Controversies concerning the anatomical definition of the conduction tissues

Anatomy of the conduction tissues 100 years on: what have we learned?

Knowledge of the anatomy of the 'conduction tissues' of the heart is a 20th century phenomenon. Although controversies still continue on the topic, most could have been avoided had greater attention been paid to the original descriptions. All cardiomyocytes, of course, have the capacity to conduct the cardiac impulse. The tissues specifically described as 'conducting' first generate the cardiac impulse, and then deliver it in such a fashion that the ventricles contract in orderly fashion. The tissues cannot readily be distinguished by gross inspection. Robust definitions for their recognition had been provided by the end of the first decade of the 20th century. These definitions retain their currency. The sinus node lies as a cigar-shaped structure subepicardially within the terminal groove. There is evidence that it is associated with a paranodal area that may have functional significance. Suggestions of dual nodes, however, are without histological confirmation. The atrioventricular node is located within the triangle of Koch, with significant inferior extensions occupying the atrial vestibules and with septal connections. The conduction axis penetrates the insulating plane of the atrioventricular junctions to continue as the ventricular pathways. Remnants of a ring of cardiomyocytes observed during development are also to be found within the atrial vestibules, particularly a prominent retroaortic remnant, although that their role has still to be determined. Application of the initial criteria for nodes and tracts shows that there are no special 'conducting tissues' in the pulmonary venous sleeves that might underscore the abnormal rhythm of atrial fibrillation.

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.

Of Tracts, Rings, Nodes, Cusps, Sinuses, and Arrhythmias-A Comment on Szili-Torok et al.'s Paper Entitled "The 'Dead-End Tract' and Its Role in Arrhythmogenesis". J. Cardiovasc. Dev. Dis. 2016, 3, 11

In the review, now published as part of the special issue devoted to the development of the conduction tissues, de Vries and his colleagues discuss the potential role of the so-called "dead-end tract" as a substrate for arrhythmogenesis [1].[...].

Estimation of Purkinje trees from electro-anatomical mapping of the left ventricle using minimal cost geodesics

The electrical activation of the heart is a complex physiological process that is essential for the understanding of several cardiac dysfunctions, such as ventricular tachycardia (VT). Nowadays, patient-specific activation times on ventricular chambers can be estimated from electro-anatomical maps, providing crucial information to clinicians for guiding cardiac radio-frequency ablation treatment. However, some relevant electrical pathways such as those of the Purkinje system are very difficult to interpret from these maps due to sparsity of data and the limited spatial resolution of the system. We present here a novel method to estimate these fast electrical pathways from the local activations maps (LATs) obtained from electro-anatomical maps. The location of Purkinje-myocardial junctions (PMJs) is estimated considering them as critical points of a distance map defined by the activation maps, and then minimal cost geodesic paths are computed on the ventricular surface between the detected junctions. Experiments to validate the proposed method have been carried out in simplified and realistic simulated data, showing good performance on recovering the main characteristics of simulated Purkinje networks (e.g. PMJs). A feasibility study with real cases of fascicular VT was also performed, showing promising results.

Evidence of specialized tissue in human interatrial septum: histological, immunohistochemical and ultrastructural findings

Background

There is a paucity of information on structural organization of muscular bundles in the interatrial septum (IAS). The aim was to investigate histologic and ultrastructural organization of muscular bundles in human IAS, including fossa ovalis (FO) and flap valve.

Methods

Macroscopic and light microscopy evaluations of IAS were performed from postmortem studies of 40 patients. Twenty three IAS specimens underwent serial transverse sectioning, and 17 - longitudinal sectioning. The transverse sections from 10 patients were immunolabeled for HCN4, Caveolin3 and Connexin43. IAS specimens from 6 other patients underwent electron microscopy.

Results

In all IAS specimens sections the FO, its rims and the flap valve had muscle fibers consisting of working cardiac myocytes. Besides the typical cardiomyocytes there were unusual cells: tortuous and horseshoe-shaped intertangled myocytes, small and large rounded myocytes with pale cytoplasm. The cells were aggregated in a definite structure in 38 (95%) cases, which was surrounded by fibro-fatty tissue. The height of the structure on transverse sections positively correlated with age (P = 0.03) and AF history (P = 0.045). Immunohistochemistry showed positive staining of the cells for HCN4 and Caveolin3. Electron microscopy identified cells with characteristics similar to electrical conduction cells.

Conclusions

Specialized conduction cells in human IAS have been identified, specifically in the FO and its flap valve. The cells are aggregated in a structure, which is surrounded by fibrous and fatty tissue. Further investigations are warranted to explore electrophysiological characteristics of this structure.

Developmental aspects of cardiac arrhythmogenesis

The transcriptional regulation orchestrating the development of the heart is increasingly recognized to play an essential role in the regulation of ion channel and gap junction gene expression and consequently the proper generation and conduction of the cardiac electrical impulse. This has led to the realization that in some instances, abnormal cardiac electrical function and arrhythmias in the postnatal heart may stem from a developmental abnormality causing maintained (epigenetic) changes in gene regulation. The role of developmental genes in the regulation of cardiac electrical function is further underscored by recent genome-wide association studies that provide strong evidence that common genetic variation, at loci harbouring these genes, modulates electrocardiographic indices of conduction and repolarization and susceptibility to arrhythmia. Here we discuss recent findings and provide background insight into these complex mechanisms.

Three-dimensional anatomy of the conduction system of the early embryonic rabbit heart

The complete embryonic cardiac conduction system is difficult to view in three dimensions, primarily because there has not been a marker of all segments of the normal system throughout all stages of development. Imaging of the conduction system components within the atria has been particularly controversial because different markers reveal different pathways that may or may not represent conduction system components. The conduction system of the adult and embryonic rabbit, however, can be labeled in its entirety with the neurofilament marker, NF-160. The conduction system of rabbit embryos at several stages of development spanning cardiac septation was therefore investigated. Optical mapping of the electrical signature of the conduction system previously revealed a close correlation between the cardiac activation patterns and the anatomy as shown by serial sections. The 3D relationship between the components of the conduction system could only be inferred from the 2D sections. The sections were consequently reconstructed using a commercial software program (AutoQuant). This is the first demonstration of the three-dimensional complete normal rabbit embryonic cardiac conduction system at several stages of development.