23-20: Left Bundle Branch Block

ECG Findings

• Wide QRS complex > 120 ms

• Dominant S wave in V₁-V₃

• Broad monophasic, M-shaped or notched R wave in lateral leads (I, aVL, V₅-V₆)

• Appropriate discordance: the ST segment and T wave are in opposite direction to the main vector of the QRS complex

• Left axis deviation

Pearls

1. In left bundle branch block (LBBB), conduction through the left bundle is delayed, whereas depolarization through the right bundle occurs at normal speed. This produces tall, M-shaped or notched R waves in the lateral leads and deep S waves in the right precordial leads.

2. The presence of an LBBB makes it challenging to diagnose an STEMI, as the typical changes such as ST elevation, ST depressions, and T-wave inversions are present. In the past, a new LBBB was considered an STEMI equivalent requiring emergent treatment. More recent data have revealed that new LBBB due to an acute MI is rare in the absence of shock, and therefore, this was removed from the STEMI criteria. Instead, use the Sgarbossa criteria to help diagnose STEMI in the presence of LBBB.