How do you tell LBBB from RBBB on an ECG?

First confirm a QRS of 120 ms or more. Then look at the end of the QRS in V1: a terminal upward R′ (an rsR′ or rSR′, the M of MaRRoW) means right bundle branch block, while a deep, mostly negative QS or rS means left bundle branch block. Confirm in leads I and V6: RBBB gives a wide S wave, and LBBB gives a broad, notched or slurred R wave with no small septal q wave.

What does WiLLiaM MaRRoW mean on an ECG?

WiLLiaM MaRRoW is a memory aid for bundle branch blocks. The middle letters name the side, and the first and last letters describe V1 and V6. WiLLiaM is left bundle branch block, with a W in V1 and an M in V6; MaRRoW is right bundle branch block, with an M in V1 and a W in V6. In LBBB, V1 is usually a deep QS or rS, not a true W, so trust the terminal deflection.

Is a new LBBB still a STEMI equivalent?

Not on its own. The 2025 ACC/AHA acute coronary syndrome guideline says a new or presumably new LBBB should not be considered diagnostic of acute MI in isolation, and a new LBBB in a patient without symptoms is not a STEMI equivalent. With ischemic symptoms, apply the Sgarbossa or modified Sgarbossa criteria. The 2023 ESC guidelines manage LBBB, RBBB or a paced rhythm with highly suspected ongoing ischemia like ST elevation.

What are the Sgarbossa and modified Sgarbossa criteria?

They detect acute MI in LBBB or a ventricular paced rhythm. The original score gives 5 points for concordant ST elevation and 3 for ST depression in V1 to V3 (each 1 mm or more), and 2 for discordant ST elevation of 5 mm or more; 3 or more points is positive but only about 49% sensitive. The modified rule needs one criterion and replaces the 5-mm rule with discordant elevation of at least 25% of the S-wave depth.

Which is more serious, LBBB or RBBB?

LBBB usually carries more weight. It is linked to structural heart disease more often than RBBB, carries a higher risk of heart failure, cardiac events and death, and calls for an echocardiogram when first found. Isolated RBBB without heart disease adds no risk and needs no treatment. Context matters, though: acute MI with RBBB is frequently caused by complete coronary occlusion, and a new RBBB carried the highest in-hospital mortality in one large registry.

How does S2 splitting differ in RBBB and LBBB?

In RBBB, late right ventricular activation delays pulmonic valve closure, so S2 is widely split and the split widens further on inspiration. In LBBB, aortic valve closure is late, so S2 splits paradoxically: the split is heard in expiration and disappears on inspiration, when the delayed pulmonic sound lands on the aortic one. A wide split that does not change with breathing points instead to an atrial septal defect.

LBBB vs RBBB: ECG Criteria, WiLLiaM MaRRoW & Sgarbossa

LBBB vs RBBB: both widen the QRS to 120 ms or more, but RBBB gives an rSR′ "M" in V1 and a wide S in V6 (MaRRoW), while LBBB gives a deep negative V1 and a broad notched R in V6 (WiLLiaM). RBBB is often benign; LBBB more often signals structural heart disease.

Read V1 first: a terminal upward R′ means right, and a deep, mostly negative QS or rS means left. Then confirm in I, aVL and V6; the full routine is in how to read an ECG.

Why it matters: LBBB hides acute and old infarction, so chest pain with LBBB is read with the Sgarbossa criteria, and a new LBBB alone is no longer a STEMI equivalent. RBBB usually leaves ST-segment criteria usable.

LBBB vs RBBB: the comparison table

Feature	RBBB	LBBB
Ventricle activated late	Right	Left
Complete block (adults)	QRS ≥120 ms	QRS ≥120 ms
V1 (and V2)	rsr′, rsR′ or rSR′, the R′ usually wider than the first r; R-peak time over 50 ms	Deep QS or rS
I, aVL, V5-V6	S wave longer than the R, or over 40 ms, in I and V6	Broad, notched or slurred R; no q in I, V5 or V6 (a narrow q may appear in aVL); R-peak time over 60 ms in V5-V6
Mnemonic	MaRRoW: M in V1, W in V6	WiLLiaM: W in V1, M in V6
ST-T waves	T opposite the terminal QRS: inverted in right precordial leads, upright in left	ST and T opposite the main QRS (discordant)
Axis	An axis of −45° or beyond suggests added left anterior fascicular block	Variable
S2	Wide split that still widens on inspiration	Paradoxical split
How common	Rises with age, up to 11.3% by 80	0.06-0.1% overall; 1-5% over 70; about 1 in 3 with heart failure
Prognosis	No added risk without heart disease	Higher risk of cardiac events and death
Infarction on the ECG	ST criteria usually still valid; ESC manages suspected ongoing ischemia like STEMI	Masks acute and old MI; use Sgarbossa
First test	Echo if heart disease is suspected (Class 2a)	Echo for every newly found LBBB (Class 1)

Ventricular and paced rhythms and pre-excitation also widen the QRS, so check for P waves, pacing spikes and a WPW delta wave first.

