From the Guidelines
Left anterior fascicular block (LAFB) on ECG is diagnosed when there is 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. When interpreting an ECG for LAFB, it's crucial to look for these specific criteria. The presence of LAFB indicates a conduction delay or block in the left anterior fascicle of the left bundle branch, leading to an abnormal sequence of left ventricular depolarization.
- Key diagnostic features include:
- 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 This condition may not require specific treatment by itself but necessitates clinical correlation to identify potential underlying heart diseases such as coronary artery disease, cardiomyopathy, or hypertensive heart disease, as suggested by the guidelines 1.
- Clinical significance is heightened when LAFB is part of a bifascicular block, combined with right bundle branch block.
- Documentation of LAFB should include any associated conduction abnormalities, ST-T wave changes, or other findings suggestive of myocardial ischemia or cardiac pathology.
From the Research
Left Anterior Fascicular Block ECG Interpretation
- The diagnosis of left anterior fascicular block (LAFB) can be evaluated using electrocardiographic criteria, including marked left axis deviation (LAD) and a delay in the time of inscription of the intrinsicoid deflection (ID) in lead aVL asynchronous to V6 2.
- Studies have shown that there is only a general relationship between the degree of LAD and delayed ID in aVL, and the incidence of delayed ID in aVL increases with the degree of LAD 2.
- The LAD alone should not be considered synonymous with LAFB, and recognition of delayed inscription of the ID in aVL is a useful supplemental criterion for diagnosis 2.
- LAFB can also be 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 lead to abnormal ECGs similar to LAFB ECG patterns 3.
- The ECG patterns of LAFB can be simulated using a whole-heart model-based simulation study, and the results show that the typical LAFB ECG patterns can also be observed from cases with slowed conduction velocity in the ventricular myocardium 3.
- Isolated and complicated left anterior fascicular block can be reviewed and illustrated using electrocardiographic criteria, including changes in repolarization and voltage criteria 4.
- Left anterior fascicular block can mask or mimic infarction and left ventricular hypertrophy, and mask right bundle branch block in the setting of an acute anterior myocardial infarction 4.