Left bundle branch block, often written as LBBB, describes a delay or block in the electrical pathway within the left bundle branch of the heart. This disruption changes the timing of ventricular activation and is typically identified on an electrocardiogram.
Understanding the definition of LBBB is important because it influences how clinicians interpret tracing, assess underlying heart conditions, and decide on monitoring or treatment strategies. The following sections detail key aspects of LBBB in a structured format.
| Term | Definition | Typical ECG Feature | Clinical Relevance |
|---|---|---|---|
| Bundle Branch | Specialized conduction fibers that deliver electrical impulses to each ventricle | Narrow QRS without block | Ensures coordinated ventricular contraction |
| Left Bundle Branch Block | Delayed conduction through the left bundle branch | Wide QRS, RsR' or M pattern in V5-V6 | May indicate structural heart disease |
| Right Bundle Branch Block | Delayed conduction through the right bundle branch | Wide QRS with characteristic Rs or rSR' pattern | Less often linked to underlying disease |
| Complete vs Incomplete | Degree of conduction delay across the branch | QRS width 120 ms or more for complete | Incomplete may be intermittent or age related |
Electrophysiology Basics of LBBB
The electrical system of the heart relies on specialized pathways to ensure orderly contraction. The left bundle branch is one of these pathways, directing impulses to the left ventricle.
How LBBB Alters Activation Sequence
In LBBB, the impulse reaches the right ventricle normally but is delayed on the left side. This delay causes the left ventricle to activate later, which is visible on the ECG as a widened QRS complex and changes in the terminal portion of the ventricular depolarization.
ECG Criteria and Diagnostic Indicators
Diagnosis of LBBB depends on specific ECG criteria that reflect delayed conduction and altered ventricular activation.
- Wide QRS complex, typically 120 milliseconds or greater
- Monophasic or notched R wave in lateral leads such as I, aVL, V5, and V6
- Deep S waves in initial septal leads V1 and V2
- T wave and ST segment changes that usually oppose the main QRS deflection
Relation to Structural Heart Disease
LBBB is often associated with underlying structural conditions that affect the heart muscle or conduction system.
Common Associated Conditions
Conditions such as hypertension, aortic valve disease, cardiomyopathy, and ischemic heart disease can lead to fibrosis or remodeling that predisposes to LBBB. Identifying these associations helps guide further evaluation and management.
Clinical Evaluation and Further Assessment
When LBBB is identified, clinicians typically evaluate symptoms and risk factors to determine the need for additional testing.
Steps in Evaluation
Assessment may include history and physical exam, echocardiography to evaluate heart structure and function, exercise stress testing when indicated, and ambulatory monitoring to assess symptom correlation and burden.
Key Takeaways for Patients and Clinicians
- LBBB reflects delayed conduction in the left bundle branch and is defined by specific ECG criteria
- It is commonly associated with structural heart disease and requires appropriate evaluation
- New onset LBBB with symptoms warrants urgent assessment for ischemia
- In asymptomatic individuals, focus remains on monitoring and managing underlying conditions
- Serial ECGs and periodic echocardiography may be used to track changes over time
FAQ
Reader questions
What does LBBB mean on an ECG report?
LBBB on an ECG report indicates that the electrical impulse is delayed in the left bundle branch, producing a wide QRS complex with specific waveform patterns that reflect altered ventricular activation.
Is LBBB a sign of a heart attack?
LBBB itself is not a heart attack, but new onset LBBB in the setting of chest symptoms may indicate acute coronary ischemia and requires urgent evaluation to rule out myocardial infarction.
Can LBBB be caused by high blood pressure?
Yes, long standing high blood pressure can lead to left ventricular hypertrophy and fibrosis, which may disrupt conduction and result in LBBB over time.
Do I need treatment if I have LBBB without symptoms?
Asymptomatic Lbbb often does not require direct treatment of the block itself, but regular follow up and management of associated conditions such as hypertension or cardiomyopathy are important.