- David Didlake
From The Vault: RBBB
Original publication date July, 2009.
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:
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? 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, which is greater than 120 ms.
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 exactly 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 this 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, 1st degree AVB, RBBB, and occasional PACs.
Generally speaking, right bundle branch block does not mimic, or obscure, the ECG diagnosis of acute STEMI the way left bundle branch block does.
But how do we know what’s abnormal for right bundle branch block? One of the things we consider is the rule of appropriate T-wave discordance. This concept usually comes up in the context of discussing left bundle branch block, but it’s also useful for right bundle branch block — with a caveat! With right bundle branch block the T-wave should be deflected opposite the terminal deflection of the QRS complex. In other words, when the terminal deflection of the QRS complex is positive, the T wave should be negative, and when the terminal deflection is negative, the T wave should be positive.
That’s a little bit different from left bundle branch block where we use the majority of the QRS complex as opposed to the terminal deflection.
The blue and red arrows show the expected relationship between the terminal deflection and the T wave with RBBB.
EMS is called to the residence of a 69 y/o M with a chief complaint of chest pain.
Onset: Gradual while driving
Provoke: Nothing makes the pain better or worse
Quality: Describes pain as pressure
Radiate: The pain does not radiate
Time: Similar episode over the weekend while gardening
Past medical history: CVA x 13 years ago, AF Past surgical history: None NKDA Medications: Warfarin, digoxin, others
On arrival the patient is found sitting in a chair. Skin presentation is pink, warm, and moist. He appears ill.
RR: 16 regular
Pulse: 130 irregular
BGL (BM): 146 (8.1)
SpO2: 99 on RA
Breath sounds are clear bilaterally and there is no obvious trauma.
An ECG is captured.
This 12-lead ECG shows acute anterior STEMI in the presence of right bundle branch block, but you really need a trained eye to see it.
Recall the concept of “appropriate T-wave discordance" with bundle branch blocks. What makes this case difficult is the fact that the T-waves are appropriately discordant.
However, the J-points are concordant in leads V1-V4!
If you look carefully you will see that the point at which the QRS complex turns into the ST-segment (the J-point or "junction" point) is elevated above the isoelectric line. That's abnormal for right bundle branch block. In fact, if the J-point isn't isoelectric in the right precordial leads it should be slightly depressed (in the same direction as the T-waves).
Lead V4 looks the most abnormal.
If you're still having doubts, consider that Q-waves are present in leads V1-V4. This is what Tomas Garcia (author, 12 Lead ECG: The Art of Interpretation) means when he says to "consider the company" that any ECG abnormality keeps.
Finally, let's look at leads III and aVF.
We are forced to assume that these are reciprocal changes even though the terminal QRS is isoelectric. You'd need an old ECG for comparison to know for certain.
This patient was, in fact, suffering LAD occlusion.
RBBB only has expected ST depression in V1-V3 when it is discordant to a positive R’ wave.
In general, RBBB does not show ST elevation. However, one may encounter ST elevation in leads with a large, slurred S wave (e.g. I, aVL, V5, V6) and this may very well be baseline for the patient. When in doubt, especially in a clinical context consistent with Acute Coronary Syndrome, consider these findings new until proven otherwise.
Attached is an excellent review of RBBB from the maestro himself, Tom Bouthillet. Do not miss this!