From the Guidelines
A left anterior fascicular block on an electrocardiogram (EKG) indicates a conduction abnormality in the left anterior fascicle of the left bundle branch of the heart, characterized by specific EKG criteria, including 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 defined by the American Heart Association 1. This condition is often associated with underlying heart disease, such as coronary artery disease, hypertensive heart disease, or cardiomyopathy, but can also occur in healthy individuals. The key EKG findings in left anterior fascicular block include:
- 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
- A QRS duration less than 120 ms, as outlined in the American Heart Association's recommendations for the standardization and interpretation of the electrocardiogram 1. The presence of a left anterior fascicular block may not require specific treatment, but it is essential to evaluate for underlying cardiac conditions, as it can be a precursor to more severe conduction abnormalities. Regular cardiac follow-up may be recommended, especially if other cardiac abnormalities or symptoms are present. The clinical significance of a left anterior fascicular block often depends on the presence of other cardiac conditions or symptoms, and its management should be guided by the overall clinical context, as suggested by the American Heart Association's guidelines 1.
From the Research
Left Anterior Fascicular Block on EKG
A left anterior fascicular block (LAFB) on an electrocardiogram (EKG) is a heart condition characterized by abnormal electrical conduction in the left anterior fascicle of the heart. The key indicators of LAFB on an EKG include:
- Marked left axis deviation (LAD) [ 2 ]
- Delayed intraventricular conduction, which can be observed as a delay in the time of inscription of the intrinsicoid deflection (ID) in lead aVL [ 2 ]
- Decreased voltage in the chest leads and increased voltage in the limb leads [ 3 ]
- Changes in repolarization, including a decrease in the T wave of leads I and AVL and an increase in leads II, III, AVF, V5, and V6 [ 3 ]
Association with Heart Failure
LAFB has been associated with an increased risk of heart failure [ 4 ]. The underlying mechanism of heart failure with LAFB is not fully understood, but it is thought to be related to impaired conduction in the ventricular myocardium [ 4 ].
Diagnostic Challenges
Diagnosing LAFB using EKG can be challenging, particularly in the presence of coexisting conditions such as inferior infarction [ 5 ]. Phase analysis has been suggested as a useful supplemental criterion for diagnosis [ 5 ]. Additionally, LAFB can mask or mimic other conditions, such as left ventricular hypertrophy and right bundle branch block [ 3 ].