Cardiac Anatomy for the Interventional Arrhythmologist: I.Terminology and Fluoroscopic Projections

Diversity and complexity of the cavotricuspid isthmus in rabbits: A novel scheme for classification and geometrical transformation of anatomical structures

The aim of this study was to describe the morphology of the cavotricuspid isthmus (CTI) in detail and introduce a comprehensive scheme to describe the topology of this region based on functional considerations. This may lead to a better understanding of isthmus-dependent flutter and fibrillation and to improved intervention strategies. We used images of the cavotricuspid isthmus from 52 rabbits of both sexes with a median weight of 3.40 ± 0.93 kg. The area of the CTI was 124.25 ± 42.14 mm² with 53.28 ± 21.13 mm² covered by pectinate muscles connecting the terminal crest and the vestibule. Isthmus length decreased from inferolateral (13.09 ±2.14 mm) to central (9.85 ± 2.14 mm) to paraseptal (4.88 ± 1.96 mm) resembling the overall human geometry. Ramification sites of pectinate muscles were identified and six levels dividing the CTI from posterior to anterior were introduced. This allowed the classification of pectinate muscle segments based on the connected ramification level. To account for the high inter-individual variations in size and shape, the CTI was projected onto a normalized reference frame using bilinear transformation. Furthermore, two measures of complexity were introduced: (i) the ramification index, which reflects the total number of muscle segments connected to a ramification site and (ii) the complexity index, which reflects the type of ramification (branching or merging site). Topological analysis showed that the complexity of the pectinate muscle network decreases from inferolateral to paraseptal and that the number of electrically uncoupled parallel pathways increases in the central section between the terminal crest and the vestibule which introduces potential reentry pathways.

Miniseries 2-septal and paraseptal accessory pathways-part II: para-Hisian accessory pathways-so-called anteroseptal pathways revisited

Surgeons, when dividing bypass tracts adjacent to the His bundle, considered them to be 'anteroseptal'. The area was subsequently recognized to be superior and paraseptal, although this description is not entirely accurate anatomically, and conveys little about the potential risk during catheter interventions. We now describe the area as being para-Hisian, and it harbours two types of accessory pathways. The first variant crosses the membranous septum to insert into the muscular ventricular septum without exiting the heart, and hence being truly septal. The second variant inserts distally in the paraseptal components of the supraventricular crest, and consequently is crestal. The site of ventricular insertion determines the electrocardiographic expression of pre-excitation during sinus rhythm, with the two types producing distinct patterns. In both instances, the QRS and the delta wave are positive in leads I, II, and aVF. In crestal pathways, however, the QRS is ≥ 140 ms, and exhibits an rS configuration in V1-2. The delta wave in V1-2 precedes by 20-50 ms the apparent onset of the QRS in I, II, III, and aVF. In the true septal pathways, the QRS complex occupies ∼120 ms, presenting a QS, W-shaped, morphology in V1-2. The delta wave has a simultaneous onset in all leads. Our proposed terminology facilitates the understanding of the electrocardiographic manifestations of both types of para-Hisian pathways during pre-excitation and orthodromic tachycardia, and informs on the level of risk during catheter ablation.

Miniseries 2-septal and paraseptal accessory pathways-part IV: inferior paraseptal accessory pathways-lessons from surgical and catheter ablation

Surgeons and electrophysiologists performing accessory pathway ablation procedures have used the term 'posteroseptal' region. This area, however, is neither septal nor posterior, but paraseptal and inferior; paraseptal because it includes the fibro-adipose tissues filling the pyramidal space and not the muscular septum itself and inferior because it is part of the heart adjacent to the diaphragm. It should properly be described, therefore, as being inferior and paraseptal. Pathways in this region can be ablated at three areas, which we term right inferior, mid-inferior, and left inferior paraseptal. The right- and left inferior paraseptal pathways connect the right and left atrial vestibules with the right and left paraseptal segments of the parietal ventricular walls. The mid-inferior paraseptal pathways take a subepicardial course from the myocardial sleeves surrounding the coronary sinus and its tributaries. Our review addresses the evolution of the anatomical concept of the inferior paraseptal region derived from surgical and catheter ablation procedures. We also highlight the limitations of the 12-lead electrocardiogram in identifying, without catheter electrode mapping, which are the pathways that can be ablated without a coronary sinus, or left heart approach.

