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.
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.
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!
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?
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?
ECG diagnosis: Borderline sinus bradycardia with 1°AVB and RBBB, occasional PACs.
Right bundle branch block – Part I