Causes of a Bundle Branch Block

 Initially, the right bundle branch off of the bundle of His travels down the interventricular septum near the endocardium. It then dives deeper into the muscular layer before re-emerging near the endocardium again. The right bundle branch receives most of its blood supply from the anterior descending coronary artery.¹

Right bundle branch block is associated with structural changes from stretch or ischemia to the myocardium. It can also occur iatrogenically from certain common cardiac procedures, such as right heart catheterization. Although there is no significant association with cardiovascular risk factors, the presence of a right bundle branch block is a predictor of mortality in myocardial infarction, heart failure, and certain heart blocks. In asymptomatic patients, isolated right bundle branch block typically does not need further evaluation.¹

A Right Bundle Branch Block is occasionally normal and is seen in 1% of young and up to 5% of elderly:

• pulmonary disease
  • chronic lung disease
  • pulmonary embolus
• cardiomyopathy
• ischaemic heart disease
• degenerative fibrosis
  • congenital heart disease – ASD, VSD, PS, Fallot’s
  • drugs / electrolyte abnormality
  • right ventriculotomy

A Left Bundle Branch Block is usually a sign of disease of the left ventricle:

• aortic stenosis / hypertension (outflow obstruction)
• cardiomyopathy
• ischaemic heart disease – e.g. AMI
• degenerative fibrosis

A simple way to diagnose a left bundle branch in an ECG with a widened QRS complex (> 120 ms) would be to look at lead V1. If the QRS complex is widened and downwardly deflected in lead V1, a left bundle branch block is present. If the QRS complex is widened and upwardly deflected in lead V1, a right bundle branch block is present.² 

The features of interventricular conduction disturbance include:

• seen with ischaemic heart disease / cardiomyopathy
• asymptomatic usually – except in complete His-bundle block
• if syncope / symptomatic:
  • ? intermittent complete heart block
  • ? ventricular tachycardia
    • ⇒ Holter monitoring / EPS

Further Reading

1. Harkness WT, Hicks M. Right Bundle Branch Block. [Updated 2020 Aug 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507872/. 
2. Left Bundle Branch Block (LBBB) ECG Review – Criteria and Examples | LearntheHeart.com (healio.com)
3. Scherbak D, Hicks GJ. Left Bundle Branch Block. [Updated 2020 Aug 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482167/.