WiLLiaM MaRRoW: what the mnemonic means

The middle letters name the side; the first and last letters describe V1 and V6. WiLLiaM = LBBB (W in V1, M in V6); MaRRoW = RBBB (M in V1, W in V6). A 2012 British Journal of General Practice tips article teaches it, but the letters are cartoons:

Letter	What the lead actually shows
M in V1 (RBBB)	rsR′ or rSR′ ("rabbit ears"); a minority show one wide, notched R
W in V6 (RBBB)	An R followed by a wide S wave (longer than the R or over 40 ms)
W in V1 (LBBB)	Usually a deep QS or rS rather than a true W
M in V6 (LBBB)	A broad R that is notched (the "M") or slurred, with no septal q

When the letters don't fit, trust the terminal deflection in V1. Two look-alikes fool the mnemonic: a ventricular paced rhythm gives an LBBB-like QRS, and the Brugada pattern resembles RBBB.

Why the patterns look this way, and how S2 splits

Normal: a septal branch of the left bundle activates the septum left to right, making small q waves in I, V5 and V6.
RBBB: the left ventricle depolarizes first and the right late, so the QRS starts normally and the late right ventricular forces come at the end: the R′ in V1 and the wide S in I and V6.
LBBB: the impulse goes down the right bundle, so the septum is activated right to left and the lateral q waves vanish. The left ventricle is then activated slowly, cell to cell, giving broad notched lateral R waves.
Discordant ST-T changes are expected in LBBB; concordant ST deviation is not, and that is the basis of the Sgarbossa criteria.
Blood supply: the right bundle is fed mostly by the left anterior descending artery, which fits anterior MI causing a new RBBB.

Pattern	S2	Why
Normal	Splits on inspiration only	Extra venous return delays P2
RBBB	Wide split, wider still on inspiration	Late right ventricular activation delays P2
Secundum ASD	Wide, fixed split	Right ventricular flow stays high through the respiratory cycle
LBBB	Paradoxical: split in expiration, single on inspiration	A2 is late, so the inspiratory delay in P2 makes them coincide
Aortic stenosis	Paradoxical	Late aortic closure

Matching murmurs are in the heart murmurs chart and the aortic stenosis question.

QRS duration thresholds: complete, incomplete and nonspecific delay

Finding	Criteria (AHA/ACCF/HRS 2009; ACC/AHA/HRS 2018)
Normal QRS	Over 110 ms is abnormal after age 16
Complete RBBB or LBBB (adults)	120 ms or more, plus the morphology criteria
Incomplete RBBB	110-119 ms with RBBB morphology
Incomplete LBBB	110-119 ms with a left ventricular hypertrophy pattern, R-peak time over 60 ms in V4-V6 and no q in I, V5 or V6
Nonspecific intraventricular conduction delay	Over 110 ms without RBBB or LBBB criteria
Children	Complete block above 100 ms at ages 4-16, above 90 ms under 4

Sources differ at the margin: StatPearls uses 100-119 ms for incomplete RBBB, but every source here uses 120 ms for complete block (normal ECG intervals).
Stricter (Strauss) LBBB criteria, used in resynchronization research, need a QRS of 140 ms or more in men and 130 ms or more in women.
Rate-dependent block: either block can appear only at faster or slower rates.
Nonspecific delay carries a risk similar to LBBB's, so it also prompts a search for structural disease.

Causes, prognosis and workup

RBBB is usually incidental. Without heart disease it adds no risk; with cardiovascular disease it independently predicts mortality. Causes:

Right heart strain: acute pulmonary embolism, where a transient RBBB signals significant right ventricular dysfunction (PE memory palace), and cor pulmonale.
Atrial septal defect: right ventricular conduction delay (rSR′ in V1 with a tall R′), right axis deviation or right ventricular hypertrophy (ASD types).
Injury: anterior MI (a new RBBB means substantial damage) and myocarditis.
Procedures: right heart catheterization; alcohol septal ablation for hypertrophic cardiomyopathy.
Chagas cardiomyopathy: RBBB, left anterior fascicular block, first-degree AV block, atrial flutter or fibrillation.
Degeneration: Lev and Lenègre disease; rarely, hyperkalemia.

