• Fascicular block (or hemiblock) arises when conduction via one of the 2 branches (anterior and posterior) of the left bundle branch is blocked resulting in delayed depolarization mediated by the blocked branch.
• ECG often shows axis deviation, qR (small q/ tall R), and rS (small r/ deep S) in lateral (lead I) and inferior (leads II, III) leads.
• Because fascicular block is caused by occlusion(s) of the supplying coronary artery such as the left anterior descending (LAD) and the right coronary artery (RCA), it often suggests underlying myocardial infarct.
• If there is combination of either left anterior or posterior fascicular block and right bundle branch block (RBBB), it’s called bifascicular block, which often presents with RBBB and axis deviation.
• If all 3 branches (right bundle, left anterior, and left posterior) are blocked, it’s called trifascicular block, which often presents with RBBB, axis deviation, and AV block.
• Left anterior fascicular block (LAFB) arises when the anterior division of the left bundle branch is blocked.
• This might suggest an occlusion of a branch of the LAD that supplies blood to the left anterior division.
• As a result, the left anterior branch’s depolarization is delayed as compared to the right bundle and left posterior branches’ depolarizations, causing:
Left axis deviation without left ventricular hypertrophy
QRS complexes with 2 components: an early component correlating with right bundle and left posterior branch and a late component correlating with the left anterior branch-- this results in “R1S3” pattern consisting of:
qR in leads I, aVL: early small q and late tall R
rS in leads II, III: early small r and late deep S
• The resulting QRS complexes are often narrow or only slightly widened; and T waves might be flat or inverted in lateral leads (I, aVL).
• LAFB is is often seen in patients with anterior STEMI.
• Left posterior fascicular block (LPFB) arises when the posterior division of the left bundle branch is blocked.
• This might suggest an occlusion of either a branch of the LAD or RCA or both since both branches supplies blood to the left posterior division.
• As a result, the left posterior branch’s depolarization is delayed as compared to the right bundle and left anterior branches’ depolarizations, causing:
Right axis deviation without right ventricular hypertrophy
QRS complexes with 2 components: an early component correlating with right bundle and left anterior branch and a late component correlating with the left posterior branch-- this results in “R3S1” pattern consisting of:
qR in leads II, III: early small q and late tall R
rS in leads I, aVL: early small r and late deep S
• The resulting QRS complexes are often narrow or only slightly widened.
• LPFB is is often seen in patients with inferior STEMI.
• Bifascicular block is fascicular block (either LAFB or LPFB) plus RBBB.
• On ECG, there is either:
RBBB + LAFB + LAD
RBBB + LPFB + RAD
• Trifascicular block is bifascicular block plus a block in the remaining fascicle either due to delayed conduction upstream (AV block) or within the fascicle itself (alternating LAFB/LPFB).
• Trifascicular block is further characterized as either “incomplete” or “complete”:
Incomplete block = bifascicular block + 1st or 2nd degree AV block or RBBB + alternating LAFB/LPFB
Complete block = bifascicular block + complete AV block (3rd degree)
