The heart’s electrical conduction system ensures that atrial impulses efficiently propagate to the ventricles, leading to synchronized contraction and optimal cardiac output. Disruptions in this system, particularly within the bundle branches or fascicles, result in bundle branch blocks (BBBs) and fascicular blocks, which are detectable on an electrocardiogram (ECG). Understanding these conduction disturbances is crucial for clinicians to interpret ECGs accurately, assess underlying cardiac pathology, and guide management.
This comprehensive post explores the types, pathophysiology, ECG patterns, clinical significance, and management of bundle branch and fascicular blocks, along with their differentiation and clinical implications.
1. Introduction to Bundle Branch Blocks
1.1 Definition
A bundle branch block occurs when there is delayed or blocked conduction along the right or left bundle branches of the His-Purkinje system. As a result, ventricular depolarization is altered, leading to characteristic changes in the QRS complex on ECG.
- Normal QRS duration: < 120 ms
- BBB QRS duration: ≥ 120 ms (0.12 seconds)
- BBBs can occur in isolation or in the context of ischemic heart disease, cardiomyopathy, hypertension, or structural heart disease.
1.2 Anatomy of the Conduction System
- SA node: Initiates electrical impulses.
- AV node: Delays conduction to allow ventricular filling.
- His bundle: Conducts impulses from AV node to ventricles.
- Right bundle branch (RBB): Conducts impulses to the right ventricle.
- Left bundle branch (LBB): Divides into:
- Left anterior fascicle (LAF)
- Left posterior fascicle (LPF)
- These fascicles transmit impulses to respective ventricular walls.
1.3 Pathophysiology
- Blockage in a bundle branch delays activation of the corresponding ventricle.
- Depolarization spreads via the remaining conduction pathways, leading to a widened QRS and secondary ST-T changes.
- Blocks can be complete or incomplete, depending on the degree of conduction delay.
2. Right Bundle Branch Block (RBBB)
2.1 Definition
RBBB is characterized by delayed right ventricular depolarization, while left ventricular conduction remains normal.
2.2 ECG Criteria for RBBB
- QRS duration ≥ 120 ms (complete); 100–119 ms for incomplete RBBB
- Lead V1–V3: rsR’ pattern (“rabbit ears”) or RSR’
- Lead I, V6: Wide S wave
- Secondary ST-T changes: T wave inversion in right precordial leads
2.3 Pathophysiology
- The impulse travels through the left bundle to depolarize the left ventricle first.
- The right ventricle is depolarized later through myocardial cell-to-cell conduction, producing the delayed R wave in V1–V3.
2.4 Clinical Causes
- Normal variant in some healthy individuals
- Ischemic heart disease (especially right ventricular infarction)
- Pulmonary embolism
- Pulmonary hypertension
- Myocarditis or cardiomyopathy
- Post-surgical (after cardiac surgery)
2.5 Clinical Significance
- Often benign if isolated
- May indicate underlying structural heart disease if symptomatic or associated with left axis deviation
- Rarely causes conduction-related complications by itself
3. Left Bundle Branch Block (LBBB)
3.1 Definition
LBBB is defined by delayed left ventricular depolarization, leading to characteristic ECG changes and prolonged QRS duration.
3.2 ECG Criteria for LBBB
- QRS duration ≥ 120 ms
- Lead V1: Predominantly negative QS or rS complex
- Lead I and V6: Broad, notched (“M-shaped”) R waves
- ST-T changes: Discordant T waves (opposite direction to QRS)
- Absence of Q waves in left-sided leads (I, V5–V6)
3.3 Pathophysiology
- Right ventricle depolarizes first.
- Impulse spreads to the left ventricle via myocardial tissue, causing prolonged QRS and altered vector orientation.
3.4 Clinical Causes
- Hypertension and left ventricular hypertrophy
- Coronary artery disease or myocardial infarction
- Cardiomyopathy (dilated or hypertrophic)
- Degenerative conduction system disease
- Post-surgical or post-transcatheter procedures
3.5 Clinical Significance
- Often indicates underlying heart disease
- LBBB can mask acute myocardial infarction on ECG
- Associated with increased risk of heart failure and mortality
- May warrant cardiac resynchronization therapy (CRT) in patients with systolic heart failure
4. Fascicular Blocks (Hemiblocks)
Fascicular blocks involve conduction delay in individual fascicles of the left bundle branch. The most common are left anterior fascicular block (LAFB) and left posterior fascicular block (LPFB).
4.1 Left Anterior Fascicular Block (LAFB)
4.1.1 ECG Features
- Left axis deviation (-45° to -90°)
- qR pattern in leads I and aVL
- rS pattern in leads II, III, and aVF
- QRS duration slightly prolonged (<120 ms)
- Normal R wave progression in precordial leads
4.1.2 Pathophysiology
- Block in left anterior fascicle delays activation of anterior and superior portions of the left ventricle.
- Electrical vector shifts leftward, producing characteristic left axis deviation.
