Cardiology

Background 

Conduction disorders refer to abnormalities in the transmission of electrical impulses through the specialized conduction pathways of the heart. Left Bundle Branch Block (LBBB) and Right Bundle Branch Block (RBBB) are intraventricular conduction delays characterized by delayed depolarization of the respective ventricles, resulting in characteristic patterns on electrocardiography (ECG). These blocks may be benign or indicate underlying structural heart disease. 

II) Classification/Types

By Anatomic Location: 

• Right Bundle Branch Block (RBBB): Delay in right ventricular activation due to blockage in the right bundle branch. 

• Left Bundle Branch Block (LBBB): Delay in left ventricular activation due to interruption in the left bundle branch. 

By Duration: 

• Intermittent Bundle Branch Block: Occurs sporadically with variable conduction. 

• Persistent Bundle Branch Block: Consistently present across ECGs. 

By Pathophysiology: 

• Rate-Dependent Block: Appears only at certain heart rates (e.g., tachycardia-dependent). 

• Fixed Block: Present regardless of heart rate. 

Pathophysiology 

Bundle branch blocks result from impaired conduction through the His-Purkinje system, delaying the normal activation of the ventricles. In RBBB, depolarization proceeds normally through the left bundle and then spreads slowly to the right ventricle via muscle-to-muscle conduction. In LBBB, the right ventricle is activated normally, but the left ventricle receives the impulse via abnormal, slow conduction, significantly altering the sequence of ventricular depolarization and contraction. 

Epidemiology 

• RBBB is more common in healthy individuals and may be incidental. 

• LBBB is often associated with structural heart disease and is a marker of worse prognosis. 

• Prevalence increases with age and comorbid cardiovascular disease. 

• LBBB is more frequently associated with heart failure, ischemic heart disease, or cardiomyopathy. 

Etiology 

I) Causes

Right Bundle Branch Block: 

• Right ventricular strain (e.g., pulmonary embolism) 

• Ischemic heart disease 

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

• Degenerative conduction system disease 

• Post-cardiac surgery or device implantation 

Left Bundle Branch Block: 

• Hypertension with left ventricular hypertrophy 

• Coronary artery disease (especially with anterior MI) 

• Dilated cardiomyopathy 

• Aortic stenosis 

• Fibrosis of the conduction system (Lenègre or Lev disease) 

II) Risk Factors

• Advanced age 

• Diabetes mellitus 

• Hypertension 

• Heart failure 

• Previous myocardial infarction 

• Structural heart abnormalities 

Clinical Presentation 

I) History (Symptoms)

• Often asymptomatic and detected incidentally on ECG. 

• May present with: 

• Syncope or presyncope (in bifascicular or trifascicular block) 

• Exertional dyspnea or fatigue (in LBBB with cardiomyopathy) 

• Chest pain (coexistent ischemia) 

II) Physical Exam (Signs)

• Normal findings if asymptomatic 

• Irregular or slow pulse in associated bradyarrhythmias 

• Signs of heart failure in LBBB with left ventricular dysfunction 

Differential Diagnosis (DDx) 

• Myocardial infarction with QRS widening 

• Ventricular pacing rhythm 

• Ventricular tachycardia 

• Wolff-Parkinson-White syndrome 

• Nonspecific intraventricular conduction delay (IVCD) 

Diagnostic Tests 

Initial Work-Up 

• 12-lead ECG: 

• RBBB: QRS ≥120 ms, rsR′ in V1, wide S wave in V6 

• LBBB: QRS ≥120 ms, broad/notched R wave in I, V5–V6; absent Q wave in I, V6; discordant ST-T changes 

• Echocardiography: Assess for structural heart disease and systolic function 

• Cardiac enzymes: If acute MI suspected, especially in new-onset LBBB 

Advanced Testing 

• Holter monitor: If intermittent symptoms (e.g., syncope or palpitations) 

• Electrophysiology study (EPS): For suspected conduction system disease or syncope 

• Cardiac MRI: Evaluate for fibrosis or infiltrative disease (e.g., sarcoidosis) 

Treatment 

I) Acute Management

• New-onset LBBB with chest pain: Treat as STEMI equivalent; consider urgent catheterization 

• Syncope with Bifascicular Block: Evaluate for high-grade AV block; temporary pacing if unstable 

II) Chronic Management

• Treat underlying condition (e.g., control hypertension, manage CAD or cardiomyopathy) 

• Permanent pacemaker if symptomatic bradycardia or advanced AV block 

• Consider Cardiac Resynchronization Therapy (CRT) for LBBB with reduced EF 

Medications 

Device Therapy 

• Permanent Pacemaker: Indicated for symptomatic bradycardia or high-grade AV block 

• Cardiac Resynchronization Therapy (CRT): Indicated in LBBB with: 

• QRS ≥150 ms 

• LVEF ≤35% 

• NYHA class II–IV on optimal therapy 

Patient Education, Screening, Vaccines 

Education 

• Report new symptoms: syncope, palpitations, exertional dyspnea 

• Importance of medication and device adherence 

• Monitoring blood pressure and blood sugar 

Screening/Prevention 

• ECG surveillance in patients with known cardiac disease 

• Echocardiography for EF monitoring in LBBB 

• Holter/event monitor for unexplained syncope 

Vaccinations 

• Influenza and pneumococcal vaccines for all cardiac patients 

Consults/Referrals 

• Cardiology: For diagnostic confirmation, echo interpretation, pacing decisions 

• Electrophysiology: If device therapy or EPS needed 

• Heart Failure Specialist: In LBBB with reduced EF 

Follow-Up 

Short-Term 

• Monitor for arrhythmias or symptom progression 

• Evaluate pacemaker function if implanted 

Long-Term 

• Periodic ECG and echo to assess progression or response to therapy 

• CRT follow-up: optimization of device settings and symptom review 

Prognosis 

• RBBB: Often benign if isolated 

• LBBB: Associated with worse prognosis due to underlying structural disease 

• Prognosis improves with early CRT in eligible LBBB patients 

• Syncope or progression to AV block increases risk for sudden cardiac death