Complete right bundle branch block (RBBB)

• In normal conduction, the ventricular septum is activated from left to right.

  • This results in the appearance of small Q waves in the lateral leads (I ,aVL, V5-V6)

• Complete block of an entire bundle branch requires that its ventricle be activated by the other ventricle

  • This results in delayed activation and prolongation of the QRS complex (≥ 120 ms)

• Complete RBBB causes right ventricular (RV) activation to occur indirectly after left ventricular (LV) activation is complete

  • The RV is no longer activated with the LV simultaneously as in normal conduction , it is activated after the LV through septal depolarization

  • In RBBB, the LV is still activated normally through the left bundle branch, hence the initial part of the QRS complex is normal and the axis remains unchanged

  • Delayed RV activation results in a secondary R wave (R' deflection) and distortion of the late portion of the QRS in the right precordial leads (V1-V3)

  • Delayed RV activation also results in a slurred and wide S wave in the lateral leads (I ,aVL, V5-V6)

  • Delayed RV activation results in repolarization abnormalities (ST depressions & T wave inversions) in the right precordial leads (V1-V3)

Incomplete right bundle branch block (IRBBB)

• • Slowed conduction through the right bundle branch, which results in ventricular conduction delay and slight prolongation of the QRS complex (100 ms - 119 ms)