Bundle of His

The bundle of His, also called the atrioventricular bundle or AV bundle, is a specialized tract of cardiac conduction tissue that transmits electrical impulses from the atrioventricular junction into the ventricular conduction system.

It forms the normal electrical connection between the atria and ventricles across the electrically insulating fibrous skeleton of the heart. After leaving the atrioventricular node, the conduction axis enters the bundle of His and then divides into the right and left bundle branches.

Rapid conduction through the His-Purkinje system allows coordinated activation of the ventricular myocardium and is essential for efficient ventricular contraction.

Location

The bundle of His begins as the continuation of the atrioventricular conduction axis distal to the AV node.

It passes through the central fibrous region of the heart and courses toward the upper interventricular septum before dividing into its major ventricular branches.

Relationship to the AV Node

The AV node lies in the inferior interatrial septal region near the triangle of Koch.

Distally, the conduction tissue continues from the compact AV nodal region into the penetrating atrioventricular bundle.

Relationship to the Cardiac Fibrous Skeleton

The fibrous skeleton electrically separates most atrial working myocardium from ventricular working myocardium.

The bundle of His traverses this insulating fibrous framework and therefore provides the normal route by which atrial electrical activity reaches the ventricles.

Electrical Connection Between Atria and Ventricles

Under normal anatomical conditions, the atrial and ventricular myocardium are not connected by broad areas of electrically conducting working muscle.

The specialized atrioventricular conduction axis provides a controlled pathway for propagation across the fibrous atrioventricular junction.

Course

The bundle of His passes from the AV nodal region through fibrous tissue toward the membranous and muscular portions of the interventricular septal region.

It subsequently divides into the right and left bundle branches, which distribute excitation to the ventricular conduction network.

Major Conduction Sequence

Order	Structure
1	Sinoatrial node
2	Atrial myocardium
3	Atrioventricular node
4	Bundle of His
5	Right and left bundle branches
6	Purkinje fibers
7	Ventricular myocardium

Penetrating Bundle

The proximal portion of the His bundle that passes through the fibrous skeleton is often described as the penetrating bundle.

This segment is anatomically important because it lies near the central fibrous body and neighboring valvular and septal structures.

Branching Portion

After traversing the fibrous region, the His bundle reaches the interventricular septal region and divides into specialized ventricular conduction pathways.

The branching pattern ultimately produces the right bundle branch and the components of the left bundle branch.

Right Bundle Branch

The right bundle branch conducts excitation along the right side of the interventricular septum toward the right ventricular apex and Purkinje network.

It contributes to rapid activation of the right ventricular myocardium.

Left Bundle Branch

The left bundle branch distributes excitation into the left ventricle.

It divides into fascicular pathways that spread through the left side of the interventricular septum and into the left ventricular Purkinje system.

Left-Sided Fascicles

The left bundle system is commonly described in terms of anterior and posterior fascicular pathways, with additional branching and anatomical variability.

These pathways permit rapid distribution of electrical activity across the large left ventricular myocardium.

Purkinje Network

The distal bundle branches give rise to an extensive network of Purkinje fibers.

Purkinje fibers conduct impulses rapidly through the subendocardial regions of the ventricles and transmit excitation to working ventricular cardiomyocytes.

Function

The primary function of the bundle of His is to conduct electrical excitation from the AV junction into the ventricular conduction system.

It forms an essential link between the AV node and the bundle branches.

Rapid Ventricular Conduction

Conduction becomes substantially faster after the impulse leaves the slowly conducting AV nodal region and enters the His-Purkinje system.

This rapid propagation helps activate large regions of ventricular myocardium within a short period.

Coordination of Ventricular Contraction

Efficient ventricular pumping requires coordinated electrical activation.

The His bundle, bundle branches, and Purkinje fibers distribute the impulse in a pattern that promotes organized ventricular contraction rather than slow cell-to-cell propagation alone.

Septal Activation

Normal ventricular activation includes early excitation of the interventricular septum followed by rapid spread through the ventricular conduction system.

The precise sequence contributes to the characteristic morphology of the normal QRS complex.

Apical Activation

The specialized conduction network rapidly delivers excitation toward the ventricular apices.

Activation then spreads through ventricular myocardium in a coordinated pattern that supports effective ejection toward the ventricular outflow tracts.

