Atrioventricular Conduction Axis Research Articles

The Atrioventricular Conduction Axis in Man and Mouse

Those using the mouse for the purposes of electrophysiological research presume that the morphology of the conduction axis is comparable with the human arrangement. As yet, however, to the best of our knowledge, no direct comparison has been made between the species. By comparing our extensive histological findings in the human heart with comparable serially-sectioned datasets prepared from adult murine hearts, we aimed to provide this information. When comparing the gross anatomy, we used three-dimensional datasets of neonatal mice hearts prepared using episcopic microscopy. The overall cardiac architecture is comparable, although the mouse has a persistent left superior caval vein draining via the coronary sinus. An inferior pyramidal space and an infero-septal recess are both present in the murine heart, although they are not as well developed as in the human heart. The overall arrangement of the conduction axis is similarly comparable, albeit with subtle differences reflecting the incomplete wedging of the subaortic outflow tract in the murine heart. Most significantly, the findings in both species reveal the presence of extensive superior septal pathways, which perhaps explain the finding of base-to-apex activation of the ventricular mass known to occur in the murine heart.

Modeling the atrioventricular conduction axis using human pluripotent stem cell-derived cardiac assembloids

The atrioventricular (AV) conduction axis provides electrical continuity between the atrial and ventricular chambers. The "nodal" cardiomyocytes populating this region (AV canal in the embryo, AV node from fetal stages onward) propagate impulses slowly, ensuring sequential contraction of the chambers. Dysfunction of AV nodal tissue causes severe disturbances in rhythm and contraction, and human models that capture its salient features are limited. Here, we report an approach for the reproducible generation of AV canal cardiomyocytes (AVCMs) with invivo-like gene expression and electrophysiological profiles. We created the so-called "assembloids" composed of atrial, AVCM, and ventricular spheroids, which effectively recapitulated unidirectional conduction and the "fast-slow-fast" activation pattern typical for the vertebrate heart. We utilized these systems to reveal intracellular calcium mishandling as the basis of LMNA-associated AV conduction block. In sum, our study introduces novel cell differentiation and tissue construction strategies to facilitate the study of complex disorders affecting heart rhythm.

Interventricular septal thickness on cardiac computed tomography as a novel risk factor for conduction disturbances in patients undergoing transcatheter aortic valve replacement.

We examined whether thickness of the basal muscular interventricular septum (IVS), as measured by pre-procedural computed tomography (CT), could be used to identify the risk of conduction disturbances following transcatheter aortic valve replacement (TAVR). The IVS is a pivotal region of the electrical conduction system of the heart where the atrioventricular conduction axis is located. Included were 78 patients with severe aortic stenosis who underwent CT imaging prior to TAVR. The thickness of muscular IVS was measured in the coronal view, in systolic phases, at 1, 2, 5, and 10 mm below the membranous septum (MS). The primary endpoint was a composite of conduction disturbance following TAVR. Conduction disturbances occurred in 24 out of 78 patients (30.8%). Those with conduction disturbances were significantly more likely to have a thinner IVS than those without conduction disturbances at every measured IVS level (2.98 ± 0.52 mm vs. 3.38 ± 0.52 mm, 4.10 ± 1.02 mm vs. 4.65 ± 0.78 mm, 6.11 ± 1.12 mm vs. 6.88 ± 1.03 mm, and 9.72 ± 1.95 mm vs. 10.70 ± 1.55 mm for 1, 2, 5 and 10 mm below MS, respectively, P < 0.05 for all). Multivariable logistic regression analysis showed that pre-procedural IVS thickness (<4 mm at 2 mm below the MS) was a significant independent predictor of post-procedural conduction disturbance (adjOR 7.387, 95% CI: 2.003-27.244, P = 0.003). Pre-procedural CT assessment of basal IVS thickness is a novel predictive marker for the risk of conduction disturbances following TAVR. The IVS thickness potentially acts as an anatomical barrier protecting the underlying conduction system from mechanical compression during TAVR.

The clinical anatomy of the atrioventricular conduction axis.

