Left anterior fascicular block (LAFB) or Left anterior hemiblock (LAH) is a type of unifascicular block of left bundle branch with typical ECG pattern – left axis deviation
ECG features
Left axis deviation between −45 and −120 degrees
QRS complex duration
qR complexes with R wave slurring in leads I and aVL
rS complexes in the inferior leads II, III, aVF
Left axis deviation
QRS is POSITIVE (dominant R wave) in Lead I
QRS is NEGATIVE (dominant S wave) in leads II, III and aVF
Picture 1 Left axis deviation
ECG 1 Left anterior fascicular block
left axis deviation -72°
qR complexes in I and aVL
rS complexes in II, III, aVF
slightly prolonged QRS complexes with duration 100ms
Causes
Left bundle branch consists of two fascicles – Left anterior fascicle and Left posterior fascicle
Normal activation of the left-ventricular free wall spreads simultaneously from two sites
When a block in either the LAF or LPF is present, activation of the free wall proceeds from one site instead of two. This changes the waveforms of the QRS complex and produces a shift of the QRS complex axis
Isolated fascicular block is often benign
The incidence of fascicular block in otherwise normal individuals is 2% to 5% and increases with age
Difference between LAFB and LVH
left axis deviation can also be seen in left ventricular hypertrophy
LAFB can be distinguished by the timing of the R wave in aVL vs. aVR
R wave in aVL precedes the late terminal R wave in aVR
References
Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Chapter 2 - Bradyarrhythmias—Conduction System Abnormalities, Editor(s): Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Arrhythmia Essentials (Second Edition), Elsevier, 2017, Pages 28-86, ISBN 9780323399685, https://doi.org/10.1016/B978-0-323-39968-5.00002-0.
CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
STRAUSS, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021
