His bundle pacing: Tips and tricks.

His bundle (HB) pacing is an established modality for achieving physiological pacing with a low risk of long-term lead-related complications. The development of specially designed lead and delivery tools has improved the feasibility and safety of HB pacing (HBP). Knowledge of the anatomy of HB region and the variations is essential for successful implantation. Newer delivery systems have further improved procedural outcomes. Challenging implant cases can be successfully performed by reshaping the current sheaths, using "sheath in sheath" technique or "two-lead implantation technique." Special attention to the lead parameters at implant, programming, and follow-up is necessary for successful long-term outcomes with HBP. Widespread use of HBP by electrophysiologists and further advances in dedicated delivery systems and leads are essential to further improve the effectiveness of the implantation.

The normal variants in the left bundle branch system.

This article reviewed the main anatomic and physiopathological aspects of the left bundle branch from its origin in the His bundle and its intraventricular distribution on the left endocardial surface. The results are based on the relevant literature and on personal observations executed on 206 hearts distributed as follows: 67 dogs, 60 humans, 45 sheep, 22 pigs, 10 cows, 2 monkeys, 1 guanaco, and 1 sea lion. The main anatomical features of the His-Purkinje conducting system may be summarized as follows: The bundle of His is composed by two segments: the penetrating and branching portions. LBB originates in the branching portion located underneath the membranous septum. There is no true bifurcation of the bundle of His in a human heart. Short after its origin the LBB gives rise to its two main fascicles, anterior and posterior, both heading the anterior and posterior papillary muscles, respectively. The anterior division is thinner and longer than the posterior one. The RBB and the most anterior fibers of the LBB arise at the end of the branching portion. In some cases a well-defined left septal fascicle can be identified, usually arising from the posterior division. Each division gives off small fibers and false tendons crossing the left ventricular cavity connecting the papillary between them or the papillary muscles with the septal surface. From each division of the LBB, their corresponding Purkinje networks emerge covering the subendocardium of the septum and the free wall of the left ventricles. There are critical relationships of the proximal segments of the His-Purkinje system with the surrounding cardiac structures whose pathologic processes may damage the conducting tissue.

Wilhelm His Junior and his bundle.

We have reviewed the evidence relative to the initial description of the penetrating atrioventricular bundle, seeking to determine whether Wilhelm His Junior is deserving of his eponym.

Anatomical study of the cardiac conduction system in swine hearts.

The cardiac conduction system (CCS) is crucial for regulating heartbeats; therefore, clinicians and comedicals involved in cardiovascular medicine treatment must have a thorough understanding of the CCS structure and function. However, anatomical education of the CCS based on actual dissection and observation is uncommon, although such educational methodology promotes three-dimensional structural understanding of the observed object. Based on previous studies, we examined the CCS structure in the heart of a swine (pig, Sus scrofa domestica) which has been used in the biological, medical and anatomical curricula as science teaching materials, by using macroscopic dissection procedures. Most CCS structures in a young pig heart were successfully identified and illustrated on a macroscopic scale. The atrioventricular bundle (His bundle) was located on the lower edge of the membranous interventricular septum and was clearly distinguished from the general myocardial fibres by its colour and fibre arrangement direction. Following the atrioventricular bundle towards the atrium or ventricle with properly removing the endocardium and myocardium, the atrioventricular node or the right and left bundles appeared respectively. In contrast, the sinoatrial node was not identified. The anatomy of the CCS in young pig hearts was essentially similar to that previously reported in humans and several domestic animals. Our findings of the CCS in young pig hearts are expected to be useful for medical and anatomical education for medical and comedical students, young clinicians and comedical workers.

Anatomical and molecular mapping of the left and right ventricular His-Purkinje conduction networks.

