Right Bundle Branch Block – Part 1

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.

QRS nomenclature rSR

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 1°AVB and RBBB, occasional PACs.

See also:

Right bundle branch block: Part 1

Right bundle branch block: Part 2

Right bundle branch block: Part 3


  • klaus says:

    Thanks for simplifying this and making it so logical. It´s so much easier to remember when one understands the underlying causes. Just a couple of questions:Will an incomplete RBBB be diagnosed from the same findings, but with a QRS <120ms then?Is there a lower limit for terminal R and S wave amplitude? Does RBBB often present with RAD?Hope to see this as a chapter in a forthcoming ECG book from you! 🙂

  • Tom B says:

    Klaus -I think your statement about incomplete RBBB is correct, although it's an ECG finding I don't waste much time thinking about.RBBB sometimes presents with a right axis deviation (especially if you measure the area of the S wave as opposed to the amplitude) but I generally think of RBBB with a right axis deviation as having bifascicular morphology RBBB/LPFB.Likewise, I think of RBBB with left axis deviation as having bifascicular morphology RBBB/LAFB (or Q waves from inferior MI).When bifascicular morphology is not present, RBBB most often seems to have either a normal axis or an indeterminate axis (meaning the leads in the frontal plane show equiphasic complexes).When this happens, the computer still calcuates a frontal plane axis, but I have no idea how it comes up with the number.Tom

  • Anonymous says:

    I think you did a really good job of explaining the identification of RBBB on the ECG. What you're missing here is an explanation of the physiological significance of such a finding. Too often we get caught up with simply finding something on the 12 lead, we forget to think about what it actually means.

  • Tom B says:

    Anonymous – Thank you for the comment! Your point is well taken with regard to the need for paramedics (and all health care providers) to understand the "why" in addition to the "what".In the case of RBBB, this is an ECG finding that may or may not be clinically significant.However, as a QRS confounder, RBBB does create a pattern of T wave discordance that should be well understood to aid in the identification of acute STEMI.We'll revisit this concept in Part II.Tom

  • klaus says:

    Tom: How about ILBBB, then? Let's say you have LBBB morphology with 110ms QRS. Shouldn't that be considered a warning of a possible ongoing/developing pathologic process although the block isn't considered complete? And if so, wouldn't an IRBBB also at least be a hint of something "going on"?

  • Tom B says:

    Klaus – The problem is that it's not specific. Whether it's incomplete or not, unless it's showing an acute injury pattern, then it's not really helpful and the ECG is nondiagnostic (the exception in my mind is changes on serially obtained ECGs).Tom

  • the slurring of the S wave in I and V6 is my favorite criterion.

  • Scarlett says:

    I would love to have a quick way to print all of these as a lesson, so that I can save them for personal review, without all the ads on the page and the comments.  I can copy and paste but that takes alot of time.  I love this website, it really teaches me alot that was never even mentioned in the class or the books.

  • ali says:

    thank you for yor nice site,when we have a Q vawe on beggining of QRS at RBBB is show a acut Anterior mi,Am i right?

  • Justina says:

    Hello, can one diagnose LPFB with LBBB?
    Thank you.

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