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Bundle of His

The Bundle of His, also known as the atrioventricular (AV) bundle, is a specialized tract of cardiac muscle fibers that serves as a critical component of the heart's electrical conduction system, transmitting depolarizing impulses from the AV node to the ventricles to coordinate their synchronous contraction.^[1] Discovered by Swiss anatomist Wilhelm His Jr. in 1893, it originates at the AV node within the central fibrous body of the heart and penetrates the membranous interventricular septum, typically measuring about 1.8 to 2.3 cm in length and 1.1 mm in thickness in adults.^[1]^[2] Composed of Purkinje-type cells, transitional cells, and pacemaker cells embedded in dense connective tissue with collagen partitioning, the bundle is insulated by a fibrous sheath to prevent lateral impulse spread and ensure rapid, directed conduction.^[1]^[2] Anatomically, the Bundle of His runs along the crest of the interventricular septum, approximately 20 mm long and 4 mm wide, before bifurcating into the right and left bundle branches, which further distribute signals via Purkinje fibers to the ventricular myocardium.^[3] It receives dual blood supply from the AV nodal artery and the first septal perforator of the left anterior descending coronary artery, and its position can vary—typically leftward on the septal crest but occasionally right-sided or subendocardial—potentially influencing procedural approaches in cardiac interventions.^[2] Embryologically, it develops from the lateral plate mesoderm during the third week of gestation, with insulation provided by fibroblasts and fibrous tissue derived from neural crest cells, forming the annulus fibrosus that electrically isolates the atria from the ventricles.^[1] Functionally, the Bundle of His rapidly propagates electrical impulses, with a conduction velocity of about 1–2 m/s, enabling the ventricles to contract in a coordinated manner that efficiently pumps blood to the body and lungs.^[1] This rapid conduction prevents asynchronous ventricular activation, maintaining normal heart rhythm and cardiac output; disruption in its pathway can lead to atrioventricular blocks, ranging from first-degree (prolonged PR interval) to third-degree (complete dissociation between atrial and ventricular activity).^[1]^[3] Clinically, the Bundle of His is significant in the diagnosis and treatment of arrhythmias, as its malfunction contributes to heart blocks and other conduction disorders in cardiovascular disease, which affects about 128 million Americans as of 2025, often detected via electrocardiogram (ECG) showing prolonged intervals or dropped beats.^[4] His-bundle pacing (HBP), a physiological alternative to traditional right ventricular pacing, directly stimulates the bundle to achieve synchronous ventricular activation, with success rates exceeding 80% in patients with AV block, improved left ventricular ejection fraction, and reduced dyssynchrony compared to apical pacing.^[2] Variations in its anatomy, such as fibrosis or positional shifts, can complicate HBP thresholds and mapping, underscoring the need for precise imaging and electrophysiological guidance in interventions.^[2]

Anatomy and Structure

Location and Gross Anatomy

The bundle of His originates from the atrioventricular (AV) node, situated within the triangle of Koch, and penetrates the insulating fibrous tissues between the mitral and tricuspid valves to enter the central fibrous body of the heart.^[5] It emerges at the top of the muscular interventricular septum, just below the right fibrous trigone, where the right fibrous trigone encircles the penetrating portion.^[5] From there, it courses as a short, non-branching tract along the crest of the interventricular septum, running through the AV component of the membranous septum before bifurcating into the left and right bundle branches, typically within 3 mm of penetration in over three-quarters of human hearts.^[5]^[6] Macroscopically, the bundle of His appears as a compact, whitish tract composed of specialized myocardial fibers, often described as a cord-like collection insulated by surrounding fibrous connective tissue.^[5] It is embedded in the subendocardial layer on the ventricular side of the membranous septum, with variable overlying myocardial covering that provides partial protection while exposing it to the left ventricular endocardium at the site where the left bundle branch arises.^[5]^[6] In terms of spatial relations, the bundle lies at the junction between the membranous septum and the crest of the muscular ventricular septum, in close proximity to the aortic root and mitral valve through their fibrous continuity.^[5] The right bundle branch originates near the interleaflet triangle between the non-coronary and right coronary cusps of the aortic valve, positioning the structure adjacent to the postero-inferior aspect of the membranous septum and the inferior border of the aortic valve.^[5]^[6]

