Bundle Branch Block: Types, Pathophysiology, Causes, Symptoms, Diagnosis, Management and Treatment

Bundle Branch Block

~Introduction

The human heart relies on a finely coordinated electrical system that ensures each heartbeat is timed and sequenced properly. Any disruption in this electrical conduction can impair cardiac efficiency and, in some cases, signal underlying disease. One such conduction disturbance is Bundle Branch Block (BBB) — a condition where there is delayed or complete interruption of the electrical impulse as it travels through one of the heart’s bundle branches.

Bundle branch blocks are common findings on electrocardiograms (ECGs). They may occur in healthy individuals without symptoms or may indicate an underlying cardiac disorder requiring further evaluation. Understanding the anatomy, types, causes, symptoms, diagnosis, and management is crucial for recognizing the clinical significance of BBB.

~Understanding the Cardiac Conduction System

To appreciate how bundle branch block arises, one must first understand the normal electrical conduction of the heart:

1. Sinoatrial (SA) Node – the natural pacemaker initiating impulses.

2. Atrioventricular (AV) Node – slows the impulse, allowing atrial emptying.

3. Bundle of His – conducts the impulse to the ventricles.

4. Right and Left Bundle Branches – electrical “highways” sending signals to each ventricle.

5. Purkinje Fibers – enable simultaneous, rapid contraction of ventricular muscle.

When either the right bundle branch or one of the left bundle branches becomes injured or impaired, the impulses travel through muscle fibers instead of the optimized conduction pathways, slowing electrical transmission.

~What Is Bundle Branch Block?

Bundle Branch Block (BBB) is a condition where there is a delay or obstruction in electrical conduction through one of the bundle branches. This delay alters the sequence of ventricular activation:

• The side with intact conduction contracts normally.

• The side with the block contracts later, receiving the impulse indirectly.

Types of Bundle Branch Block

1. Right Bundle Branch Block (RBBB)

2. Left Bundle Branch Block (LBBB)

3. Left Anterior Fascicular Block (LAFB)

4. Left Posterior Fascicular Block (LPFB)

5. Bifascicular Block

6. Trifascicular Block

While RBBB and LBBB are the most discussed, fascicular blocks are equally important components of the left bundle branch pathology.

~Right Bundle Branch Block (RBBB)

Pathophysiology

In RBBB, the electrical impulse cannot travel through the right bundle branch. The left ventricle activates normally, and the impulse then spreads to the right ventricle via myocardial cell-to-cell conduction. This results in delayed right ventricular depolarization.

ECG Features

• QRS duration ≥ 120 ms

• RSR' pattern in V1 and V2 (“rabbit ears”)

• Wide, slurred S waves in leads I and V6

Causes

RBBB may appear in:

• Normal individuals (benign in many cases)

• Congenital heart disease (e.g., atrial septal defect)

• Pulmonary embolism

• Right ventricular hypertrophy

• Myocarditis

• Ischemic heart disease

Clinical Significance

Isolated RBBB often has minimal clinical consequences. However, if associated with symptoms or underlying heart disease, further evaluation is necessary.

~Left Bundle Branch Block (LBBB)

Pathophysiology

LBBB results from delayed conduction along the left bundle branch, causing the right ventricle to depolarize first. The left ventricle receives impulses later, leading to significant dyssynchrony of ventricular contraction.

ECG Features

• QRS duration ≥ 120 ms

• Broad, notched or slurred R waves in leads I, V5, V6

• Deep, broad S waves in V1–V3

• Absence of normal septal Q waves in lateral leads

Causes

LBBB is commonly associated with structural heart disease:

• Hypertension with left ventricular hypertrophy

• Coronary artery disease or myocardial infarction

• Cardiomyopathy (dilated or hypertrophic)

• Aortic stenosis

• Degenerative conduction system disease

Clinical Significance

LBBB is usually more concerning than RBBB because it often points to significant underlying cardiac pathology. It may also interfere with diagnosing myocardial infarction since ST-segment patterns are altered.

~Fascicular Blocks

The left bundle branch divides into two fascicles: anterior and posterior.

Left Anterior Fascicular Block (LAFB)

• Causes left axis deviation

• Often due to ischemia, hypertension, or fibrosis

Left Posterior Fascicular Block (LPFB)

• Causes right axis deviation

• Rarer due to strong blood supply to the posterior fascicle

Bifascicular Block

Involves any two fascicles (example: RBBB + LAFB).

Trifascicular Block

Impairment of all three fascicles, potentially leading to complete heart block.

~Causes of Bundle Branch Block

1. Degenerative Fibrosis

Age-related fibrosis is one of the most common causes, especially in LBBB.

2. Ischemic Heart Disease

• Myocardial infarction

• Chronic coronary artery disease
  LBBB may appear suddenly during acute MI.

3. Hypertensive Heart Disease

Long-standing hypertension causes ventricular hypertrophy and conduction delays.

4. Cardiomyopathies

Dilated or infiltrative cardiomyopathies (amyloidosis, sarcoidosis) impair conduction pathways.

5. Congenital Heart Disease

• Tetralogy of Fallot repairs may cause RBBB.

• ASD often associated with RBBB.

