The Bundle of His (atrioventricular bundle) is the specialized conduction pathway that connects the AV node to the ventricular myocardium. It divides into left and right bundle branches that distribute the electrical impulse to both ventricles.
Bundle of His (AV Bundle)
Location
The Bundle of His originates from the distal AV node (NH zone) and penetrates the central fibrous body of the cardiac skeleton.
Course:
- Passes through the right fibrous trigone (central fibrous body)
- Crosses the membranous interventricular septum
- Travels along the posterior-inferior border of the membranous septum
- Divides into left and right bundle branches at the septal crest
Relations
| Structure | Relationship |
|---|---|
| Superior | Membranous interventricular septum |
| Inferior | Muscular interventricular septum |
| Left | Left ventricle (subendocardial) |
| Right | Right atrium (tricuspid septal leaflet) |
| Anterior | Aortic valve (right and non-coronary cusps) |
Dimensions
- Length: 10-20 mm (from AV node to bifurcation)
- Diameter: 1-3 mm
- Fiber count: Several thousand specialized conduction cells
Microscopic Anatomy
The Bundle of His consists of specialized Purkinje-like cells:
- Larger than working myocytes
- Pale appearance (few myofibrils)
- Rich in glycogen
- Well-developed gap junctions (connexin 40, 43)
- Surrounded by collagen sheath (electrical insulation)
Penetrating vs. Non-Penetrating Portion
Penetrating portion:
- Passes through the central fibrous body
- Electrically insulated by fibrous tissue
- Protected from surrounding atrial and ventricular activity
Non-penetrating (branching) portion:
- Emerges on the ventricular side of the fibrous skeleton
- Begins to divide into bundle branches
- More susceptible to compression
Blood Supply
Arterial supply:
- Primary: AV nodal artery (RCA or LCx)
- Secondary: First septal perforator of the LAD
Clinical significance:
- Dual blood supply protects against complete heart block
- Both LAD and RCA disease may be needed to cause His bundle ischemia
Right Bundle Branch (RBB)
Origin and Course
The right bundle branch is a direct continuation of the Bundle of His. It courses along the right side of the interventricular septum.
Course segments:
- Proximal: Passes inferior to the membranous septum
- Mid: Courses through the septal myocardium (intramyocardial)
- Distal: Emerges at the base of the anterior papillary muscle
- Terminal: Continues via the moderator band to the anterior right ventricular wall
Moderator Band
The moderator band (septomarginal trabecula) is a muscular band that carries the right bundle branch from the septum to the anterior papillary muscle.
Function:
- Conducts the impulse to the anterior right ventricular wall
- Provides mechanical support (prevents overdistension of the RV)
Supply Territory
| Region | Activation |
|---|---|
| Interventricular septum (right side) | Right bundle branch |
| Right ventricular free wall | Right bundle branch (via moderator band) |
| Right ventricular apex | Distal right bundle branch |
| Anterior papillary muscle | Right bundle branch |
Conduction Properties
- Conduction velocity: 1-2 m/s
- Thinner and more fragile than the left bundle branch
- More susceptible to block from minor insults
Left Bundle Branch (LBB)
Origin and Bifurcation
The left bundle branch emerges from the Bundle of His as a broad, fan-like structure on the left side of the interventricular septum. It typically divides into three fascicles:
Anterior Fascicle:
- Courses to the anterior papillary muscle
- Supplies the anterior and superior left ventricle
- Thin, long fascicle (more susceptible to block)
Posterior Fascicle:
- Courses to the posterior papillary muscle
- Supplies the posterior and inferior left ventricle
- Short, thick fascicle (less susceptible to block)
Septal Fascicle:
- Courses to the mid-septum
- Supplies the septal wall
- Variable presence (present in 50-70% of hearts)
Supply Territory
