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Left Anterior Fascicular Block

Left Anterior Hemiblock (LAHB), Left Anterior Fascicular Block

Left Anterior Fascicular Block

• In left anterior Fascicular Block:
• The lower part of the left ventricle is activated first
• Through the posterior fascicle
• A small initial vector (V_I) is created
• Then the upper lateral part is activated
• Through the myocardium, because the left anterior fascicle is blocked
• A main cardiac vector (V_H) is created and it turns upward to the left

ECG and Left Anterior Fascicular Block

• Narrow QRS complex < 0.12s
• The QRS is slightly widened by about 20ms but does not exceed 0.12s
• Left axis deviation > -30º (most commonly -45º to -90º)
• Lead I is positive
• Leads II and III are negative
• Prolonged R wave peak time in aVL > 45ms
• The time from the beginning of the QRS complex to the peak of the R wave is more than 45ms
• Due to prolonged activation of the left ventricle
• Specifically, the part that should be activated through the blocked anterior fascicle

Left Anterior Fascicular Block and Old Anteroseptal Infarct

• Precordial Leads (V1-V2) are placed parasternally in the 4th intercostal space
• Physiological P wave (V1-V2) is equifasic (positive-negative)
• Leads (V1-V2) are used for differential diagnosis:
• Left Anterior Fascicular Block vs. Old Anteroseptal Infarct

Differential Diagnosis Using V1-V2

• Properly placed V1-V2 electrodes in the 4th intercostal space
• P wave is equifasic
• Sometimes there is a QS configuration in V1-V2, which can indicate:
• Left Anterior Fascicular Block, or Old Anteroseptal Infarct
• If V1-V2 are placed lower (6th intercostal space)
• P wave is monofasic positive
• rS configuration (V1-V2) appears
• This indicates Left Anterior Fascicular Block
• Old infarct would still have QS configuration (without r wave)
• If V1-V2 are placed higher (2nd intercostal space)
• P wave is monofasic negative

Left Anterior Fascicular Block

• Narrow QRS complexes (< 0.12s)
• Left Axis Deviation (> -30º)
• Positive QRS in lead I
• Negative QRS in leads II and III