Left bundle branch block

In left bundle branch block (LBBB) a block in the left bundle branch causes the left ventricle to depolarise later than the right. This causes certain characteristic ECG features.

Etiology

See bundle branch block.

Clinical features

The characteristic ECG features include:

• Wide QRS (> 120 ms)
• Wide and notched R wave in leads I, aVL, V5, V6 (forming an “M” shape)
• Absent Q waves in leads I, aVL, V5, V6
• Deep (sometimes notched) S wave in leads V1, V2, V3 (sometimes forming a “W” shape)

The ECG findings can be explained. The delay in depolarisation of the left ventricle causes the Q wave to be absent in leads corresponding to the left ventricle (I, aVL, V5, V6). The delayed activation of the left ventricle prolongs the leftward progression of the depolarisation, leading to a positive and widened R wave in the same leads. The deep S wave in the leads corresponding to the right ventricle may perhaps be explained by the lack of a counterpart depolarisation in the left ventricle.

LBBB is a sort of “ST-elevation-equivalent” in the diagnosis of acute myocardial infarction. An acute myocardial infarction can manifest as LBBB instead of ST elevation. As such, a person with a new onset LBBB, coronary chest pain and positive troponin dynamics is considered to have STEMI, even in the absence of ST elevation.