal pathologic lesions which may act as the substrates for ventricular tachyarrhythmias and sudden death. Sample case studies are used to illustrate the suggested criteria and techniques for examination, and commonly-encountered pathologic lesions and normal variants are discussed.

Stereologic study of the sinoatrial node of rats -- age related changes

Changes in the sinoatrial node represent the major mechanism of sudden death in humans, and because of the sparse knowledge about the effects of aging on this structure, light microscopic and quantitative studies of the sinoatrial node were undertaken. Twenty-one hearts were studied, seven rat hearts from each of the following age groups: three months of age, twelve months of age and eighteen months of age. In the stereologic study, the following parameters were studied: Vv([nc]) and Vv([interstitium]) % (the volume densities of the nodal cell and interstitium, determined by the point-counting method), and Nv([nc]) (1/mm(3)) (the numerical density of the nodal cell, determined by the disector method). The mean volume of the nodal cell (V([nc])) (microm(3)) was also determined. The comparisons showed that in the oldest animals, the volume density of the nodal cells decreased, while the volume density of the interstitium increased. Although numerical density of the nodal cell per volume of sinus node decreased, the nodal cells displayed increased mean volume with age. In conclusion, the aging process implies changes in the cell and fiber content of the sinoatrial node.

[Vascularization of the sinoatrial segment in the heart conduction system in bovine and canine hearts] ]

Vascularisation of the conductive heart musculature was investigated by means of the serial coronorography and injective-corrosive method in bovine and canine hearts. The S-A system of the bovine heart was in 50% of the investigated hearts was supplied by the anterior left by the anterior right sinus artery (Arteria nodus sinuatrialis anterior sinistra), and in 30% of the cases by the anterior right sinus artery (Arteria nodus sinuatrialis anterior dextra). Both anterior sinus arteries vascularize the S-A bovine heart system in 20% of the investigated cases. The S-A canine heart system, in 50% of the investigated cases, receive vascularization from the anterior left sinus artery, from the right anterior artery in 40% of the cases, and only in 10% of the investigated hearts is the S-A system supplied by the right and left posterior sinus arteries (Arteria nodus sinuatrialis posterior dextra et sinistra). The S-A system vascularisation of the conductive heart musculature is not conditional by the type of the arterial vascularisation of the heart.

[Structural organization of the acellular region of the sinoauricular nerve plexus in the dog heart at maximal motor activity]

The reactions of the dog sinoatrial node artery under hypo- and hyperkinesia were revealed by the serial semi-thin section morphometric analysis. Single maximal physical load may be a cause of significant changes in the main source of blood supply of the dog heart sinoauricular region. During 4 weeks period hypokinezia leads to decrease of structural and functional reserve of the artery. The following physical load causes acute reduction of the blood volume in the region of the right atrium of the heart. The individual peculiarities of the artery adaptive reactions are determined by the level of the heart functional activity and by the cardiac muscle metabolism, which should be taken into account when using maximal physical loads in clinic and in experimental studies.

The sinoatrial node, a heterogeneous pacemaker structure

This article focuses on the regional heterogeneity of the mammalian sinoatrial (SA) node in terms of cell morphology, pacemaker activity, action potential configuration and conduction, densities of ionic currents (i(Na), i(Ca,L), i(to), i(K,r), i(K,s) and i(f)), expression of gap junction proteins (Cx40, Cx43 and Cx45), autonomic regulation, and ageing. Experimental studies on the single SA node cell to the whole animal are reviewed. The heterogeneity is considered in terms of the gradient model of the SA node, in which there is gradual change in the intrinsic properties of SA node cells from periphery to centre, and the alternative mosaic model, in which there is a variable mix of atrial and SA node cells from periphery to centre. The heterogeneity is important for the dependable functioning of the SA node as the pacemaker for the heart, because (i) via multiple mechanisms, it allows the SA node to drive the surrounding atrial muscle without being suppressed electrotonically; (ii) via an action potential duration gradient and a conduction block zone, it promotes antegrade propagation of excitation from the SA node to the right atrium and prevents reentry of excitation; and (iii) via pacemaker shift, it allows pacemaking to continue under diverse pathophysiological circumstances.

Posterior sinus node artery and accessory atrioventricular node artery arising by a common origin: a case report

We describe herein a rare and hitherto not reported variation, found in a Japanese male cadaver, in which a posterior sinus node (SN) artery and an accessory atrioventricular node (AN) artery originate from a common trunk branching from the posterior segment of the circumflex artery. After arising in this manner, the posterior SN artery passed in a clockwise direction around the posterior, lateral, and finally anterior wall of the left atrium to the sinus venosus, giving off a branch to the SN from posteriorly. The accessory AN artery coursed in a counterclockwise direction on the posterior wall of the left atrium as far as the crux of the heart, where it bent anterosuperiorly and continued within the interatrial septum. It entered the AN from superiorly and, crossing deep to the principal AN artery, reached the inferior and superficial portion of this node. It could be considered that the accessory AN artery in this study is a modified version of arteries entering and coursing in the interatrial septum, as exemplified by Kugel's anastomotic artery.

