Bundle Branch Block on ECG: How to Reliably Distinguish LBBB and RBBB

Bundle branch block patterns are something you encounter constantly in clinical practice – whether in the emergency department, in prehospital emergency medicine, or in the ICU. While a right bundle branch block (RBBB) often appears as an incidental finding without immediate therapeutic consequences, a new-onset left bundle branch block (LBBB) can force a time-critical treatment decision. This is because LBBB masks classic ST elevations and turns MI diagnosis into a real challenge. If you have a solid command of the morphology criteria for both bundle branch blocks and know the Sgarbossa criteria, you'll make better decisions in emergencies. This article gives you the tools to systematically distinguish LBBB from RBBB, avoid pitfalls, and correctly derive the clinical implications.

Anatomical Fundamentals of the Cardiac Conduction System

The bundle of His divides after the AV node into the right bundle branch and the left bundle branch. The left bundle branch further divides into an anterior and a posterior fascicle. Conduction through these branches ensures that both ventricles are depolarized nearly simultaneously – the entire process normally takes less than 120 ms.

If one of the branches is blocked, the ipsilateral ventricle is no longer depolarized via the specialized conduction system but instead propagates in a delayed fashion through myocardial tissue from cell to cell. The consequences:

• Widened QRS complex (≥ 120 ms in complete block, 100–119 ms in incomplete block)
• Characteristic morphology changes that indicate which branch is affected
• Secondary repolarization abnormalities (ST segment and T wave are discordant to the dominant QRS deflection)

Understanding these fundamentals is essential for correctly interpreting the typical ECG patterns and avoiding false diagnosis of ischemia or other pathology.

Right Bundle Branch Block: Morphology and Criteria

In RBBB, the right ventricle is depolarized with a delay. The initial depolarization via the left bundle branch and the septum remains normal; only the terminal component – the delayed right ventricular depolarization – alters the QRS morphology.

Diagnostic Criteria for Complete RBBB

• QRS duration ≥ 120 ms
• V1/V2: rsR' configuration (the classic "M-shape" or "rabbit ears") – the terminal R' wave represents the delayed right ventricular depolarization
• V5/V6 and lead I: Broad, slurred S wave as the terminal component (qRS pattern)
• Discordant ST depression and T wave inversion in V1–V3 (secondary repolarization abnormality, not an ischemic sign per se)

Mnemonic

"WiLLiaM MaRRoW" – a popular memory aid:

• MaRRoW → in Right precordial leads (V1) an M-shaped pattern (RBBB)
• WiLLiaM → in V1 a W-shaped pattern (LBBB)

Clinical Significance of RBBB

RBBB is common and often benign. It can occur in:

• Healthy hearts (especially in younger patients, athletes)
• Right heart strain (pulmonary embolism, pulmonary hypertension, cor pulmonale)
• Right ventricular hypertrophy
• Myocarditis
• Ischemic heart disease (especially with involvement of the right coronary system)

Important in emergencies: A new-onset RBBB in the context of dyspnea and tachycardia should raise suspicion for pulmonary embolism. Unlike LBBB, RBBB generally does not significantly mask classic infarction signs – ST segment assessment in leads I, II, aVL, aVF, and V4–V6 remains largely unaffected.

Left Bundle Branch Block: Morphology and Criteria

In LBBB, the entire initial and terminal depolarization becomes abnormal. The septum is no longer depolarized from left to right as usual, but from right to left. The left ventricle depolarizes in a delayed fashion through myocardial tissue, resulting in massive changes to QRS morphology and repolarization.

Diagnostic Criteria for Complete LBBB

• QRS duration ≥ 120 ms
• V1/V2: Broad, deep QS or rS configuration (the "W-shape") – no septal q and no positive terminal component
• V5/V6 and leads I, aVL: Broad, often notched or slurred R wave without Q wave and without S wave (monophasic R)
• Discordant ST segment and T wave: In leads with a positive QRS, ST depression and T wave inversion are normal; in leads with a negative QRS, ST elevation and positive T waves are the expected secondary repolarization abnormality
• Absent septal Q waves in I, aVL, V5, V6

Clinical Significance of LBBB

LBBB is rarely harmless and is almost always associated with structural heart disease:

• Hypertensive heart disease
• Dilated cardiomyopathy
• Coronary artery disease / acute myocardial infarction
• Aortic valve disease (especially aortic stenosis)
• Degenerative conduction disease (Lenegre syndrome)

The central problem in emergencies: LBBB alters the entire ST segment morphology so dramatically that classic STEMI criteria are no longer applicable. This is precisely where the Sgarbossa criteria become relevant.

