Definition

Normally the septum is activated from left to right, producing small Q waves in the lateral leads

In LBBB, the normal direction of septal depolarisation is reversed (becomes right to left), as the impulse spreads first to the RV via the right bundle branch and then to the LV via the septum

This sequence of activation extends the QRS duration to > 120 ms and eliminates the normal septal Q waves in the lateral leads

The overall direction of depolarisation (from right to left) produces tall R waves in the lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3), and usually leads to left axis deviation

As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (‘M’-shaped) R wave in the lateral leads

ECG features of LBBB

Diagnostic Criteria:

- QRS duration of > 120 ms
- Dominant S wave in V1
- Broad monophasic R wave in lateral leads (I, aVL, V5-V6)
- Absence of Q waves in lateral leads (I, V5-V6; small Q waves are still allowed in aVL)
- Prolonged R wave peak time > 60ms in left precordial leads (V5-6)

QRS Morphology in the Lateral Leads

The R wave in the lateral leads may be either:

‘M’-shaped

Notched

Monophasic

RS complex

Left Bundle Branch Block ST elevation

In left bundle branch block, the ST segments and T waves show “appropriate discordance” — i.e. they are directed opposite to the main vector of the QRS complex

This produces ST elevation and upright T waves in leads with a negative QRS complex (dominant S wave), while producing ST depression and T wave inversion in leads with a positive QRS complex (dominant R wave)

Example of LBBB

Causes of LBBB

- Aortic stenosis
- Ischaemic heart disease
- Hypertension
- Dilated cardiomyopathy
- Anterior MI
- Primary degenerative disease (fibrosis) of the conducting system (Lenegre disease)
- Hyperkalaemia
- Digoxin toxicity

LBBB Quiz

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