Electrical Properties

His bundle cells are specialized cardiomyocytes capable of conducting action potentials more rapidly than AV nodal tissue.

Their electrical properties are adapted for transmission rather than generation of significant contractile force.

Fast-Response Action Potentials

Cells within the His-Purkinje system generally exhibit fast-response action potentials.

Rapid sodium influx contributes importantly to the steep phase 0 depolarization and relatively high conduction velocity of this tissue.

Conduction Velocity

Conduction through the His-Purkinje system is rapid compared with conduction through the AV node.

This difference allows the AV node to provide a physiological delay while the distal conduction system distributes excitation efficiently through the ventricles.

Pacemaker Capability

Cells within the His-Purkinje system can possess latent pacemaker activity.

Under normal conditions, this activity is suppressed by faster impulses arriving from the sinoatrial node through the AV conduction system.

Escape Rhythms

If impulses from higher pacemakers fail to reach the distal conduction system, subsidiary pacemaker tissue may generate an escape rhythm.

Escape rhythms originating lower in the conduction system are generally slower than normal sinus rhythm.

Relationship to the ECG

Electrical conduction through the bundle of His occurs after atrial depolarization and AV nodal delay but before widespread ventricular depolarization.

Its activity is not represented by a distinct visible wave on the standard surface electrocardiogram.

PR Interval

The PR interval reflects conduction from the onset of atrial depolarization to the onset of ventricular depolarization.

This includes conduction through atrial tissue, the AV node, and the His-Purkinje system proximal to ventricular activation.

QRS Complex

The QRS complex represents ventricular depolarization.

Normal rapid conduction through the His bundle and its branches helps produce a relatively narrow QRS complex.

His Bundle Electrogram

During invasive electrophysiological studies, electrical activity from the His bundle can be recorded directly using intracardiac electrodes.

This allows conduction intervals to be measured more precisely than is possible from the surface ECG alone.

AH and HV Intervals

Interval	General Meaning
AH interval	Conduction from atrial activation through the AV nodal region to His bundle activation
HV interval	Conduction from His bundle activation to ventricular activation through the distal His-Purkinje system

Blood Supply

The blood supply of the His bundle and neighboring conduction tissue can arise from branches of the coronary circulation supplying the septal and atrioventricular junctional regions.

Because conduction tissue lies near territories supplied by both right and left coronary branches, ischemia in relevant regions can impair conduction.

AV Nodal Artery

The proximal atrioventricular conduction axis may receive blood from the AV nodal artery, which most commonly arises from the right coronary artery in right-dominant circulation.

The exact vascular contribution varies among individuals.

Septal Branches

More distal portions of the conduction system receive blood from septal perforating branches, particularly branches associated with the anterior interventricular artery.

This vascular arrangement has clinical relevance in ischemic injury involving the interventricular septum.

Relationship to the Membranous Septum

The bundle of His is closely related to the membranous portion of the interventricular septum.

This anatomical relationship places the conduction axis near structures frequently encountered during cardiac surgery and transcatheter procedures.

Relationship to the Aortic Root

The atrioventricular conduction axis lies close to the aortic root and adjacent fibrous structures.

Procedures involving the aortic valve region can therefore affect the His bundle or nearby conduction tissue.

Relationship to the Tricuspid Valve

The proximal conduction axis is also closely related to the septal aspect of the tricuspid valve and central fibrous body.

Knowledge of this relationship is important during surgery involving the atrioventricular septal region.

Relationship to the Ventricular Septum

After passing through the fibrous skeleton, the conduction axis reaches the upper interventricular septum.

Its branches then distribute along the septal surfaces toward their respective ventricles.

Cardiac Conduction System Components

Structure	Principal Role
SA node	Normal initiation of cardiac impulses
AV node	Delays atrioventricular conduction
Bundle of His	Conducts excitation across the fibrous skeleton into the ventricular system
Bundle branches	Distribute impulses along the interventricular septum
Purkinje fibers	Rapidly distribute excitation through ventricular myocardium

Clinical Significance

His-Purkinje Conduction Disease

Disease involving the His bundle or its distal branches can slow or interrupt ventricular conduction.

Depending on the location and severity of the abnormality, this may produce bundle branch block, fascicular block, or advanced atrioventricular block.

