How do you distinguish between Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB) in an adult patient with a history of cardiovascular disease or risk factors?

Distinguishing RBBB from LBBB on ECG

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

Diagnostic Criteria for Complete RBBB

Look at lead V1 first - this is where RBBB is most obvious:

• QRS duration ≥120 ms in adults (>100 ms in children 4-16 years, >90 ms in children <4 years) 1, 2
• rSR', rsR', or rSR pattern in leads V1 or V2 - the classic "M-shaped" or "rabbit ears" appearance, with the R' deflection usually wider than the initial R wave 1, 2
• Wide S wave in leads I and V6 - S wave duration greater than R wave or >40 ms 1, 2
• R peak time >50 ms in lead V1 but normal in V5 and V6 1

Diagnostic Criteria for Complete LBBB

Look at leads V5 and V6 - this is where LBBB is most obvious:

• QRS duration ≥120 ms in adults (>100 ms in children 4-16 years, >90 ms in children <4 years) 1
• Broad notched or slurred R wave in leads I, aVL, V5, and V6 - monophasic appearance without the typical q wave 1
• Absent q waves in leads I, V5, and V6 (though a narrow q may appear in aVL without pathology) 1
• R peak time >60 ms in leads V5 and V6 but normal in V1-V3 1
• Discordant ST-T waves - ST segments and T waves typically opposite in direction to the QRS complex 1

Algorithmic Approach to Differentiation

Step 1: Measure QRS Duration

• If QRS <120 ms in adults, neither complete RBBB nor LBBB is present 1
• If QRS 110-119 ms, consider incomplete bundle branch block 1

Step 2: Examine Lead V1

• If rSR' pattern (M-shaped) → Think RBBB 1, 2
• If deep S wave or QS pattern → Think LBBB 1

Step 3: Examine Leads V5-V6

• If wide S waves present → Confirms RBBB 1
• If broad monophasic R waves without q waves → Confirms LBBB 1

Step 4: Check for Absent Q Waves

• Absent q waves in I, V5, V6 → Strongly suggests LBBB 1
• Normal q waves preserved → More consistent with RBBB 1

Critical Pitfalls and Caveats

Atypical Presentations

Beware of axis-dependent morphology changes: An unusual RBBB with inferior QRS axis (+90°) can show LBBB-like morphology in V5-V6 due to the geometric relationship between the heart's electrical axis and electrode position 3. In such cases, the wide S waves in leads I and aVL help confirm RBBB 3.

Bilateral bundle branch block exists: A rare entity showing RBBB pattern in V1 (terminal R wave) but LBBB pattern in leads I and aVL (absent S wave) 4. This unusual conduction disorder may confer increased cardiovascular risk 4.

Clinical Significance Differs

LBBB is more ominous than RBBB: LBBB is very rare in healthy individuals and represents a strong marker of underlying structural cardiovascular disease, potentially indicating ischemic heart disease or cardiomyopathy years before structural changes are detectable 1. In contrast, RBBB may represent an isolated, clinically benign finding in up to 1% of the general population 1, 2.

Complete bundle branch block warrants workup: The European Society of Cardiology recommends that demonstration of complete bundle branch block should lead to cardiological evaluation including exercise testing, 24-hour ECG monitoring, and echocardiography to exclude underlying pathology 1, 2.

Impact on Other ECG Interpretations

RBBB preserves ST-segment interpretation for acute MI diagnosis, unlike LBBB which significantly complicates ECG diagnosis 2. However, RBBB reduces sensitivity of voltage criteria for left ventricular hypertrophy by decreasing S-wave amplitude in right precordial leads 1.

LBBB makes LVH diagnosis nearly impossible: Standard ECG voltage criteria for LVH are unreliable in LBBB, with up to 90% of LBBB patients having anatomic LVH at autopsy 1, 5. The diagnosis should generally not be attempted unless QRS duration exceeds 155 ms with left atrial abnormality present 1.

Quick Reference Table

Feature	RBBB	LBBB
V1 morphology	rSR' (M-shaped) [1]	Deep S or QS [1]
V6 morphology	Wide S wave [1]	Broad notched R [1]
Q waves in I, V5-V6	Present [1]	Absent [1]
R peak time V1	>50 ms [1]	Normal [1]
R peak time V5-V6	Normal [1]	>60 ms [1]
Clinical significance	Often benign [1,2]	Usually pathologic [1]
ST interpretation for MI	Preserved [2]	Complicated [2]