Aberrancy and a Complete LBBB That Isn’t!

Jerry W. Jones, MD FACEP FAEM


This post is about aberrancy, retrograde P′ waves and disease of the conducting system. This ECG was recorded in 2011, so the patient is an 80 year old female who presented to the emergency room for an unknown complaint. There’s a lot to learn on this 12-lead ECG…

First, the patient is in sinus rhythm and therein lies our first problem. We see obvious P waves in the inferior leads (II, III, aVF), but not much in Leads I, aVL or aVR. This tells us that the P wave axis is very vertical: the impulse is traveling almost straight down toward the recording electrode on the left foot – which happens to be the positive pole for Leads II, III and aVF. Leads I and aVL are left-sided and Lead aVR is right-sided. They are seeing little to none of the P wave vector, so that tells us that the vector is directed very vertically and downward.

There is no suggestion of a P wave in Lead I, which tells us that the P wave vector is perpendicular to that lead. Any atrial impulse sent out toward Lead aVR is apparently cancelled by the atrial impulse traveling toward Lead aVL.


But let’s move over to the precordial leads. We see very little suggestion of a P wave in those leads except in Lead V1. Why is that? Think about it. What is the relationship between the frontal plane (limb leads) and the horizontal plane (precordial leads)? You did know they were related, didn’t you?

The frontal plane is perpendicular to the horizontal plane. They only thing they share is the horizontal “X” axis. Lead I and Lead V6 share the same axis (basically). That’s why they should look similar under normal circumstances.
What this means is that when a particular deflection is large in the frontal (vertical) plane, it will be correspondingly smaller in the horizontal plane. Certainly, there are other factors that can affect this relationship. The proximity of some of the precordial leads to the cardiac surface is one factor.
OK… so we see the effect that a vector perpendicular to not only a lead but an entire plane can have on a 12-lead ECG. Now let’s turn our attention to disease in the ventriculr conduction system.
We learned from the Ashman phenomenon that each R-R interval sets the refractory period for the following R-R interval. This can change from beat to beat and it is what allows a tachycardia to occur without any beats falling within a refractory period and blocking – the faster the rate, the shorter the refractory period. But what happens when a beat arrives early – such as a premature atrial complex? It does manage to fall within that refractory period and conducts with a bundle branch block. We call that aberrant conduction – and that’s what aberrancy is – a bundle branch block caused by an impulse arriving in the ventricles too soon, while one of them is refractory.
When aberrancy occurs it’s almost always of the RBBB type. The reason for this is that, even under normal circumstances, the refractory period of the right bundle branch is longer than the left except at faster rates. With faster rates, the refractory period of the left bundle branch becomes longer. But the rate on this ECG is just 87/minute. That’s not fast!
Let’s look at one of the rhythm strips from our ECG:


This is the V1 rhythm strip and I’ve numbered each ventricular depolarization (that’s QRS for you newbies). We see two early beats caused by a premature atrial complex and another caused by a premature junctional complex (PJC). If you look at the first premature beat (#6) you will see no P wave. Could it be hidden in the preceding T wave? One might say, “Not likely because the T wave looks exactly like all the other T waves.” That’s true for Lead V1 – but look down in the Lead II rhythm strip, recorded simultaneously with Lead V1: the T wave between the 5th and 6th QRS looks very different than the other T waves in the strip! That T wave is much larger than the others in Lead II. That means that a positive voltage has been added to another positive voltage: an upright P′ wave has been added to an upright T wave. When two positive deflections are “added together,” i.e., superimposed on each other, the resulting deflection will be larger than either deflection individually. This premature beat is a premature atrial complex (PAC) and the PAC is hidden in the preceding T wave.

But let’s look at beat (#9) on the Lead II rhythm strip. You will see that there is an inverted P′ wave preceding the QRS and separate from the T wave.  It is inverted in Lead II (meaning that it is retrograde) and the P′-R interval is only 80 msec which is much too short to have conducted through the AV node and then excited the ventricles. That P′-R interval is just too short for a low atrial PAC. That is a premature junctional complex (JPC).

But let’s focus our attention on the beat following the PJC, beat #10. QRS #10 is normal! Why? The black lines at the top of the strip give us the answer. The black line on the left measures the coupling interval of the first PJC.

I copied that coupling interval and then aligned it with the onset of premature ectopic beat #9. As you can see the coupling interval between beat #9 and beat #10 is slightly longer. That means that beat #10 had just a little bit more time for the left bundle branch to recover from

its abnormally prolonged refractory period and conduct normally. This is a functional left bundle branch block. It’s also called a rate-related LBBB and, even more specifically, a tachycardia-dependent LBBB. There – we’ve gone from very general to very specific – but all three terms are valid.


The second coupling interval gives us an indication of what the heart rate would have to be for normal conduction through the left bundle branch.

This brings up another issue. Were it not for the PJCs, would we have known that this was a functional LBBB and not a block due to a fixed defect? No, we wouldn’t have known! This leads to the question: how many “complete LBBBs” are real blocks and how many are just conduction delays? The consensus at this time is that – though we really can’t know for certain – it is suspected that most bundle branch blocks are not total, complete blocks, but instead are conduction delays in one of the bundle branches.


To learn more, visit: https://medicusofhouston.com








Location: Houston, Texas

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