Bundle Branch Blocks and Axis Deviation

Left Bundle Branch Block LBBB Overview

• 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 depolarization (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.

Dominant S wave in V1 with broad notched (‘M’-shaped) R wave in V6

Right Bundle Branch Block (RBBB) Overview

• In RBBB, activation of the right ventricle is delayed as depolarisation has to spread across the septum from the left ventricle.
• The left ventricle is activated normally, meaning that the early part of the QRS complex is unchanged.
• The delayed right ventricular activation produces a secondary R wave (R’) in the right precordial leads (V1-3) and a wide, slurred S wave in the lateral leads.
• Delayed activation of the right ventricle also gives rise to secondary repolarization abnormalities, with ST depression and T wave inversion in the right precordial leads.
• In isolated RBBB the cardiac axis is unchanged, as left ventricular activation proceeds normally via the left bundle branch.

Tall R’ wave in V1 (“M” pattern) with wide, slurred S wave in V6 (“W” pattern)