Editor's note: This article was listed in the LITFL Review 154's "Best of #FOAMed" section.
The left bundle branch arises from the Bundle of His, and subsequently is divided into the anterior and posterior fascicles. The anterior fascicle is usually supplied by septal perforators from the Left Anterior Descending artery, and the posterior fascicle typically has a dual supply from septal perforators from the Left Anterior Descending artery and the Posterior Descending artery (arising from the Right Coronary).
Electrocardiographically, a LBBB is defined as QRS duration greater than or equal to 120 ms; a broad-notched or slurred R wave in leads I, aVL, V5, and V6; absent Q waves in leads I, V5, and V6
; and an R peak time >60 ms in leads V5 and V6 but normal in leads V1 to V3 (1). LBBB can be transient and/or rate-related (1). These morphologic changes make it difficult to discern whether or not a patient presenting to the emergency department with chest pain is experiencing a STEMI.