Fascicular Block or Hemiblocks (LAFB and LPFB) understanding simplified.

Left bundle divides into two fascicules i.e. anterior and posterior.

Anterior fascicle activates anterior and superior part of left ventricle and posterior fascicle activates inferior and posterior part of left ventricle

Electrical current passing through fascicle is faster and inscribes smaller wave unlike through muscle.

For simplification purpose lets us, only consider frontal plane i.e. up and down left and right and not worry about posterior and anterior.

Anterior fascicle activates superiorly and left along the lead 1 and aVL, and posterior fascicle activates along inferior leads i.e. aVF and lead 3

Left anterior fascicular block (LAFB)

Conduction system of heart

Hexagonal reference of ECG

If anterior fascicle is blocked there is delay in activation along lead 1 and aVL, along with there is unopposed (vector) along inferior leads because posterior fascicle activation is fast and normal, so initially in leads 1 an aVL electrical vector moves away, leading initial “q” waves in lead 1 and aVL, and in inferior leads initially positive “r” waves is inscribed, i.e. lead aVF and lead 2 and 3.

After initial inferior activation by posterior fascicle superior part of the left ventricle gets activated leading formation of positive “R” waves in lead 1 and aVL and negative “S” waves inferior leads ie aVF lead 2 and lead 3

As later activation produces larger deflection compared to initial activation by fascicle, frontal plane axis is determined by later deflection i.e. left and superior axis (“R” in lead 1 and aVL). So in LAFB frontal plane axis is left ward -45 to -90 (axis in between -30 to -45 is considered not due to LAHB but instead LVH)

Later activation is more anterior in LAFB so lateral precardial leads inscribes negative S waves

LAFB occurs frequently because it is delicate structure, and can be present in normal persons. In CAD its occurrences carries bad prognosis

Inferior wall MI is masked by LAFB

Diagnostic ECG finings in LAFB

1. Frontal plane axis -45 to -90

2. qR pattern in lead 1 and aVL

3. QRS duration less than 120 msecs

4. Time to peak R wave in lead aVL more than 45 msecs

Left posterior fascicular block (LPFB)

In LPHB electrical forces are just opposite to LAHB ie initial vector is towards left and suproior later vector moves to inferior, so initially in LPHB, small “r” waves in lead 1 and aVL, in inferior leads aVF, lead 2 and 3 “q” waves are produced. Later vector is reversed so “S” wave in lead 1 and aVL and “R” wave in lead aVF lead 2 and 3

Later vector is bigger so it determines frontal plane axis ie right axis +90 to +180

Diagnostic ECG findings in LPFB

1. Frontal plane axis +90 to +180

2. rS pattern in lead 1 and aVL with qR pattern in 3 and aVF

3. QRS duration less than 120 msecs

4. exclude other causes for right axis deviation

LPFB is less common because fascicle is thick, it is more protected in location ie LVOT