LBBB is linked to structural heart disease more often than RBBB. Causes: coronary disease, hypertension, dilated, hypertrophic and infiltrative cardiomyopathies (sarcoidosis, amyloidosis), aortic stenosis or regurgitation, TAVR, aortic valve surgery, septal myectomy, Lev and Lenègre disease, myocarditis, hyperkalemia and antiarrhythmic drugs.

Workup (2018 ACC/AHA/HRS):

New LBBB: transthoracic echo (Class 1); if disease is still suspected and the echo is unrevealing, cardiac MRI, CT or nuclear imaging (Class 2a).
RBBB and other conduction delays: echo if structural disease is suspected (Class 2a).
Asymptomatic LBBB with suspected coronary disease: stress testing with imaging may be considered (Class 2b); LBBB and paced rhythms obscure ischemia on the exercise ECG, so imaging is added.
Symptoms of intermittent bradycardia: ambulatory monitoring (Class 1) or an electrophysiology study (Class 2a).
Resynchronization (CRT): Class 1 with LVEF 35% or less, sinus rhythm, LBBB with QRS 150 ms or more, and NYHA class II to ambulatory IV symptoms on guideline-directed therapy. RBBB and other non-LBBB patterns respond less: CRT "can be useful" (Class 2a) only with QRS 150 ms or more and class III or ambulatory IV symptoms, and is not recommended with QRS under 150 ms and class I-II symptoms (2023 HRS). See HFrEF vs HFpEF.

LBBB and acute MI: Sgarbossa and modified Sgarbossa

The Sgarbossa criteria look for ST changes that break LBBB's discordance rule; they also apply to ventricular paced rhythm.

Criterion	Original Sgarbossa (1996)	Modified Sgarbossa (Smith, 2012)
Concordant ST elevation ≥1 mm in any lead	5 points	Positive
ST depression ≥1 mm in V1, V2 or V3 (concordant, since the QRS is negative there)	3 points	Positive
Excessively discordant ST elevation	≥5 mm where the QRS is negative: 2 points	≥1 mm and at least 25% of the S-wave depth (ST/S ratio −0.25 or less)
Rule positive when	Score ≥3	Any one criterion

The weighted rule is specific but insensitive: specificity over 90%, sensitivity about 49%. Discordant elevation alone scores 2, so it is never positive by itself.
The modified rule swaps the arbitrary 5-mm threshold for a proportion. In a validation study (45 acute coronary occlusions, 249 controls), sensitivity was 80% vs 49%, specificity 99% vs 100%.
Paced rhythm: across 16 centers, the modified criteria were more sensitive for occlusion MI (81% vs 56%), with similar specificity against emergency department controls (96% vs 97%).
Barcelona algorithm (2020): counts concordant ST deviation of 1 mm in any lead, and discordant deviation of 1 mm in low-voltage leads (largest R or S 6 mm or less); it reported 93-95% sensitivity in its own cohorts.
A negative score does not exclude MI: repeat the ECG and troponin. Echo can help but should not delay the catheterization lab when occlusion is suspected.
RBBB: the usual ST criteria apply (AHA 2009); a qR in V1 with other RBBB criteria means RBBB plus septal infarction. Reperfusion is covered in STEMI vs NSTEMI management.

Is a new LBBB a STEMI equivalent? What current guidelines say

Source	Position
1996 and 2004 ACC/AHA STEMI guidelines	Symptoms plus new or presumably new LBBB within 12 hours: emergency reperfusion (Class 1)
2013 ACCF/AHA STEMI guideline	New or presumably new LBBB "should not be considered diagnostic of AMI in isolation"
2023 ESC ACS guidelines	High suspicion of ongoing ischemia plus LBBB, RBBB or a paced rhythm: manage like ST elevation, whether or not the block is already known
2025 ACC/AHA/ACEP/NAEMSP/SCAI ACS guideline	New LBBB is infrequent and not diagnostic in isolation; clinical correlation is required; in an asymptomatic patient it is not a STEMI equivalent
2026 Fifth Universal Definition of MI	New or presumed new LBBB and/or RBBB with other signs of ischemia may be considered equivalent to STEMI

Why it changed: the old rule caused frequent false catheterization-lab activations and inappropriate fibrinolysis, because most such patients had no acute occlusion. In one emergency department cohort, acute MI was not significantly more common with new LBBB than with old LBBB or none (7.3% vs 5.2% vs 6.1%).

