- 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 > 120ms 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 can produce a broad or notched (‘M’-shaped) R wave in the lateral leads.
ECG Diagnostic Criteria for LBBB
- 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)
Causes of LBBB
- Aortic stenosis
- Ischaemic heart disease
- Dilated cardiomyopathy
- Anterior MI
- Primary degenerative disease (fibrosis) of the conducting system (Lenegre disease)
- Digoxin toxicity
NB. It is unusual for left bundle branch block to exist in the absence of organic disease.
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