Bundle Branch Block on ECG: LBBB and RBBB Criteria
A bundle branch block widens the QRS to 120 ms or more because one ventricle is activated late, through muscle rather than the conduction system. Right bundle branch block gives an rsR' in V1 and a wide S wave in I and V6; left bundle branch block gives a broad, notched R wave in I, aVL, V5 and V6 and a deep S wave in V1.
At a glance
- RBBB: QRS of 120 ms or more, rsR' in V1 or V2, and a wide, slurred S wave in I and V6. Often benign, but a new RBBB can come from pulmonary embolism or anterior MI.
- LBBB: QRS of 120 ms or more, a broad notched or slurred R wave in I, aVL, V5 and V6 with no Q waves there, and a deep QS or rS in V1. It usually reflects heart disease, so get an echocardiogram.
- The same shapes with a QRS of 110 to 119 ms are incomplete bundle branch blocks.
- ST segments and T waves normally point away from the main QRS. Changes that point the same way in LBBB suggest MI: use the Sgarbossa criteria.
- In heart failure with a low ejection fraction, LBBB with a wide QRS marks the patients who gain most from cardiac resynchronization therapy.
Normal values
| Measure | Normal | Abnormal |
|---|---|---|
| QRS duration, adults | Under 110 ms | 110 to 119 ms: incomplete block; 120 ms or more: complete block or another conduction delay |
Criteria
Complete right bundle branch block
QRS of 120 ms or more; rsr', rsR' or rSR' in V1 or V2, with the terminal R usually wider than the first; an S wave in I and V6 longer than the R wave or longer than 40 ms; R-peak time in V1 over 50 ms.
Complete left bundle branch block
QRS of 120 ms or more; broad notched or slurred R waves in I, aVL, V5 and V6, sometimes an RS pattern in V5 and V6; no Q waves in I, V5 and V6; R-peak time over 60 ms in V5 and V6; ST segments and T waves usually opposite to the QRS.
Strict LBBB
QRS of 140 ms or more in men or 130 ms or more in women, QS or rS in V1 and V2, and mid-QRS notching or slurring in two or more of V1, V2, V5, V6, I and aVL. Proposed to pick out true LBBB before resynchronization therapy.
Incomplete RBBB or LBBB
The same shape with a QRS of 110 to 119 ms in adults.
Nonspecific intraventricular conduction delay
A QRS over 110 ms that fits neither pattern.
Bifascicular block
RBBB with left anterior or left posterior fascicular block.
What it looks like
RBBB in V1: rsR'
RBBB in V6: wide slurred S
LBBB in V1: deep wide QS or rS
LBBB in V6: broad notched R, no Q
How to measure it
- Measure the QRS in the lead where it is widest.
- Look at V1. An rsR', the rabbit ears, points to RBBB; a deep, wide QS or rS points to LBBB.
- Look at I and V6. A wide S wave points to RBBB; a broad R wave with no Q wave points to LBBB.
- Check that the ST segments and T waves point away from the end of the QRS. That is the expected secondary change.
- Compare with an old ECG. A new block needs an explanation.
Causes
Right bundle branch block
- A normal finding in some healthy people
- right ventricular strain or hypertrophy
- pulmonary embolism
- atrial septal defect and other congenital heart disease
- ischemic heart disease
- degenerative conduction disease
- after right heart catheterization
Left bundle branch block
- Hypertension and left ventricular hypertrophy
- ischemic heart disease
- dilated and other cardiomyopathies
- aortic stenosis, and after TAVR or aortic valve surgery
- degenerative conduction disease
What to do
- RBBB alone without symptoms needs no treatment. Look for right heart strain or an atrial septal defect when the history or exam suggests it.
- New LBBB: get an echocardiogram to look for structural heart disease.
- Chest pain with LBBB or a paced rhythm: read the ST segments with the modified Sgarbossa criteria. Standard STEMI criteria still apply in RBBB.
- Bifascicular block with syncope: look for intermittent high-grade AV block with monitoring or an electrophysiology study.
- Heart failure with an ejection fraction of 35% or less and LBBB with a QRS of 150 ms or more: refer for cardiac resynchronization therapy.
Pitfalls and mimics
- Calling a wide-complex rhythm a bundle branch block before ruling out ventricular tachycardia, hyperkalemia and pre-excitation.
- Missing rate-related bundle branch block, which appears only at faster rates.
- V1 and V2 electrodes placed too high can create an rSr' that looks like incomplete RBBB.
- The Brugada pattern can look like RBBB in V1 and V2, but without the wide S wave in I and V6.
Key studies
Zareba 2011, Circulation (MADIT-CRT)
Among 1,817 patients with mild heart failure, resynchronization with a defibrillator cut heart failure events or death in those with LBBB (hazard ratio 0.47) but not in those with RBBB or a nonspecific conduction delay.
Strauss 2011, Am J Cardiol
Argued that conventional criteria label many hearts as LBBB that don't have true left bundle block, and proposed stricter criteria built on QRS duration and mid-QRS notching.
Surawicz 2009, AHA/ACCF/HRS ECG standards, part III
Sets the standard ECG criteria for right and left bundle branch block, incomplete blocks and fascicular blocks.
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Questions clinicians ask
What is the difference between LBBB and RBBB on an ECG?
Both widen the QRS to 120 ms or more. In RBBB, V1 shows an rsR' and I and V6 show a wide S wave. In LBBB, V1 shows a deep QS or rS, and I, aVL, V5 and V6 show a broad, notched R wave with no Q wave.
Is right bundle branch block serious?
Often not. Isolated RBBB is common in healthy people. A new RBBB, or one with symptoms, can point to pulmonary embolism, right heart strain or ischemia and needs a look for the cause.
Is left bundle branch block serious?
It usually reflects underlying heart disease, such as hypertension, coronary disease or cardiomyopathy, so a new LBBB should prompt an echocardiogram. It also hides the usual ECG signs of MI.
Can you diagnose a heart attack with LBBB?
Yes, with the modified Sgarbossa criteria: concordant ST elevation, concordant ST depression in V1 to V3, or discordant ST elevation that is at least 25% of the preceding S wave.
What is a bifascicular block?
Right bundle branch block plus a block of one of the left fascicles, usually the anterior one. Add a long PR and it is sometimes called trifascicular block, though that name is imprecise.
Sources
- Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235-e240. doi.org/10.1161/CIRCULATIONAHA.108.191095
- Strauss DG, Selvester RH, Wagner GS. Defining left bundle branch block in the era of cardiac resynchronization therapy. Am J Cardiol. 2011;107(6):927-934. doi.org/10.1016/j.amjcard.2010.11.010
- Zareba W, Klein H, Cygankiewicz I, et al. Effectiveness of cardiac resynchronization therapy by QRS morphology in the Multicenter Automatic Defibrillator Implantation Trial-Cardiac Resynchronization Therapy (MADIT-CRT). Circulation. 2011;123(10):1061-1072. doi.org/10.1161/CIRCULATIONAHA.110.960898
- Moss AJ, Hall WJ, Cannom DS, et al. Cardiac-resynchronization therapy for the prevention of heart-failure events. N Engl J Med. 2009;361(14):1329-1338. doi.org/10.1056/NEJMoa0906431
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
Written in our own words by 2 Minute Medicine's physician editors from the sources listed here. The diagrams are schematic drawings made for 2MM, not patient tracings. This page supports, and does not replace, a full ECG reading and clinical judgment.
For health care professionals. Values are for adults unless a page says otherwise.
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