Bundle Branch Blocks



  • 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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