1) Right bundle branch block (RBBB) is characterized by a QRS duration ≥ 120ms and signs of delayed right ventricular activation. It is commonly benign but worsens prognosis when seen in acute myocardial infarction, especially of the proximal left anterior descending artery. 2) Left bundle branch block (LBBB) has a QRS duration ≥ 120ms and signs of delayed left ventricular activation. New onset LBBB indicates the need for thrombolysis in myocardial infarction and worsens prognosis when seen in acute myocardial infarction. 3) Hemiblocks involve delayed activation of only one side of the ventricles. Left anterior hemiblock may mask inferior wall myocardial infarction. Left posterior hemiblock in anterior myocardial infarction

1. Bundle Branch Blocks
Dr.W A P S R Weerarathna

3. Blood supply
• RCA (proximal)  Sinus nodal artery
• LAD  Septal branch to proximal RBB
and anterior LBB.
• LAD (septal) & RCA (terminal) 
Posterior fascicle of LBB
• PDA  AV nodal branch and bundle of
HIS supply

4. Right Bundle Branch Block (RBBB)
• QRS duration ≥ 120ms
• rSR’ pattern or notched R wave in V1
• Wide S wave in I and V6

5. Causes
• Normal variant in 0.2% of adults.
• CAD  Acute anterior MI (occlusion of proximal
LAD)
• Pulmonary hypertension (COPD)
• Acute pulmonary embolism
• Congenital heart disease e.g. ASD, Ebstein’s
anomaly
• Rate dependent RBBB
• Rare: Brugada syndrome

6. Clinical significance
• RBBB is commonly seen and is usually
benign
• RBBB in the setting of an acute MI
worsens the prognosis (indicates proximal
LAD occlusion)
• Presence of RBBB on ECG is not a
contraindication for TMT

7. Right Bundle Branch Block

9. Left Bundle Branch Block (LBBB)
• QRS duration ≥ 120ms
• Broad R wave in I and V6
• Prominent QS wave in V1
• Absence of q waves (including physiologic
q waves) in I and V6

10. Causes
• CAD  Acute AWMI (new onset LBBB)
• Dilated Cardiomyopathy
• Aortic stenosis
• Long-standing hypertension
• Rate dependent LBBB
• RV paced rhythm

11. Clinical significance
• New onset LBBB is an indication for thrombolytic
therapy
• LBBB in the setting of an acute MI worsens the
prognosis
• Standard LVH criteria are not valid in presence
of LBBB
• LBBB may mask signs of myocardial infarction
• LBBB on ECG is a contraindication for TMT
• Presence of LBBB in heart failure indicates
ventricular dyssynchrony

12. Left Bundle Branch Block

14. Diagnosis of MI in the presence of
LBBB
• Sgarbossa criteria: Points
• ST segment elevation of ≥1mm 5
concordant with QRS complex
• ST depression ≥ 1mm in leads V1-V3 3
• ST segment elevation ≥5mm and 2
discordant with QRS complex
• Score ≥ indicates Acute MI

15. Left anterior hemiblock (LAHB)
• Left axis deviation (> -30 degrees)
• Duration of QRS complex < 120msec
• qR morphology in Lead I, aVL
• rS morphology in Leads II, III, aVF
• ‘q1S3’ pattern
• Slurred S waves in left precordial leads

16. LAHB

17. LAHB - significance
• May be normal variant
• Occurs in HTN, Cardiomyopathy
• May be seen in acute MI (LAD territory)
• Masks old inferior wall MI by abolishing
the diagnostic Q-waves in II, III, and aVF
LAHB is more common than LPHB

18. Left Posterior hemiblock
• The duration of the QRS complex axis is
normal (<120msec)
• QRS axis is ≥ +120° (RAD)
• Prominent Q wave in leads II, III, and aVF
• rS complexes in Leads I, aVL
• ‘S1Q3’ pattern

19. LPHB

20. LPHB – significance
• LPHB may mimic old IWMI due to Q
waves in II, III, aVF
• LPHB in the setting of Anterior MI
indicates extensive damage to the
conduction system and poor prognosis
• Other causes of RVH are to be excluded
before diagnosing LPHB