ECG Level 3 Tutorial: Bundle Branch Blocks
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Tutorial: Bundle Branch Blocks
Understand why a right bundle branch block and left bundle branch block pattern are created. Learn also the diagnostic criteria.
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Tutorial: Bundle Branch Blocks RBBB
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Right bundle branch block (RBBB) is not a mirror image of LBBB. Some important differences include:
  • RBBB is common in normal healthy hearts (LBBB is not)
  • RBBB does not cause big changes in the ST segment or the T wave. This is because LV depolarization and repolarization are both normal because the left bundle is conducting normally.
  • RBBB therefore does not cause diagnostic interference with infarction, ischemia, or LVH

Let's look at what happens to depolarization and the ECG in RBBB. Remember that the LV will be depolarized first and that LV depolarization will be normal. The septum is the first to depolarize:
This will produce an early upgoing R wave in V1/V2 and it might produce a small downgoing (Q wave) in V6:
Next, the rest of the LV depolarizes, mostly toward V6:
This will produce a downward deflection (S wave) in V1/Vand an upward deflection (R wave) in V6
Finally, the RV is SLOWLY depolarized as the electric impulse is transmitted without the fast conducting fibers:RBBB ECG

This will produce a second upward deflection in V1/Vand we call this waveform R prime (R').
In V6 we have a wide downgoing waveform and this is called a "slurred" S wave:

We ran out of letters for upgoing waves in the QRS, so that is why we use "R prime" which is designated as R' ... not to be confused with Optimus Prime. Remember that similar to LBBB, one entire ventricle is depolarized slowly. Therefore, one of the diagnostic criteria is the same: QRS duration > 120 ms. RBBB has the RSR' configuration in V1/2. You can remember RBBB: it has RaBBBit ears.

RBBB also has the slurred S wave in V5/6 (and usually also lead I since it is similar to V6). The slurred S wave needs to be either wider that the R wave or > 40 ms (1 little square). The slurred S wave has a rounded "bottom", as opposed to a pointy bottom, and it is wide.

You might see lower case "r" and "s" with upper case "R" and "S". These simply designate a small waveform (lower case) and a large waveform (upper case). For example, in V1 above, we might say it is rSR' because the second R wave is larger than the first R wave.

Another point that is important when you compare LBBB vs. RBBB : remember that in RBBB in V6 the upgoing R wave is caused by FAST depolarization of the LV. This means that we would expect this portion of the QRS to be narrow and the V6 intrinsicoid deflection to be normal/narrow.

Sumary of diagnostic criteria for RBBB:
  • QRS > 120 ms
  • RSR' configuration in V1/2 
  • slurred (wide) S wave in V5/6
Exceptions to the rule:
  • if the patient has had an old anterior infarct with Q waves, then the first R wave in V1/2 might be missing (because the R wave is replaced by a Q wave)

1. Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS Recommendationsfor the Standardization and Interpretation of the Electrocardiogram. J Am Coll Cardiol 2009;53:976–981.