left bundle branch
Summary
The left bundle branch (LBB) is part of the cardiac conduction system that carries electrical impulses from the bundle of His to the left ventricle, splitting into anterior and posterior fascicles. It depolarizes the interventricular septum and left ventricle, enabling coordinated contraction. Left bundle branch block (LBBB) produces a characteristic wide QRS pattern on ECG and is often associated with underlying structural heart disease.
Detail
The left bundle branch arises from the bundle of His after it passes through the membranous interventricular septum, dividing into the left anterior fascicle and left posterior fascicle (a trifascicular system when combined with the right bundle branch). It rapidly conducts impulses via Purkinje fibers to the left ventricular myocardium, ensuring near-simultaneous depolarization of both ventricles for efficient, synchronized contraction. Normally, septal depolarization proceeds left-to-right, contributing to the initial small R wave in V1 and Q wave in lateral leads.
In left bundle branch block (LBBB), conduction through the left bundle is delayed or blocked, so the left ventricle is depolarized indirectly via slow cell-to-cell spread from the right ventricle, causing delayed, dyssynchronous left ventricular activation. This produces a widened QRS (>120 ms), broad/notched R waves in lateral leads (I, aVL, V5-V6), deep S waves in V1-V2, and absence of septal Q waves in lateral leads. LBBB obscures the ability to diagnose myocardial infarction using standard ST-segment criteria (Sgarbossa criteria are used instead) and can itself indicate underlying structural heart disease such as coronary artery disease, dilated cardiomyopathy, hypertensive heart disease, aortic valve disease, or myocarditis. New LBBB in the setting of chest pain is treated as a STEMI-equivalent when associated with clinical suspicion of ischemia (per some guidelines, though this has evolved with Sgarbossa criteria use). LBBB can cause dyssynchronous ventricular contraction, contributing to reduced ejection fraction and heart failure; cardiac resynchronization therapy (biventricular pacing) is used in select heart failure patients with LBBB and reduced EF to improve synchrony and symptoms. Differentiating LBBB from right bundle branch block (RBBB) is a key ECG skill: LBBB shows a broad, often notched R wave in V6 ("M" shaped) and deep S in V1, whereas RBBB shows RSR' pattern ("rabbit ears") in V1 and wide S wave in lateral leads.
Sources
- Goldberger's Clinical Electrocardiography: A Simplified Approach
- Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine
- First Aid for the USMLE Step 1
- UpToDate: Left bundle branch block
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.