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Left Posterior Fascicular Block

caution
Variant

Bifascicular block: the right bundle fails too

HR75bpm
RR800msQT402ms
Draws the normal ECG in grey behind every lead. Shortcut: N.

Left posterior fascicular block with right bundle branch block

Bifascicular block: the right bundle and the left posterior fascicle have both failed, and the whole ventricular mass depends on the left anterior fascicle. The tracing is RBBB on the axis of LPFB.

On the trace

Try each step, then check it.
  1. Measure the QRS.
    Show 143 ms: the right bundle branch block widens it.
  2. Look at V1 and V2.
    Show qR in V1 and rsR′ in V2, with inverted T waves: the RBBB, here with its late forces starting from a small q.
  3. Find the axis.
    Show +140°, far to the right: the LPFB. See +140° on the axis page.
  4. Look at the limb leads.
    Show rS in I and aVL, the S 12.2 mm in aVL; qR in II, III and aVF, 15.6 mm in III. The posterior fascicular block's shape.
  5. Look at V5 and V6.
    Show rS: here the right axis and the late rightward forces meet, and the left chest leads end negative.

Telling it apart

LPFB with RBBBLPFB aloneRBBB alone
QRS143 ms89 ms143 ms
Axis+140°+120°+80°
V1qRqrrSR′
I, aVLrSrSqRs
II, III, aVFqRqRRs, R

As with LPFB alone, the right axis counts only once right ventricular hypertrophy, lung disease and a lateral infarct are excluded.

What it means

Less common than RBBB with LAFB, and more serious: the left posterior fascicle is short, broad and doubly supplied, so blocking it takes extensive disease, and with the right bundle gone too, the whole ventricle depends on the left anterior fascicle. It is more likely to progress to complete heart block, particularly during a myocardial infarction, and syncope with it calls for monitoring and often a pacemaker.

Full articleLeft Posterior Fascicular BlockHow to recognise it, why it happens, what it means and what to do.