Conduction disease
Left bundle branch block
Broad QRS with a notched R in the lateral leads — and repolarisation you can no longer interpret at face value.
What you see
- QRS ≥ 120 ms.
- Broad, notched or slurred R wave in I, aVL, V5 and V6 — the 'M'.
- Deep, broad S wave in V1 — the 'W' of the old WiLLiaM mnemonic.
- Appropriate discordance: the ST segment and T wave point opposite to the main QRS deflection.
- No septal Q waves in the lateral leads.
Why it looks like that
The left bundle fails, so the septum and left ventricle are activated late and slowly from the right. Depolarisation is abnormal, and repolarisation is therefore abnormal too — which is why ST and T changes cannot be read as ischaemia in the usual way.
What to do
- New LBBB with ischaemic symptoms is treated as an acute coronary syndrome and discussed with the cath lab.
- Use the Sgarbossa criteria to identify infarction within LBBB: concordant ST elevation ≥ 1 mm, concordant depression ≥ 1 mm in V1–V3, or discordant elevation disproportionate to the QRS.
- LBBB with heart failure and a QRS over 150 ms is a strong indication for cardiac resynchronisation.
The trap
'New LBBB equals STEMI' has been retired — most LBBB found in the emergency department is old, and the old teaching sent many patients to the cath lab unnecessarily. What matters is the clinical picture plus Sgarbossa, not the block alone.
More in conduction disease
First degree AV block
A PR interval over 200 ms with every P wave still conducted.
Mobitz I (Wenckebach)
The PR interval lengthens beat by beat until one P wave fails to conduct.
Mobitz II AV block
P waves drop without warning while the PR interval stays fixed — infranodal disease, and a pacemaker indication.
Complete (third degree) heart block
P waves and QRS complexes marching independently, with a slow escape rhythm keeping the patient alive.