Left Bundle Branch Block (LBBB)

Left Bundle Branch Block LBBB

  • Normally the septum is activated from left to right, producing small Q waves in the lateral leads.
  • In LBBB, the normal direction of septal depolarisation is reversed (becomes right to left), as the impulse spreads first to the RV via the right bundle branch and then to the LV via the septum.
  • This sequence of activation extends the QRS duration to > 120 ms and eliminates the normal septal Q waves in the lateral leads.
  • The overall direction of depolarisation (from right to left) produces tall R waves in the lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3), and usually leads to left axis deviation.
  • As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (‘M’-shaped) R wave in the lateral leads.
Left Bundle Branch Block LBBB W and M
Dominant S wave in V1 with broad, notched (‘M’-shaped) R wave in V6

ECG Diagnostic Criteria

  • QRS duration of > 120 ms
  • Dominant S wave in V1
  • Broad monophasic R wave in lateral leads (I, aVL, V5-V6)
  • Absence of Q waves in lateral leads (I, V5-V6; small Q waves are still allowed in aVL)
  • Prolonged R wave peak time > 60ms in left precordial leads (V5-6)
Associated Features
  • Appropriate discordance: the ST segments and T waves always go in the opposite direction to the main vector of the QRS complex
  • Poor R wave progression in the chest leads
  • Left axis deviation

ECG QRS Morphology

QRS Morphology in the Lateral Leads

The R wave in the lateral leads may be either:

  • M‘-shaped
  • Notched
  • Monophasic
  • RS complex

QRS Morphology in V1

The QRS complex in V1 may be either:

  • rS complex (small R wave, deep S wave)
  • QS complex (deep Q/S wave with no preceding R wave)

QRS Morphology Examples

ECG LBBB R wave changes in I aVL V6
  • Lead aVL: ‘M’-shaped QRS complex
  • Lead I: Notched R wave
  • V6: Monophasic R wave

ECG LBBB changes in precordial leads
  • V1: rS complex (tiny R wave, deep S wave) and appropriate discordance (ST elevation and upright T wave)
  • V5: RS complex
  • V6: Monophasic R wave

Causes of Left Bundle Branch Block

NB. It is unusual for left bundle branch block to exist in the absence of organic disease. New LBBB in the context of chest pain is traditionally considered part of the criteria for thrombolysis.

However, more recent data suggests that chest pain patients with new LBBB have little increased risk of acute myocardial infarction at the time of presentation.

ECG Examples of LBBB

Example 1
ECG Left Bundle Branch Block LBBB 2

Example 2

Example 3
Left bundle branch block LBBB 4

Incomplete LBBB

Incomplete LBBB is diagnosed when typical LBBB morphology is associated with a QRS duration < 120ms.

ECG Incomplete LBBB 1

Differential Diagnosis

Left ventricular hypertrophy may produce a similar appearance to LBBB, with QRS widening and ST depression / T-wave inversion in the lateral leads.

Related Topics


  • Da Costa D, Brady WJ, Edhouse J. Bradycardias and atrioventricular conduction block. BMJ. 2002 Mar 2;324(7336):535-8. PMID: 11872557.

LITFL Further Reading

Advanced Reading




Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

One comment

  1. Dr Md Mahbubor Rahman
    Dr Md Mahbubor Rahman

    Nice presentation

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