How do you identify RBBB on the 12 lead ECG?
Forget about turn signals and bunny ears!
All you need for the ECG diagnosis of RBBB are the following:
- A supraventricular rhythm
- QRS duration equal or greater than 120 ms (0.12 s)
- Terminal R wave in lead V1
- S wave in lead I
It’s that easy!
Let’s look at an example.
What’s the rhythm?
Borderline sinus bradycardia with 1°AVB and occasional PACs.
Is that a supraventricular rhythm? Yes!
Let’s move on.
Is the QRS duration equal to or greater than 120 ms (o.12 s)?
In other words, are the QRS complexes “wide”?
Be careful! 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 QRS duration is > 120 ms. Just barely, but it’s like being pregnant. It either is or it isn’t!
So we have a supraventricular rhythm with wide QRS complexes. This process is important because one of the most important and basic rules of electrocardiography is:
Wide complex rhythms are ventricular until proven otherwise!
Once you have determined that a supraventricular rhythm is wide, you can examine QRS moprhology to figure out what kind of intraventricular conduction delay is present.
Let’s look at the 12 lead ECG.
Is there a terminal R wave in lead V1?
Yes!
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. Compare it to the rsR’ complex in this PowerPoint slide.
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!
Why?
Ask yourself a question. If the right bundle branch is blocked, which ventricle depolarizes first?
The left ventricle!
So 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, 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.
All we have to do now is search lead I for a terminal S wave.
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 I
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!
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
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.
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
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"?
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.