The Atrioventricular Conduction Axis and its Implications for Permanent Pacing

Extensive knowledge of the anatomy of the atrioventricular conduction axis, and its branches, is key to the success of permanent physiological pacing, either by capturing the His bundle, the left bundle branch or the adjacent septal regions. The inter-individual variability of the axis plays an important role in underscoring the technical difficulties known to exist in achieving a stable position of the stimulating leads. In this review, the key anatomical features of the location of the axis relative to the triangle of Koch, the aortic root, the inferior pyramidal space and the inferoseptal recess are summarised. In keeping with the increasing number of implants aimed at targeting the environs of the left bundle branch, an extensive review of the known variability in the pattern of ramification of the left bundle branch from the axis is included. This permits the authors to summarise in a pragmatic fashion the most relevant aspects to be taken into account when seeking to successfully deploy a permanent pacing lead.

Three-dimensional visualization of the bovine cardiac conduction system and surrounding structures compared to the arrangements in the human heart

In the human heart, the atrioventricular node is located toward the apex of the triangle of Koch, which is also at the apex of the inferior pyramidal space. It is adjacent to the atrioventricular portion of the membranous septum, through which it penetrates to become the atrioventricular bundle. Subsequent to its penetration, the conduction axis is located on the crest of the ventricular septum, sandwiched between the muscular septum and ventricular component of the membranous septum, where it gives rise to the ramifications of the left bundle branch. In contrast, the bovine conduction axis has a long non-branching component, which penetrates into a thick muscular atrioventricular septum having skirted the main cardiac bone and the rightward half of the non-coronary sinus of the aortic root. It commonly gives rise to both right and left bundle branches within the muscular ventricular septum. Unlike the situation in man, the left bundle branch is long and thin before it branches into its fascicles. These differences from the human heart, however, have yet to be shown in three-dimensions relative to the surrounding structures. We have now achieved this goal by injecting contrast material into the insulating sheaths that surround the conduction network, evaluating the results by subsequent computed tomography. The fibrous atrioventricular membranous septum of the human heart is replaced in the ox by the main cardiac bone and the muscular atrioventricular septum. The apex of the inferior pyramidal space, which in the bovine, as in the human, is related to the atrioventricular node, is placed inferiorly relative to the left ventricular outflow tract. The bovine atrioventricular conduction axis, therefore, originates from a node itself located inferiorly compared to the human arrangement. The axis must then skirt the non-coronary sinus of the aortic root prior to penetrating the thicker muscular ventricular septum, thus accounting for its long non-branching course. We envisage that our findings will further enhance comparative anatomical research.

What is the real cardiac anatomy?

The heart is a remarkably complex organ. Teaching its details to medical students and clinical trainees can be very difficult. Despite the complexity, accurate recognition of these details is a pre‐requisite for the subsequent understanding of clinical cardiologists and cardiac surgeons. A recent publication promoted the benefits of virtual reconstructions in facilitating the initial understanding achieved by medical students. If such teaching is to achieve its greatest value, the datasets used to provide the virtual images should themselves be anatomically accurate. They should also take note of a basic rule of human anatomy, namely that components of all organs should be described as they are normally situated within the body. It is almost universal at present for textbooks of anatomy to illustrate the heart as if removed from the body and positioned on its apex, the so‐called Valentine situation. In the years prior to the emergence of interventional techniques to treat cardiac diseases, this approach was of limited significance. Nowadays, therapeutic interventions are commonplace worldwide. Advances in three‐dimensional imaging technology, furthermore, now mean that the separate components of the heart can readily be segmented, and then shown in attitudinally appropriate fashion. In this review, we demonstrate how such virtual dissection of computed tomographic datasets in attitudinally appropriate fashion reveals the true details of cardiac anatomy. The virtual approach to teaching the arrangement of the cardiac components has much to commend it. If it is to be used, nonetheless, the anatomical details on which the reconstructions are based must be accurate. Clin. Anat. 32:288–309, 2019. © 2019 The Authors. Clinical Anatomy published by Wiley Periodicals, Inc. on behalf of American Association of Clinical Anatomists.