[Sinus node vascularization in the black African]

The aim of this study is to identify in 43 adults African hearts the different arteries of the sinus node. The method used is the injection corrosion technic of the coronary vessels. This study conclude that the right coronary system is the principal way of irrigation of sinus node in Africans (46.6% of cases).

Tridimensional mapping: guided modification of the sinus node

Radiofrequency (RF) catheter modification of the sinus node appears to be a promising therapeutic modality for the treatment of inappropriate sinus tachycardia. Modification, as opposed to total obliteration, of the atrial pacemaker requires precise localization of the sinus node. This has been successfully achieved with a multicatheter approach guided by intracardiac echocardiography. This article describes the first clinical use of a tridimensional nonfluoroscopic mapping system to guide successful RF modification of the sinus node in two cases of inappropriate sinus tachycardia. This system simplifies the current approach and greatly reduces the fluoroscopy time.

The architecture of the sinus node, the atrioventricular conduction axis, and the internodal atrial myocardium

Concomitant with the development of catheter ablation techniques for the treatment of atrial arrhythmias, there is renewed interest in the morphologic arrangement of the cardiac conduction system. In this article, we revisit the anatomy of the specialized tissues, making special reference to the descriptions given at the time of their discovery. According to criteria for histologic distinction of morphologically specialized tracts set nearly 100 years ago, the penetrating bundle (of His) and the ventricular bundle branches are tracts of specialized cells encased by insulating sheaths of fibrous tissue. In contrast, the sinus and AV nodes are recognized histologically but are not insulated from the working atrial myocardium. At its distal extent, the AV node is distinguished from the penetrating bundle not so much by cellular characteristics, but by the presence of a fibrous collar that surrounds the specialized cells. At the atrial part, a zone of histologically transitional cells interposes between the compact node and the working atrial myocardium. Transitional cells enter the triangle of Koch to join the compact node from superiorly, inferiorly, posteriorly, and from the left. Transitional cells of the sinus node, in contrast, are limited to short tongues that interdigitate with musculature of the terminal crest. Apart from a variable extension of its tail, there are no prominent histologically discrete extensions from the sinus node into the working atrial musculature. The internodal myocardium does not contain discrete conducting tracts comparable with the ventricular bundle branches. Preferential conduction more likely reflects the arrangement of the working internodal cells and their related cellular properties.

Regional differences in effects of 4-aminopyridine within the sinoatrial node

4-Aminopyridine (4-AP)-sensitive transient outward current (Ito) has been observed in the sinoatrial node, but its role is unknown. The effect of block of Ito by 5 mM 4-AP on small ball-like tissue preparations (diameter approximately 0.3-0.4 mm) from different regions of the rabbit sinoatrial node has been investigated. 4-AP elevated the plateau, prolonged the action potential, and decreased the maximum diastolic potential. Effects were greater in tissue from the periphery of the node than from the center. In peripheral tissue, 4-AP abolished the action potential notch, if present. 4-AP slowed pacemaker activity of peripheral tissue but accelerated that of central tissue. Differences in the response to 4-AP were also observed between tissue from more superior and inferior regions of the node. In the intact sinoatrial node, 4-AP resulted in a shift of the leading pacemaker site consistent with the regional differences in the response to 4-AP. It is concluded that 4-AP-sensitive outward current plays a major role in action potential repolarization and pacemaker activity in the sinoatrial node and that its role varies regionally.

Clinical importance of intramural blood vessels in the sino-atrial segment of the conducting system of the heart

We studied the morphology and histology of intramural vessels in the sino-atrial [S-A] segment of the cardiac conducting system because of the clinical relevance of the anatomy of the microvascular system for those performing cardiac surgery or coronary angiography, catheterization or arterialization. In 25 human and 25 canine hearts the vasculature was fixed by the Komahidze method, the anatomy was studied and sections were stained and studied by microscopy. Of clinical importance is a finding of considerable variation in the exact anatomy of the arterial supply. The sinonodal artery is a large artery originating from the right or left coronary arteries. In some cases the coronary arterial course could not be demonstrable by coronary angiography thus increasing the risk of coronary artery surgery at the site. Cross connections also permit the extra-cardiac circulation to supply the nodal tissue. The S-A node has a rich intramural network of anastomosing blood vessels which is of surgical importance in the case of damage and surgical procedures. The density of the capillary network in the node is greater than that in adjacent atrial tissue. The distribution and density of the smallest anastomotic blood vessels were different in various parts of the node, the capillary net was most dense in sections from dorsal parts of the S-A node. No marked differences were found when human and canine systems were compared. Canine preparations could be used for analysis of the S-A node vascularization because of their similarity to the human heart. These findings are clinically relevant to the S-A node dysfunction that may follow cardiac surgical procedures. Surgeons should be aware that there is considerable variation in the exact anatomy of the arterial supply of the S-A node which might be important for cardiological examination and treatment (cardiac surgery, coronary angiography, etc.).

