From the Guidelines
Left anterior fascicular block (LAFB) is a cardiac conduction abnormality that can be diagnosed on an electrocardiogram (ECG) with a frontal plane axis between 45° and 90°, a qR pattern in lead aVL, an R-peak time in lead aVL of 45 ms or more, and a QRS duration less than 120 ms, as outlined in the 2009 AHA/ACCF/HRS recommendations 1.
Diagnosis and Characteristics
The diagnosis of LAFB is based on specific ECG criteria, including:
- Frontal plane axis between 45° and 90°
- qR pattern in lead aVL
- R-peak time in lead aVL of 45 ms or more
- QRS duration less than 120 ms These criteria are essential for accurately identifying LAFB and distinguishing it from other cardiac conduction abnormalities.
Clinical Implications
LAFB is often an incidental finding that does not require specific treatment when isolated. However, it is crucial to evaluate for underlying heart disease, as LAFB can be associated with coronary artery disease, hypertension, cardiomyopathy, or valve disease 1. When LAFB occurs with right bundle branch block, it may indicate more extensive conduction system disease and carries a higher risk of progressing to complete heart block.
Management and Prognosis
Patients with isolated LAFB generally have a good prognosis and do not require specific therapy. However, they should undergo cardiac evaluation to identify any underlying structural heart disease. Regular cardiac follow-up is recommended, especially if other conduction abnormalities develop, as this may indicate progressive cardiac disease requiring intervention. The 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities provides guidance on the management of patients with bifascicular block, including those with LAFB 1.
Key Considerations
- Evaluate for underlying heart disease in patients with LAFB
- Regular cardiac follow-up is recommended, especially if other conduction abnormalities develop
- Consider device-based therapy in patients with bifascicular block, including those with LAFB, according to the 2012 ACCF/AHA/HRS guidelines 1
From the Research
Left Anterior Fascicular Block
- Left anterior fascicular block (LAFB) is a type of heart block that can be identified by an abnormal electrocardiogram (ECG) 2.
- LAFB is associated with an increased risk of heart failure, and non-specific intraventricular conduction delay due to lesions of the conduction bundles and slow cell to cell conduction may also contribute to heart failure 2.
- The electrocardiographic criteria for isolated LAFB include decreased voltage in the chest leads and increased voltage in the limb leads, as well as changes in repolarization such as decreased T wave in leads I and AVL and increased T wave in leads II, III, AVF, V5, and V6 3.
Risk of Adverse Cardiovascular Outcomes
- LAFB is associated with an increased risk of incident higher degree atrioventricular block (AVB), syncope, pacemaker implantation, and death 4.
- The risk of developing third-degree AVB is increased by 0%-2% over 10 years in patients with isolated LAFB, with a hazard ratio (HR) of 1.6 (95% confidence interval [CI] 1.25-2.05) 4.
- The combination of right bundle branch block, LAFB, and first-degree AVB is associated with a higher risk of adverse cardiovascular outcomes, including up to 23% increased 10-year risk of developing third-degree AVB (HR 11.0; 95% CI 7.7-15.7) 4.
Diagnosis and Masking of Other Conditions
- LAFB may mask or mimic infarction and left ventricular hypertrophy, and may also mask right bundle branch block in the setting of an acute anterior myocardial infarction 3.
- The simulation results of a whole-heart model-based study suggest that impaired conduction in the ventricular myocardium may lead to abnormal ECGs similar to LAFB ECG patterns, which may provide a potential reference for LAFB diagnosis 2.