Mahaim Revisited.

The name Ivan Mahaim is well-known to electrophysiologists. However, alternative anatomical substrates can produce the abnormal rhythms initially interpreted on the basis of the pathways he first described. These facts have prompted suggestions that Mahaim should be deprived of his eponym. It is agreed that specificity is required when describing the pathways that produce the disordered cardiac conduction, and that the identified pathways should now be described in an attitudinally appropriate fashion. The authors remain to be convinced that understanding will be enhanced simply by discarding the term 'Mahaim physiology' from the lexicon. It is fascinating to look back at the history of accessory atrioventricular junctional conduction pathways outside the normal accessory atrioventricular conduction system, and their possible role in rhythm disturbances. It took both the anatomist and the clinical arrhythmologist quite some time to understand the complex anatomical architecture and the ensuing electrophysiological properties. Over the years, the name Mahaim was often mentioned in those discussions, although these pathways were not the ones that produced the eponym. The reason for this review, therefore, is to present relevant information about the person and what followed thereafter.

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

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

Critical Assessment of the Concepts and Misconceptions of the Cardiac Conduction System over the Last 100 Years: The Personal Quest of Robert H. Anderson.

Anatomical concepts regarding the conduction system of the heart have been a matter of debate since pioneering work done at the beginning of the 20th century. Robert H. Anderson was actively involved in this field for half a century. We aimed to investigate how his own concepts evolved over time. We have assessed anatomical concepts relating to the cardiac conduction system appearing since the key contributions made in the initial decade of the 20th century, analyzing them from the perspective of Robert H. Anderson, particularly focusing on the anatomical aspects of structures such as accessory atrioventricular pathways, including the so-called Mahaim-type fibers, connections between the atrioventricular node and the atrial myocardium, and so-called "specialized" internodal atrial tracts. To accomplish this task, we have taken as our starting point the initial concepts published in the first decade of the century, along with those subsequently reported up to 1976, and assessing them in the light of our most recently published works. The concepts put forward by Robert Anderson with regard to atrioventricular nodal bypass tracts, atrioventricular nodal inputs, decrementally conducting accessory pathways, and "tracts" for internodal atrial conduction, have remained consistent along the time frame of half a century.

The membranous septum revisited: A glimpse of our anatomical past.

The so-called membranous septum is the fibrous component of the septal structures within the heart. It is relatively subtle in its appearance, but of considerable significance to the understanding of cardiac function and cardiac disease, both congenital and acquired. Surprisingly, its existence was seemingly unknown until the early decades of the 19th century. At this time, those writing in the English language described it as the "undefended space," recognizing its importance in the setting of its aneurysmal dilation, and as the site of septal defects. By the initial decade of the 20th century, it had come to be recognized as the landmark to the site of atrioventricular bundle. Over the first decade of the 21st century, its clinical significance has been emphasized in the context of transcutaneous replacement of the aortic valve. In this review, we describe our own recent investigations of this fibrous part of the septal structures. At the same time, we provide a glimpse of our anatomic past, explaining how its initial description relied on the observations of young physicians taking their first steps in the investigation of cardiac anatomy.

Sunao Tawara : further musings on his tribulations in providing the basis for the modern-day understanding of cardiac electrophysiology.

Sunao Tawara, who was born in 1873 and died in 1952, is considered the father of modern cardiac electrophysiology. He published his monumental monograph describing the atrioventricular conduction axis in 1906. He achieved this task in the face of multiple tribulations as a doctoral student working in a cultural environment that was not his own. Although his letters underscoring the publication of the monograph have been published, little emphasis has been placed on the potential problems he encountered in bringing his task to fruition. For example, it was not until the final 6 months of his studies that he resolved the issue of the connection between the atrioventricular bundle and the so called "Purkinje cardiomyocytes". His exchanges with his mentor, Ludwig Aschoff, emphasized that the difficulties he encountered in making the connection caused him quite some turmoil. We believe that this issue, and others that he identified in his correspondence, are worthy of further attention.

The ox atrioventricular conduction axis compared to human in relation to the original investigation of sunao tawara.

