Bundle Branch Blocks
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- Blockage of intraventricular electrical impulses through the right and left bundles
- Complete bundle branch block:
- Absence or delay of conduction down one bundle, with normal conduction down the other bundle
- Affected ventricle depolarizes from muscle to muscle in a slower and more disorganized fashion.
- Quasi-random signal (QRS) complex at 120 msec or longer
- Incomplete bundle branch block:
- Delayed depolarization, but less than complete bundle branch block
- QRS complex duration 100–120 msec
- Right bundle branch block (RBBB):
- Delayed depolarization of the right ventricle
- Left bundle branch block (LBBB):
- Delayed depolarization of the left ventricle
- LBBB can be caused by delay of conduction in main left bundle or delay in both fascicles of the left bundle.
- Causes early activation of the right side of the septum and the right ventricular myocardium (so explaining loss of “septal Q” on ECG)
- Left bundle branches into 2 fascicles:
- Left anterior fascicle: Initial septal activation proceeds inferiorly, anteriorly, and to the right.
- Left posterior fascicle: Isolated blockage rare; activation begins in the midseptum and finishes in inferior and posterior walls.
- Bifascicular block:
- RBBB with concomitant block of the left anterior or left posterior fascicle
- Myocardial infarction
- Age-related fibrosis of Purkinje fibers
- Valvular disease
- Exercise induced
- Congenital/atrial septal defect
- Brugada syndrome (RBBB): Cause of sudden cardiac death in otherwise healthy patients.
- Chagas disease (especially Central/South America)
- Postoperative, following cardiac surgery
- Calcium blockers
- Tricyclic antidepressants
- Type Ia and Ic antiarrhythmics