Systematic Differentiation: LBBB vs. RBBB at a Glance

Criterion	RBBB	LBBB
QRS duration	≥ 120 ms	≥ 120 ms
V1	rsR' (M-shape)	QS or rS (W-shape)
V6	qRS with broad S wave	Monophasic R, no Q
Septal Q (I, V5, V6)	Present	Absent
ST discordance	V1–V3	All leads
MI diagnosis	Largely possible	Massively impaired
Axis	Can be normal	Often left axis deviation

Practical tip: Always start your assessment in V1. The morphology in V1 is the key to rapid differentiation. Broad terminal R' wave → RBBB. Deep, broad S/QS configuration → LBBB.

Sgarbossa Criteria: MI Diagnosis in the Presence of LBBB

The major clinical challenge with LBBB is recognizing an acute ST-elevation myocardial infarction (STEMI). The expected secondary repolarization abnormalities in LBBB mimic or mask ischemic ST changes. The Sgarbossa criteria provide a validated method to identify acute MI in the presence of LBBB.

Original Sgarbossa Criteria

The criteria use a point system:

1. Concordant ST elevation ≥ 1 mm in leads with a positive QRS complex → 5 points
2. Concordant ST depression ≥ 1 mm in V1, V2, or V3 → 3 points
3. Discordant (excessive) ST elevation ≥ 5 mm in leads with a negative QRS complex → 2 points

A score ≥ 3 points indicates acute myocardial infarction with high specificity.

Important: Concordant ST elevation (criterion 1) is the most specific and strongest individual criterion. If you see ST elevation in a lead with a positive QRS complex, this is highly pathological – because normally in LBBB you expect discordant repolarization (i.e., ST depression with a positive QRS).

Modified Sgarbossa Criteria (Smith Criteria)

The modified criteria improve sensitivity, particularly for the third criterion. Instead of an absolute threshold of 5 mm, a proportional approach is used:

• Modified criterion 3: Discordant ST elevation with a ratio of ST elevation to S wave depth ≥ 0.25 (i.e., ≥ 25% of the preceding S wave amplitude)

This ratio is significantly more sensitive than the rigid 5 mm cutoff because it accounts for individual QRS amplitude. A small QRS complex with 3 mm of ST elevation and an S wave depth of 8 mm is far more concerning than 5 mm of ST elevation with an S wave depth of 30 mm.

Clinical Application in Practice

The Sgarbossa criteria are not a substitute for clinical judgment. In emergencies, the following applies:

• Sgarbossa positive + consistent clinical presentation (chest pain, dyspnea, shock) → Treat as STEMI, activate the cardiac catheterization lab
• Sgarbossa negative, but high clinical suspicion → A negative Sgarbossa score does not rule out MI (limited sensitivity). Further workup: serial ECGs, troponin, echocardiography
• Unclear situation → Maintain a low threshold for cardiology consultation and advanced imaging

Special Case: New vs. Pre-existing LBBB

For a long time, a new-onset LBBB in the setting of acute coronary syndrome was considered a STEMI equivalent with an indication for primary percutaneous coronary intervention (PCI). Current evidence has tempered this blanket recommendation:

• A new LBBB alone – without ischemic symptoms and without positive Sgarbossa criteria – does not automatically justify emergent catheterization lab activation
• What matters is the combination of new LBBB, symptoms suggestive of ischemia, and ideally positive Sgarbossa criteria
• A pre-existing, chronic LBBB has no acute therapeutic consequence per se – here, application of the Sgarbossa criteria is particularly valuable for distinguishing acute ischemia from the pre-existing repolarization abnormality

Practical tip: Whenever possible, always compare with a prior ECG. Dynamic changes compared to previous findings are a strong indicator of acute pathology, even when the individual Sgarbossa criteria are not met.