Histology and Cellular Composition

The bundle of His is composed primarily of specialized cardiac conduction myocytes, known as Purkinje-type cells, along with transitional cells that bridge the atrioventricular (AV) nodal cells and the Purkinje fibers of the ventricles. These cells exhibit fewer sarcomeres and a higher glycogen content compared to the working myocardium, contributing to their pale-staining appearance under light microscopy and reduced contractile function. The tissue forms an elongated structure approximately 1.8 cm in length in adult humans, embedded within dense connective tissue of the interventricular septum.^[1]^[7] Histologically, the bundle consists of elongated, pale-staining fibers with central nuclei, arranged in a braided or anastomosing pattern of multiple strands partitioned by longitudinally oriented collagen septa. This organization minimizes lateral impulse spread and is surrounded by a protective connective tissue sheath derived from the central fibrous body. Intercalated discs in these fibers are oriented obliquely, featuring tongue-and-groove joints and abundant tight junctions, differing from the perpendicular, jagged-line discs in ventricular myocytes. Electron microscopy reveals sparse, randomly oriented myofibrils, abundant mitochondria, Golgi apparatus, and occasional binucleate cells within the Purkinje-type fibers.^[7]^[1] The primary cell types include Purkinje-like cells, which predominate and support rapid conduction through abundant gap junctions expressing connexin 40 (Cx40) and connexin 43 (Cx43), facilitating efficient ion flow between cells. Slender and broad transitional cells are present in varying proportions, providing continuity from the AV node, while pacemaker (P) cells at the proximal end are oval-shaped with minimal sarcoplasmic reticulum and simple cell membranes. In contrast to ventricular myocytes, these conduction cells have fewer T-tubules, emphasizing their role in electrical rather than mechanical activity.^[8]^[7]^[1] Variations in histology exist between humans and animal models; for instance, the human bundle of His features more discrete, compact fiber bundles compared to the longer, less partitioned structures in horses (mean length 2.85 mm) and pigs (1.77 mm), while canine bundles (1.53 mm) show similarities to humans but with subtle differences in nerve fiber innervation. These species-specific traits influence the interpretation of animal studies in human cardiac research.^[9]

Physiology and Function

Electrical Conduction Mechanism

The Bundle of His functions as a rapid conduit for action potentials transmitted from the atrioventricular (AV) node to the ventricles, ensuring efficient propagation of the cardiac impulse. This structure exhibits a conduction velocity of approximately 1.5 m/s (range 1.3-1.7 m/s),^[10] which is facilitated by the high expression of voltage-gated sodium channels (Nav1.5) in its Purkinje-like cardiomyocytes, enabling fast depolarization.^[11] Action potentials within the Bundle of His display characteristics typical of fast-conducting myocardial fibers, with a rapid upstroke in phase 0 driven by sodium ion (Na⁺) influx through Nav1.5 channels. The upstroke velocity is around 400 V/s in Purkinje fibers.^[12] The plateau phase (phase 2) is sustained by inward calcium ion (Ca²⁺) currents via L-type channels, while repolarization in phase 3 occurs through potassium ion (K⁺) efflux mediated by delayed rectifier channels. The effective refractory period in these fibers is approximately 250 ms. The impulse travels through the compact Bundle of His before bifurcating into the left and right bundle branches, which promotes near-simultaneous activation of the ventricular myocardium and minimizes dyssynchrony. Originating from the penetrating portion of the AV node, this pathway maintains the integrity of ventricular depolarization timing. Conduction velocity in the Bundle of His depends on biophysical properties such as fiber diameter and axial resistivity, following approximations from cable theory.

