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