Left Bundle Branch Block (LBBB)

ECG Diagnostic criteria

  • QRS duration ≥ 120ms
  • Dominant S wave in V1
  • Broad monophasic R wave in lateral leads (I, aVL, V5-6)
  • Absence of Q waves in lateral leads
  • Prolonged R wave peak time > 60ms in leads V5-6
Left-Bundle-Branch-Block-LBBB ECG-Strip LITFL
LBBB: Left Bundle Branch Block
V1: Dominant S wave
V6: broad, notched (‘M’-shaped) R wave

Associated features include:

  • Left axis deviation (LAD);
  • Poor R wave progression in precordial leads, and
  • Appropriate discordance (discussed below)


In normal cardiac conduction, impulses travel equally down the left and right bundles, with the septum activated from left to right and the formation of small Q waves in lateral leads

  • In LBBB, conduction delay means that impulses travel first via the right bundle branch to the RV, and then to the LV via the septum
  • Septal activation is thus reversed eliminating lateral Q waves
  • The overall depolarisation vector from the right to left ventricle produces tall R waves in lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3). The delay between activation of the RV and LV produces the characteristic “M-shaped” R wave seen in lateral leads
  • Delayed overall conduction time to the LV extends the QRS duration to ≥ 120 ms
Sequence of conduction in LBBB
Sequence of conduction in LBBB:
1) Conduction delay means impulses travel first via the right bundle branch (black arrow)
2) Septum is activated from right-to-left (yellow arrows)
3) Overall depolarisation vector is directed towards lateral leads (red arrow)

ECG QRS Morphology

QRS Morphology in the Lateral Leads

The R wave in the lateral leads may be either “M-shaped”, notched, monophasic, or an 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)

ECG LBBB changes in precordial leads
Left bundle branch block (LBBB):
1) rS complex in V1 (tiny R wave, deep S wave)
2) Characteristic lateral lead morphology in V5-6
3) Note appropriate discordance in V1 with ST elevation and upright T wave

What about the ST elevation?
  • Appropriate discordance refers to the fact that abnormal depolarisation should be followed by abnormal repolarisation, which appears discordant to the preceding QRS complex
  • Lateral leads with tall, broad R waves will often have associated ST-segment depression and T-wave inversion, and those with deep S waves can have an allowable amount of ST elevation that does not indicate ischaemia (generally viewed as < 25% of the size of the preceding S wave)
  • Any concordant ST segment change is concerning for ischaemia. For further reading, see LITFL: Sgarbossa Criteria
LBBB with AF: Note the appropriate discordance manifesting as ST depression in leads V5-6, as well as mild ST elevation in leads V2-3.

Causes of Left Bundle Branch Block

It is unusual for LBBB to exist in the absence of organic disease. Causes are varied and include:

New LBBB in the context of chest pain was once considered a “STEMI-equivalent” and part of the criteria for thrombolysis. However, more up-to-date data suggests that chest pain patients with new LBBB have little increased risk of acute myocardial infarction at the time of presentation.

Practice has now evolved to examining for excessive discordance or concordant ST segment changes indicative of infarction.

ECG Examples of LBBB
Example 1

Broad notched R waves are best appreciated in leads aVL and I here. There is absence of Q waves in leads V5-6.

Example 2

LBBB with AF. Note deep S waves in leads V1-3 and tall broad R waves laterally. Appropriate discordance is present.

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
  • Right ventricular paced rhythms will produce a similiar morphology, as impulse conductions originate from the RV and travel across the septum to the LV as is the case in LBBB. Pacing spikes will be present. The same concepts regarding appropriate discordance apply.
  • Left ventricular hypertrophy may produce a similar appearance to LBBB, with QRS widening and ST depression / T-wave inversion in the lateral leads.


Advanced Reading



LITFL Further Reading



MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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


  1. Thank you. So, when you have a left bundle branch block, can this ever be a normal scenario? That is – nothing to worry about?

    • LBBB always indicates pathology, but is common in elderly patients and an incidental finding of LBBB on ECG in asymptomatic older patients does not warrant acute concern.

  2. The vector moving towards the left does not explain the notch in V1 and V 6 , but it does explain a monophasic S and R wave in V1 and V 6. Explanation for the notch will be useful .

    • Hi Dr Ramesh,

      Thank you for your comment.

      The normal sequence of electrical excitation in cardiac muscle occurs as follows:
      1) Left to right across the interventricular septum
      2) Down the septum to the apex
      3) Along the ventricular walls to the AV groove, proceeding from endocardial to epicardial surface
      4) The last parts to be depolarised are the posterobasal portion of the LV, pulmonary conus, and uppermost portion of the septum

      Thus, if we take a notched V6 as the example:
      1) The initial R wave is due to (abnormal) right-to-left septal depolarisation
      2) Impulses subsequently travel down the septum towards the apex, producing a small S wave
      3) The second R wave is due to subsequent spread of excitation along the ventricular walls from endocardium to epicardium.

      I hope this explanation helps.


  3. Is it right that in BBB normally T waves should be oriented opposite to the QRS terminal forces? What happens if this doesn’t happen – e.g. negative terminal forces and inverted T waves or positive terminal forces and upright T waves? Are these changes significant?

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