How do left bundle branch block (LBBB) and right bundle branch block (RBBB) differ on a 12‑lead ECG and in their clinical significance?

LBBB vs RBBB: ECG Differentiation and Clinical Significance

ECG Diagnostic Criteria

The key to distinguishing RBBB from LBBB is examining QRS morphology in leads V1 and V6: RBBB shows an rSR' (M-shaped) pattern in V1 with wide S waves in V6, while LBBB shows broad notched R waves in V6 with deep S waves in V1. 1

RBBB ECG Features

• QRS duration ≥120 ms 1
• rSR', rsR', or rSR' pattern in V1-V2 (M-shaped or "rabbit ears") 1, 2
• Wide S waves in leads I and V6 (>40 ms or longer than R wave) 2
• R peak time >50 ms in V1 but normal in V5-V6 1, 2
• Septal Q waves preserved in leads I, V5-V6 1

LBBB ECG Features

• QRS duration ≥120 ms 1
• Broad notched or slurred R waves in leads I, aVL, V5, and V6 1, 2
• Deep S waves or QS pattern in V1 1
• Absent Q waves in leads I, V5, and V6 (due to abnormal right-to-left septal activation) 1, 3
• R peak time >60 ms in V5-V6 1, 2

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Clinical Significance: A Critical Distinction

LBBB is far more ominous than RBBB and represents a strong marker of underlying structural cardiovascular disease, potentially indicating ischemic heart disease or cardiomyopathy years before structural changes become detectable. 1, 2

LBBB Clinical Implications

• Very rare in healthy individuals and almost always pathologic 4, 2
• Strong association with structural heart disease: may be the first manifestation of ischemic heart disease, cardiomyopathy, hypertensive heart disease, or diffuse myocardial disease 4, 2, 3
• Up to 90% of LBBB patients have anatomic left ventricular hypertrophy at autopsy 4, 1
• Induces ventricular dyssynchrony leading to abnormal contraction patterns, asymmetric hypertrophy, and potential LV dilatation 3
• Higher acute complications: In acute MI, LBBB patients have the highest rates of pulmonary edema (18.8%) and cardiogenic shock compared to RBBB (12.0%) or isolated STEMI (7.9%) 5
• Even asymptomatic LBBB mandates cardiology evaluation given strong disease association 2

RBBB Clinical Implications

• May represent benign isolated finding in up to 1% of the general population, particularly in younger individuals 1, 2
• Can be idiopathic and clinically benign or represent underlying pathology (ischemic disease, cardiomyopathies, congenital heart disease, ARVC) 4, 2
• Less commonly associated with structural disease than LBBB 4
• Prognostic significance depends on LV function: RBBB predicts increased mortality primarily in patients with reduced LV systolic function (wall motion index ≤1.5), whereas LBBB predicts mortality even with preserved LV function 6

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Diagnostic Workup Requirements

Both complete RBBB and LBBB require comprehensive cardiological evaluation, but the urgency and concern are greater with LBBB. 4, 2

Mandatory Evaluation for Bundle Branch Block

• Exercise stress testing to assess for exercise-induced arrhythmias or conduction worsening 4, 2
• 24-hour Holter monitoring to detect intermittent conduction abnormalities 4, 2
• Echocardiography to evaluate for structural heart disease including cardiomyopathies, ischemic disease, hypertensive heart disease, and ARVC 4, 2

Special Considerations

• In young athletes with bifascicular block (LBBB or RBBB with left posterior hemiblock), obtain ECG in siblings to exclude genetically determined progressive cardiac conduction disease (Lenegre disease) 2
• Intermittent or rate-dependent LBBB has the same clinical and prognostic significance as stable LBBB 4, 2

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Impact on Other ECG Interpretations

Acute MI Diagnosis

• RBBB preserves ST-segment interpretation for acute MI diagnosis 1
• LBBB significantly complicates ECG diagnosis of acute MI 1
• RBBB with concomitant ST elevation carries particularly high mortality risk (OR 1.77) compared to isolated STEMI 5

LVH Diagnosis

• LBBB makes LVH diagnosis nearly impossible on ECG 1
• Only diagnose LVH with LBBB if: QRS duration >155 ms, left atrial abnormality present, AND precordial voltage criteria met—though sensitivity remains low 4, 2
• RBBB reduces sensitivity for LVH detection by reducing S wave amplitude in right precordial leads 4, 2
• With RBBB, use ancillary features: left atrial abnormality, left axis deviation, and specific criteria (SV1 >2 mm, RV5-6 >15 mm) 4

Left Anterior Fascicular Block Considerations

• QRS vector shifts posteriorly and superiorly with larger R waves in I and aVL, deeper S waves in V5-V6 4, 2
• R-wave amplitude in I and aVL becomes unreliable for LVH diagnosis 4, 2
• Prevalence 0.5-1.0% in general population under age 40 4, 2

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Prognostic Implications

Mortality Data

• Long-term mortality after MI: RBBB carries HR 1.23 while LBBB HR 1.05 in multivariable analysis, but this varies significantly by LV function 6
• In-hospital MACE rates: highest with LBBB (13.9%) vs RBBB (9.9%) vs isolated STEMI (6.7%), driven primarily by mortality 5
• Bifascicular blocks (combinations of bundle branch block and hemiblock) reflect extensive conduction system involvement and carry increased risk of clinically significant AV block 4, 2

Mechanical Dyssynchrony

• LBBB shows higher prevalence of mechanical dyssynchrony (54-63% depending on criteria) compared to pure RBBB (29-33%) 7
• This dyssynchrony may lead to "latent cardiomyopathy": normal LV dimensions and ejection fraction at rest but abnormal pulmonary artery pressure during exercise, lactate production during pacing, and ischemic scintigraphic findings without coronary disease 3