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Bundle Branch Block

A bundle branch block (BBB) is a medical condition characterised by a partial or complete interruption in the flow of electrical impulses in either of the bundle branches of the heart's electrical system. The heart's electrical activity begins in the sinoatrial node, travels through the atria, and summates at the atrioventricular node.

From there, it moves down the bundle of His and divides into the right and left bundle branches.

Disruption in these pathways can lead to altered ventricular depolarisation and loss of synchrony.

ECG showing a bundle branch block
ECG showing a bundle branch block


When a bundle branch or fascicle is injured due to underlying heart disease, myocardial infarction, or cardiac surgery, it may cease to conduct electrical impulses properly. This results in altered pathways for ventricular depolarisation, causing a slowdown in electrical movement and changes in impulse direction. Consequently, ventricular synchrony is lost, ventricular depolarisation is prolonged, and there may be a drop in cardiac output. In heart failure patients, a specialised pacemaker can be used to resynchronize the ventricles, shortening the QRS interval and improving the ejection fraction.


A bundle branch block can be diagnosed when the QRS complex duration on an electrocardiogram (ECG) exceeds 120 ms.

  • Right Bundle Branch Block (RBBB) typically causes prolongation of the last part of the QRS complex and may shift the heart's electrical axis slightly to the right. The ECG will show a terminal R wave in lead V1 and a slurred S wave in lead I.
  • Left Bundle Branch Block (LBBB) widens the entire QRS and typically shifts the heart's electrical axis to the left. The ECG will show a QS or rS complex in lead V1 and a monophasic R wave in lead I.

Appropriate T wave discordance, where the T wave is deflected opposite the terminal deflection of the QRS complex, is also a normal finding with bundle branch blocks. The simultaneous occurrence of left and right bundle branch blocks leads to total AV block.

Image showing the ECG characteristics of a right bundle branch block and a left bundle branch block.
Image showing the ECG characteristics of a right bundle branch block and a left bundle branch block.


Bundle branch blocks are classified based on the anatomical location of the defect:

  • Right Bundle Branch Block (RBBB) can be incomplete (IRBBB) or complete (CRBBB).
  • Left Bundle Branch Block (LBBB) can be incomplete (iLBBB) or complete (cLBBB).

The left bundle branch block can be further sub-classified into:

  • Left Anterior Fascicular Block: Involves only the anterior half of the left bundle branch (fascicle).
  • Left Posterior Fascicular Block: Involves only the posterior part of the left bundle branch.

Other classifications include:

  • Bifascicular Block: A combination of RBBB and either left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB).
  • Trifascicular Block: A combination of RBBB with either LAFB or LPFB along with a first-degree AV block.
  • Tachycardia-dependent Bundle Branch Block

To differentiate between the two main types, observe the deflection of the QRS complex in the V1 lead: a downward deflection indicates LBBB, while an upward deflection indicates RBBB. In both cases, the QRS is wide (>0.12 seconds).


Some individuals are born with bundle branch blocks, while others acquire them due to heart disease. People with bundle branch blocks may remain asymptomatic and have only an abnormal ECG appearance. However, complex and diffuse bundle blocks, or those associated with significant ventricular muscle damage, may indicate severe heart disease. In severe cases, a pacemaker may be required to restore optimal electrical conduction to the heart muscle.

Self-assessment MCQs (single best answer)

What is a bundle branch block (BBB)?

Which part of the heart's electrical system initiates the electrical activity?

What is the characteristic ECG finding for a Right Bundle Branch Block (RBBB)?

Which type of bundle branch block typically shifts the heart's electrical axis to the left?

Which of the following classifications involves a combination of RBBB and a first-degree AV block?

What is the QRS complex duration on an ECG that indicates a bundle branch block?

Which of the following can be an underlying cause of a bundle branch block?

In the diagnosis of bundle branch block, which lead is very important for differentiating between RBBB and LBBB?

What does an upward deflection of the QRS complex in lead V1 indicate?

Which treatment may be required for severe bundle branch block cases with significant ventricular muscle damage?


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