LBBB vs RBBB: How to Tell Them Apart on ECG
Both LBBB and RBBB are intraventricular conduction delays that widen the QRS to ≥120 ms because one bundle branch fails to conduct, forcing slow cell-to-cell depolarization of that ventricle. They are told apart by the QRS morphology across the precordium: the V1-V2 pattern and its mirror image in the lateral leads. The key axis to remember is that RBBB in isolation is frequently benign, whereas LBBB usually signals underlying heart disease and obscures ischemic ST-T changes.
How to tell them apart
| Feature | Left Bundle Branch Block (LBBB) | Right Bundle Branch Block (RBBB) |
|---|---|---|
| V1-V2 pattern | Broad S wave with an rS or QS pattern (dominant downward deflection) | RSR' pattern, classically 'M-shaped' |
| Lateral leads (I, aVL, V5-V6) | Broad, monophasic R wave that is notched or slurred at its peak — there is no true intervening S wave (a 'M'-shaped or plateaued R) | Broad, slurred terminal S wave following a normal-onset R |
| Mnemonic | WiLLiaM — 'W' in V1 and 'M' in V6 | MaRRoW — 'M' in V1 and 'W' in V6 |
| Axis | Left axis deviation is common | Isolated RBBB typically has a normal axis; a rightward or leftward shift usually signals a concomitant fascicular block or underlying structural disease rather than the RBBB itself |
| Clinical significance | Usually indicates underlying heart disease (ischemic, hypertensive, or cardiomyopathic) | Often benign when isolated; may reflect right heart disease when pathologic |
| Relationship to acute MI | Obscures the ECG diagnosis of MI because its ST-T changes are discordant to the QRS; ischemia is assessed using the Sgarbossa criteria rather than standard ST-elevation thresholds | Does not distort the ST segments the way LBBB does, so acute MI can generally be read on ECG in the usual manner |
| Fascicular block associations | Reflects block of the entire left bundle | RBBB plus left anterior or posterior fascicular block = bifascicular block, which risks progression to complete heart block |
The reasoning
Start by confirming the QRS is ≥120 ms — this establishes a bundle branch block and excludes narrow-complex mimics. Then anchor on V1: an 'M-shaped' RSR' in V1 with a broad terminal S laterally is RBBB (MaRRoW), whereas a downward, broad S-wave complex in V1 with a broad, monophasic notched R wave laterally is LBBB (WiLLiaM). Axis provides supportive evidence — left axis deviation leans toward LBBB, while isolated RBBB is typically normal-axis. Finally, arbitrate clinical significance: an isolated RBBB in an asymptomatic patient is often benign, but LBBB should raise concern for underlying heart disease and, because it distorts the ST segments, requires the Sgarbossa criteria plus troponin to evaluate for acute ischemia.
Key tests
- 12-lead ECG — V1-V2 morphology: LBBB shows a broad S wave (rS/QS), while RBBB shows an RSR' ('M-shaped') complex; this is the single most discriminating finding.
- 12-lead ECG — lateral leads (I, aVL, V5-V6): LBBB produces a broad, monophasic notched or slurred R wave (no true intervening S), whereas RBBB produces a broad terminal S wave; apply WiLLiaM (LBBB) vs MaRRoW (RBBB).
- QRS duration measurement: both are ≥120 ms, confirming a bundle branch block before pattern analysis distinguishes left from right.
- Axis determination (leads I and aVF): left axis deviation favors LBBB, whereas isolated RBBB is usually normal-axis and any axis shift with RBBB suggests an added fascicular block.
- Comparison with a prior ECG and clinical correlation: in an LBBB patient with chest pain, apply the Sgarbossa criteria and objective ischemia markers (troponin) to detect acute MI, since new LBBB alone is nonspecific.
What they share
- Widened QRS duration ≥120 ms
- A bundle branch conduction defect producing a broad, abnormal QRS complex
- Both can be recognized systematically by comparing V1-V2 morphology with the lateral leads
- Both can coexist with fascicular blocks and other conduction disease
Pitfalls
- Assuming any new LBBB with chest pain is automatically a STEMI equivalent — this is outdated; the 2013 ACCF/AHA STEMI guidelines removed new/presumed-new LBBB as an independent STEMI criterion because it lacks specificity. Use clinical context, troponin, and the Sgarbossa criteria to identify true ST-elevation MI in the setting of LBBB.
- Mixing up the mnemonics: WiLLiaM is LBBB (W in V1, M in V6) and MaRRoW is RBBB (M in V1, W in V6) — reversing them flips the diagnosis.
- Describing the LBBB lateral-lead complex as an 'RSR'' or 'RsR'' — that triphasic pattern belongs to RBBB in V1. The typical LBBB lateral morphology is a broad, monophasic R wave that is notched or slurred at its peak, without a true intervening S wave.
- Dismissing every RBBB as benign; while isolated RBBB often is, RBBB with left anterior fascicular block is bifascicular block and, with added first-degree AV block, a 'trifascicular' high-risk pattern that can progress to complete block.
- Interpreting axis deviation as an intrinsic feature of RBBB — isolated RBBB is normal-axis, so a leftward or rightward shift points to a coexisting fascicular block or structural heart disease.
- Confusing a bundle branch block pattern with ventricular ectopy — PVC and VT morphology are described by LBBB/RBBB patterns (LBBB pattern = RV origin, RBBB pattern = LV origin), but the conduction block itself follows every sinus beat.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.