Fascicular Blocks: The Smaller Siblings

In addition to complete bundle branch blocks, there are fascicular blocks (hemiblocks) that affect the left bundle branch:

Left Anterior Hemiblock (LAHB)

• Marked left axis deviation (axis < −30°, often < −45°)
• QRS duration usually < 120 ms
• Small q in I and aVL, small r in II, III, aVF
• Most common fascicular block, often clinically benign

Left Posterior Hemiblock (LPHB)

• Marked right axis deviation (axis > +90°)
• QRS duration usually < 120 ms
• Rare, as the posterior fascicle is more robust (dual blood supply)
• Diagnosis of exclusion: right heart strain and right ventricular hypertrophy must be considered in the differential diagnosis

Bifascicular Block

The combination of RBBB + LAHB (common) or RBBB + LPHB (rare) is termed a bifascicular block. This becomes clinically relevant when a first-degree AV block is also present – this is sometimes referred to as a "trifascicular block" (although this term is pathophysiologically imprecise). The danger: progression to complete AV block with hemodynamic instability.

Bundle Branch Blocks in the ACLS Algorithm

In the context of Advanced Cardiac Life Support, bundle branch blocks are relevant in several scenarios:

Bradycardia Algorithm

• A new-onset bundle branch block with bradycardia may indicate progression to higher-degree AV block
• Bifascicular block with first-degree AV block and symptoms → early pacemaker readiness (transcutaneous pacing)
• Alternating bundle branch block (alternating LBBB and RBBB) is a warning sign for impending complete AV block

Tachycardia Algorithm

• A wide complex tachycardia (QRS ≥ 120 ms) can be ventricular tachycardia (VT) or supraventricular tachycardia (SVT) with aberrant conduction (= functional bundle branch block)
• Brugada criteria and the Vereckei algorithm help with differentiation
• When in doubt: wide complex tachycardia = VT until proven otherwise
• With a pre-existing bundle branch block, the QRS morphology during tachycardia can be compared to the baseline ECG – if the morphology matches, this favors SVT with aberrant conduction

STEMI Management

• LBBB with suspected acute MI → apply Sgarbossa criteria (see above)
• RBBB → standard ST segment assessment in the unaffected leads is largely possible
• Ventricular paced rhythm produces an LBBB-like pattern → Sgarbossa criteria are also applicable here

Common Errors and Pitfalls

• Confusing secondary repolarization abnormalities with ischemia: Discordant ST changes in bundle branch block are the norm, not pathology. Only concordant changes or excessive discordance are alarming.
• Forgetting to compare with prior ECGs: A chronic LBBB without a prior ECG for comparison can trigger unnecessary catheterization lab activations. Document the bundle branch block and ensure prior ECGs are accessible.
• Missing an alternating bundle branch block: If the block alternates between RBBB and LBBB, there is an acute risk of complete AV block.
• Misinterpreting QRS width: Measure QRS duration in the lead with the widest complex (often V2 or V3), not in limb leads.
• Overlooking or overinterpreting incomplete bundle branch block: An incomplete RBBB (QRS 100–119 ms, rSr' in V1) is often a normal finding, especially in young people.

Summary: Your Approach to Bundle Branch Block in Emergencies

1. Measure QRS width – Complete (≥ 120 ms) or incomplete (100–119 ms)?
2. Assess V1 – M-shape (RBBB) or W-shape (LBBB)?
3. If LBBB: Apply Sgarbossa criteria – Concordant ST elevation? Excessive discordance?
4. Integrate clinical context – Symptoms? Hemodynamics? Prior ECG available?
5. If RBBB: Consider pulmonary embolism if the clinical context fits
6. Identify fascicular blocks – Check axis deviation, recognize bifascicular block
7. In wide complex tachycardia – When in doubt, treat as VT

Practical Training

Reliable interpretation of bundle branch blocks and correct application of the Sgarbossa criteria require regular practice with real ECG examples and in simulated emergency scenarios. In the ACLS course from Simulation Tirol, you systematically train the differentiation of LBBB and RBBB, practice applying the Sgarbossa criteria using clinical cases, and integrate your ECG findings into the ACLS algorithms – including the time-critical decision to activate the catheterization lab. This way, you gain the confidence you need in emergencies.