Left anterior fascicular block (LAFB)


  • 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

1) rS complexes in the inferior leads II, III, aVF
2) Left axis deviation between −45 and −120 degrees
3) QRS complex duration normal or slightly prolonged (80-110ms)
4) qR complexes with R wave slurring in leads I and aVL   

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


  • 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


  1. 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.
  2. CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
  3. STRAUSS, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021