Sections View Full Chapter Figures Tables Videos Full Chapter Figures Tables Videos Supplementary Content +++ 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 ++ 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. 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. ++ FIGURE 23.20A Left Bundle Branch Block. (ECG contributor: James V. Ritchie, MD.) Graphic Jump LocationView Full Size|Favorite Figure|Download Slide (.ppt) ++ FIGURE 23.20B The QRS is wider than 120 ms (double arrow). The T-wave deflection is in the opposite direction from the QRS deflection (arrowhead). Graphic Jump LocationView Full Size|Favorite Figure|Download Slide (.ppt) GET ACCESS TO THIS RESOURCE Sign In Username Error: Please enter User Name Password Error: Please enter Password Forgot Username? Forgot Password? Sign in via OpenAthens Sign in via Shibboleth Get Free Access Through Your Institution Contact your institution's library to ask if they subscribe to McGraw-Hill Medical Products. Access My Subscription GET ACCESS TO THIS RESOURCE Subscription Options Pay Per View Timed Access to all of AccessEmergency Medicine 24 Hour $34.95 (USD) Buy Now 48 Hour $54.95 (USD) Buy Now Best Value AccessEmergency Medicine Full Site: One-Year Individual Subscription $595 USD Buy Now View All Subscription Options