It was Sunao Tawara who, in 1906, established the foundations for knowledge of the arrangement of the atrioventricular conduction axis in man and other mammals. Study of the hearts of ungulates was a central part in his investigation, which assessed other species, including man. He described several subtle differences between the mammals. We have now ourselves studied the cardiac conduction tissue of the ox heart, comparing our findings with our knowledge of the arrangement in man, and providing new insights into the differences illustrated by Tawara. It is, perhaps, surprising that these differences, although subtle, have not attracted more attention. We show that the major difference is the fact that the noncoronary aortic sinus in the ox heart is mainly supported by the myocardium of the ventricular septum, whereas in the human heart the sinus, and its leaflet, are in fibrous contiguity with the aortic leaflet of the mitral valve. It is this feature that determines the difference in the arrangement of the conduction axis between the species. We also show that the emergence of the left bundle branch on the left ventricular aspect of the muscular septum is more variable than previously described. Clin. Anat. 33:383-393, 2020. © 2019 Wiley Periodicals, Inc.

Unusual variants of pre-excitation: From anatomy to ablation: Part III-Clinical presentation, electrophysiologic characteristics, when and how to ablate nodoventricular, nodofascicular, fasciculoventricular pathways, along with considerations of permanent junctional reciprocating tachycardia.

The recognition of the presence, location, and properties of unusual accessory pathways for atrioventricular conduction is an exciting, but frequently a difficult, challenge for the clinical cardiac arrhythmologist. In this third part of our series of reviews, we discuss the different steps required to come to the correct diagnosis and management decision in patients with nodofascicular, nodoventricular, and fasciculo-ventricular pathways. We also discuss the concealed accessory atrioventricular pathways with the properties of decremental retrograde conduction that are associated with the so-called permanent form of junctional reciprocating tachycardia. Careful analysis of the 12-lead electrocardiogram during sinus rhythm and tachycardias should always precede the investigation in the catheterization room. When using programmed electrical stimulation of the heart from different intracardiac locations, combined with activation mapping, it should be possible to localize both the proximal and distal ends of the accessory connections. This, in turn, should then permit the determination of their electrophysiologic properties, providing the answer to the question "are they incorporated in a tachycardia circuit?". It is this information that is essential for decision-making with regard to the need for catheter ablation, and if necessary, its appropriate site.

Part II-Clinical presentation, electrophysiologic characteristics, and when and how to ablate atriofascicular pathways and long and short decrementally conducting accessory pathways.

Recognition of the presence, location, and properties of unusual accessory pathways for atrioventricular conduction is an exciting, frequently difficult, challenge for the clinical cardiac arrhythmologist. In this second part of our series of reviews relative to this topic, we discuss the steps required to achieve the correct diagnosis and appropriate management in patients with the so-called "Mahaim" variants of pre-excitation. We indicate that, nowadays, it is recognized that these abnormal rhythms are manifest because of the presence of atriofascicular pathways. These anatomical substrates, however, need to be distinguished from the other long and short accessory pathways which produce decremental atrioventricular conduction. The atriofascicular pathways, along with the long decrementally conducting pathways, have their atrial components located within the vestibule of the tricuspid valve. The short decremental pathways, in contrast, can originate in the vestibules of either the mitral or tricuspid valves. As a starting point, careful analysis of the 12-lead electrocardiogram, taken during both sinus rhythm and tachycardias, should precede any investigation in the catheterization room. When assessing the patient in the electrophysiological laboratory, the use of programmed electrical stimulation from different intracardiac locations, combined with entrainment technique and activation mapping, should permit the establishment of the properties of the accessory pathways, and localization of its proximal and distal ends. This should provide the answer to the question "is the pathway incorporated into the circuit underlying the clinical tachycardia". That information is essential for decision-making with regard to need, and localization of the proper site, for catheter ablation.

A finite element approach for modeling micro-structural discontinuities in the heart.

The presence of connective tissue as well as interstitial clefts forms a natural barrier to the electrical propagation in the heart. At a microscopic scale, such uncoupling structures change the pattern of the electrical conduction from uniform towards complex and may play a role in the genesis of cardiac arrhythmias. The anatomical diversity of conduction structures and their topology at a microscopic size scale is overwhelming for experimental techniques. Mathematical models have been often employed to study the behavior of the electrical propagation at a sub-cellular level. However, very fine and computationally expensive meshes are required to capture all microscopic details found in the cardiac tissue. In this work, we present a numerical technique based on the finite element method which allows to reproduce the effects of microscopic conduction barriers caused by the presence of uncoupling structures without actually resolving these structures in a high resolution mesh, thereby reducing the computational costs significantly.