Beta-adrenergic and muscarinic receptor mRNA accumulation in the sinoatrial node area of adult and senescent rat hearts

The sinoatrial (SA) node is the cardiac pacemaker and changes in its adrenergic-muscarinic phenotype have been postulated as a determinant of age-associated modifications in heart rate variability. To address this question, right atria were microdissected, the SA node area was identified by acetylcholinesterase staining, and, using a RT-PCR method, the accumulation of mRNA molecules encoding beta1- and beta2-adrenergic (beta1- and beta2-AR) and muscarinic (M2-R) receptor was quantified to define the proportion between beta-AR and M2-R mRNAs within the sinoatrial area of adult (3 months) and senescent (24 months) individual rat hearts. In adult hearts, the highest M2-R/beta-AR mRNA ratio was observed within the sinoatrial area compared with adjacent atrial myocardium, while in the senescent hearts, no difference was observed between sinoatrial and adjacent areas. This change was specific of the sinoatrial area since adult and senescent whole atrial or ventricular myocardium did not differ in their M2-R/beta-AR mRNA ratio, and was associated with a fragmentation of acetylcholinesterase staining of the senescent SA node. Quantitative changes in the expression of genes encoding proteins involved in heart rate regulation specifically affect the sinoatrial area of the senescent heart.

Anatomical and clinical aspects of the blood supply of the sinoatrial node

BACKGROUND

The recent study of the variations of the origin of the sinoatrial node and on the "arterial network of the perinodal sinusal area" in normal hearts points out the importance of this network.

PURPOSE

Report on a case of patient with syncope of ischemic etiology.

CONCLUSION

In this patient the arterial network did not protect the node from the ischemia caused by the obstruction of the artery of the sinoatrial node.

The relationship between the sinus node and the right atrial appendage

BACKGROUND

The developmental anatomy of the right atrium, particularly the union between its sinus venarum (sinus) and atrial portions, has not been completely settled. Invagination of the embryonic sinus into the atrium brings about electrical and structural union between these primitive chambers but results in the formation of the terminal crest, a seemingly bulky and intrusive band of muscle separating sinus from atrium. However, it is unlikely that a structure that can be found in all mammalian species is merely a functionless remnant. Structural differences between the smooth sinus and the deeply trabeculated right atrial appendage emphasize the two-part shape of the right atrium, but it is hypothesized that the sinus and atrium form a single, well-knit chamber shaped for the supply of nutrient blood to atrial conducting tissues rather than for downstream bloodflow.

METHODS AND RESULTS

The morphology of the right atrium was studied grossly and microscopically in 54 adult hearts. The distribution of Thebesian sinusoids arising from the right atrial appendage was identified by ink perfusion and clearing of several additional adult and fetal hearts. The sinus node was found to be precisely co-extensive with the undercut portion of the terminal crest and with pectinate muscles lining the pyramidal portion of the right atrial appendage. Sinusoids and interpectinate spaces related to the undercut portion of the terminal crest are limited to the area of the sinus node.

CONCLUSION

These findings support the hypothesis that the right atrial appendage, pectinate muscles and terminal crest evolved to supply nutrient blood to conducting myocardium of the sinus portion of the right atrium, and that this chamber, like the right ventricle, is structured as a single and completely finished unit. Interpectinate spaces and Thebesian sinusoids offer clues to the location of conducting pathways, including the sinus node.