Functioning of the cardiac conduction system depends critically on its structure and its complement of ion channels. Therefore, the aim of this study was to document both the structure and ion channel expression of the left and right ventricular His-Purkinje networks, as we have previously done for the sinoatrial and atrioventricular nodes. A three-dimensional (3D) anatomical computer model of the His-Purkinje network of the rabbit heart was constructed after staining the network by immunoenzyme labelling of a marker protein, middle neurofilament. The bundle of His is a ribbon-like structure and the architecture of the His-Purkinje network differs between the left and right ventricles. The 3D model is able to explain the breakthrough points of the action potential on the ventricular epicardium during sinus rhythm. Using quantitative PCR, the expression levels of the major ion channels were measured in the free running left and right Purkinje fibres of the rabbit heart. Expression of ion channels differs from that of the working myocardium and can explain the specialised electrical activity of the Purkinje fibres as suggested by computer simulations; the expression profile of the left Purkinje fibres is more specialised than that of the right Purkinje fibres. The structure and ion channel expression of the Purkinje fibres are highly specialised and tailored to the functioning of the system. The His-Purkinje network in the left ventricle is more developed than that in the right ventricle and this may explain its greater clinical importance.

Relationship of paced left bundle branch pacing morphology with anatomic location and physiological outcomes.

BACKGROUND: Left bundle branch pacing (LBBP) is an emerging physiological pacing modality. However, little is known about pacing at different locations on the left bundle branch (LBB). OBJECTIVE: The purpose of this study was to explore pacing and physiological characteristics associated with different LBBP locations. METHODS: The study included 68 consecutive patients with normal unpaced QRS duration and successful LBBP implantation. Patients were divided into 3 groups according to the paced QRS complex as left bundle branch trunk pacing (LBTP), left posterior fascicular pacing (LPFP), or left anterior fascicular pacing (LAFP). Electrocardiographic (ECG) characteristics, pacing parameters, and fluoroscopic localization were collected and analyzed. RESULTS: There were 17 (25.0%), 35 (51.5%), and 16 (23.5%) patients in the LBTP, LPFP, and LAFP groups, respectively. All subgroups had relatively narrow paced QRS complex (128.6 ± 9.1 ms vs 133.7 ± 11.2 ms vs 134.8 ± 9.6 ms; P = .170), fast left ventricular activation (70.4 ± 9.0 ms vs 70.6 ± 10.2 ms vs 71.0 ± 9.0 ms; P = .986), as well as low and stable pacing thresholds. Delayed right ventricular activation and interventricular dyssynchrony were similar between groups. Fluoroscopic imaging indicated that the lead tip was located most commonly in the basal-middle region of the septum (67.7%), and this was independent of paced QRS morphology group (88.2% vs 57.1% vs 68.8%; P = .106). CONCLUSION: Pacing at different sites of the LBB resulted in similar intraventricular and interventricular electrical synchrony in patients with an intact conduction system. Fluoroscopic imaging alone could not predict specific LBBP paced ECG morphology.

The anatomy of the cardiac conduction system.

All the myocytes within the heart have the capacity to conduct the cardiac impulse. A population of myocytes is specialized so as to generate the cardiac impulse and then to conduct it from the atrial to the ventricular chambers. This population has become known as the conduction system. Anatomists who seek to demonstrate the location of the components of this system must contend with the fact that the components of the system cannot be distinguished from the working myocardial elements by gross dissection. In important presentations to the German Pathological Society in 1910, rules were suggested for the histological distinction of these conducting cells. These rules proposed that the myocytes, to be considered as part of the conduction system, should be histologically discrete, traceable from section to section in serially prepared material, and if to be considered as tracts, should be insulated by fibrous tissue from the adjacent myocytes. Immunohistochemical techniques have now been developed that better demonstrate the distinction between the cells specialized to conduct from working myocytes. These new techniques, for the most part, confirm the accuracy of the initial descriptions. They also reveal additional areas with the characteristics of conduction tissues. These additional areas are located in a paranodal area adjacent to the sinus node, in the vestibules of both atrioventricular valvar orifices, and in a partial ring around the aortic root. In this review, we describe all these features, emphasizing the relationship of the newly recognized components to the established parts of the cardiac conduction system, and how the new findings need to be assessed in the light of the old criteria.