Role in Cardiac Synchronization

The bundle of His ensures sequential activation of the cardiac chambers by receiving electrical impulses from the atrioventricular (AV) node after a conduction delay of approximately 0.1 to 0.2 seconds, which permits complete atrial systole and optimal ventricular filling prior to ventricular contraction.^[13]^[14] This timing is essential for efficient hemodynamics, as the delay originates primarily in the AV node, while the bundle of His then rapidly propagates the signal to prevent premature ventricular activation.^[1] To achieve interventricular synchrony, the bundle of His bifurcates shortly after penetrating the ventricular septum into the left bundle branch and the right bundle branch, with the left bundle further dividing into anterior (superior) and posterior (inferior) fascicles that distribute the impulse across the left ventricular endocardium.^[1]^[2] This anatomical arrangement promotes near-simultaneous depolarization of the left and right ventricles from apex to base, coordinating their contraction for effective ejection of blood into the aorta and pulmonary artery.^[1] On the electrocardiogram (ECG), the bundle of His contributes to the initial phase of the QRS complex, representing the onset of ventricular septal depolarization as the impulse emerges from the bundle branches and enters the Purkinje fiber network.^[1] The conduction velocity through the bundle, approximately 2 m/s, supports this rapid spread, ensuring the QRS duration remains narrow (typically 0.08-0.10 seconds) in normal hearts.^[13] Physiological adaptations in bundle of His conduction maintain synchronization during varying demands, such as exercise, where sympathetic activation accelerates overall heart rate but the His-Purkinje system exhibits rate-dependent changes in refractoriness to sustain coordinated ventricular activation.^[1]

Clinical Significance

Associated Pathologies

Dysfunction of the bundle of His and the distal His-Purkinje system can lead to various conduction abnormalities, primarily atrioventricular (AV) blocks and bundle branch blocks, which disrupt the normal synchronous activation of the ventricles. These pathologies often manifest as delayed or blocked electrical impulses below the AV node, resulting in widened QRS complexes on electrocardiography and potential hemodynamic instability. Infra-Hisian AV block, for instance, refers to second- or third-degree AV block occurring below the bundle of His, typically involving the bundle branches or Purkinje fibers, and is characterized by a stable PR interval with sudden dropped beats in Mobitz type II or complete dissociation in third-degree block. This condition arises from interruptions in the His-Purkinje system, leading to risks of ventricular asystole or reliance on slower escape rhythms, often presenting with symptoms like syncope or fatigue.^[15]^[16] Bundle branch blocks (BBBs) represent another key pathology, where conduction delay or blockade occurs in the left or right bundle branches emanating from the bundle of His. Left bundle branch block (LBBB) results from damage to the left bundle branch, often the anterior or posterior fascicles, causing asynchronous left ventricular depolarization with a QRS duration exceeding 120 ms and leftward axis deviation. Symptoms may include exacerbation of heart failure due to dyssynchrony or syncope from associated bradyarrhythmias. Right bundle branch block (RBBB), conversely, stems from septal or right ventricular issues affecting the right bundle branch, producing a rightward axis shift and rSR' pattern in V1 on ECG, and is frequently asymptomatic but can contribute to split S2 sounds on auscultation. Both LBBB and RBBB increase the risk of adverse outcomes, particularly when new-onset during acute events.^[17]^[18]^[19] Bifascicular and trifascicular blocks involve combinations of fascicular damage within the His-Purkinje system, heightening the propensity for complete heart block. Bifascicular block typically combines RBBB with left anterior or posterior fascicular block, reflecting involvement of two of the three main distal pathways from the bundle of His, and may present with syncope or minimal symptoms if compensated. Trifascicular block extends this to all three fascicles, often indicated by bifascicular block plus first-degree AV block (prolonged PR interval), signaling advanced conduction disease. The annual incidence of progression to complete heart block in bifascicular block cases is approximately 1% to 4%, underscoring the need for monitoring in symptomatic patients. These blocks disrupt the rapid ventricular activation that normally occurs within 40-60 ms post-His deflection.^[20]^[21]^[22] Common etiologies of these pathologies include age-related degeneration, myocardial infarction, and congenital anomalies. Age-related fibrosis, as in Lev's disease, causes sclerotic degeneration of the bundle of His and proximal left bundle branch, typically onsetting in the fourth decade and progressing to LBBB or AV block through fibro-fatty replacement of conduction fibers. Myocardial infarction, particularly anterior infarcts affecting the septal perforators of the left anterior descending artery, commonly induces new RBBB or LBBB due to ischemic damage to bundle branches, while inferior infarcts may impact the right bundle via the posterior descending artery, leading to higher mortality rates. Congenital anomalies like Lev's disease represent developmental defects in the conduction axis, often familial and progressive, manifesting as bundle branch disorders without overt structural heart disease.^[23]^[24]^[17]^[25]

