Bundle Branch Block


  • Bundle branch block and other ventricular conduction disorders are usually primarily conditions affecting the his-purkinje system.
  • Normally, the spread of conduction down this system allows a coordinated activity of ventricular contraction.
  • If there is dysfunction of this system, this cannot occur
  • Bundle branch block is primarily a diagnosis made on ECG.

Left Bundle Branch Block


  • Normally, the septum is activated from left to right, producing small Q waves in the lateral leads (I, aVL and V5-6)
  • In complete LBBB, this is reversed as electrical activity first has to pass through the right ventricle before spreading to the left side


  • LBBB is normally associated with pathology e.g.
    • Aortic stenosis
    • Ischaemic heart disease
    • Hypertension
    • Dilated cardiomyopathy
    • Anterior MI
    • Primary degenerative disease (fibrosis) of the conducting system (Lenegre disease)
    • Hyperkalaemia
    • Digoxin toxicity
  • Older people are more likely to have LBBB due to increased risk of causative conditions

Presentation and ECG findings

  • Patients may have symptoms of associated cause e.g. chest pain from ischaemic heart disease
  • In patients with heart failure with a left ventricular ejection fraction of <40%, LBBB may worsen symptoms
  • ECG
    • Diagnostic criteria
      • Wide QRS >120ms (3 small squares)
      • Dominant S wave in V1 (‘W’ shape)
      • Absence of Q waves in lateral leads (I, V5-6) and prolonged R wave peak times >60ms in the left precordial leads (V5-6) (‘M’ shape)
        • WiLLiaM
      • There may also be left axis deviation

NB If a patient is found to have LBBB, then (depending on clinical status) further investigations e.g. echocardiography; exercise tolerance ECG etc may be done to look for underlying heart disease


  • In patients with heart failure, cardiac resynchronisation therapy (pacemaker) can be considered
  • If patients are symptomatic e.g. syncope/chest pain etc; patients may be treated, but usually in line with guidelines for syncope/chest pain etc


Right Bundle Branch Block


  • In RBBB, spread of electrical activity to the right ventricle is delayed as it has to spread across the septum from the left ventricle


  • In contrast to LBBB, RBBB may be seen as part of the aging process (fibrosis) and may be a ‘functional’ sign
  • Other pathological causes include
    • Right ventricular hypertrophy/ cor pulmonale
    • Pulmonary embolus
    • Ischaemic heart disease
    • Rheumatic heart disease
    • Myocarditis or cardiomyopathy
    • Degenerative disease of the conduction system
    • Congenital heart disease e.g. atrial septal defect


  • Diagnostic criteria
    • Broad QRS >120ms
    • RSR pattern in V1-3 (‘M’ shaped QRS)
      • Caused by a delay in right ventricular depolarisation
      • NB Occasionally this is instead a broad monophasic R wave (QR)
    • Wide, slurred S wave in the lateral leads (I, aVL, V5-6) (‘W’ shape QRS complex)
  • Other features include ST depression and T wave inversion in the right precordial leads (V1-3)
    • Can occasionally be misinterpreted at right sided MI (consider patient presentation also)


  • Treat any suspected underlying cause


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s