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

Electrophysiology

In a normal heart conduction, electrical impulses move down both the left and right bundle branches, causing the septum to activate from left to right and generating small Q waves in the lateral leads.

However, in left bundle branch block (LBBB), conduction delay occurs, causing impulses to travel first through the right bundle branch to the right ventricle (RV) before reaching the left ventricle (LV) through the septum. This reversed septal activation eliminates lateral Q waves. The right-to-left overall depolarization creates tall R waves in the lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3). The delay in activation between the RV and LV results in the recognizable “M-shaped” R wave in lateral leads. The extended overall conduction time to the LV leads to a QRS duration longer than 120 ms.

 

QRS Morphology in the Lateral Leads

In the lateral leads, the R wave can present as an “M-shaped”, notched, monophasic, or RS complex pattern.

QRS Morphology in V1

In lead V1, the QRS complex may take the form of either:

  1. An rS complex, which has a small R wave and a deep S wave.
  2. A QS complex, characterized by a deep Q/S wave with no preceding R wave.