Bundle Branch Block

A bundle branch block occurs when conduction through one of the major ventricular bundle branches is delayed or interrupted.

The affected ventricle is then activated partly through slower spread from the opposite side, producing delayed depolarization and widening of the QRS complex.

Right Bundle Branch Block

Right bundle branch block delays electrical activation of portions of the right ventricle.

The left ventricle is activated through the left bundle system, after which excitation spreads toward the delayed right ventricular myocardium.

Left Bundle Branch Block

Left bundle branch block alters the normal sequence of left ventricular activation.

It can produce substantial electrical and mechanical dyssynchrony because much of the left ventricle is activated through abnormal pathways.

Fascicular Block

Conduction can be impaired selectively within fascicles of the left bundle system.

These abnormalities alter the direction and sequence of ventricular depolarization and may change the electrical axis of the heart.

Bifascicular Disease

Conduction abnormalities may involve combinations of the major ventricular conduction pathways.

The significance depends on which pathways are affected and whether disease progresses to more extensive conduction block.

Complete Heart Block

Complete atrioventricular block can occur when conduction between the atria and ventricles is interrupted at or below the AV junction.

If the block involves the His-Purkinje system, ventricular activation may depend on a slower distal escape pacemaker.

Infra-Hisian Block

Infra-Hisian block refers to conduction disease located below the His bundle recording site within the distal His-Purkinje system.

Electrophysiological testing can help distinguish nodal from His-Purkinje conduction abnormalities.

Myocardial Infarction

Ischemic injury involving the interventricular septum can damage the His bundle or bundle branches.

This may result in new conduction abnormalities, including bundle branch block or higher-grade atrioventricular block.

Degenerative Conduction Disease

Fibrosis and degeneration of the cardiac conduction system can impair conduction through the His-Purkinje network.

Progressive disease may produce bradyarrhythmias or intermittent atrioventricular block.

Aortic Valve Procedures

The His bundle lies close to the aortic annulus and membranous septal region.

Surgical or transcatheter aortic valve procedures can therefore cause temporary or permanent conduction disturbances in some patients.

Ventricular Septal Surgery

Operations involving the membranous or upper muscular interventricular septum require careful attention to the conduction axis.

Direct injury or postoperative edema near the His bundle can produce atrioventricular block.

Pacemaker Therapy

Permanent pacing may be required when disease of the atrioventricular conduction system causes clinically significant bradycardia or advanced heart block.

The pacing strategy depends on the level of conduction disease and the patient's overall cardiac condition.

His Bundle Pacing

His bundle pacing is a cardiac pacing technique that attempts to stimulate the native His-Purkinje conduction system directly.

When effective, activation can use the heart's specialized conduction pathways to produce a more physiological ventricular activation pattern than conventional ventricular myocardial pacing in selected patients.

Key Features of the Bundle of His

Feature	Key Point
Alternative name	Atrioventricular bundle
Origin	Continuation of the AV conduction axis distal to the AV node
Location	Central fibrous region and upper interventricular septal region
Primary function	Conducts impulses from the AV junction into the ventricular conduction system
Major branches	Right and left bundle branches
Distal network	Purkinje fibers
Conduction	Rapid compared with AV nodal conduction
ECG relationship	Contributes to conduction preceding the QRS complex
Clinical importance	Heart block, bundle branch disease, electrophysiological localization and conduction-system pacing

Anatomical and Physiological Importance

The bundle of His is a crucial anatomical bridge between the atrial and ventricular components of the cardiac conduction system. By traversing the fibrous skeleton, it provides the normal route through which impulses leaving the AV node can reach the ventricles.

Its rapid conduction properties differ from the slow conduction of the AV node. The AV node delays the impulse, while the His bundle and its branches rapidly distribute excitation throughout the ventricular conduction system. This combination allows atrial contraction to precede ventricular contraction while still producing highly coordinated ventricular activation.

The bundle's close relationships with the central fibrous body, membranous septum, aortic root, tricuspid valve, and interventricular septum also make it clinically important. Ischemia, fibrosis, congenital abnormalities, surgery, and transcatheter procedures in these regions can disturb His-Purkinje conduction and produce clinically significant rhythm and conduction abnormalities.