From the Guidelines
Left bundle branch block (LBBB) and right bundle branch block (RBBB) are distinct electrical conduction abnormalities in the heart, with LBBB being associated with a higher risk of underlying heart conditions and worse mortality outcomes, as evidenced by a study showing LBBB was associated with significantly worse mortality (HR: 1.17; 95% CI: 1.00–1.36) and a decrease in LVEF to #35% (HR: 1.34; 95% CI: 1.09–1.63) 1.
Definition and Diagnosis
LBBB is characterized by a widened QRS complex (>120 ms) with a dominant R wave in lead V6 and absence of Q waves in leads I and V6, while RBBB is characterized by a widened QRS complex with an RSR' pattern (rabbit ears) in lead V1 and a slurred S wave in leads I and V6 1.
Clinical Implications
Both conditions may be asymptomatic and discovered incidentally, but treatment focuses on addressing any underlying heart disease rather than the conduction abnormality itself. Regular cardiac follow-up is important, especially for LBBB, which generally has more serious implications than RBBB.
Key Differences
The key differences between LBBB and RBBB lie in their electrocardiographic (ECG) patterns and associated underlying heart conditions. LBBB is often associated with coronary artery disease, hypertension, or cardiomyopathy, while RBBB can occur in healthy individuals but may also indicate pulmonary embolism, congenital heart disease, or right ventricular strain.
Management
Management of conduction disorders, including LBBB and RBBB, involves addressing any underlying heart disease and may include permanent pacing or cardiac resynchronization therapy (CRT) in certain cases, as outlined in the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.
- LBBB with QRS >150 ms and Class II or greater HF symptoms may benefit from CRT.
- RBBB with syncope or alternating BBB may require permanent pacing.
- Regular cardiac follow-up is essential to monitor for any changes in conduction or underlying heart disease.
From the Research
Left Bundle Branch Block (LBBB)
- LBBB is a condition where the left bundle branch of the heart's electrical conduction system is blocked, causing the left ventricle to contract later than the right ventricle 2, 3, 4, 5.
- The electrocardiogram (ECG) criteria for LBBB include a QRS duration ≥140 ms for men and ≥130 ms for women, QS or rS in leads V1 and V2, and mid-QRS notching or slurring in ≥2 of leads V1, V2, V5, V6, I, and aVL 2, 3.
- LBBB can be caused by various factors, including primary conduction disease, underlying pathology, or iatrogenic factors 5.
- Cardiac resynchronization therapy (CRT) is a treatment option for patients with LBBB, which can improve left ventricular mechanical function by changing the sequence of electrical activation 2, 3.
Right Bundle Branch Block (RBBB)
- RBBB is a condition where the right bundle branch of the heart's electrical conduction system is blocked, causing the right ventricle to contract later than the left ventricle 6.
- RBBB is associated with an increased risk of pacemaker implantation, heart failure, and cardiovascular death 6.
- The electrocardiogram (ECG) criteria for RBBB are not as well-defined as those for LBBB, but it is often characterized by a QRS duration ≥120 ms and a specific pattern of QRS complex morphology in the right precordial leads 6.
Comparison of LBBB and RBBB
- Both LBBB and RBBB can have significant clinical implications, including an increased risk of heart failure and cardiovascular death 6.
- However, LBBB is more strongly associated with heart failure and cardiovascular death, particularly in men 6.
- The QRS interval duration is an important factor in determining the risk of heart failure and cardiovascular death in patients with LBBB, with longer QRS intervals associated with a higher risk 6.