Spectroscopic studies of the human heart conduction system ex vivo: implication for optical visualization.

Fluorescence excitation and emission spectra of the heart tissues specimens have been measured ex vivo with the aim of finding out the optical differences characteristic for the human heart conduction system (the His bundle) and ventricular myocardium. The optimal conditions enhancing the spectral differences between the His bundle and myocardium were found by recording the fluorescence signal in the range from 420 nm to 465 nm under the excitation at wavelengths starting from 320 nm to 370 nm. In addition, the spectral differences between the His bundle and the connective tissue, which is often present in the heart, could be displayed by comparing the ratios of fluorescence intensities being measured at above 460 nm under the preferred excitation of elastin and collagen. The left and right branches of the His bundle were visualized ex vivo in the interventricular septum of the human heart under illumination at 366 nm.

An Electro-Anatomic Atlas of His Bundle Pacing: Combining Fluoroscopic Imaging and Recorded Electrograms.

Permanent His bundle pacing (PHBP) has gained significant popularity given improved implant success rates given better tools and increasing data on the clinical benefits of PHBP. In this article, the authors hope to review the relevant anatomy of the bundle of His (HB) and help correlate PHBP implant characteristics with patient anatomy using fluoroscopic and electro-anatomic correlations.

His Bundle Pacing.

Traditional right ventricular (RV) pacing for the management of bradyarrhythmias has been pursued successfully for decades, although there remains debate regarding optimal pacing site with respect to both hemodynamic and clinical outcomes. The deleterious effects of long-term RV apical pacing have been well recognized. This has generated interest in approaches providing more physiological stimulation, namely, His bundle pacing (HBP). This paper reviews the anatomy of the His bundle, early clinical observations, and current approaches to permanent HBP. By stimulating the His-Purkinje network, HBP engages electrical activation of both ventricles and may avoid marked dyssynchrony. Recent studies have also demonstrated the potential of HBP in patients with underlying left bundle branch block and cardiomyopathy. HBP holds promise as an attractive mode to achieve physiological pacing. Widespread adaptation of this technique is dependent on enhancements in technology, as well as further validation of efficacy in large randomized clinical trials.

Changes in fascicle length from rest to maximal voluntary contraction affect the assessment of voluntary activation.

The purpose of this study was to investigate the effect of the differences between the actual fascicle length during a voluntary contraction and the fascicle length at rest of the triceps surae muscle on the determination of the voluntary activation (VA) by using the interpolated twitch technique. Twelve participants performed isometric voluntary maximal (MVC) and submaximal (20%, 40%, 60% and 80% MVC) contractions at two different ankle angles (75 degrees and 90 degrees ) under application of the interpolated twitch technique. Two ultrasound probes were used to determine the fascicle length of soleus, gastrocnemius medialis and gastrocnemius lateralis muscles. Further, the MVCs and the twitches were repeated for six more ankle angles (85 degrees , 95 degrees , 100 degrees , 105 degrees , 110 degrees and 115 degrees ). The VA of the triceps surae muscle were calculated (a) using the rest twitch force (RTF) measured during the same trial as the interpolated twitch force (ITF; traditional method) and (b) using the RTF at an ankle angle where the fascicle length showed similar values between ITF and RTF (fascicle length consideration method). The continuous changes in fascicle length from rest to MVC affect the accuracy of the assessment of the VA. The traditional method overestimates the assessment of the VA on average 4% to 12%, especially at 90 degrees ankle angle (i.e. short muscle length). The reason for this influence is the unequal force-length potential of the muscle at twitch application by the measure of ITF and RTF. These findings provide evidence that the fascicle length consideration method permits a more precise prediction (an improvement of 4-12%) of the voluntary contraction compared to the traditional method.

