Left Posterior Fascicular Block (LPFB)

In left posterior fascicular block (previously 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.

ECG Criteria for Left Posterior Fascicular Block (LPFB)

Right axis deviation (RAD) (> +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

Example ECG

Left Posterior Fascicular Block (LPFB) lqR rS complexesLeft Posterior Fascicular Block (LPFB) qR rS complexes

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

Left Posterior Fascicular Block (LPFB) Right axis deviation

Right Axis Deviation (RAD)

Leads II, III and aVF are POSITIVE;
Leads I and aVL are NEGATIVE

Prolonged R-wave peak time

R-wave peak time: Time from onset of the QRS to the peak of the R wave in aVF > 45 ms

Handy Tips

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: Example: acute pulmonary embolus; tricyclic overdose; lateral STEMI; and right ventricular hypertrophy.