How do you diagnose a bundle branch block (BBB) on an electrocardiogram (ECG)?

How to Diagnose Bundle Branch Block on ECG

Bundle branch block is diagnosed on ECG by identifying a QRS duration ≥120 ms combined with specific morphologic patterns that differ between right and left bundle branch blocks.

Right Bundle Branch Block (RBBB) Diagnostic Criteria

Complete RBBB requires QRS duration ≥120 ms with characteristic morphologic features in specific leads. 1

Morphologic Features:

• rsr', rsR', or rSR' pattern in leads V1-V2 (the R' or r' deflection is typically wider than the initial R wave) 1
• Prolonged S waves in leads I and V6 (>40 ms or longer than the R wave) 1
• R peak time >50 ms in V1 but normal in V5-V6 1
• Rarely, a qR pattern may appear in leads V1 or V2 1

Clinical Pitfall:

A QR pattern in V1 with RBBB has high positive predictive value for high-risk pulmonary embolism and may indicate impending cardiac arrest, requiring urgent consideration of thrombolytic therapy. 2

Left Bundle Branch Block (LBBB) Diagnostic Criteria

Complete LBBB requires QRS duration ≥120 ms with broad notched or slurred R waves in lateral leads and absent septal Q waves. 1

Morphologic Features:

• Broad notched or slurred R waves in leads I, aVL, V5, and V6 1
• Absent Q waves in leads I, V5, and V6 (loss of normal septal depolarization) 1
• R peak time >60 ms in V5-V6 1
• The QRS prolongation should not be gradual 3

Critical Clinical Context:

LBBB is very rare in healthy individuals and is a strong ECG marker of underlying structural cardiovascular disease. 1 Even asymptomatic LBBB requires cardiology evaluation given its strong association with structural disease. 1 LBBB may occur as an early manifestation of ischemic heart disease or cardiomyopathy, many years before structural changes in the left ventricle can be detected. 1

Fascicular Blocks (Hemiblocks)

Left Anterior Fascicular Block (LAFB):

• QRS vector shifts posteriorly and superiorly 1
• Larger R waves in leads I and aVL 1
• Smaller R waves but deeper S waves in leads V5 and V6 1
• QRS duration usually normal or only slightly prolonged (<120 ms) 4
• Prevalence in general population (age <40 years) is 0.5–1.0% 1

Left Posterior Fascicular Block (LPFB):

• Very rare finding, usually associated with RBBB 1

Bifascicular Block Patterns

Bifascicular block shows impaired conduction in two of the three fascicles, most commonly RBBB combined with LAFB. 1

Recognition:

• RBBB pattern (as described above) plus left axis deviation indicating LAFB 1
• Less commonly: RBBB with LPFB 1

Critical Warning:

When first-degree AV block accompanies bifascicular block, this suggests trifascicular involvement and carries higher mortality risk. 1 Alternating bundle branch block (block in all three fascicles on successive ECGs) is a Class I indication for permanent pacemaker implantation. 1

Mandatory Workup Algorithm

When complete bundle branch block or hemiblock is identified, the following evaluation is required: 5

1. Exercise testing to assess for exercise-induced arrhythmias or conduction worsening 5
2. 24-hour ECG monitoring to detect intermittent conduction abnormalities 5
3. Cardiac imaging (echocardiography) to evaluate for structural heart disease including:
   • Ischemic heart disease 5
   • Cardiomyopathies 5
   • Hypertensive heart disease 5
   • Arrhythmogenic right ventricular cardiomyopathy (ARVC) 5
   • Congenital heart disease 5

Special Consideration for Athletes:

In young athletes with bifascicular block (LBBB, RBBB with left posterior hemiblock), obtain ECG in siblings to exclude genetically determined progressive cardiac conduction disease (Lenegre disease). 5

Important Diagnostic Pitfalls

Masquerading Bundle Branch Block:

Precordial leads showing RBBB pattern while limb leads resemble LBBB is a rare but critical finding indicating severe diffuse conduction system disease with poor prognosis. 6 This pattern is almost invariably associated with severe underlying heart disease. 6

Intermittent LBBB:

Rate-dependent or intermittent LBBB has the same clinical and prognostic significance as stable LBBB and requires the same thorough evaluation. 1

Impact on Other ECG Diagnoses:

• RBBB reduces S wave amplitude in right precordial leads, decreasing sensitivity for detecting left ventricular hypertrophy 1
• Diagnosing LVH in the presence of LBBB is unreliable unless specific criteria are met: QRS duration >155 ms, left atrial abnormality, and precordial voltage criteria 1
• With LAFB, R wave amplitude in leads I and aVL becomes unreliable for LVH diagnosis 1