Left Anterior Fascicular Block (LAFB)

ECG criteria

Typical ECG of LAFB, demonstrating:

• rS complexes in leads II, III, aVF, with small R waves and deep S waves
• qR complexes in leads I, aVL, with small Q waves and tall R waves
• Left Axis Deviation (LAD): Leads II, III and aVF are NEGATIVE; Leads I and aVL are POSITIVE

Associated features include:

• QRS duration normal or slightly prolonged (80-110ms)
• Increased QRS voltage in limb leads

Electrophysiology

In LAFB, impulses are conducted to the left ventricle (LV) via the posterior fascicle, which inserts into the inferoseptal wall of the LV along its endocardial surface

• Depolarisation spreads from endocardium to epicardium and thus the initial vector is directed downwards and rightwards, towards inferior leads. This produces small R waves in leads II, III and aVF, and small Q waves in leads I and aVL
• The major wave of depolarisation is slightly delayed and spreads upwards and leftwards, producing tall R waves in left-sided leads and deep S waves in inferior leads
• This overall delay of approximately 20ms (compared with normal simultaneous conduction via both fascicles) results in a slight widening of the QRS
• Because impulses reach the left-sided leads later than normal, there is an increase R wave peak time in aVL (time from onset of QRS to peak of R wave)

Sequence of conduction in LAFB

1. YELLOW line: Impulses are conducted to the left ventricle via the posterior fascicle
2. RED arrow: Initial depolarisation vector is directed towards inferior leads
3. BLUE arrow : Major depolarisation vector, which is slightly delayed, is directed towards left-sided leads

ECG Morphology

qR complexes in lateral limb leads, and rS complexes in inferior leads

Left Axis Deviation (LAD)

• Leads I and aVL are POSITIVE;
• Leads II, III and aVF are NEGATIVE

Prolonged R-wave peak time

• Prolonged R-wave peak time (= the time from onset of the QRS to the peak of the R wave) in aVL > 45 ms

Examples of LAFB

Example 1

• Left axis deviation
• qR complexes in I, aVL
• rS complexes in II, III, aVF
• Prolonged R wave peak time in aVL
• Increased QRS voltage in limb leads

Example 2

• Left axis deviation
• qR complexes in I, aVL; rS complexes in inferior leads
• Prolonged R wave peak time in aVL
• Slightly prolonged QRS duration

Example 3

Handy Tips

In LAFB, the QRS voltage in lead aVL may meet voltage criteria for LVH (R wave height > 11 mm), but there will be no LV strain pattern.

Related Topics

• Left bundle branch block LBBB
• Right Bundle Branch Block RBBB
• Left anterior fascicular block LAFB
• Left posterior fascicular block LPFB
• Interventricular Conduction Delay IVCD
• Bifascicular block
• Trifascicular block
• Complete Heart block CHB

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest