When the right bundle branch of the electrical conduction system is blocked, it can no longer directly activate the right ventricle (RV). RV activation is delayed, and occurs indirectly through the ventricular septum from an intact left bundle branch.
ECG criteria and findings in RBBB
QRS ≥ 120 ms
Late R' or R peak, M-shaped QRS (RSR') pattern or sometimes a wide R or qR in lead V1
Early R peak, wide S wave in lead V6
Wide, slurred S wave in the lateral leads (I,aVL,V5-V6)
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)
Right Bundle Branch Block in MI
In a typical RBBB, leads V1-V2 can have mild discordant ST-segment depression, which is a normal finding
RBBB should have no ST-segment elevation in any lead, The ST-segments should be isoelectric, and any ST-segment elevation is ABNORMAL
Beware STEMI’s in RBBB…often missed by the computer and the clinician
Don’t trust the computer to make the diagnoses when the STE is subtle
Look at all the leads and trace out where the QRS ends and where the J-point begins to evaluate for ST-segment deviations
To learn more about how to diagnose MI in RBBB in MI check out MI in BBB & Paced Rhythm Criteria