Diagnostic and Therapeutic Approaches

The diagnosis of Bundle of His disorders primarily relies on electrocardiography (ECG), which identifies conduction abnormalities such as bundle branch block (BBB) through QRS complex widening exceeding 120 milliseconds.^[17] In BBB, surface ECG reveals characteristic patterns, including broad, notched R waves in left-sided leads for left BBB or rSR' complexes in right precordial leads for right BBB, allowing localization of the conduction delay.^[17] Invasive electrophysiological studies provide more precise assessment via His bundle electrography, an intracardiac recording technique using electrode catheters inserted through the femoral vein to measure the His-to-ventricular (H-V) interval, with normal values ranging from 35 to 55 milliseconds; prolongation beyond 70 milliseconds indicates infra-Hisian block and heightened risk of progression to complete heart block.^[17] These studies evaluate atrioventricular conduction sites, response to pacing maneuvers, and drug effects, aiding in differentiating supra-Hisian from infra-Hisian delays.^[26] Echocardiography assesses mechanical consequences of Bundle of His dysfunction, such as ventricular dyssynchrony in BBB, through pulsed-wave Doppler measurement of septal-to-posterior wall motion delay exceeding 60 milliseconds or strain imaging to quantify dyssynchrony indices.^[17] Cardiac magnetic resonance imaging (MRI) detects underlying structural issues like septal fibrosis contributing to conduction pathology, using late gadolinium enhancement to visualize scar tissue in the interventricular septum that may impair His bundle function.^[27] Therapeutic interventions for Bundle of His disorders emphasize physiological pacing to restore synchronized ventricular activation. His bundle pacing (HBP) involves transvenous lead placement at the His bundle site to achieve direct capture of the conduction system, serving as an alternative to traditional right ventricular pacing with success rates of approximately 85-95% in experienced centers.^[28] In a 2018 multicenter study of 106 patients with indications for cardiac resynchronization therapy, HBP was successful in 90%, with QRS duration narrowing by approximately 46 ms on average and improvement in left ventricular ejection fraction by 12% at 1 year in responders, reducing heart failure hospitalization risk compared to biventricular pacing.^[28] As per 2023 international guidelines, conduction system pacing, encompassing HBP and emerging left bundle branch area pacing (with success rates >90%), is preferred (Class I recommendation) for physiological synchrony in patients with AV block to minimize dyssynchrony and heart failure progression.^[29] Pharmacological management targets underlying conditions rather than directly repairing conduction defects; for acute symptomatic bradycardia associated with His bundle blocks, atropine (0.5-1 mg IV every 3-5 minutes, max 3 mg) can transiently improve AV conduction in vagally mediated or reversible cases, while isoproterenol infusion is used for refractory bradycardia pending pacing.^[30] Surgical options are reserved for rare cases where obstructive pathologies impinge on the Bundle of His, such as in hypertrophic cardiomyopathy; septal myectomy removes excess septal myocardium to alleviate left ventricular outflow tract obstruction, with operative techniques emphasizing preservation of the nearby His bundle to avoid iatrogenic conduction disturbances, achieving low mortality (<1%) in high-volume centers.^[31]

