What are the characteristic ECG (electrocardiogram) findings in patients with bundle branch blocks, including left bundle branch block (LBBB) and right bundle branch block (RBBB)?

ECG Characteristics of Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

Complete RBBB is diagnosed when QRS duration is ≥120 ms with characteristic morphologic features including an rsr', rsR', or rSR' pattern in leads V1-V2, prolonged S waves in leads I and V6 (>40 ms or longer than the R wave), and an R peak time >50 ms in V1 but normal in V5-V6. 1

Specific ECG Features of RBBB:

• QRS duration: ≥120 ms 1
• Lead V1-V2 morphology: rsr', rsR', rSR', or rarely qR pattern, where the R' or r' deflection is typically wider than the initial R wave 1
• Lateral leads (I, V6): Prolonged S waves (>40 ms or longer than the R wave) 1
• R peak time: >50 ms in V1 but remains normal in V5-V6 1
• Presence of S waves: S waves are present in leads I and aVL in pure RBBB 2

Clinical Implications:

• RBBB reduces S wave amplitude in right precordial leads, which decreases the sensitivity of ECG criteria for detecting left ventricular hypertrophy 1
• Complete RBBB is uncommon in healthy individuals and may represent either an idiopathic, isolated, clinically benign conduction delay or underlying cardiac pathology 3

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Left Bundle Branch Block (LBBB)

Complete LBBB requires QRS duration ≥120 ms with broad notched or slurred R waves in leads I, aVL, V5, and V6, absent Q waves in leads I, V5, and V6, and R peak time >60 ms in V5-V6. 1

Specific ECG Features of LBBB:

• QRS duration: ≥120 ms (≥140 ms in men or ≥130 ms in women for strict criteria) 1, 4
• Lateral leads (I, aVL, V5, V6): Broad notched or slurred R waves 1
• Absence of septal Q waves: No Q waves in leads I, V5, and V6 due to abnormal septal activation from right to left 1, 5
• Lead V1/V2 morphology: Always either rS or QS pattern 4
• R peak time: >60 ms in V5-V6 1
• QRS notching: Notched or slurred QRS complexes are identified in at least 1 of the 4 leads (I, aVL, V5, V6) in 100% of true LBBB cases 4

Strict Strauss Criteria for LBBB:

• QRS duration ≥140 ms (men) or ≥130 ms (women) 4, 6
• QS or rS in leads V1 and V2 4, 6
• Mid-QRS notching or slurring in ≥2 of leads V1, V2, V5, V6, I, and aVL 4, 6

Clinical Implications:

• LBBB is very rare in otherwise healthy individuals and is a strong ECG marker of underlying structural cardiovascular disease 3, 7
• LBBB may occur as an early and isolated manifestation of ischemic heart disease or cardiomyopathy, many years before structural changes in the left ventricle can be detected 3
• 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
• A voltage criterion of SV2 + RV6 >4.5 mV demonstrates 86% sensitivity and 100% specificity for LVH in the presence of LBBB 8

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Bilateral Bundle Branch Block

An ECG pattern of RBBB in lead V1 with absent S waves in leads I and aVL indicates concomitant left bundle branch delay (bilateral bundle branch block). 2

Specific ECG Features:

• RBBB pattern in lead V1 combined with absent S waves in leads I and aVL 2
• This pattern is 100% specific and 64% sensitive for the presence of pre-existing left bundle branch block 2
• Pure RBBB and bifascicular blocks are associated with S waves in leads I and aVL 2

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Fascicular Blocks

Left Anterior Fascicular Block:

• QRS vector shifts in a posterior and superior direction 3
• Larger R waves in leads I and aVL 3
• Smaller R waves but deeper S waves in leads V5 and V6 3
• R-wave amplitude in leads I and aVL are not reliable criteria for LVH in this setting 3
• Estimated prevalence in the general population (age <40 years) is 0.5–1.0% 3

Left Posterior Fascicular Block:

• Very rare finding, usually associated with RBBB 3

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Important Clinical Caveats

• Never assume LBBB is benign: Even asymptomatic LBBB requires cardiology evaluation given its strong association with structural disease 3, 7
• Intermittent LBBB: Rate-dependent or intermittent LBBB has the same clinical and prognostic significance as stable LBBB 3, 7
• Athletes: Complete bundle branch block or hemiblock in athletes mandates cardiological work-up including exercise testing, 24-hour ECG, and imaging to evaluate underlying pathological causes 3, 7
• Combinations of blocks: Bifascicular blocks (LBBB, RBBB with left posterior hemiblock) reflect more extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 3