6. Pulmonary Disorders

RBBB may occur in:

• Pulmonary embolism

• Chronic obstructive pulmonary disease (COPD)

• Pulmonary hypertension

7. Iatrogenic Causes

• Cardiac surgery

• Catheter-based interventions

• Ablations involving septal areas

8. Medications

Some antiarrhythmics (e.g., flecainide) may widen QRS and mimic BBB.

9. Electrolyte Disturbances

Severe hyperkalemia may cause conduction delays resembling BBB.

~Symptoms and Clinical Presentation

Many individuals with BBB have no symptoms, especially with isolated RBBB. When symptoms occur, they may include:

Common Symptoms

• Fatigue

• Dizziness

• Presyncope or syncope

• Palpitations

• Dyspnea

LBBB-Specific Symptoms

LBBB can cause:

• Exercise intolerance

• Worsening heart failure due to dyssynchronous ventricular contraction

• Chest pain, especially if associated with ischemia

When Is Bundle Branch Block Dangerous?

A BBB can become concerning when:

• It develops suddenly with chest pain (possible myocardial infarction)

• It accompanies syncope (suggests conduction system disease)

• It is part of bifascicular or trifascicular block

• It occurs in patients with known cardiomyopathy

~Diagnosis

1. Electrocardiogram (ECG)

This is the primary tool for diagnosing BBB. Each type has characteristic QRS patterns and durations. The QRS widening (>120 ms) is a hallmark.

2. Echocardiography

To assess:

• Ejection fraction

• Wall motion abnormalities

• Structural heart disease

3. Cardiac MRI

Useful for detecting:

• Scar tissue

• Myocardial fibrosis

• Infiltrative diseases

4. Stress Testing

Stress ECG may be unreliable in LBBB; imaging modalities (stress echo or nuclear scan) are preferred.

5. Electrophysiological (EP) Study

In selected cases, to evaluate conduction impairment severity.

6. Blood Tests

To rule out:

• Myocardial infarction

• Electrolyte disturbances

• Thyroid dysfunction

~Complications of Bundle Branch Block

1. Progression to Complete Heart Block
   Especially in bifascicular or trifascicular blocks.

2. Heart Failure Exacerbation
   LBBB causes dyssynchrony, reducing cardiac efficiency.

3. Arrhythmias
   Atrial fibrillation or ventricular arrhythmias may coexist.

4. Increased Mortality in Certain Conditions
   LBBB in patients with MI or heart failure carries worse prognostic implications.

~Management and Treatment

Treatment depends on:

• Type of BBB

• Presence of symptoms

• Underlying cardiac conditions

1. Management of Underlying Disease

• Treat hypertension, coronary artery disease, or valvular diseases.

• Manage heart failure with standard therapy (ACE inhibitors, beta-blockers, diuretics).

2. Pacemaker Therapy

Indicated when BBB contributes to significant conduction block or symptoms.

Pacemaker Indications

• Symptomatic bradycardia with BBB

• Bifascicular or trifascicular block with syncope

• Alternating bundle branch block

• High-grade AV block with wide QRS

3. Cardiac Resynchronization Therapy (CRT)

CRT uses a biventricular pacemaker to correct electrical dyssynchrony, especially in LBBB.

Indications for CRT

• LBBB with QRS ≥150 ms

• Left ventricular ejection fraction ≤35%

• Symptomatic heart failure (NYHA II–IV)
  CRT significantly improves survival and symptoms.

4. Medication Adjustments

Avoid drugs that slow conduction in susceptible individuals.

5. Monitoring

• Regular ECGs

• Holter monitoring for intermittent conduction issues

• Follow-up echocardiograms in structural disease

~Prognosis

Right Bundle Branch Block

• Often benign in healthy individuals.

• Prognosis depends on underlying heart disease.

Left Bundle Branch Block

• Usually signifies structural heart disease.

• Associated with higher risk of heart failure, arrhythmias, and mortality.

Bifascicular and Trifascicular Blocks

• Higher risk of progressing to complete heart block.

• Often require pacemaker placement.

~Bundle Branch Block in Special Populations

1. Athletes

RBBB may appear due to increased vagal tone and enlarged right ventricles. Usually benign, but requires evaluation.

2. Pregnant Women

BBB may appear due to hemodynamic and hormonal changes. Evaluation typically includes ECG and echo.

3. Elderly

Degenerative conduction system disease is common; LBBB increases heart failure risk.

4. Patients with Acute MI

New LBBB may mask ST-segment elevation. Such cases are managed as potential emergencies.

~Preventive Measures

While BBB itself may not always be preventable, preventing underlying heart disease helps reduce risk.

Recommendations

• Control blood pressure

• Manage diabetes and cholesterol

• Maintain a heart-healthy lifestyle

• Avoid tobacco and excessive alcohol

• Regular cardiac checkups if at risk

~Conclusion

Bundle Branch Block represents a spectrum of conduction disturbances ranging from benign variants to markers of serious cardiac disease. While many individuals, especially those with isolated RBBB, may lead normal lives without symptoms, LBBB and multifascicular blocks warrant careful assessment due to their association with structural heart disease and risk of progression to complete heart block.

Modern diagnostic tools, especially ECG and echocardiography, allow precise detection and evaluation of BBB. Treatments range from managing underlying causes to advanced therapies such as cardiac resynchronization therapy for patients with LBBB and heart failure.

Early recognition and appropriate management are key to preventing complications, improving symptoms, and optimizing the quality of life for individuals with bundle branch block.