Bundle Branch Blocks



Left BBB

• Block in left bundle leads to reversal of normal direction of activation of septum
• Normal - L to R, LBBB - R to L
• Left venricle contracts later in than the right ventricle
  
• Increases duration of QRS (> 120 ms), tall R waves in lateral leads (I, V5-6), deep S waves in anterior leads (V1-3), LAD

Causes of LBBB (unusual to have LBBB w/out underlying cardiac pathology)

• Aortic valve disease (stenosis, regurgitation, root dilatation)
  
• CAD / Ischemic heart disease / MI
  
• Dilated CM
• Hyperkalaemia
• Digoxin toxicity
• Prior cardiac surgery
• Degenerative diseases of the conduction system (e.g. Lenegre-Lev syndrome)

Left AFB

• Blockage of Anterior fascicle leads to impulses to LV conducted via L posterior fascicle (delayed conduction widens QRS)
• Initial electrical vector is down and right (small R waves inferior leads and small Q waves in left-sided leads)
• As depolarization progresses electrical vector moves leftward (large R waves in left-sided leads and large S wave in inferior leads)
• EKG criteria include:
• LAD
• Small Q waves and tall R waves in I and aVL
• Small R waves with deep S waves in II, III, aVF
• QRS duration normal to slightly long
• Prolonged R wave peak time (time from the beginning of the QRS complex to the peak of R wave) in aVL
  

Causes of LAFB

• CAD / IHD / MI
• Hypertension
• Aortic valve disease
• CM / cardiac degenerative diseases

Left PFB

• Blockage of Posterior fascicle leads to impulses to LV conducted via L Anterior fascicle (delayed conduction widens QRS)
• Initial electrical vector is up and leftwards (small R waves in Ieft-sided leads and small Q waves in inferior leads)
• As depolarization progresses electrical vector moves down and to the right (RAD with tall R waves in inferior leads and deep S waves in left-sided leads)
• EKG criteria include:
• RAD (with no other obvious cause)
  
• Small R waves with deep S waves in I and aVL
• Small Q waves with Tall R waves in II, III, and aVF
• Prolonged R wave peak time (time from the beginning of the QRS complex to the peak of R wave) in aVL

Causes

• LPFB rare because it has redundant blood supplies
• Do not diagnose without considering other causes of RAD

Right BBB

• Block in right bundle leads to delay in activation of RV so depolarization spreads through mycardium rather than conduction system
  
• LV activated normally so early part of QRS is normal
• Delayed activation of RV produces second R wave (R`) in right precordial leads (V1-V3) and wide, slurred S wave laterally
• Incomplete RBBB - QRS < 120 ms
  

Causes

• Congenital heart disease (e.g. atrial septal defect)
• Brugada syndrome
  
• Right ventricular hypertrophy / cor pulmonale
• PE
  
• Ischaemic heart disease
• Rheumatic heart disease
• Myocarditis or cardiomyopathy
• Degenerative diseases of the conduction system (e.g. Lenegre-Lev syndrome)
• Incomplete RBBB may be normal variant

Bi-Fascicular Block

• Combination of RBBB and LAFB (most common) or LPFB
• Conduction to ventricles is via sole remaining fascicle
• Could progress to Tri-fascicular block (i.e. CHB)
  

Causes

• Overlaps with individual causes of blocks

Sources: http://lifeinthefastlane.com/ecg-library/basics/right-bundle-branch-block/
Sources: http://lifeinthefastlane.com/ecg-library/basics/left-bundle-branch-block/
Sources: https://lifeinthefastlane.com/ecg-library/basics/left-anterior-fascicular-block/
Sources: https://lifeinthefastlane.com/ecg-library/basics/left-posterior-fascicular-block/