History and Nomenclature

Discovery and Historical Context

The atrioventricular (AV) bundle, now known as the bundle of His, was first described in 1893 by Swiss anatomist Wilhelm His Jr., who conducted meticulous dissections of human hearts and identified a distinct muscular tract providing continuity between the atrial and ventricular septa.^[32] This discovery challenged prevailing notions of complete electrical isolation between the atria and ventricles, proposing instead a pathway for impulse transmission that unified atrial and ventricular contractions.^[33] His's observations, detailed in his anatomical study, emphasized the bundle's role in bridging the insulating fibrous tissue at the AV junction, marking a pivotal shift in understanding cardiac electrophysiology.^[34] Early 20th-century confirmations solidified His's findings, with Japanese pathologist Sunao Tawara providing a comprehensive histological mapping of the cardiac conduction system in his 1906 monograph Das Reizleitungssystem des Säugetierherzens.^[35] Tawara delineated the bundle as part of a treelike network originating from the AV node, extending through the His bundle to the Purkinje fibers, thus integrating it into the full excitatory pathway across mammalian hearts.^[36] Concurrently, British cardiologist Thomas Lewis advanced correlations between electrocardiographic patterns and conduction disruptions in the 1910s, using capillary electrometers to link AV dissociation in heart block to potential bundle involvement, thereby validating the functional implications of His's and Tawara's anatomical descriptions.^[37] Subsequent electrophysiological investigations in the mid-20th century further confirmed the bundle's rapid conduction properties, notably through studies by Jesús Alanís and colleagues in 1958, who recorded the bundle's electrical activity in isolated canine hearts, revealing biphasic potentials between atrial and ventricular deflections.^[38] These recordings established the bundle's velocity at approximately 2 meters per second, underpinning its role in synchronized ventricular activation.^[39] Modern validations emerged in the 2000s with advancements in three-dimensional electroanatomical mapping, such as high-resolution imaging techniques that precisely localized the bundle within intact human hearts, corroborating historical anatomy with non-invasive visualizations.^[40] Initial acceptance of His's structure faced debates in the early 1900s, with critics questioning whether the identified tract truly represented the conduction pathway or merely an artifact of dissection, amid uncertainties over AV impulse transmission. These controversies were largely resolved by the 1920s through refined histological techniques, including serial sectioning by researchers, which provided unequivocal evidence of the bundle's specialized myocardial composition and continuity.^[41]

Etymology and Terminology

The Bundle of His is named after the Swiss anatomist and cardiologist Wilhelm His Jr. (1863–1934), who first described this specialized cardiac conduction pathway in his seminal 1893 publication, Die Tätigkeit des embryonalen Herzens und deren Bedeutung für die Lehre von der Herzbewegung beim Erwachsenen, where he identified it as a muscular tract connecting the atrial and ventricular septa during embryonic heart development.^[34]^[37] The term "His" directly derives from his surname, honoring his pioneering histological observations that clarified the pathway's role in coordinating heart contractions.^[42] The word "bundle" in the name originates from the Latin fasciculus, meaning a small bundle or cluster of fibers, a term traditionally applied in anatomy to describe compact tracts of nerve or muscle fibers, reflecting the structure's appearance as a cohesive group of specialized myocardial cells.^[43]^[44] This linguistic root underscores the anatomical emphasis on its fibrous composition, distinguishing it from broader cardiac musculature.^[45] Historically, the structure has been referred to by several alternative names, including atrioventricular (AV) bundle, common bundle.^[1] In modern standardized nomenclature, it is officially designated the "atrioventricular bundle" according to the Nomina Anatomica (1983), which prioritizes descriptive terminology over eponyms to promote international consistency in anatomical education and research.^[46]^[47] The terminology evolved significantly in the early 20th century, initially appearing as the "Kent-His bundle" to acknowledge concurrent descriptions by Albert Frank Stanley Kent, whose observations of myocardial connections were later recognized as accessory pathways rather than the primary AV conduction tract, leading to a post-1920s consensus favoring the exclusive "His bundle" or "bundle of His" to avoid conflation with pathological variants like Kent bundles.^[48]^[49]^[33] This refinement aligned with advancing electrophysiological understanding, solidifying His Jr.'s description as the foundational reference for the normal conduction system.^[50]

References

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