54  Bundle Branch Blocks

54.1 Left BBB

  • V1: wide QRS, negative S deflection (‘deep S wave’)
  • V6: wide QRS, positive R deflection (‘tall R wave’); may have dicrotic notch (‘M-shaped’)
  • Other features (Source):
    • Appropriate discordance: the ST segments and T waves always go in the opposite direction to the main vector of the QRS complex
    • Poor R wave progression in the chest leads
    • LAD (typically)
  • Causes:
    • LBBB/paced rhythm/ventricular ectopy
    • LVH
    • LAFB
    • WPW
    • inferior MI

54.2 Right BBB

  • V1: wide QRS, RSR’ pattern (‘M-shaped pattern’ or ‘bunny ear pattern’)
  • V6: wide QRS, small upward deflection (R wave) followed by ‘wide, slurred S wave’
  • Other features:
    • ST depression and T wave inversion in the right precordial leads (V1-3)
    • often normal axis deviation (unless other cause, e.g. RVH, which leads to RAD)
  • Causes:
    • LPFB
    • lateral MI
    • RVH
    • PE
    • COPD
    • ventricular ectopy
    • WPW
    • hyperK
    • Na-channel blocker toxicity
    • could be normal in those with horizontally positioned heart