It is axiomatic that the chances of achieving accurate capture of the conduction axis and its fascicles will be optimized by equally accurate knowledge of the relationship of the components to the recognizable cardiac landmarks, and we find it surprising that acknowledged experts should continue to use drawings that fall short in terms of anatomical accuracy. The accuracy achieved by Sunao Tawara (1906) in showing the location of the atrioventricular conduction axis is little short of astounding. Our purpose in bringing this to current attention is to question the need of the experts to have produced such inaccurate representations, since the findings of Tawara have been extensively endorsed in very recent years. The recent studies do no more than point to the amazing accuracy of the initial account of Tawara. At the same time, we draw attention to the findings described in the middle of the 20th century by Ivan Mahaim (1947). These observations have tended to be ignored in recent accounts. They are, perhaps, of equal significance to those seeking specifically to pace the left fascicles of the branching atrioventricular bundle.

Transfiguration of Human Atrioventricular Conduction Axis with His bundle Pacing: Effect on Underlying Bundle Branch Block

His bundle pacing comes with the expectation of avoiding morbidity and mortality associated with pacing the ventricular myocardium. The paced complex typically exhibits a delta wave and resembles the relatively normal QRS complex of fasciculoventricular preexcitation. In patients with atrioventricular conduction disease, His bundle pacing ‘corrects’ both right and left bundle branch block. Conduction block is known to have a diffuse substrate and mechanism of correction remains unclear. In 51 patients with normal QRS complex, 35 patients with right bundle branch block and left axis deviation, and 41 patients with left bundle branch block we analyzed the changes in the paced QRS complex. In all patients His bundle pacing caused a decrease in ventricular activation time, an increase in lead 1 voltage, an inferior shift in the frontal QRS axis, and a rightward shift in R wave transition in the precordial leads. In patients with normal QRS, despite these changes, the paced QRS remained in normal range. In left bundle branch block, these changes resulted in decrease of prolonged ventricular activation time, increased lead 1 voltage , and normalized the delayed R wave transition in precordial leads. The correction of right bundle branch block suggested that the right ventricular free wall was being activated earlier. This was consistent with early rightward transition of precordial R waves and the inferior QRS axis and the known base to apex activation of right ventricular wall in fasciculoventricular preexcitation. We conclude that the QRS complex in normal range, seen in fasciculoventricular preexcitation, is caused by rapidly conducting, base-to-apex, and septum to lateral wall activation in both left and right ventricles. This would explain the correction of left axis deviation and reversal of delayed lateral wall activation in right and left bundle branch block, by His bundle pacing.

The Atrioventricular Conduction Axis Revisited for the 21st Century.

Although first described in the final decade of the 19th century, the axis responsible for atrioventricular conduction has long been the source of multiple controversies. Some of these continue to reverberate. When first described by His, for example, many doubted the existence of the bundle we now name in his honour, while Kent suggested that multiple pathways crossed the atrioventricular junctions in the normal heart. It was Tawara who clarified the situation, although many of his key definitions have not universally been accepted. In key studies in the third decade of the 20th century, Mahaim then suggested the presence of ubiquitous connections that provided "paraspecific" pathways for atrioventricular conduction. In this review, we show the validity of these original investigations, based on our own experience with a large number of datasets from human hearts prepared by serial histological sectioning. Using our own reconstructions, we show how the atrioventricular conduction axis can be placed back within the heart. We emphasise that newly emerging techniques will be key in providing the resolution to map cellular detail to the gross evidence provided by the serial sections.

Multi-modality imaging evaluation and pre-surgical planning for aortic valve-sparing operations in patients with aortic root aneurysm.

Cardiac computed tomography (CT) and magnetic resonance (CMR) supplement echocardiography in the evaluation of those with aortic root and ascending aortic dilation, determining timing for intervention, guiding pre-surgical planning and post-operative surveillance. The dynamic, three-dimensional complexity of the aortic root and how it relates to the base of the left ventricle must be understood in any surgical approach addressing the aneurysmal aortic root. With improved imaging technology and the importance for proper patient counseling, it is no longer acceptable to enter the operating theater without a detailed blue print of what the problem is, and how best to address it. In addition, reliance on surgical expertise alone for intraoperative evaluation and decision making could be suboptimal due to the unloaded condition of the aortic root and the variance of experience of the surgeons to successfully repair the aortic valve. This is exemplified by the selective surgeons and centers who have the ability to tackle these aortic valve and root pathologies, compared to mitral valve repair techniques that have been codified and are generalizable. This review discusses a multimodality imaging approach in the patient with aortic root aneurysm, focusing on the precision added with pre-surgical CT assessment to guide aortic-valve sparing operations. This precision is afforded with a detailed understanding of the anatomy of the aortic root and underlying support, and its accurate evaluation by standard two- and three-dimensional imaging. Furthermore, we describe the evolving ability to predict the location of ventricular components of the atrioventricular conduction axis with further clinical imaging to personalize surgical strategies.

