Left Anterior Fascicular Block (LAFB)

ECG criteria
Left Anterior Fascicular Block (LAFB) overview 1
Left Anterior Fascicular Block (LAFB) overview 2
Scroll to annotate: Left Anterior Fascicular Block (LAFB)

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

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
Left Anterior Fascicular Block (LAFB) qR rS complexes
qR complexes

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


Left Anterior Fascicular Block (LAFB) Left axis LAD
Left Axis Deviation (LAD)

Left Axis Deviation (LAD)

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

Prolonged R-wave peak time aVL
Prolonged R-wave peak time

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 Anterior Fascicular Block (LAFB)
  • 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
ECG 2 Left Anterior Fascicular Block (LAFB)
  • 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
ECG 3 Left Anterior Fascicular Block (LAFB)

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.



Advanced Reading

Online

Textbooks


LITFL Further Reading

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

Emergency Medicine Specialist MBChB FRCEM FACEM. Medical Education, Cardiology and Web Based Resources | @jjlarkin78 | LinkedIn |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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