Histological topography of the atrioventricular node and its extensions in relation to the cardiothoracic surgical landmarks in normal human hearts.

BACKGROUND: Atrioventricular (AV) nodal injury which results in cardiac conduction disorders is one of the potential complications of heart valve surgeries and radiofrequency catheter ablations. Understanding the topography of the AV conduction system in relation to the tricuspid and mitral valves will help in reducing these complications. METHODS: A tissue block of 3cmx4cm, which contain the AV node, bundle of His and the AV nodal extensions, was excised at the AV septal junction in 20 apparently normal human hearts. The block was divided into three equal segments through vertical incisions perpendicular to the insertion of the septal leaflet of the tricuspid valve. Each segment was processed and stained with H&E and Gomori to study the different parts of the AV conduction system. RESULTS: The lower pole of the AV node was located vertically above the tricuspid septal leaflet (TSL) in 100% (20/20) of cases and at the level of the muscular interventricular septum in 65% (13/20) of cases. The upper pole of the compact AV node was located at the level of the mitral valve leaflet (MVL) in 50% (10/20) of cases. The penetrating bundle of His was seen at the level of the TSL, while the branching bundle of His was situated 1.9±1.5 mm inferior to the TSL. The right and left posterior extensions of the AV node spanned from the MVL to 2.9±1.3 mm above the TSL. CONCLUSIONS: A rectangular area (2.5 mm × 12 mm) in the Koch's triangle was devoid of AV nodal tissue and could be labeled as a safe area with no risk of conduction defects during valve surgeries. Information on the separation of AV nodal extensions from the TSL, MVL and muscular interventricular septum may play a crucial role in guiding and improving the safety of radiofrequency ablations.

Location and functional characterization of the right atrioventricular pacemaker ring in the adult avian heart.

Previous histological studies showed that in addition to a sinus node, an atrioventricular (AV) node, an AV bundle, left and right bundle branches, birds also possess a right AV-Purkinje ring that is located in the atrial sheet of the right muscular AV-valve along all its base length. The functionality of the AV-Purkinje ring is unknown. In this work, we studied the topology of pacemaker myocytes in the atrial side of the isolated chicken spontaneously contracting right muscular AV-valve using the method of microelectrode mapping of action potentials. We show that AV-cells having the ability to show pacemaking reside in the right muscular AV-valve. Pacemaker action potentials were exclusively recorded close to the base of the valve along its whole length from dorsal to the ventral attachment to the interventricular septum. These action potentials have much slower rate of depolarization, lower amplitude, and higher diastolic depolarization than action potentials of Purkinje (conducting) cells. We conclude the right AV-valve has a ring bundle of pacemaker cells (but not Purkinje cells) in the adult chicken heart.

The Cardiac Conduction System: Generation and Conduction of the Cardiac Impulse.

In this article, the authors outline the key components behind the automated generation of the cardiac impulses and the effect these impulses have on cardiac myocytes. Also, a description of the key components of the normal cardiac conduction system is provided, including the sinoatrial node, the atrioventricular node, the His bundle, the bundle branches, and the Purkinje network. Finally, an outline of how each stage of the cardiac conduction system is represented on the electrocardiogram is described, allowing the reader of the electrocardiogram to translate background information about the normal cardiac conduction system to everyday clinical practice.

Living anatomy of the atrioventricular junctions. A guide to electrophysiologic mapping. A Consensus Statement from the Cardiac Nomenclature Study Group, Working Group of Arrhythmias, European Society of Cardiology, and the Task Force on Cardiac Nomenclature from NASPE.

