How do you identify right bundle branch block (RBBB) on the 12 lead ECG?
Most of us were told to look for “bunny ears” or to use the “turn signal” method but all you really need for the ECG diagnosis of RBBB are the following:
- Supraventricular rhythm
- QRS duration equal or greater than 120 ms (0.12 s)
- Terminal R wave in lead V1
- Slurred S wave in lead I
Let’s look at an example.
What’s the rhythm?
We have borderline sinus bradycardia with 1°AVB and occasional PACs.
Is that a supraventricular rhythm? Yes.
Let’s look at the 12-lead ECG.
Is the QRS duration equal to or greater than 120 ms (o.12 s)? In other words, are the QRS complexes “wide”?
It’s easy to fixate on the tight R-wave and discount the S-wave with RBBB. If this was a tachycardia at a rate of 150, it might appear to be a narrow complex tachycardia, when in fact, it would be a wide complex tachycardia!
The computer is measuring the QRS duration here at 132 ms (0.132 s) which is greater than 120 ms (0.12 s).
So we have a supraventricular rhythm with wide QRS complexes.
Once you have determined that a supraventricular rhythm is wide you should go to lead V1 and see if you can classify it as a right or left bundle branch block. For right bundle branch block we look for a terminal R-wave.
What do we mean by “terminal R wave”?
The last wave of a QRS complex is the terminal wave, or terminal deflection. If a QRS complex ends in an R wave, then it has a terminal R-wave. It can also be said that the terminal deflection is positive.
I would call the QRS complex in this 12 lead ECG an rsR’ complex.
It’s important to think in terms of the terminal deflection (or terminal R wave) in lead V1 with RBBB because the QRS morphology can be quite variable!
Consider these examples.
All of these QRS complexes are different. Most are positively deflected but some are negatively deflected. Most start with an R-wave, but a few start with a Q-wave. However, they all share one important feature.
They all have a terminal R-wave!
Ask yourself a question. If the right bundle branch is blocked, which ventricle depolarizes first? The left ventricle! Which ventricle depolarizes last? The right ventricle!
What is the only precordial lead on the right side of the chest? Lead V1!
A terminal R-wave in lead V1 represents late right ventricular depolarization.
The terminal S-wave in lead I represents the same thing, because the positive electrode for lead I is on the left shoulder. So, late left-to-right ventricular depolarization moves away from the positive electrode for lead I and toward the positive electrode for lead V1.
Remember when I said that the first step was to establish that you were dealing with a supraventricular rhythm?
The QRS complex in the top row, far right, was cropped from a run of VT (lead MCL-1 which is a surrogate for lead V1). The QRS complex in the bottom row, far right, was also taken from a run of VT.
So, when you have a supraventricular rhythm, with wide QRS complexes, and a terminal R-wave in lead V1, you’re 99% of the way toward calling this a RBBB.
To confirm look for a terminal S-wave (some textbooks call it a “slurred” S-wave) in lead I.
Does lead I show a terminal S wave? Yes!
ECG diagnosis: Borderline sinus bradycardia with 1Â°AVB and RBBB, occasional PACs.
Right bundle branch block: Part 1