4.1.3 Clinical Causes
- Hypertension with left ventricular hypertrophy
- Ischemic heart disease
- Cardiomyopathy
- Degenerative conduction system disease
4.1.4 Clinical Significance
- Often asymptomatic
- May indicate underlying cardiac pathology
- LAFB + RBBB = bifascicular block, which has prognostic significance
4.2 Left Posterior Fascicular Block (LPFB)
4.2.1 ECG Features
- Right axis deviation (+90° to +180°)
- rS pattern in leads I and aVL
- qR pattern in leads II, III, and aVF
- QRS duration slightly prolonged (<120 ms)
- Rare compared to LAFB
4.2.2 Pathophysiology
- Block in left posterior fascicle delays activation of the posterior and inferior left ventricle.
- Electrical vector shifts rightward, producing right axis deviation.
4.2.3 Clinical Causes
- Rarely occurs in isolation
- Usually associated with structural heart disease (ischemia, infarction)
- Degenerative conduction disease
4.2.4 Clinical Significance
- Less common but may signal severe conduction disease
- LPFB + RBBB = bifascicular block, similar prognostic implications as LAFB + RBBB
5. Bifascicular and Trifascicular Blocks
5.1 Bifascicular Block
- Involves two conduction pathways:
- RBBB + LAFB (most common)
- RBBB + LPFB (less common)
- ECG shows:
- RBBB pattern (V1–V3)
- Axis deviation corresponding to fascicular block
5.2 Trifascicular Block
- Involves all three pathways:
- RBBB + LAFB or LPFB + first-degree AV block
- May be intermittent or complete
- Associated with high risk of complete heart block
- Symptomatic patients often require permanent pacing
6. Causes of Bundle Branch and Fascicular Blocks
6.1 Structural Heart Disease
- Coronary artery disease
- Myocardial infarction
- Cardiomyopathies (dilated, hypertrophic)
- Valvular heart disease
6.2 Degenerative Conduction System Disease
- Lenègre’s disease
- Lev’s disease
- Age-related fibrosis of the His-Purkinje system
6.3 Inflammatory or Infiltrative Diseases
- Myocarditis
- Sarcoidosis
- Amyloidosis
6.4 Iatrogenic Causes
- Post-cardiac surgery
- Post-catheter ablation
- Post-device implantation
6.5 Electrolyte and Drug-Induced Causes
- Hyperkalemia
- Sodium-channel blocking drugs
- Beta-blockers (rarely cause conduction delay)
7. ECG Interpretation and Diagnosis
7.1 Systematic Approach
- Check QRS duration: >120 ms indicates BBB
- Identify bundle branch: RBBB vs LBBB based on precordial leads
- Check frontal plane axis: Detect fascicular blocks
- Look for associated AV blocks: First-degree or higher
- Assess for secondary ST-T changes: Opposite to QRS deflection
7.2 Differential Diagnosis
- Ventricular hypertrophy (can mimic BBB)
- Ventricular ectopy or paced rhythm
- Hyperkalemia
- Acute myocardial infarction (especially LBBB masks MI)
8. Clinical Implications
8.1 Right Bundle Branch Block
- Often benign if isolated
- May indicate pulmonary embolism or RV strain
- In acute MI, RBBB is a marker of extensive infarction if associated with LAD occlusion
8.2 Left Bundle Branch Block
- Usually indicates underlying structural heart disease
- Complicates ECG diagnosis of MI
- Associated with increased risk of heart failure and mortality
- CRT is considered in symptomatic LBBB with heart failure
8.3 Fascicular Blocks
- Often asymptomatic but markers of conduction disease
- Presence with RBBB (bifascicular block) may precede complete AV block
- Requires close monitoring for symptomatic bradycardia or syncope
9. Management
9.1 General Principles
- Treat underlying cause (ischemia, hypertension, heart failure)
- Monitor asymptomatic patients
- Symptomatic patients require evaluation for pacing if high-grade block suspected
9.2 Pacing Indications
- Symptomatic bifascicular block with syncope
- Trifascicular block with intermittent AV block
- Complete heart block or advanced conduction disease
9.3 Medical Therapy
- Optimize heart failure treatment
- Avoid drugs that worsen conduction delay (beta-blockers, antiarrhythmics) unless necessary
10. Prognosis
- Isolated RBBB or LAFB: Generally benign if asymptomatic
- LBBB: Higher mortality, especially with heart failure or ischemic heart disease
- Bifascicular or trifascicular blocks: Risk of progression to complete heart block
- Symptomatic blocks: Require permanent pacing for survival and quality of life
11. Key ECG Patterns Summary
| Block Type | ECG Features | Clinical Significance |
|---|---|---|
| RBBB | QRS ≥120 ms, rsR’ in V1, wide S in I, V6 | Usually benign, may indicate RV strain |
| LBBB | QRS ≥120 ms, broad notched R in I, V5-V6 | Often structural heart disease, may mask MI |
| LAFB | Left axis deviation, qR in I, aVL | Often benign, marker of conduction disease |
| LPFB | Right axis deviation, qR in II, III, aVF | Rare, marker of underlying disease |
| Bifascicular Block | RBBB + LAFB or RBBB + LPFB | Risk of progression to complete block |
| Trifascicular Block | Bifascicular + 1° AV block | High risk, often requires pacing |
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