Left Anterior Fascicular Block (LAFB)

In left anterior fascicular block (previously left anterior hemiblock), impulses are conducted to the left ventricle via the left posterior fascicle, which inserts into the infero-septal wall of the left ventricle along its endocardial surface.

• In reaching the left ventricle, the initial electrical vector is therefore directed downwards and rightwards (as excitation spreads outwards from endocardium to epicardium), producing small R waves in the inferior leads (II, III, aVF) and small Q waves in the left-sided leads (I, aVL).
• The major wave of depolarisation then spreads in an upwards and leftwards direction, producing large positive voltages (tall R waves) in the left-sided leads and large negative voltages (deep S waves) in the inferior leads.
• This process takes about 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
• The impulse reaches the left-sided leads later than normal, resulting in a increased R wave peak time (the time from onset of the QRS to the peak of the R wave) in aVL.

ECG Criteria for Left Anterior Fascicular Block (LAFB)

• Left axis deviation (usually between -45 and -90 degrees)
• Small Q waves with tall R waves (= ‘qR complexes’) in leads I and aVL
• Small R waves with deep S waves (= ‘rS complexes’) in leads II, III, aVF
• QRS duration normal or slightly prolonged (80-110 ms)
• Prolonged R wave peak time in aVL > 45 ms
• Increased QRS voltage in the limb leads

Example ECG

• qR complexes in leads I and aVL
• rS complexes in II, III and aVF

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

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

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

Advanced Reading

• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
• Wagner GS. Marriott’s Practical Electrocardiography 12e
• Chan TC. ECG in Emergency Medicine and Acute Care
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e