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Left bundle branch

Bundle branches · LAD septals + AVN artery

Short, broad sheet of fibres leaving the His bundle almost immediately and cascading down the left septal surface, where it divides into anterior and posterior fascicles (plus a variable septal fascicle). It depolarises the septum from left to right first, which is why normal ECGs carry small septal q waves in the lateral leads.

Traced from the start

  1. Sinoatrial node
  2. Internodal pathways (anterior, middle, posterior)Right atrial myocardium
  3. Atrioventricular node
  4. Bundle of His (atrioventricular bundle)
  5. Left bundle branch

Detail

Conduction velocity
~2 m/s; the earliest ventricular activation is the left mid-septum about 10 ms after the His bundle
Arterial supply
Proximal LBB by LAD septal perforators and the AV nodal artery (dual); its broad fan-like origin cannot be interrupted by a single small lesion
Divisions
Left anterior (superior) fascicle to the anterolateral papillary muscle; left posterior (inferior) fascicle to the posteromedial papillary muscle; septal fibres to the mid-septum

When it goes wrong

Interruption of the broad left bundle — almost always structural: ischaemic heart disease, anterior MI, dilated cardiomyopathy, hypertensive LVH, aortic stenosis, Lev/Lenègre fibrosis, post-TAVI

LBBB: QRS 120 ms or more, dominant S in V1, broad notched monophasic R in I, aVL, V5–V6, absent septal q laterally, left axis, ST–T changes discordant to the QRS; never a normal variant

New LBBB with ischaemic chest pain

Treat as an infarct equivalent when Sgarbossa criteria are met — concordant ST elevation of 1 mm or more; concordant ST depression of 1 mm or more in V1–V3; discordant ST elevation of 5 mm or more (or above 25% of the S wave); otherwise LBBB conceals infarction

LBBB with LVEF 35% or less and heart failure despite optimal drugs

Dyssynchronous, late-activated lateral wall — cardiac resynchronisation (biventricular or conduction-system pacing) improves symptoms and mortality, most of all with QRS 150 ms or more

Right ventricular apical pacing

LBBB-shaped paced QRS because the septum is activated right to left; a pacing spike precedes each QRS

Practise this structure

2 questions in the bank tagged Cardiac Conduction.

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