Bundle branch block
Also calledBBB · bundle-branch block
Definition
A conduction delay or block in the right or left bundle branch of the His-Purkinje system, which changes the order in which the ventricles are activated and widens the QRS complex; in left bundle branch block, for example, the left ventricle is activated late by slow conduction across the septum from the right side. A complete block requires a QRS duration of at least 120 ms in adults (incomplete block is 110 to 119 ms). Right bundle branch block shows an rsR' or rSR' pattern in V1 or V2 with wide S waves in leads I and V6, while left bundle branch block shows broad, notched or slurred R waves in leads I, aVL, V5 and V6 with no q waves in I, V5 and V6.
Clinical noteLeft bundle branch block alters the ST segment and T wave by itself (they usually point opposite to the QRS), so it can hide the ECG changes of acute myocardial infarction; the Sgarbossa criteria, and a later modified version by Smith and colleagues, were developed to recognize infarction in its presence.
Sources
- Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235-e240. doi.org/10.1161/CIRCULATIONAHA.108.191095
- Ponnusamy SS, Vijayaraman P, Ellenbogen KA. Left bundle branch block-associated cardiomyopathy: a new approach. Arrhythm Electrophysiol Rev. 2024;13:e15. doi.org/10.15420/aer.2024.14
- Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. N Engl J Med. 1996;334(8):481-487. doi.org/10.1056/NEJM199602223340801
- Smith SW, Dodd KW, Henry TD, et al. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012;60(6):766-776. doi.org/10.1016/j.annemergmed.2012.07.119
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.