Lack of evidence for longitudinal dissociation of the atrioventricular conduction axis.

Longitudinal dissociation of the aggregated specialized cardiomyocytes within the non-branching portion of atrioventricular conduction axis has proved a controversial topic for both morphologists and electrophysiologists. We have now used morphological methods, including three-dimensional assessment, to revisit, in human, canine, and bovine hearts, the presence or absence of interconnections between the aggregated cardiomyocytes making up the non-branching bundle. We analyzed three datasets from human and canine hearts, and two from bovine hearts, using longitudinal and orthogonal serial histological sections. In addition, we assessed three hearts using translucent India ink injected specimens, permitting assessment of the three-dimensional arrangement of the cardiomyocytes. Using the longitudinal sections, we found numerous oblique interconnections between the groups of specialized cardiomyocytes. When assessing orthogonal sections, we noted marked variation in the grouping of the cardiomyocytes. We interpreted this finding as evidence of bifurcation and convergence of the groups seen in the longitudinal sections. The three-dimensional assessment of the bovine material confirmed the presence of the numerous interconnections. The presence of multiple connections between the cardiomyocytes in the non-branching bundle rules out the potential for longitudinal dissociation. This article is protected by copyright. All rights reserved.

Evidence for concealed fasciculo-ventricular connections as revealed by His bundle pacing.

It is almost 100 years ago since Mahaim described the so-called paraspecific connections between the ventricular conduction axis and the crest of the muscular ventricular septum, believing such pathways to be ubiquitous. These pathways, however, have yet to be considered as potential pathways for septal activation during His bundle pacing. So as to explore the hypothesis that specialised septal pathways might provide the substrate for septal activation during His bundle pacing, we compared the findings from 22 serially sectioned histological datasets and 34 different individuals undergoing His bundle pacing. We found histologically specialised pathways connecting the branching component of the atrioventricular conduction axis with the crest of the muscular ventricular septum in almost four-fifths of the histological datasets. In 32 of 34 patients undergoing His bundle pacing, the QRS complex closely resembled published images of known conduction through fasciculo-ventricular pathways. In only two patients was a delta wave not seen at any pacing voltages. Capture of these connections varied according to pacing voltage, a finding which correlated with the distance of the pathways from the site of penetration of the ventricular conduction axis. Ventricular activation times remained normal in the presence of the delta wave at higher pacing voltage but were prolonged at lower voltages. Our histologic findings confirm fasciculo-ventricular connections, initially described by Mahaim as being paraspecific, are likely ubiquitous. Analysis of 12-lead electrocardiograms leads us to conclude that fasciculo-ventricular pathways, concealed during sinus rhythm, become manifest with His bundle pacing.

New insights into the mechanisms of fast and slow conduction in the atrioventricular node