Don't forget RBBB: among 6,742 patients with acute MI, the infarct artery was totally occluded more often with RBBB than with LBBB (51.7% vs 39.4%), and in-hospital mortality was highest (18.8%) with new or presumably new RBBB.

Exam rule: asymptomatic new LBBB, echo; ischemic symptoms with any LBBB, apply Sgarbossa; a positive rule, instability or high suspicion of ongoing ischemia, emergency angiography.

Fascicular, bifascicular and "trifascicular" block

The left bundle divides into anterior, posterior and septal fascicles. A fascicular block (hemiblock) shifts the axis but keeps the QRS under 120 ms.

Pattern	ECG	Meaning
Left anterior fascicular block	Axis −45° to −90°; qR in aVL, R-peak time ≥45 ms; rS in II, III and aVF	Common; alone, not independently linked to more events
Left posterior fascicular block	Axis +90° to +180° (Merck: beyond +120°); rS in I and aVL; qR in III and aVF	Rare in isolation
Bifascicular block	RBBB plus either fascicular block	No treatment unless intermittent second- or third-degree AV block appears
Bifascicular block with a long PR	Plus first-degree AV block	Often mislabeled "trifascicular"; the long PR is usually AV nodal
True trifascicular block	Alternating RBBB and LBBB, or RBBB with alternating fascicular blocks	Permanent pacing

The 2009 AHA/ACCF/HRS recommendations advise naming each defect instead of "bifascicular" or "trifascicular," but Merck and the 2018 guideline still use the terms.

When to pace (2018 ACC/AHA/HRS):

Class 1: syncope with bundle branch block and an HV interval of 70 ms or more (or infranodal block); alternating bundle branch block.
Class 3 (harm): asymptomatic isolated conduction disease with 1:1 AV conduction and no other pacing indication; a new bundle branch or fascicular block after MI without second- or third-degree AV block.
Neuromuscular disease: reasonable in Kearns-Sayre syndrome with conduction disease (Class 2a); may be considered in myotonic dystrophy type 1 with PR over 240 ms, QRS over 120 ms or fascicular block (Class 2b).
Irreversible Mobitz II, high-grade or third-degree AV block is paced regardless of symptoms (AV block cheat sheet).

Paced rhythm: the LBBB pattern that pacing creates

Right ventricular pacing activates the right ventricle first, so the QRS is wide and LBBB-like. Look for pacing spikes; unlike true LBBB, a paced rhythm often has no R wave in V6.
Infarction: use Sgarbossa or modified Sgarbossa; the 2023 ESC guidelines manage a paced rhythm with highly suspected ongoing ischemia like ST elevation.
Pacing-induced cardiomyopathy: right ventricular pacing causes dyssynchrony like LBBB's. Risk rises with pacing burdens of 20-40% or more; cohorts report it in 5.9-39% of patients, usually defined as an LVEF fall of 10% or more from a baseline above 50%. Upgrading to CRT is recommended (Class 1).
Prevention: with LVEF 36-50% and AV block needing pacing over 40% of the time, CRT or His bundle pacing is reasonable instead (2018, Class 2a).
Conduction system pacing: left bundle branch area pacing typically leaves an RBBB-like delay in V1 (qR, Qr or rSR′).
Unexpected RBBB morphology with a right ventricular lead: check for lead perforation or accidental left ventricular placement.

USMLE traps: classic teaching vs current evidence

Trap	What's actually true
New LBBB plus chest pain equals STEMI	Not on its own; use Sgarbossa, and manage like STEMI when ongoing ischemia is highly suspected, old block or new
A Sgarbossa score under 3 rules out MI	The weighted rule misses about half of occlusions
RBBB hides a STEMI like LBBB	Classically no: ST criteria still apply (AHA 2009). But the 2023 ESC says RBBB prevents accurate ST assessment and manages suspected ongoing ischemia like STEMI; a new RBBB in ACS is high risk
A wide split S2 means ASD	ASD's split is fixed; RBBB's still moves with breathing
Bifascicular block plus a long PR is "trifascicular" and needs a pacemaker	The long PR is usually AV nodal; no treatment unless intermittent second- or third-degree AV block appears
An exercise ECG can test for ischemia in LBBB	LBBB obscures ischemic ST changes; add imaging
LBBB needs a temporary pacer before an intraoperative pulmonary artery catheter	Catheter-induced RBBB can cause complete heart block, but routine prophylactic pacing is Class 3 (harm)

More cardiology walkthroughs are on the Cardiology hub.