Demonstration of living anatomy clarifies the morphology of interatrial communications

Inferences made regarding the postnatal anatomy of the atrial septum still tend to be based on developmental evidence. Although atrial septation is a well-defined process, it is remarkably complex and remains poorly understood. It is now established, however, that the process involves the conjugation of several myocardial structures and mesenchymal tissues of both intracardiac and extracardiac origin. The resultant postnatal morphology is equally complex, evidenced by the fact that, in the normal heart, only the floor of the oval fossa, along with its anteroinferior muscular buttress, are true anatomical septums. In this regard, septums can be defined as partitions that can be removed without creating communications with the extracavitary space. The true septal components are surrounded by grooves, which themselves largely represent infolding of the atrial walls. These anatomical features can now accurately be revealed using virtual dissection of CT data sets. These images, when carefully reconstructed, demonstrated the anatomy with as much accuracy as when hearts are dissected in the autopsy room. Such virtual dissection, furthermore, shows the components as they are seen within the chest, thus facilitating understanding for those intending to undertake interventional therapeutic procedures. By preparing such images, we show the complexity of the normal atrial septum and its surrounds. We show that it is only defects within the oval fossa, or the much rarer vestibular defects, which can appropriately be illustrated in the context of a normally constructed heart.

Anatomy for Ventricular Tachycardia Ablation in Structural Heart Disease

Ablation of ventricular tachycardia (VT) in the setting of structural heart disease, previously reserved for highly experienced specialized centers, is being performed at more centers internationally as cardiac electrophysiologists gain advanced training. Interventional cardiac electrophysiologists need a high level of anatomic knowledge to guide a procedure that can carry significant risk. Understanding cardiac anatomy improves the chance of procedural success and also the likelihood of appropriate decision making if complications are encountered. This article focuses on selected anatomic regions where complex anatomy can be an impediment to successful VT ablation.

Reappraisal of classical electrocardiographic criteria in detecting accessory pathways with a strict para-Hisian location

BACKGROUND

Identification of electrocardiographic (ECG) criteria for para-Hisian accessory pathways (APs) is based on a small series of patients. The presence of a negative delta wave in leads V(1) and V(2) has been suggested as an ECG marker of this AP location.

OBJECTIVE

To validate these ECG findings in a large series of patients with strict invasive criteria for that location.

METHODS

We included 105 patients (39 women, 66 men; mean age 26 ± 12 years, range 5-82 years) with an ECG pattern compatible with preexcitation through an anteroseptal or midseptal AP following established ECG criteria. A para-Hisian AP was defined when the location of its successful catheter ablation coincided with either the largest recordable His bundle electrogram or a His bundle potential of>0.1 mV. Patients without that definition were included in the control group.

RESULTS

A para-Hisian location of the AP was found in 52 patients. AP locations of the remaining 53 patients (control group) were anteroseptal (n = 39), midseptal (n = 9), and fasciculoventricular (n = 5). A negative delta wave in leads V(1) and V(2) was observed in 13 patients with para-Hisian APs (sensitivity 25%; specificity 92%). However, the sum of initial r-wave amplitudes in those leads was<0.5 mV in 44 of the patients with para-Hisian APs and in 13 patients of the control group (sensitivity 85%; specificity 75.5%; area under receiver-operator characteristic curve 0.85).

CONCLUSIONS

The presence of negative delta waves in leads V(1) and V(2) indicates a poor sensitivity and high specificity to detect APs with a strict definition of para-Hisian location. The sum of initial r-wave amplitudes in those ECG leads could be a useful, adjunctive marker in the noninvasive identification of these challenging APs.