Regional differences in the response of the isolated sino-atrial node of the rabbit to vagal stimulation

1. The effects of brief postganglionic vagal nerve stimulation on electrical activity in different regions of the rabbit sino-atrial node and surrounding atrial muscle were recorded. 2. At the centre of the node (the leading pacemaker site), the brief stimulation resulted in a large hyperpolarization followed by a depolarization and a shortening of the action potential. All effects were short lasting (time to 90% recovery of membrane potential, 0.8 s). 3. At other sites within the node and in the surrounding atrial muscle, although there was still a substantial action potential shortening, the hyperpolarization was smaller and the depolarization was small or absent. All effects were longer lasting (time to 90% recovery in atrial muscle, 11.4s). 4. The depolarization in the centre of the node was abolished by block of the hyperpolarization-activated current (i(f)) by Cs+ or zatebradine (UL-FS 49). It could, therefore, result from the activation of i(f) during the preceding hyperpolarization. 5. Block of acetylcholinesterase by eserine greatly slowed recovery from vagal stimulation at all sites, demonstrating that recovery is dependent on acetylcholinesterase. The longer lasting effects of vagal stimulation in atrial muscle, therefore, result from lower acetylcholinesterase activity. 6. Vagal stimulation resulted in a short lasting initial slowing of spontaneous action potentials followed by a long-lasting secondary slowing. Whereas the initial slowing coincided with the effects of vagal stimulation on the centre of the node, the secondary slowing coincided with the slower effects of vagal stimulation on the surrounding atrial muscle. The secondary slowing was reduced by 68 +/- 11% (n = 5) by cutting the atrial muscle away from the node. 7. It is concluded that the short-lasting initial slowing of spontaneous action potentials is the direct effect of vagal stimulation on the centre of the sino atrial node, whereas the secondary slowing is the result of the longer lasting effects of vagal stimulation on the surrounding atrial muscle and the electrotonic suppression of the node by the muscle.

The artery of the sinuatrial node: anatomic considerations based on 45 injection-dissections of the heart

The origin, course and mode of termination of the artery of the sinuatrial node was studied in 45 anatomic specimens by injection-dissection. It was solitary in 88.89% of the cases and double in 11.11%. It arose from the right coronary a. in 64.45% of cases, from the left coronary a. in 24.44%, and from both in 11.11%. The side of origin was not significantly influenced by the coronary dominance. When it arose from the right coronary a., the course of the sinuatrial a. varied greatly with its site of origin from the coronary vessel, either from an atrial a. or from one of its collateral branches, and also depended on its relations with the interatrial septum. When it arose from the left coronary a., its course was relatively uniform, except for arteries arising from the inferior atrial aa., which characteristically involve the posterior wall of the left atrium. Three modes of termination were found: precaval, retrocaval, and in a pericaval arterial circle. These observations made it possible to understand the possible origin of disorders of rhythm observed following disturbance of the arterial supply to the sinuatrial node during certain stages of cardiac surgery, particularly during atriotomies and the surgical correction of certain valvular disorders and congenital malformations, which expose the a. of the sinuatrial node.

Anatomy of the sinus node of camels (Camelus dromedarius)

Anatomy of the sinus node was studied in six camel hearts (Camelus dromedarius) with serial histologic sectioning. The sinus node in this species of animal was located 0.5 mm beneath the epicardium, near the junction between the cranial vena cava and the right atrium at the sulcus terminalis. Its shape was elongated, bent oblong with 28.25 mm length, 5.75 mm width and 5.38 mm thickness. The maximum section area was 101.66 mm2. The central artery of the node originated from the circumflexus branch of the left coronary artery and, throughout its length in the substance of the sinus node, had an internal elastic membrane. Histologically, the sinus node of this animal contained a central artery and a framework of collagen fibres, which were distributed around the central artery. The nodal cells were irregularly organized around the central artery and two types, i.e. "p' cells and transitional cells were present. The "p' cells had a perinuclear clear zone but the transitional cells contained more myofibrils. The intercalated discs were not present. At the periphery of the sinus node there were many nerve fibres and a ganglion. The purkinje fibres were present within atrial myocardium, as well as within ventricular myocardium. The glycogen content of the sinus nodal cells was higher than that of the atrial myocardial cells.

Age-related changes in structure and relative collagen content of the human and feline sinoatrial node. A comparative study

Age-related changes in the structure and size of the human and cat sinoatrial nodes were studied by light microscopy, with emphasis on changes in relative collagen volume. Sinoatrial nodes from 41 humans (aged 0-94 years) and 21 cats (aged 6 weeks-18 years) were used. It was found that there were no changes in the dimensions of the sinoatrial node during adult life in either species. In Sirius Red F3 Ba stained sections, the relative volume of collagen was measured using an interactive image analysis system. The relative volume of collagen in the human sinoatrial node increases from 38% during childhood to 70% during adulthood. Once adulthood is reached, there are no further changes in the relative volume of collagen. In the cat sinoatrial node the relative volume of collagen is only 27% and does not change with age. The organisation of collagen in the sinoatrial node, however, demonstrates an age-dependent change in both humans and cats. From coarse strands between clusters of nodal cells it gradually changes into a fine network of isolated collagen fibres which surround individual nodal cells. This process is more pronounced in humans. It is concluded that age-related changes in sinoatrial node function are not related to an increase in collagen content in the sinoatrial node.