LEFT BUNDLE BRANCH BLOCK

The diagnostic criteria for LBBB are:

• Broad QRS complex: >120 ms (3 small squares)
• Dominant S wave in V1
• Broad, monophasic R wave in lateral leads: I, aVL, V5-V6
• Absence of Q waves in lateral leads
• Prolonged R wave >60ms in leads V5-V6

Pathophysiology

When viewed from the right-hand side (V1), net depolarization travels away (towards the left), resulting in negative ECG deflections. The first downward deflection represents the right ventricle, and the slightly delayed 2^nd downward deflection corresponds to the depolarization of the left ventricle.

When viewed from the left-hand side (V6), where the net depolarization is travelling towards the detector, deflections are positive on the ECG. Again, there will be two peaks (RR) due to the delay in left ventricular depolarization.

In left bundle branch block (Figure 4):

1. The sino-atrial node acts as the initial pacemaker
2. Depolarization reaches the atrioventricular node
3. Depolarization down the bundle of His occurs only via the right bundle branch. The septum is abnormally depolarized from right to left.
4. The right ventricular wall is depolarized as normal.
5. The left ventricular walls are eventually depolarized by the right bundle branch, this occurs by a slower, less efficient pathway.

Clinical relevance

LBBB is always pathological. LBBB may be due to conduction system degeneration or myocardial pathologies such as ischemic heart dz, cardiomyopathy & valvular heart disease.

LBBB may also occur after cardiac procedures, which damage the left bundle branch or His bundle. A STEMI presenting as chest pain with LBBB is exceedingly rare.

Branches of the left bundle branch

Due to the relatively greater mass of the left ventricle, disruptions in the depolarization of the left ventricular muscle can cause cardiac axis changes. The left bundle branch splits into anterior and posterior fascicles.

LBBB = Left anterior fascicular block (LAFB) + Left posterior fascicular block (LPFB)

Each branch of the left bundle branch may be damaged in isolation. Anterior fascicle block, which is much more common, causes left axis deviation. Posterior fascicle block may cause right axis deviation. However, the posterior fascicle does much less work than the anterior fascicle, so it can be blocked without any obvious ECG changes.

The right ventricular muscle does not have enough mass to significantly deviate the cardiac axis.

Key points

• Consider bundle branch block in an ECG trace with broad complexes.
• A broad complex tachycardia requires differentiation between ventricular tachycardia or sinus tachycardia with concurrent bundle branch block.
• Left bundle branch block is always pathological and indicates significant damage to the cardiac conduction system.
• When assessing whether a broad QRS complex is LBBB or RBBB, the appearances of V1 and V6 are often enough to provide the answer using the WiLliaM and MaRroW technique.

Sources

https://geekymedics.com/bundle-branch-block/