Cardiac pathologists and electrophysiologists have studied the atrioventricular (AV) node since 1906, when Tawara 1 Tawara S. Das Reitzleitungssystem des Saugetierherzens: Eine anatomisch-histologische Studie uber das Atrioventrikularbundel und die Purkinjeschen Faden. Gustav Fischer, Jena, Germany1906: 135-136 Google Scholar described its inferior extensions in the human heart. One century later, this important cardiac structure still carries the reputation of a profound enigma, invoked as the riddle of Sphinx, by means of its electrophysiological properties during both normal conduction and reentrant tachycardia. 2 Katritsis D.G. The human atrioventricular node: oedipus and the riddle of the Sphinx. Arrhythm Electrophysiol Rev. 2020; 9: 52-53 Crossref PubMed Scopus (0) Google Scholar In recent years, considerable evidence has accrued pertaining to its anatomic structure, 3 Inoue S. Becker A.E. Posterior extensions of the human compact atrioventricular node: a neglected anatomic feature of potential clinical significance. Circulation. 1998; 97: 188-193 Crossref PubMed Scopus (271) Google Scholar , 4 Hikspoors J. Macias Y. Tretter J.T. et al. Miniseries 1–part I: the development of the atrioventricular conduction axis. Europace. 2022; 24: 432-442 Crossref PubMed Scopus (1) Google Scholar , 5 Macias Y. de Almeida M.C. Tretter J.T. et al. Miniseries 1–part II: the compara-tive anatomy of the atrioventricular conduction axis. Europace. 2022; 24: 443-454 Crossref PubMed Scopus (3) Google Scholar , 6 Anderson R.H. Sanchez-Quintana D. Mori S. Cabrera J.A. Back Sternick E. Re-evaluation of the structure of the atrioventricular node and its connections with the atrium. Europace. 2020; 22: 821-830 Crossref PubMed Scopus (32) Google Scholar along with the electrophysiological properties, which have allowed identification of the so-called fast and slow pathways involved in atrioventricular nodal reentrant tachycardia (AVNRT). 7 Katritsis D.G. Josephson M.E. Classification, electrophysiological features and therapy of atrioventricular nodal reentrant tachycardia. Arrhythm Electrophysiol Rev. 2016; 5: 130-135 Crossref PubMed Scopus (29) Google Scholar , 8 Katritsis D.G. A unified theory for the circuit of atrioventricular nodal re-entrant tachycardia. Europace. 2020; 22: 1763-1767 Crossref PubMed Scopus (7) Google Scholar , 9 Katritsis D.G. Calkins H. Anderson R.H. The specialized atrioventricular ring tissues participate in the circuit of atrioventricular nodal reentrant tachycardia. J Am Heart Assoc. 2021; 10e022811 Crossref Scopus (0) Google Scholar But still the nature of these patterns of conduction in the context of sinus rhythm, as opposed to those observed during tachycardia, remains obscure. In the great Sophoclean tragedy, in light of its insurmountable difficulty, the denouement of the riddle of Sphinx by mortal Oedipus was initially perceived by the Athenians as a form of hubris against the gods by a human. 10 Sophocles. Oedipus Tyrannus. 429 BC. Google Scholar We believe that the accumulation of recent anatomic and electrophysiological evidence now enables us to provide the rationale for solving the nodal riddle without committing scientific hubris.

Aneurysm of the membranous septum. A potential cause of sudden death

Aneurysms of the membranous septum are extremely rare malformations that are usually asymptomatic. However, they have the potential for varied complications including sudden cardiac death. Reported here is a case of sudden death in a 25-year-old woman. A medico-legal autopsy revealed a large aneurysm of the membranous septum. It is speculated that the infiltration of fibrous tissue from the aneurysm into the atrioventricular conduction axis may have led to the observed mortality.

Anatomy of the heart with the highest heart rate.

Shrews occupy the lower extreme of the seven orders of magnitude mammals range in size. Their hearts are large relative to body weight and heart rate can exceed a thousand beats a minute. It is not known whether traits typical of mammal hearts scale to these extremes. We assessed the heart of three species of shrew (genus Sorex) following the sequential segmental analysis developed for human hearts. Using micro‐computed tomography, we describe the overall structure and find, in agreement with previous studies, a large and elongate ventricle. The atrial and ventricular septums and the atrioventricular (AV) and arterial valves are typically mammalian. The ventricular walls comprise mostly compact myocardium and especially the right ventricle has few trabeculations on the luminal side. A developmental process of compaction is thought to reduce trabeculations in mammals, but in embryonic shrews the volume of trabeculations increase for every gestational stage, only slower than the compact volume. By expression of Hcn4, we identify a sinus node and an AV conduction axis which is continuous with the ventricular septal crest. Outstanding traits include pulmonary venous sleeve myocardium that reaches farther into the lungs than in any other mammals. Typical proportions of coronary arteries‐to‐aorta do not scale and the shrew coronary arteries are proportionally enormous, presumably to avoid the high resistance to blood flow of narrow vessels. In conclusion, most cardiac traits do scale to the miniscule shrews. The shrew heart, nevertheless, stands out by its relative size, elongation, proportionally large coronary vessels, and extent of pulmonary venous myocardium.

Miniseries 2-Septal and paraseptal accessory pathways-Part I: The anatomic basis for the understanding of para-Hisian accessory atrioventricular pathways.

