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Left Anterior Fascicular Block

caution
Variant

Bifascicular block: the right bundle fails too

HR75bpm
RR800msQT400ms
Draws the normal ECG in grey behind every lead. Shortcut: N.

Left anterior fascicular block with right bundle branch block

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

On the trace

Try each step, then check it.
  1. Measure the QRS.
    Show 141 ms: the right bundle branch block widens it.
  2. Look at V1 and V2.
    Show rSR′, with an R′ of 7.5 mm in V1 and an inverted T: the RBBB.
  3. Find the axis.
    Show −78°, far to the left: the LAFB. See −78° on the axis page.
  4. Look at the limb leads.
    Show qR in I and aVL, now ending in a small s; rS in II, III and aVF, the S 15.6 mm deep in III. The fascicular block's shape, with RBBB's late forces added at the end.
  5. Look at I and V6.
    Show qRs: the broad terminal s of RBBB, 3.4 mm in I and 4.3 mm in V6.

Telling it apart

LAFB with RBBBLAFB aloneRBBB alone
QRS141 ms96 ms143 ms
Axis−78°−54°+80°
V1rSR′rSrSR′
I, aVLqRsqRqRs
II, III, aVFrSrSRs, R

What it means

This is the commonest bifascicular block. The right bundle and the left anterior fascicle share their blood supply, from the septal branches of the left anterior descending artery, so they often fail together, in degenerative disease of the conduction system or in an anterior myocardial infarction. Without symptoms, it needs no pacemaker, only a watch on the tracing over time; with syncope, intermittent complete heart block has to be looked for, because only one fascicle is left.

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