Right and Left Bundle Branches

The right and left bundle branches are specialized pathways of the ventricular cardiac conduction system. They arise from the bundle of His and conduct electrical excitation along the interventricular septum toward the Purkinje networks of the right and left ventricles.

The bundle branches provide rapid electrical transmission to the ventricular myocardium. Their organization allows the two ventricles to be activated in a coordinated sequence, which is essential for efficient ventricular contraction and ejection.

The right bundle branch primarily supplies the right ventricular conduction network. The left bundle branch supplies the left ventricle and divides into major fascicular pathways before merging with the extensive left ventricular Purkinje system.

Position in the Cardiac Conduction System

The bundle branches form the connection between the bundle of His and the distal Purkinje network.

They therefore occupy an intermediate position within the specialized ventricular conduction system.

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

Origin

The bundle of His descends from the atrioventricular junction toward the upper interventricular septum.

Its branching portion separates into right-sided and left-sided conduction pathways that distribute electrical excitation into the respective ventricles.

Interventricular Septum

The interventricular septum provides the principal anatomical route along which the proximal bundle branches travel.

The conduction pathways then branch toward the ventricular apices, papillary muscles, and free walls through the Purkinje system.

Right Bundle Branch

The right bundle branch is the specialized conduction pathway that carries excitation toward the right ventricle.

It descends along the right side of the interventricular septum and continues toward the right ventricular apex.

Course of the Right Bundle Branch

The right bundle branch follows a relatively discrete course through the interventricular septal region.

Distally, its fibers distribute into the right ventricular Purkinje network and communicate with ventricular working myocardium.

Moderator Band

A portion of the right bundle branch travels within the septomarginal trabecula, commonly called the moderator band.

This muscular band extends from the interventricular septum toward the anterior papillary muscle and provides a rapid route for conduction across part of the right ventricular cavity.

Anterior Papillary Muscle

Conduction tissue reaching the anterior papillary muscle through the moderator band allows early electrical activation of this region.

Timely papillary muscle contraction helps tension the chordae tendineae as right ventricular pressure rises.

Right Ventricular Purkinje Network

The distal right bundle branch divides into Purkinje fibers beneath the endocardium of the right ventricle.

These fibers rapidly distribute excitation to the septal, apical, and free-wall myocardium.

Left Bundle Branch

The left bundle branch carries excitation from the His bundle into the left ventricle.

Compared with the right bundle branch, the left-sided conduction system spreads into a broader branching network along the left side of the interventricular septum.

Course of the Left Bundle Branch

The left bundle branch passes beneath the endocardium on the left side of the interventricular septum.

It divides into fascicular pathways that distribute electrical impulses to different regions of the left ventricle.

Left Anterior Fascicle

The left anterior fascicle conducts excitation toward anterior and superior regions of the left ventricle.

It is generally a relatively slender pathway and is clinically important because selective conduction block can alter the direction of ventricular depolarization.

Left Posterior Fascicle

The left posterior fascicle distributes excitation toward posterior and inferior regions of the left ventricle.

It is generally broader than the anterior fascicle and has a somewhat different anatomical and vascular distribution.

Septal Conduction Fibers

Additional septal fibers may contribute to the left ventricular conduction network.

The exact organization of the left bundle system is anatomically more complex than a simple two-branch division and can vary among individuals.

Left Bundle Organization

Component	General Distribution
Anterior fascicle	Anterior and superior left ventricular regions
Posterior fascicle	Posterior and inferior left ventricular regions
Septal fibers	Interventricular septal regions with variable organization

Left Ventricular Purkinje Network

The fascicular pathways branch extensively into the left ventricular Purkinje network.

This network rapidly distributes excitation throughout the thick left ventricular myocardium.

Histological Structure

The bundle branches consist of specialized conducting cardiomyocytes rather than ordinary nerve fibers.

Their cells are adapted for rapid electrical propagation and ultimately become continuous with the Purkinje network.

Specialized Cardiomyocytes

Cells of the His-Purkinje system contain contractile elements but are functionally specialized for conduction.

They possess electrophysiological properties that allow impulses to travel much faster than through AV nodal tissue.

Electrical Coupling

Specialized conduction cells are electrically coupled through gap junctions.