| Fascicle | Region | ECG Leads |
|---|---|---|
| Anterior | Anterior LV, anterolateral papillary muscle | I, aVL |
| Posterior | Inferior LV, posteromedial papillary muscle | II, III, aVF |
| Septal | Interventricular septum (left side) | V1, V2 |
Conduction Properties
- Conduction velocity: 2-4 m/s
- Broad, fan-like origin (redundant connections)
- More resistant to block than the right bundle branch
Conduction Velocity
| Structure | Conduction Velocity |
|---|---|
| Bundle of His | 1-2 m/s |
| Right bundle branch | 1-2 m/s |
| Left bundle branch | 2-4 m/s |
| Purkinje fibers | 2-4 m/s |
Ventricular Activation Sequence
- Septal activation: Left bundle branch activates the septum first (20 ms)
- Apical activation: Impulse spreads to the apex (40 ms)
- Free wall activation: Spreads to the lateral walls (60 ms)
- Basal activation: Last regions activated (80-100 ms)
Blood Supply
| Structure | Primary Supply | Secondary Supply |
|---|---|---|
| Bundle of His | AV nodal artery | First septal perforator |
| Right bundle branch | First septal perforator (LAD) | AV nodal artery (proximal) |
| Left anterior fascicle | First septal perforator (LAD) | Multiple perforators |
| Left posterior fascicle | Posterior descending artery | LAD septal perforators |
Clinical Significance
Right Bundle Branch Block (RBBB)
Causes:
- Normal variant (especially in young athletes)
- Right ventricular hypertrophy (cor pulmonale)
- Pulmonary embolism
- Congenital heart disease (ASD, VSD)
- Ischemic heart disease (LAD occlusion)
- Cardiac surgery
ECG findings:
- QRS > 120 ms
- rSR pattern in V1-V2 (rabbit ears)
- Wide, slurred S wave in I, V6
- ST-T discordance (appropriate discordance)
Prognosis:
- New RBBB: May indicate underlying heart disease
- RBBB with anterior MI: Higher mortality
- RBBB alone: Generally benign
Left Bundle Branch Block (LBBB)
Causes:
- Hypertension (most common)
- Aortic stenosis
- Ischemic heart disease
- Dilated cardiomyopathy
- Lenegre disease (primary conduction disease)
- Cardiac surgery
ECG findings:
- QRS > 120 ms
- Broad, notched R wave in I, V5, V6
- Absent Q waves in I, V5, V6
- ST-T discordance
- Left axis deviation (not always)
Prognosis:
- LBBB independently predicts mortality
- Associated with structural heart disease
- May indicate underlying cardiomyopathy
- LBBB induces dyssynchronous LV contraction
Hemifascicular Blocks
Left Anterior Fascicular Block (LAFB):
- Left axis deviation (-45 to -90 degrees)
- Normal QRS duration
- qR in I, rS in II, III, aVF
Left Posterior Fascicular Block (LPFB):
- Right axis deviation (+90 to +180 degrees)
- Normal QRS duration
- rS in I, qR in II, III, aVF
Bifascicular and Trifascicular Block
Bifascicular block: RBBB + LAFB (most common) or RBBB + LPFB
Trifascicular block: Bifascicular block + first-degree AV block
Prognosis:
- Trifascicular block has higher risk of progression to complete heart block
- May require prophylactic pacemaker
- Annual risk of complete heart block: 1-5%
Cardiac Resynchronization Therapy (CRT)
LBBB causes dyssynchronous ventricular contraction:
- Septum contracts before the lateral wall
- Lateral wall stretches during septal contraction
- Reduced LV ejection fraction
CRT improves synchrony by pacing both ventricles simultaneously:
- LV lead placed via coronary sinus (posterolateral vein)
- RV lead placed in the apex or septum
- Indicated in LBBB with EF < 35%
Catheter Ablation Near the His Bundle
His bundle ablation:
- For atrial fibrillation with rapid ventricular rates
- Creates complete heart block
- Pacemaker-dependent
His bundle pacing:
- Physiologic pacing (preserves normal ventricular activation)
- More difficult than RV pacing
- Higher pacing thresholds, lower sensed R waves
Congenital Complete Heart Block
- Complete AV block present at birth
- Associated with maternal lupus (anti-Ro/SSA antibodies)
- The Bundle of His is replaced by fibrous tissue
- Requires pacemaker implantation