Bundle Branch Blocks

Basics

Impaired ventricular conduction through His–Purkinje bundles
Incomplete bundle branch block:
- Partial delay in conduction down one bundle
- Quasi-random signal (QRS) complex duration of 100–120 ms
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
- QRS complex at 120 ms or longer
Right bundle branch block (RBBB):
- Delayed depolarization of the right ventricle causing the right ventricle to depolarize via slower, muscle-to-muscle conduction
Left bundle branch block (LBBB):
- Delayed depolarization of the left ventricle
- 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
- 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)
Bifascicular block:
- RBBB with concomitant block of the left anterior or left posterior fascicle

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