Notes on a Randomly-Selected ECG
Jerry W. Jones, MD FACEP FAAEM


Figure 1

What is the rhythm on this ECG? Don’t be too quick to respond because I just asked you a trick question! Which rhythm are we discussing – the atrial rhythm or the ventricular rhythm? Although this looks like a “single” ECG, it is really two ECGs – an atrial ECG and a ventricular ECG – one superimposed upon the other.

Let’s begin with the atrial rhythm. Do you see any P waves? I don’t. That – by definition – is either atrial fibrillation, sinus arrest or third degree SA block.

Now let’s consider the ventricular rhythm. It appears irregular. It is always best to assume that a ventricular rhythm that is based on an ectopic junctional or ventricular escape pacemaker will be very regular – even though it may be very slow. That isn’t always the case, but it will be about 99% of the time in my experience.

OK… let’s put these two observations together: if the atrial rhythm (or lack of an atrial rhythm) were due to a sinus arrest or third degree SA block, then we would expect an escape rhythm that would be very regular – whether junctional or ventricular. That is not the case here.

Therefore, it is most likely that this represents atrial fibrillation with a ventricular rhythm is NOT a ventricular escape rhythm. The AV node is conducting the fibrillatory impulses from the atria to the ventricles. This ECG demonstrates a fine atrial fibrillation. In this case, the word fine means very small and almost imperceptible. A fine atrial fibrillation usually implies an atrial fibrillation of long duration. New atrial fibrillation is usually coarse and very easily seen (and much easier to treat).

If we look in Lead V1, we see a classic complete left bundle branch block (cLBBB) – or do we? The morphology of the QRS complexes in Leads I, aVL, V1 and V6 are classic for cLBBB! But there’s just one problem…

If you look at the third QRS from the left, you will see a normal, narrow QRS complex. That cannot happen with a true complete left bundle branch block… but there it is! My point here is that many – if not most! – bundle branch blocks are NOT blocks at all but instead represent a conduction delay in a bundle branch that is still able to conduct under more favorable circumstances.

If you look more closely at the ECG – and use some calipers – you will see that the R-R interval from the second (wide) QRS to the third (normal) QRS is the longest R-R interval on the tracing. This is a rate-related (or acceleration dependent) left bundle branch block. As the rate slows down, the intervals between the QRS complexes widen, giving the left bundle branch more time to repolarize and conduct normally.

Because the refractory period of the right bundle branch is normally longer than that of the left bundle branch, aberrant conduction (make no mistake – this represents aberrant conduction) usually appears with a cRBBB morphology. At faster heart rates, the left bundle branch develops a longer refractory period and aberrant conduction will appear with a cLBBB morphology. But there is yet another problem…

The heart rate on this ECG isn’t particularly fast – and yet the acceleration dependent bundle branch block appears with a cLBBB morphology. This indicates serious disease of the conduction system in the left ventricle. So, atrial fibrillation plus an acceleration dependent cLBBB at a normal heart rate is very indicative of extensive heart disease.

Is there anything else we can deduce from this ECG? Absolutely! Did you know that around 92% of people with cLBBB have left ventricular hypertrophy which is the principal cause of cLBBB? Now that’s with a fixed cLBBB, so I am not sure of the percentage when the cLBBB is acceleration dependent.

There is another issue here: has there been a previous MI that has resulted in these findings? Probably, but we can only surmise. There is a very poor R wave progression in the precordial leads with transition occurring in Lead V6. But does that represent an old (remote) anterior MI? Not necessarily. If you look at Leads II, III and aVF you will see QS complexes suggestive of an anterior fascicular block pattern. Anterior fascicular block is one of the more common causes of “poor R wave progression,” much more common than a remote MI. However, an ECG such as this one likely indicates a much higher probablility of a previous MI.

This pattern however is not actually due to a block – or even a delay in the anterior fascicle. It is due to the fact that the first structure activated in the left ventricle following the on-time activation of the right ventricle was the posterior fascicle. While the normal beat has an rS morphology in Lead III, it has a small monophasic R in Lead II.

The QRS duration (not shown) was 130 msec. That really isn’t very wide for a cLBBB morphology, although it certainly qualifies for cLBBB. When the cLBBB is not especially wide, always be careful that you aren’t mislabeling a “nonspecific intraventricular conduction defect” which can often look just like a cLBBB. You can check for this by making sure that there aren’t any septal q waves in Leads I or V6. (Careful! Lead aVL can have what appears to be a septal q even during a cLBBB!)

I hope you learned a bit about left bundle branch block in this article. Why not come join us in one of my Masterclasses? These are four day, live and in-person presentations with lots of active participation by all. The atmosphere is very casual and friendly but the learning is very immersive and intense. AMA PRA Category 1 Credit is awarded.

These are not beginner’s courses and they aren’t review courses. Just be aware that many of the “advanced” ECG courses are not advanced at all, but simply a rehash of the usual introductory material. Some are still teaching concepts that were debunked almost 60 years ago!

We do not offer “board certification” because THERE IS NO RECOGNIZED BOARD CERTIFICATION IN ELECTROCARDIOGRAPHY!

 

The Masterclass in Advanced Electrocardiography
Halifax, Nova Scotia
April 21 – 24, 2025

The Masterclass in Advanced Dysrhythmias
Houston, Texas
June 9 -12, 2025

https://medicusofhouston.com

jwjmd@medicusofhouston.com




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