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Bifascicular Block

Bifascicular Block (RBBB with either LAFB or LPFB)

Bifascicular Block

• In bifascicular block, 2 ventricular pathways are simultaneously blocked
• Bifascicular block has 2 combinations:
• RBBB + LAH (Right Bundle Branch Block + Left Anterior Hemiblock)
• RBBB + LPH (Right Bundle Branch Block + Left Posterior Hemiblock)
• The ventricles are activated only through one intact fascicle, which arises from the left bundle branch
• If the intact fascicle is blocked
• It results in AV block III degree (complete electrical isolation of atria from ventricles)
• It does not result in trifascicular block
• The left posterior fascicle is thicker than the anterior and has dual blood supply
• If the left posterior fascicle is damaged, it usually involves extensive vascular damage, and almost always results in RBBB
• Therefore, LPH is almost always part of bifascicular block (RBBB + LPH)
• Left posterior hemiblock has a poor prognosis, regardless of whether it is part of a bifascicular block
• LAH + LPH is not bifascicular block
• Two pathways are blocked, but it is a left bundle branch block (LBBB)
• The left bundle branch can be blocked at 2 locations
• At the trunk (just above the bifurcation of fascicles)
• At the level of the left and right fascicles, which is not considered bifascicular block

Right Bundle Branch Block (RBBB)

• Wide QRS ≥ 0.12s
• If the QRS is 0.10-0.12s wide, it indicates incomplete RBBB
• V1 (rsR')
• Resembles the letter "M," with the second ear of the rabbit being larger
• V1 (Negative T waves)
• V6 (Wide S wave)
• Resembles the letter "W"

Left Posterior Hemiblock

• Narrow QRS complexes (< 0.12s)
• Right axis deviation (>90°)
• Negative QRS in lead I
• Positive QRS in lead III

Bifascicular Block (RBBB + Left Posterior Hemiblock)

• Right Bundle Branch Block (RBBB)
• Wide QRS ≥ 0.12s
• rSR' in V1 (Second rabbit ear is larger)
• Wide S wave in V6
• Left Posterior Fascicular Block (ĽZH)
• Negative QRS in lead I
• Positive QRS in lead III

Bifascicular Block (iRBBB + Left Posterior Hemiblock)

• Incomplete Right Bundle Branch Block (iRBBB)
• Narrow QRS < 0.12s
• RSR' in V1
• Wide S wave in V6
• Left Posterior Hemiblock (ĽZH)
• Negative QRS in lead I
• Positive QRS in lead III
• Sinus Tachycardia: 150/min.

Bifascicular Block (RBBB + Left Posterior Hemiblock)

• Right Bundle Branch Block (RBBB)
• Wide QRS ≥ 0.12s
• rSR' in V1 (The second rabbit ear is larger)
• Wide S wave in V6
• Left Posterior Hemiblock (ĽZH)
• Negative QRS in lead I
• Bifasic QRS in lead III
• Extreme right axis deviation