Current nomenclature for the atrioventricular (AV) junctions derives from a surgically distorted view, placing the valvar rings and the triangle of Koch in a single plane with antero-posterior and right-left lateral coordinates. Within this convention, the aorta is considered to occupy an anterior position, although the mouth of the coronary sinus is shown as being posterior. Although this nomenclature has served its purpose for the description and treatment of arrhythmias dependent on accessory pathways and atrioventricular nodal reentry, it is less than satisfactory for the description of atrial and ventricular mapping. To correct these deficiencies, a consensus document has been prepared by experts from the Working Group of Arrhythmias of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. It proposes a new anatomically sound nomenclature that will be applicable to all chambers of the heart. In this report, we discuss its value for description of the AV junctions, establishing the principles of this new nomenclature.

Proximal atrioventricular bundle, atrioventricular node, and distal atrioventricular bundle are distinct anatomic structures with unique histological characteristics and innervation.

BACKGROUND: Direct 3D analysis (ie, stereotaxic analysis of 3 planes) has shown that the atrioventricular (AV) node (AVN) is continuous with only specialized myocardium of the proximal AV bundle (PAVB) and distal AV bundle (DAVB) or His bundle. The purpose of the present study was to determine whether the PAVB, AVN, and DAVB possess histological features distinct from each other and from the ordinary myocardium. METHODS AND RESULTS: A protocol that preserves the cytoplasmic and interstitial integrity of the tissue and permits serial sections of the AV junction region to be made in 3 orthogonal planes showed that the PAVB, AVN, and DAVB are characterized by myocardium aggregated into fascicles containing approximately 8 myofibers. Myofibers within the fascicles are coiled or spiraled about each other; and spiraling is most compact in the PAVB. Collagen encases individual fascicles and segregates primary fascicles into secondary fascicles. Fascicles, and not myofibers, are in parallel array in the PAVB, interwoven in the AVN, and parallel in the DAVB. Narrow junctions of parallel fascicles separate the AVN from the PAVB and DAVB. Myocytes, which are largest in DAVB, possess clear perinuclear regions; thin finger-like end processes, which are most numerous in the AVN; uniform, delicate cross-striations; and intercalated disks, which are broader in the PAVB and form short stacks in the AVN. Sheaves of nerve terminals are found, including boutons as in skeletal muscle [corrected]. CONCLUSIONS: The PAVB, AVN, and DAVB have distinct histological features. Collagen septation of primary and secondary fascicles presents natural barriers within the tissues and to surrounding myocardium and structures. These findings confirm that the AV junction region contains a specialized conduction system that is anatomically isolated from ordinary myocardium.

Three-dimensional reconstruction of the rabbit atrioventricular conduction axis by combining histological, desmin, and connexin mapping data.

BACKGROUND: The 3D structure of the atrioventricular conduction axis incorporating detailed cellular and molecular composition, especially that relating to gap-junctional proteins, is still unclear, impeding mechanistic understanding of cardiac rhythmic disorders. METHODS AND RESULTS: A 3D model of the rabbit atrioventricular conduction axis was reconstructed by combining histological and immunofluorescence staining on serial sections. The exact cellular boundaries, especially those between transitional cells and atrial myocardium, were demarcated by a dense and irregular desmin-labeling pattern in conductive myocardium. The model demonstrates that the atrioventricular conduction axis is segregated into 2 connecting compartments, 1 predominantly expressing connexin45 (compact node and transitional cells) and the other predominantly coexpressing connexin43 and connexin45 (His bundle, lower nodal cells, and posterior nodal extension). The transitional zone shows unique features of spatial complexity, including a bridging bilayer structure (a deep transitional zone connecting with a superficial atrial-transitional overlay) and asymmetrical continuity (wider atrial-transitional interfaces and shorter atrial-axial distances in the hisian portion than in the ostial portion). In the latter compartment, the His bundle, lower nodal cells, and posterior nodal extension form a continual axis and longitudinal transitional-axial interface. CONCLUSIONS: Key findings of the present study are the demonstration of a distinct anatomical border between transitional and atrial cells, connection between transitional cells and both lower nodal cells and posterior nodal extension, and distinctive connexin expression patterns in different compartments of the rabbit atrioventricular conduction axis. These features, synthesized in a novel 3D model, provide a structural framework for the interpretation of nodal function.

