Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android

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


  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.


Left Bundle Branch Block. (ECG contributor: James V. Ritchie, MD.)


The QRS is wider than 120 ms (double arrow). The T-wave deflection is in the opposite direction from the QRS deflection (arrowhead).

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.