Understanding left bundle branch block (LBBB) vs right bundle branch block (RBBB)
Your heart’s rhythm depends on a precise electrical system. When one of the two main electrical pathways, called the bundle branches, is blocked or delayed, this is called a Bundle Branch Block (BBB). These blocks alter how efficiently the heart beats and are typically identified through an electrocardiogram (ECG) showing a widened QRS complex (over 120 milliseconds).
There are two main types:
- Left Bundle Branch Block (LBBB)
- Right Bundle Branch Block (RBBB)
Though they may appear similar on ECG, they have different causes, implications, and risks.2,6,8
What is a bundle branch block?
Bundle branch blocks occur when the normal electrical signals travelling through the heart’s ventricles are interrupted.
- Left Bundle Branch Block (LBBB):
"In LBBB, the activation of the left ventricle is delayed compared to the right ventricle."
- Right Bundle Branch Block (RBBB):
"In RBBB, the right ventricle receives electrical activation after the left ventricle."
This electrical delay creates uncoordinated contractions between the ventricles and may or may not produce symptoms. The diagnosis is usually made through an ECG.8
Left bundle branch block (LBBB)
How it affects the heart
LBBB slows or blocks the signal to the left ventricle, the heart's main pumping chamber. This affects how efficiently the heart can pump blood to the body and may worsen existing heart conditions.
Common causes of LBBB
- Coronary artery disease
- High blood pressure
- Cardiomyopathy (weakened heart muscle)
- Aortic valve disease
- Prior heart attack
LBBB is rarely seen in healthy individuals and usually indicates underlying structural heart disease.1
Symptoms and risks
Many people with LBBB have no symptoms, but it may cause:
- Fatigue
- Dizziness
- Shortness of breath
- Exercise intolerance
- Palpitations
Most importantly, LBBB is associated with
- Increased risk of heart failure
- Higher mortality rates
- Difficulty diagnosing acute myocardial infarction (AMI) due to masked ECG changes5
ECG features of LBBB
- QRS duration >120 ms
- No Q waves in leads I, V5, V6
- Broad, notched R waves in lateral leads
- Discordant ST-T changes
These changes can interfere with the detection of heart attack signs on an ECG.8
Treatment options
The focus is on treating the underlying heart disease. In patients with heart failure and LBBB, Cardiac Resynchronisation Therapy (CRT) is often used to improve heart function.
Cardiac resynchronisation therapy (CRT) uses a specialised pacemaker to synchronise the contractions of the right and left ventricles, resulting in:
Right bundle branch block (RBBB)
How it affects the heart
Right bundle branch block (RBBB) causes a delay in the electrical impulses reaching the right ventricle, which sends blood to the lungs. This delay is generally less concerning than a left bundle branch block (LBBB), particularly when discovered incidentally.
Common causes of RBBB
- May occur in healthy individuals
- Chronic lung diseases (e.g., COPD)
- Pulmonary embolism
- Congenital heart disease (e.g., atrial septal defect)
- Post-surgical changes
RBBB is more common with age and may be found during routine ECGs.2,3
Symptoms and risks
Like LBBB, many people are asymptomatic. RBBB is more concerning when it develops during a heart attack.
- Increased in-hospital mortality (~94% higher risk)
- Increased long-term mortality (~50% higher)
These findings are based on large-scale meta-analyses.2,3,6
ECG features of RBBB
- QRS duration >120 ms
- “Rabbit ears” (rsR') in lead V1
- Wide S wave in leads I and V6
- Normal axis
RBBB does not usually interfere with detecting heart attacks on ECG, unlike LBBB.8
Treatment options
An isolated RBBB without symptoms or structural heart disease does not require treatment. Management involves:
- Monitoring
- Treating underlying lung or cardiac conditions
- Prompt intervention if it develops during acute coronary syndromes.6
Clinical implications: when to worry
Feature LBBB RBBB
- Side affected: Left ventricle, Right ventricle
- Common cause: Heart disease is often benign, sometimes lung or congenital disease
- ECG clues: Wide QRS, absent Q waves, broad R in lateral leads. Wide QRS, rsR′ in V1, wide S in I, V6
- Asymptomatic possible? Yes Yes
- Risk in heart failure: High to low to moderate
- The risk of a heart attack may mask MI on ECG. Increased risk of new-onset
- Requires CRT? Often, yes, if EF is low, rarely needed2,5
Conclusion
- Left and Right Bundle Branch Blocks both disrupt the heart's electrical system, but have different implications:
- LBBB is more likely to indicate serious heart disease and may lead to heart failure. It often requires advanced treatment such as CRT
- RBBB is often harmless when found in healthy people, but it can indicate serious problems if it appears suddenly during a heart attack
- Both types of BBB require accurate ECG interpretation, especially in emergency settings. Patients diagnosed with either should follow up with a cardiologist to determine if further evaluation or intervention is needed
References
- Liddle D, Hutchinson A, Wellings H, Power K, Robinson L, Monk J. Integrated Immunomodulatory Mechanisms through which Long-Chain n-3 Polyunsaturated Fatty Acids Attenuate Obese Adipose Tissue Dysfunction. Nutrients. 2017 Nov 27;9(12):1289.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5748740
- Xiang L, Zhong A, You T, Chen J, Xu W, Shi M. Prognostic Significance of Right Bundle Branch Block for Patients with Acute Myocardial Infarction: A Systematic Review and Meta-Analysis. Medical science monitor: international medical journal of experimental and clinical research [Internet]. 2016 Mar 27;22:998–1004. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4811299
- Xiong Y, Wang L, Liu W, Hankey GJ, Xu B, Wang S. The Prognostic Significance of Right Bundle Branch Block: A Meta-analysis of Prospective Cohort Studies. Clinical Cardiology. 2015 Oct;38(10):604–13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6490823
- Zurmeyer S, Fotopoulou C, Braicu E, Schlichting U, Sehouli J. Clostridium septicum can cause distant myonecrosis in patients with ovarian cancer. Anticancer research [Internet]. 2013 Apr;33(4):1585–9. https://pubmed.ncbi.nlm.nih.gov/23564801
- Schneider JF, Emerson Thomas H, Sorlie P, Kreger BE, McNamara PM, Kannel WB. Comparative features of newly acquired left and right bundle branch block in the general population: The framingham study. The American Journal of Cardiology. 1981 Apr;47(4):931–40. https://pubmed.ncbi.nlm.nih.gov/6452050
- Francia P, Balla C, Paneni F, Volpe M. Left bundle-branch block—pathophysiology, prognosis, and clinical management. Clinical Cardiology. 2007;30(3):110–5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6653265
- Arora R, Chattopadhyay S, Agrawal S, Chatterjee S. Self-inflicted herpetic whitlow. BMJ case reports [Internet]. 2014 Dec;2014:bcr2013201817. https://pubmed.ncbi.nlm.nih.gov/24729108
- R. Andrew Archbold, Sayer J, Ray S, Wilkinson P, Kulasegaram Ranjadayalan, Timmis A. Frequency and prognostic implications of conduction defects in acute myocardial infarction since the introduction of thrombolytic therapy. European Heart Journal. 1998 Jun 1;19(6):893–8. https://pubmed.ncbi.nlm.nih.gov/9651713
