Left Bundle Branch Block (LBBB)


Left Bundle Branch Block or LBBB is the result of a delay or interruption of conduction within the left bundle branch. In complete LBBB, the duration of QRS is >120ms and in incomplete LBBB it is between 100 and 120 ms.

Left Bundle Branch Block
Left Bundle Branch Block

Complete LBBB ECG features :-

  • Prolonged QRS duration >120ms and may be as long as 200ms
  • Wide RR’ deflections in lead V6 and standard lead i
  • QS complex in lead V1
  • rS complexes- small initial r wave with a deep and wide ensuing S wave in leads V2 to V4.

Significance of complete LBBB-

  • Complete LBBB indicates ORGANIC HEART DISEASE. It is commonly associated with ischaemic heart disease, cardiomyopathies, intrinsic disease of the conduction system and hypertensive heart disease.
  • Acute MI can present as new onset LBBB and results in diagnostic dilemma. Presence of concordant ST segment elevation of >1mm, ST segment depression in leads V1-V3 and discordant ST elevation of more than 5mm with LBBB suggests acute MI
  • Presence of LBBB in patients with left ventricular dysfunction is an indication for biventricular pacing cardiac resynchronisation therapy.

Incomplete Left Bundle branch Block-

In the case of incomplete LBBB, conduction through the left bundle branch and its ramifications is still possible but will be delayed. This is in cintrast to complete LBBB where conduction through the left sided specialised conduction system is no longer possible but occurs through ordinary myocardial tissue. The type of electrocardiographic manifestation that occurs with incomplete LBBB will depend on the degree of delay within the left bundle branch. Progressively increasing delay of conduction within the left bundle branch will result in a progressive sequence of electrocardiographic manifestations.

Incomplete Left bundle branch block
Incomplete Left Bundle Branch Block


The earliest manifestations of incomplete LBBB-

  • The first manifestations of incomplete LBBB are the following-
  • The small initial q wave of the normal qR complex in the left-oriented leads- leads V5 and V6 and standard lead I disappears. This will result in a single tall R wave.
  • The small initial r wave of the normal; rS complex in lead VI disappears. this will result in a QS complex.
  • These manifestations are due to delay in the formation and inscription of the normal left sided septal vector. This means that the normal right sided septal vector now has more time to develop so that it equals the magnitude of the left sided septal vector. The two septal vectors thus cancel or nullify each other, thereby resulting in the disappearance of the effective or resultant normal left to right septal vector.

The manifestations of more advanced forms of incomplete LBBB-

  • With further progression of the incomplete LBBB, a slur appears on the upstroke of the QRS complex.
  • This slur becomes increasingly prominent and is accompanied by widening and eventual notching of the QRS complex until the fully developed manifestation of complete LBBB is attained.
  • uch manifestations are due to increasing dominance of the right septal vector that penetrates the electrophysiological intraseptal barrier to an increasing degree. It is permitted to play a more dominant role due to an increasing delay within the left bundle branch.

Clinical implications of Left bundle branch Block-

  • Presence of LBBB indicates underlying heart disease.
  • Acute myocardial infarction can present as new LBBB.
  • Presence of LBBB in patients with left ventricular dysfunction is an indication for biventricular pacing cardiac resynchronisation therapy.

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