Although the anatomy of the atrioventricular conduction axis was well described over a century ago, the precise arrangement in the regions surrounding its transition from the atrioventricular node to the so-called bundle of His remain uncertain. We aimed to clarify these relationships. We have used our various datasets to examine the development and anatomical arrangement of the atrioventricular conduction axis, paying particular attention to the regions surrounding the point of penetration of the bundle of His. It is the areas directly adjacent to the transition of the atrioventricular conduction axis from the atrioventricular node to the non-branching atrioventricular bundle that constitute the para-Hisian areas. The atrioventricular conduction axis itself traverses the membranous part of the ventricular septum as it extends from the node to become the bundle, but the para-Hisian areas themselves are paraseptal. This is because they incorporate the fibrofatty tissues of the inferior pyramidal space and the superior atrioventricular groove. In this initial overarching review, we summarize the developmental and anatomical features of these areas along with the location and landmarks of the atrioventricular conduction axis. We emphasize the relationships between the inferior pyramidal space and the infero-septal recess of the subaortic outflow tract. The details are then explored in greater detail in the additional reviews provided within our miniseries. Our anatomical findings, described here, provide the basis for our concomitant clinical review of the so-called para-Hisian arrhythmias. The findings also provide the basis for understanding the other variants of ventricular pre-excitation.

Miniseries 1—Part IV: How frequent are fasciculo-ventricular connections in the normal heart?

Abstract Aims Seeking to account for accessory atrioventricular conduction potentially leading to ventricular pre-excitation, Mahaim in the mid-20th century had described pathways between the atrioventricular conduction axis and the muscular ventricular septum. We aimed to look for such ‘paraspecific’ connections in adult human hearts. Methods and results We serially sectioned 21 hearts, covering the triangle of Koch and the aortic root, and assessing the atrioventricular node, the penetration of the conduction axis, and the bundle branches in our search for fasciculo-ventricular connections. We also calculated the length of the non-branching bundle, and if present the origin of the fasciculo-ventricular connections. The non-branching bundle was 3.6 ± 1.7 mmin length, varying from 1.7 mm to 7.2 mm. Fasciculo-ventricular connections were found in more than half of the hearts, making direct contact with the muscular septum at an average of 3.5 ± 1.7 mm from the origin of the left bundle branch, with the site of origin varying from 1.1 mm to 5.5 mm from the first fascicle of the left bundle branch. In three hearts, additional fasciculo-fascicular connections were observed in the left bundle branch. Two loops were small, but one loop extended over 9.5 mm. Conclusion We endorse the finding of Mahaim that fasciculo-ventricular pathways exist in most human hearts. We presume the identified connections had the capability of producing ventricular pre-excitation. More studies are needed to determine the potential clinical manifestations.

Miniseries 1-Part II: the comparative anatomy of the atrioventricular conduction axis.

The arrangement of the conduction axis is markedly different in various mammalian species. Knowledge of such variation may serve to question the validity of using animals as prospective models for design of systems for clinical use. We compared the arrangement of the atrioventricular conduction axis in human, murine, canine, porcine, and bovine hearts, examining serially sectioned datasets from 20 human, 16 murine, 3 porcine, 5 canine, and 1 bovine hearts. We also analysed computed tomographic datasets obtained from bovines and one human heart. Unlike the situation in the human heart, there is no formation of an atrioventricular fibrous membranous septum in the murine, canine, porcine, nor bovine hearts. Canine, porcine, and bovine hearts also lack an infero-septal recess, when defined as a fibrous plate supporting the buttress of the atrial septum. In these species, half of the non-coronary leaflet is directly opposed to the ventricular septal surface. There is a long right-sided non-branching component of the axis, which skirts the attachment of the non-coronary sinus of the aortic root. In the bovine heart, moreover, the left bundle branch usually extends intramyocardially as a solitary tape before surfacing and ramifying on the left ventricular septal surface. The difference in the atrioventricular conduction axis between species may influence the anatomical substrates for atrioventricular re-entry tachycardia, as well as providing inferences for assessing the risks of transcatheter implantation of the aortic valve. Further studies are now needed to assess these possibilities.

Miniseries 1-Part I: the Development of the atrioventricular conduction axis.

