Left bundle branch block (LBBB), a pattern seen on the surface electrocardiogram (ECG), results when normal electrical activity in the His-Purkinje system is interrupted (figure 1). The normal sequence of activation is altered dramatically in LBBB, with a resultant characteristic appearance on the ECG (waveform 1).
LBBB most often occurs in patients with underlying heart disease and may be associated with progressive conducting system disease. However, LBBB can also be seen in asymptomatic patients with a structurally normal heart. The presence of LBBB complicates the diagnosis of myocardial ischemia/infarction and interferes with the interpretation of exercise testing. In patients with significant LV dysfunction, LBBB results in left ventricular dyssynchrony and may contribute to heart failure.
The anatomy, clinical manifestations, differential diagnosis, prognostic implications, and treatment of LBBB will be reviewed here. Additional details regarding the ECG manifestations of LBBB are discussed separately. (See "ECG tutorial: Intraventricular block", section on 'Left bundle branch block'.)
ANATOMY AND ELECTROPHYSIOLOGY
Anatomy — The bundle of His divides at the juncture of the fibrous and muscular boundaries of the interventricular septum into the right and left bundle branches. The main left bundle branch penetrates the membranous portion of the interventricular septum under the aortic ring and then divides into several fairly discrete branches. The components of the left bundle branch are (figure 1) [1-5]:
- A pre-divisional segment.
- An anterior fascicle that crosses the left ventricular outflow tract and terminates in the Purkinje system of the anterolateral wall of the left ventricle.
- A posterior fascicle that fans out extensively inferiorly and posteriorly into Purkinje fibers.
- In some hearts, a median fascicle to the interventricular septum.