- In left posterior fascicular block (aka left posterior hemiblock), impulses are conducted to the left ventricle via the left anterior fascicle, which inserts into the upper, lateral wall of the left ventricle along its endocardial surface.
- On reaching the ventricle, the initial electrical vector is therefore directed upwards and leftwards (as excitation spreads outwards from endocardium to epicardium), causing small R waves in the lateral leads (I and aVL) and small Q waves in the inferior leads (II, III and aVF).
- The major wave of depolarisation then spreads along the free LV wall in a downward and rightward direction, producing large positive voltages (tall R waves) in the inferior leads and large negative voltages (deep S waves) in the lateral leads.
- This process takes up to 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
- The impulse reaches the inferior 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 aVF.
Diagnostic Criteria for LPFB
- Right axis deviation (> +90 degrees)
- Small R waves with deep S waves (= ‘rS complexes’) in leads I and aVL
- Small Q waves with tall R waves (= ‘qR complexes’) in leads II, III and aVF
- QRS duration normal or slightly prolonged (80-110ms)
- Prolonged R wave peak time in aVF
- Increased QRS voltage in the limb leads
- No evidence of right ventricular hypertrophy
- No evidence of any other cause for right axis deviation
- LPFB is much less common than LAFB, as the broad bundle of fibres that comprise the left posterior fascicle are relatively resistant to damage when compared with the slim single tract that makes up the left anterior fascicle.
- It is extremely rare to see LPFB in isolation. It usually occurs along with RBBB in the context of a bifascicular block.
- Do not be tempted to diagnose LPFB until you have ruled out more significant causes of right axis deviation: i.e. acute PE, tricyclic overdose, lateral MI, right ventricular hypertrophy.
Examples of LPFB
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
- Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.