Left bundle branch block (LBBB)
Also calledLBBB · left bundle branch block · CLBBB · complete LBBB
Definition
Complete block of the left bundle branch, so the left ventricle is activated late, by slow conduction across the septum from the right side. Diagnosed by a QRS duration of 120 ms or more with broad, notched or slurred R waves in leads I, aVL, V5 and V6 (R-peak time over 60 ms in V5 and V6), absent q waves in leads I, V5 and V6, small r waves or QS complexes in V1 to V3, and ST segments and T waves that usually point opposite to the main QRS deflection.
Clinical noteLeft bundle branch block alters the ST segment and T wave by itself, 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
- Samesima N, God EG, Kruse JCL, et al. Brazilian Society of Cardiology guidelines on the analysis and issuance of electrocardiographic reports - 2022. Arq Bras Cardiol. 2022;119(4):638-680. doi.org/10.36660/abc.20220623
- 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.