This permits ionic current to spread rapidly from one cell to another along the ventricular conduction pathways.

Function

The primary function of the bundle branches is to carry impulses rapidly from the His bundle toward the ventricular Purkinje networks.

This minimizes delays between activation of different ventricular regions.

Rapid Ventricular Activation

Rapid conduction through the bundle branches and Purkinje fibers allows the ventricles to depolarize over a relatively short period.

This coordinated electrical activation is represented by the QRS complex on the surface electrocardiogram.

Septal Depolarization

Normal ventricular activation includes early depolarization of the interventricular septum.

The left-sided septal conduction system contributes importantly to the normal sequence of septal activation.

Direction of Septal Activation

In normal conduction, initial septal activation generally proceeds from the left side of the septum toward the right.

This early electrical vector contributes to the normal morphology of the QRS complex in several ECG leads.

Apical Activation

The conduction system rapidly delivers excitation toward the ventricular apices.

Electrical activity then spreads through the Purkinje network into the ventricular myocardium.

Free-Wall Activation

Purkinje fibers distribute impulses from the distal bundle branches to the ventricular free walls.

The coordinated activation pattern helps produce effective contraction of both ventricles.

Ventricular Mechanical Coordination

Electrical synchronization is essential for efficient ventricular pressure generation.

When one bundle branch is blocked, delayed activation of the affected ventricle can create mechanical dyssynchrony as well as an abnormal ECG pattern.

Electrical Properties

Bundle branch cells are part of the fast-response His-Purkinje conduction system.

Their action potentials have rapid depolarizing upstrokes that depend importantly on fast sodium channels.

Fast-Response Action Potential

Rapid sodium influx during phase 0 produces a steep depolarization in healthy His-Purkinje tissue.

This property contributes to the high conduction velocity of the bundle branches.

Refractory Period

After depolarization, conduction cells enter a refractory period during which their ability to respond to another impulse is reduced.

Differences in refractoriness can influence conduction during premature beats and tachyarrhythmias.

Automaticity

Distal His-Purkinje tissue possesses latent pacemaker capability.

Normally, this activity is suppressed by faster impulses originating from the sinoatrial node.

Relationship to the ECG

The bundle branches are critical determinants of the normal sequence of ventricular depolarization.

Intact bilateral conduction contributes to a relatively narrow QRS complex because both ventricles are activated rapidly through specialized pathways.

QRS Complex

The QRS complex represents ventricular depolarization.

Interruption of a major bundle branch forces part of the ventricle to be activated through slower myocardial spread, typically prolonging the QRS duration.

Normal Versus Abnormal Conduction

Condition	Activation Pattern
Normal bundle conduction	Rapid activation through both ventricular conduction systems
Right bundle branch block	Delayed right ventricular activation
Left bundle branch block	Delayed and altered left ventricular activation
Fascicular block	Altered left ventricular activation vector through remaining pathways

Blood Supply

The bundle branches receive arterial blood from vessels supplying the interventricular septum.

Septal perforating branches of the anterior interventricular artery provide an important contribution, with additional supply to portions of the conduction system from other coronary branches.

Anterior Interventricular Artery

The anterior interventricular artery, also called the left anterior descending artery, gives rise to septal perforating branches.

These branches supply much of the interventricular septum and important portions of the ventricular conduction system.

Posterior Septal Supply

Posterior septal regions may receive arterial supply from branches associated with the posterior interventricular artery.

The precise vascular pattern depends partly on coronary dominance and individual anatomy.

Clinical Significance

Right Bundle Branch Block

Right bundle branch block (RBBB) occurs when electrical conduction through the right bundle branch is delayed or interrupted.

The left ventricle is activated through the intact left bundle system, while the right ventricle is subsequently activated through slower spread from the left side.

ECG in Right Bundle Branch Block

RBBB alters the terminal portion of ventricular depolarization because right ventricular activation is delayed.

Complete RBBB is associated with a widened QRS complex and characteristic changes in right precordial and lateral leads.

Left Bundle Branch Block

Left bundle branch block (LBBB) occurs when conduction through the major left bundle system is substantially delayed or interrupted.

The right ventricle is activated first through the right bundle branch, followed by delayed spread of excitation into the left ventricle.

