Incomplete bundle branch block
Also calledIRBBB · ILBBB · incomplete RBBB · incomplete LBBB · partial bundle branch block · iRBBB · iLBBB
Definition
A right or left bundle branch block pattern in which the QRS is prolonged but falls short of the complete-block threshold of 120 ms; in adults the QRS duration is 110 to 119 ms. Incomplete RBBB keeps the right bundle branch block shape (an rsr', rsR' or rSR' pattern in V1 or V2 and a wide S wave in leads I and V6). Incomplete LBBB shows an R-peak time over 60 ms in V4 to V6, no q waves in leads I, V5 and V6, and a left ventricular hypertrophy pattern.
Clinical noteAn incomplete RBBB pattern can be present without any heart disease.
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
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.