Architectural and functional asymmetry of the His-Purkinje system of the murine heart.

OBJECTIVE: The aim of this work was to target a vital reporter gene in the mouse cardiac conduction system (CS) to distinguish this tissue from the surrounding myocardium in the adult heart. METHODS: A transgenic mouse line has been created in which EGFP is expressed under the control of the Cx40 gene. Correlative investigations associating EGFP imaging and electrophysiological techniques were carried out on the adult heart and isolated cardiomyocytes. RESULTS: In the heart of the Cx40(EGFP/+) mice, EGFP signal was seen in the coronary arteries, the atria, the atrioventricular (AV) node and the His-Purkinje system. The latter was found to be structurally and functionally asymmetrical. The anatomical asymmetry was apparent in both the number of strands or fasciculi making up the His bundle branches (BBs) (1 strand on the right, 20 or so on the left), and the density (low on the right, high on the left) of the network of Purkinje fibers (PFs) that extends over the ventricular wall surfaces. The profiles of the electrical activation patterns recorded on the right and left flanks of the septum were also asymmetrical, mirroring the architecture of the branches. EGFP made it easy to identify the Purkinje cells in populations of dissociated cardiomyocytes and they were investigated using the patch-clamp technique. The hyperpolarization-activated current (If) was recorded in all spontaneously active Purkinje cells. CONCLUSIONS: This investigation provides positive evidence of the asymmetry of the His-Purkinje system of the adult mouse, and the first patch-clamp recording data on murine cardiac Purkinje cells. This mouse model opens up new perspectives for investigating the contribution of specific genes to the morphology and function of the His-Purkinje system.

The conduction system of the swine heart.

Although the pig has been used as an experimental model for ischemic heart disease and sudden death, relatively little is known about the anatomy of the conduction system (CS) of this animal. We attempted to correlate electrophysiologic and anatomic differences between the pig and human CS. Invasive electrophysiologic studies were performed in five healthy anesthetized pigs. In contrast to the adult human, the pig has sinus tachycardia, shortened PR and H-V intervals, and a relatively short sinoatrial conduction time. Compared with the human CS, serial sections of the CS of pig hearts showed the following differences: (1) the atrioventricular node is located more to the right of the summit of the ventricular septum; (2) the penetrating bundle is very short, and the bifurcation of the bundle into bundle branches occurs more proximally; (3) there is more connective tissue and less elastic tissue; and (4) there is a copious amount of nerve fibers (about 50 percent throughout the CS). The presence of the abundant neural tissue implies that there is an important neurogenic component to conduction in the pig. Because of the above differences from the human, the pig should be used with caution as an experimental model in ischemic heart disease and sudden death where arrhythmias are studied.

His bundle interruption for reentry tachycardia in Wolff-Parkinson-White syndrome.

This report relates the experience with 16 patients with Wolff-Parkinson-White syndrome in whom His bundle interruption was performed for reentry atrioventricular (AV) tachycardia caused by a circuit composed of the His and Kent bundles. A review of the surgical anatomy of the area encompassing the AV node and the His bundle is included. The reasons for selection of His bundle interruption in the 16 patients were as follows: (1) it is safer in the poor-risk patient; (2) the His bundle was adjacent to the Kent bundle and could not be avoided; or (3) the His bundle was divided after attempted interruption of the Kent bundle failed. The methods used for interruption, either alone or in combination, included suture ligation, electrocautery, incision, and cryothermia. This study showed that in order to interrupt the His bundle with minimum physiological impairment, ablation should be done at the AV node--His bundle junction. This can be achieved in most patients with carefully applied cryothermia. If cryothermic ablation fails, then an incision must be made that separates the inferior aspect of the atrial septum from the right fibrous trigone. His bundle interruption at the AV node--His bundle junction was accomplished in 13 of the 16 patients.