Mechanical Effects of LBBB

Because the left ventricle is activated abnormally, LBBB can produce significant ventricular mechanical dyssynchrony.

This can reduce pumping efficiency, particularly in patients with underlying ventricular dysfunction.

Left Anterior Fascicular Block

Block of the left anterior fascicle redirects ventricular activation through the remaining left-sided conduction pathways.

This commonly produces a characteristic shift in the frontal-plane electrical axis.

Left Posterior Fascicular Block

Block of the left posterior fascicle alters the sequence of left ventricular activation in the opposite distribution.

Isolated left posterior fascicular block is less common than isolated left anterior fascicular block.

Bifascicular Block

Bifascicular block refers to conduction disease affecting two major fascicular pathways.

A common pattern combines right bundle branch block with block of one left-sided fascicle.

Trifascicular Conduction Disease

The term trifascicular disease is used clinically in discussions of conduction abnormalities involving the right bundle and both major left fascicular pathways.

The exact electrophysiological abnormality should be defined carefully because surface ECG findings do not always establish the precise anatomical level of conduction delay.

Complete Heart Block

Extensive disease of the His-Purkinje system can prevent atrial impulses from reaching the ventricular myocardium.

When complete atrioventricular block occurs distally, ventricular activity depends on an escape pacemaker below the level of block.

Myocardial Infarction

Infarction involving the interventricular septum can damage one or more bundle branch pathways.

New bundle branch or fascicular conduction abnormalities may therefore occur with significant ischemic injury.

Degenerative Conduction Disease

Fibrosis and degeneration of the His-Purkinje system can progressively impair conduction through the bundle branches.

This process can produce bundle branch block, fascicular block, bradyarrhythmia, or advanced atrioventricular block.

Aortic Valve Disease and Procedures

The proximal ventricular conduction system lies close to the membranous septum and aortic root.

Valve disease, surgical aortic valve replacement, or transcatheter aortic valve procedures can therefore affect nearby conduction tissue.

Cardiac Resynchronization Therapy

Marked ventricular conduction delay, particularly certain patterns of left bundle branch block, can produce ventricular dyssynchrony.

In appropriately selected patients with heart failure, cardiac resynchronization therapy can improve the timing of ventricular contraction.

Conduction System Pacing

Conduction system pacing techniques can target the His bundle or left bundle branch region.

The aim is to recruit the native His-Purkinje system and achieve a more physiological pattern of ventricular activation in selected patients.

Right and Left Bundle Branch Comparison

Feature	Right Bundle Branch	Left Bundle Branch
Primary ventricle	Right ventricle	Left ventricle
General configuration	Relatively discrete pathway	Broader branching system
Major fascicles	Not conventionally divided into anterior and posterior fascicles	Anterior and posterior fascicles, with additional septal branching
Special landmark	Associated with moderator band	Broad distribution along left interventricular septum
Distal continuation	Right ventricular Purkinje network	Left ventricular Purkinje network

Key Features

Feature	Key Point
Origin	Bundle of His
Location	Interventricular septal conduction system
Primary function	Rapid distribution of impulses toward both ventricles
Right branch destination	Right ventricular Purkinje system
Left branch destination	Left ventricular fascicles and Purkinje system
Action potential type	Fast-response
ECG relationship	Essential to normal rapid QRS activation
Major abnormalities	RBBB, LBBB and fascicular blocks

Anatomical and Physiological Importance

The right and left bundle branches form the principal rapid-conduction pathways connecting the bundle of His with the ventricular Purkinje networks. Their location along the interventricular septum permits electrical impulses to reach the apical and ventricular free-wall regions rapidly.

The right bundle branch follows a relatively focused pathway and has an important relationship with the moderator band. The left bundle branch forms a broader network with anterior, posterior, and variable septal pathways that distribute excitation throughout the larger left ventricle.

Normal conduction through both branches is essential for coordinated ventricular depolarization and contraction. Block in either branch alters the normal activation sequence, changes QRS morphology, and can produce mechanical dyssynchrony. Their anatomy is therefore important in electrocardiography, electrophysiology, ischemic heart disease, cardiac surgery, valve interventions, and modern conduction-system pacing.