The left bundle branch is a broad sheet of conducting fibres that leaves the bundle of His and fans out over the left side of the interventricular septum. It divides into a thin left anterior fascicle and a broader left posterior fascicle, with septal fibres in between. Through these it activates the thick left ventricle.
| Location | Beneath the endocardium on the left side of the interventricular septum, spreading towards the bases of the two left ventricular papillary muscles. |
|---|---|
| Parts |
|
| Blood Supply | Septal perforators of the left anterior descending artery (anterior fascicle); Left anterior descending and posterior descending arteries (posterior fascicle) |
Rapidly spreads excitation over the left ventricular surface of the septum and towards the papillary muscles and free wall, so the left ventricle contracts efficiently from apex to base.
Left bundle branch block makes the left ventricle contract in an uncoordinated way and is often a sign of underlying heart disease. In heart failure with left bundle branch block, cardiac resynchronisation therapy (biventricular pacing) can improve symptoms and survival. Left anterior fascicular block is common and shifts the ECG axis to the left.
Failure of conduction in the left bundle, so the left ventricle is activated late from the right. It produces a broad QRS and is associated with hypertension, coronary disease and cardiomyopathy.
Block of the left anterior or posterior fascicle alone, seen as an axis shift on the ECG. Combined with right bundle branch block it can progress to complete heart block.
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