Left anterior fascicular block (LAFB)

• Leftward axis
• qR in I and aVL
• rS in III
• QRS may be normal or slightly prolonged
• Poor R-wave progression is common
• No other reasons for leftward axis

Differential diagnosis for leftward axis

• Left ventricular hypertrophy (LVH)
• Left bundle branch block (LBBB)
• Left anterior fascicular block (LAFB)
• Paced rhythm
• Ventricular beats/ectopy
• Pre-excitation (e.g. WPW)
• Inferior MI (from Q-waves)
• Hyperkalemia
• Normal Variant

Left posterior fascicular block (LPFB)

• Rightward axis
  • Rarely a normal variant
    • Left posterior fascicle receives blood supply from both left and right coronary arteries
    • Almost always occurs in presence of CAD: if acute MI, there is usually multivessel disease, extensive infarct and poor prognosis
  • rS in I and aVL
  • qR in III
  • QRS may be normal or slightly prolonged
  • No other reasons for rightward axis

Differential diagnosis for rightward axis

• Right ventricular hypertrophy (RVH)
• Left posterior fascicular block (LPFB)
• Lateral MI (from Q-waves in lead I)
• Ventricular ectopy (VT)
• Hyperkalemia
• Na⁺channel blocker toxicity
• Pulmonary HTN – Acute (PE) & chronic lung disease (COPD)
• Lead misplacement / Dextrocardia
• Normal variant in young kids & thin adults with horizontally positioned hearts​

Type of Fascicular Block	LAFB	LPFB
Axis	Left Axis Deviation	Right Axis Deviation
Leads I & aVL
Lead III

Bonus Pearls:

Bifascicular Block

• Conduction abnormality where two of the three main fascicles of the His/Purkinje system are blocked

• Usually a RBBB + LAFB
• Or RBBB + LPFB

Trifascicular Block

• 1st degree AV Block + RBBB + LAFB
• Or 1st degree AV Block + RBBB + LPFB

References:

1. Chung EK. ECG Diagnosis: A Self-Assessment Workbook. Blackwell Science. 2000. Amazon Link.