Despite years of research, many details of the formation of the atrioventricular conduction axis remain uncertain. In this study, we aimed to clarify the situation. We studied three-dimensional reconstructions of serial histological sections and episcopic datasets of human embryos, supplementing these findings with assessment of material housed at the Human Developmental Biological Resource. We also examined serially sectioned human foetal hearts between 10 and 30 weeks of gestation. The conduction axis originates from the primary interventricular ring, which is initially at right angles to the plane of the atrioventricular canal, with which it co-localizes in the lesser curvature of the heart loop. With rightward expansion of the atrioventricular canal, the primary ring bends rightward, encircling the newly forming right atrioventricular junction. Subsequent to remodelling of the outflow tract, part of the primary ring remains localized on the crest of the muscular ventricular septum. By 7 weeks, its atrioventricular part has extended perpendicular to the septal parts. The atrioventricular node is formed at the inferior transition between the ventricular and atrial parts, with the transition itself marking the site of the penetrating atrioventricular bundle. Only subsequent to muscularization of the true second atrial septum does it become possible to recognize the definitive node. The conversion of the developmental arrangement into the definitive situation as seen postnatally requires additional remodelling in the first month of foetal development, concomitant with formation of the inferior pyramidal space and the infero-septal recess of the subaortic outflow tract.

Miniseries 1-Part III: 'Behind the scenes' in the triangle of Koch.

To take full advantage of the knowledge of cardiac anatomy, structures should be considered in their correct attitudinal orientation. Our aim was to discuss the triangle of Koch in an attitudinally appropriate fashion. We reviewed our material prepared by histological sectioning, along with computed tomographic datasets of human hearts. The triangle of Koch is the right atrial surface of the inferior pyramidal space, being bordered by the tendon of Todaro and the hinge of the septal leaflet of the tricuspid valve, with its base at the inferior cavotricuspid isthmus. The fibro-adipose tissues of the inferior pyramidal space separate the atrial wall from the crest of the muscular interventricular septum, thus producing an atrioventricular muscular sandwich. The overall area is better approached as a pyramid rather than a triangle. The apex of the inferior pyramidal space overlaps the infero-septal recess of the subaortic outflow tract, permitting the atrioventricular conduction axis to transition directly to the crest of the muscular ventricular septum. The compact atrioventricular node is formed at the apex of the pyramid by union of its inferior extensions, which represent the slow pathway, with the septal components formed in the buttress of the atrial septum, thus providing the fast pathway. To understand its various implications in current cardiological catheter interventions, the triangle of Koch must be considered in conjunction with the inferior pyramidal space and the infero-septal recess. It is better to consider the overall region in terms of a pyramidal area of interest.

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

Subtle differences exist between dog and human, despite use of the dog as a model for cardiac surgical and electrophysiological research. The purpose of this study was to investigate the differences in the atrioventricular conduction axis and adjacent structures between dogs and humans. 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. All canine hearts lacked an inferoseptal recess, with the noncoronary leaflet of the aortic valve and the right fibrous trigonehaving 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. 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.

Identification of the atrioventricular conduction axis and its positional relationship with anatomical landmarks of a heart with tricuspid atresia

Previous reports of the atrioventricular conduction axis (AVCA) in patients with tricuspid atresia (TA) have focused on its anatomical association with the classical Fontan operations and adjunctive procedures, such as enlargement of the bulboventricular foramen and closure of the atrial septal defect.1 However, surgical techniques for TA have evolved in recent decades, and the total cavopulmonary connection and the Damus–Kaye–Stansel procedure have been widely adopted. Thus, the previous anatomical findings have become less relevant.

The atrioventricular conduction axis and the aortic root-Inferences for transcatheter replacement of the aortic valve.

Conduction problems still occur following transcatheter aortic valvar replacement. With this in mind, we have assessed the relationship of the conduction axis to the aortic root. We used serial histological sections, made perpendicular to the base of the triangle of Koch in nine hearts, and perpendicular to the aortic root in 11 hearts. We first defined the extent of the fibrous tissues forming the boundaries of an infero-septal recess of the subaortic outflow tract, found in all datasets but one. When the recess was present, the axis penetrated through its rightward wall, giving rise to the left bundle branch prior to entering the outflow tract. The axis itself was usually on the crest of the ventricular septum, but could be deviated leftward or rightward. Its proximity to the virtual basal plane reflected the angulation of the muscular septum. On average, the superior edge of the left bundle was within 3.3 mm of the hinge of the right coronary leaflet, with a range from 0.4 to 10.2 mm. The arrangement was markedly different in the case lacking an infero-septal recess. Our findings necessitated a redefinition of the right fibrous trigone and the central fibrous body. The atrioventricular conduction axis, having entered the aortic root, is usually closest at the hinge of the right coronary leaflet. Knowledge of the depth of the infero-septal recess, and the angulation of the muscular ventricular septal, may help to avoid conduction problems following transcatheter implantation of the aortic valve.