Worked USMLE-style vignettes

Vignette 1: the preoperative ECG. A 64-year-old man with hypertension and no symptoms has an ECG before knee surgery; one 3 years ago was normal. It shows PR 170 ms, QRS 148 ms, broad notched R waves in I, aVL, V5 and V6 without q waves, a deep QS in V1 and discordant ST-T changes. S2 is single on inspiration and split on expiration. Next step? A. Transthoracic echo; B. Emergency angiography; C. Fibrinolysis; D. Permanent pacemaker; E. Electrophysiology study

Answer: A. A new, asymptomatic LBBB with paradoxical splitting needs an echo (Class 1). B: asymptomatic new LBBB is not a STEMI equivalent. C: nothing suggests coronary occlusion. D: asymptomatic block with 1:1 conduction is not paced (Class 3). E: that study is for symptoms such as syncope.

Vignette 2: chest pain with an old LBBB. A 71-year-old woman with LBBB on last year's ECG has 45 minutes of crushing chest pain. Her ECG shows LBBB with 2 mm ST elevation in I, V5 and V6, where the QRS is upright, and 1.5 mm ST depression in V2 and V3. Next step? A. Emergency angiography; B. Repeat troponin in 3 hours; C. Treat as chronic LBBB; D. Exercise treadmill test; E. Coronary CT angiography

Answer: A. Concordant ST elevation (5 points) plus ST depression in V1-V3 (3 points) scores 8, and the modified rule is positive. B: waiting delays reperfusion. C: the block's age no longer decides. D: acute MI and unstable angina are absolute contraindications to exercise testing, and LBBB obscures the ST changes anyway. E: CT generally has no role when acute occlusion is suspected.

Vignette 3: the split that doesn't move. A 29-year-old woman has exertional dyspnea, a 2/6 midsystolic murmur at the left upper sternal border and a widely split S2 that does not change with breathing. The ECG shows right axis deviation and rSR′ in V1; QRS is 112 ms. Most likely diagnosis? A. Secundum ASD; B. Complete RBBB; C. Ventricular septal defect; D. LBBB; E. Pulmonic stenosis

Answer: A. A fixed split, a flow murmur and right ventricular conduction delay point to a secundum ASD; echo confirms it. B: the QRS is under 120 ms, and RBBB's split varies with breathing. C: a ventricular septal defect leaves right ventricular volume normal, so the split is not fixed. D: LBBB splits paradoxically. E: pulmonic stenosis widens the split, but it still varies.

Vignette 4: syncope with bifascicular block. A 76-year-old man has fainted twice without warning; LVEF is 55%. The ECG shows QRS 136 ms with rSR′ in V1, a wide S in I and V6, axis −60° and qR in aVL. Monitoring captured no event; an electrophysiology study shows an HV interval of 82 ms and no inducible ventricular tachycardia. Next step? A. Permanent pacemaker; B. Observation; C. 30 more days of monitoring; D. Implantable defibrillator; E. CRT

Answer: A. RBBB plus left anterior fascicular block with syncope and an HV interval of 70 ms or more is a Class 1 pacing indication. B: observation suits asymptomatic block. C: the study has already shown the indication. D: no ventricular tachycardia was inducible, and LVEF is normal. E: CRT requires a reduced LVEF.

Key takeaways

Complete block is a QRS of 120 ms or more; incomplete is 110-119 ms. V1 decides (terminal R′ is RBBB; deep QS or rS is LBBB); confirm in I and V6.
RBBB: often incidental; think ASD, pulmonary embolism, anterior MI, cor pulmonale and Chagas disease. Its split moves with breathing; ST criteria usually still work.
LBBB: think structural heart disease; echo every new LBBB; paradoxical splitting; CRT with LVEF 35% or less and QRS 150 ms or more.
LBBB alone is not a STEMI equivalent. With ischemic symptoms, apply Sgarbossa (5, 3, 2; positive at 3) or the modified rule (any criterion; ST/S −0.25 or less).
Pace alternating bundle branch block or syncope with an HV interval of 70 ms or more, not asymptomatic block. Right ventricular pacing looks like LBBB and is read with Sgarbossa.