ECG education

Left bundle branch block

Vivienne Miller



An ECG of a healthy young man shows a left bundle branch block. What are the potential causes and significance of this ECG pattern?

Key Points

  • Generally, in patients with LBBB, the QRS complex is more than 120 msec in duration with upright monomorphic R waves in V5, V6, aVL and lead I without Q waves. There is no secondary R wave inV1.
  • Incomplete LBBB is diagnosed when a patient has the ECG criteria for LBBB but the QRS duration is less than 120 msec.
  • It is difficult to be definite of an acute myocardial infarction in the presence of an LBBB without a rise in the serum troponin level and/or classic symptoms and signs.
  • Patients with incomplete or complete LBBB require full investigation, including a cardiac echocardiography, assessment of cardiovascular risk factors, consideration of functional stress testing and a cardiology consultation.
  • Patients with no impairment of exercise tolerance and no evidence of cardiac failure do not require any treatment other than reducing their cardiovascular risk factors as appropriate.
  • Pacemaker cardiac resynchronisation therapy is required for patients with LBBB who have a reduced ejection fraction, those who are symptomatic with exertion and those who have cardiomyopathy.