Bundle Branch Blocks

Basics

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 ms or longer
Incomplete bundle branch block:
- Delayed depolarization, but less than complete bundle branch block
- QRS complex duration 100–120 ms
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
Cardiomyopathy
Cor pulmonale
Hypertension
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 or TAVR procedure
Drugs:
- β-blockers
- Calcium blockers
- Tricyclic antidepressants
- Type Ia and Ic antiarrhythmics
- Digitalis

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