03/29/2021
Ectopy or Aberrancy? Or Something Else?
Jerry W. Jones, MD FACEP FAAEM
One of the most difficult issues for beginners in ECG interpretation involves deciding whether a beat is ectopic or aberrant. That is the topic of the discussion in this monograph. I think you will find there is something here for both newbies and more experienced electrocardiographers alike.
An ectopic beat is an impulse that did not arise in the sinus node. Ectopic impulses can arise just about anywhere in either atrium, the AV node/junctional area or in either ventricle. They can originate in regular working myocardium or in specialized conducting fibers.
This ECG is perfect for this discussion because it presents us with two (spoiler-alert!) aberrantly-conducted beats – one is a classic RBBB and the other a non-classic RBBB. We will begin with a short discussion about atrial fibrillation and the Ashman-Gouaux phenomenon, commonly known as the Ashman phenomenon because so few people can pronounce Gouaux correctly (Goo-OH or GWO, for short).
Drs. Ashman and Gouaux, while doing some research in New Orleans, La. noted that during atrial fibrillation, if a long R-R interval was followed by a short R-R interval, the QRS ending the short R-R interval was frequently abnormally widened.
What they discovered was that each R-R interval determined the length of the refractory period of the following R-R interval. What better environment for many occurrences of long-short R-R intervals than a heart in atrial fibrillation?
But this concept is not limited to atrial fibrillation – it can also occur during sinus rhythm. Let’s start with the aberrantly-conducted QRS labeled “B.”
If we look in Lead V1, we see a classic rSR′ of complete RBBB. The r wave is very tiny, but it’s there! Now let’s look at Lead V2. Again, we see a RBBB, but this is not so classic – it consists of an R-R′-S; that is, an R wave followed by an R′ wave followed by an S wave. It doesn’t matter how many R-primes (R′) are present, if the QRS interval ends with an S wave of any size – it is not a RBBB morphology!
While Lead V2 often continues the RBBB morphology with perhaps a small amount of variation, it occasionally doesn’t, and this is one of those times. Leads V1 and V2 are considered “right-sided” septal leads with regard to precordial leads V1 – V6. Lead V3 is more of an apicolateral lead and shouldn’t be considered a right-sided precordial lead. As you can see, the aberrantly-conducted QRS (“B”) in Lead V3 is a monophasic R. This is also a variant morphology for RBBB, but it is certainly not classic.
If we look below QRS “B” in the V5 rhythm strip, we see a classic morphology of RBBB that typically appears in Leads V5 and V6: a tall R wave with a wide, slurred s wave. The R/S ratio in Leads V5 and V6 should be > 1.0.
So, what has happened here? The R-R interval that began with beat “A” and ended with the next QRS resulted in a relatively long R-R interval. Then beat “B” appeared, resulting in a much shorter R-R interval. By appearing
when it did, the impulse arrived during the relative refractory period of the right bundle branch and conduction in the right bundle branch was slowed long enough for the left ventricle to complete depolarization and then cross the septum to the right ventricle, which was then depolarized after the left ventricle. Alternatively, it is also possible that the impulse arrived in the right bundle branch during the absolute refractory period and conduction in the right bundle branch failed completely.
Remember: there cannot be a terminal S wave in a classic RBBB in the right-sided, septal leads (V1 and V2). The R′ represents depolarization of the right ventricle. A terminal S wave – in this case – would suggest that the left ventricle is being depolarized again without any repolarization (indicated by the T wave) having occurred yet! Impossible!
Now let’s look at aberrant QRS “A.” It is a monophasic R wave. If you enlarge the tracing considerably, you might detect a very tiny q wave preceding the R wave. The monophasic R wave is followed by an inverted T wave (as expected in any bundle branch block that ends with a positive deflection).
Now let’s look at what is happening in Leads V2 and V3. In those leads we also see a tall R wave. But both also terminate with an easily visible S wave. And then, both QRS intervals are followed by an inverted T wave. These T waves are significantly inverted.
As a general (but not absolute) rule, a T wave following an aberrant or ectopic beat is usually opposite the main or terminal deflection. When it appears otherwise, you must consider the presence of another pathological process, such as ischemic heart disease.
Here are examples of classic RBBB from Lead V1 and classic LBBB from Lead V6.
Look at the ST segments and T waves of both complexes. In RBBB (Figure 1), the ST segment begins sloping down from the J point which is usually located on or within about 1 mm below the baseline. In LBBB (Figure 2), the ST segment begins sloping down from the J point which is usually located below the baseline. This is called a secondary repolarization abnormality; but in spite of its name, it is not really abnormal in the presence of a bundle branch block. It is expected over right-sided leads in RBBB and over left-sided leads in LBBB. Note that the polarity of the T waves is opposite the last deflection of the QRS.
Now let’s focus on the repolarization abnormalities for QRS “A” in Leads V2 and V3. The T wave is indeed inverted, but the terminal deflection of the QRS complexes is an S wave. If this were an s wave less than 1 mm, I wouldn’t be too concerned, but these are significant, easily-seen S waves! This most likely represents a primary repolarization abnormality.
What’s the difference between a primary and a secondary repolarization abnormality? A secondary repolarization abnormality, as seen in bundle branch blocks, premature ventricular complexes and some instances of ventricular hypertrophy and ventricular pre-excitation, is due to alterations in the path that repolarization must take due to the fact that depolarization also took an abnormal path. A primary repolarization abnormality is due to pathology at the cellular level, such as ischemia, acidosis, substance toxicity and severe electrolyte derangements, to name a few. Primary repolarization abnormalities result in a T wave that is concordant with the main or terminal deflection of the QRS.
Please bear in mind: when we speak of primary or secondary repolarization abnormalities, we are referring only to those instances where depolarization (the QRS interval) is abnormal – not to regular, normal QRS complexes.
Both these instances represent aberrant beats. Aberrancy – about 99.9% of the time – refers to bundle branch block. RBBB aberrancy is more common because the right bundle branch has a longer refractory period than the left bundle branch. That is why a premature beat is more likely to encounter the relative refractory period of the right bundle branch.
Because aberrant beats begin normally, the first half of an aberrant beat – especially if classic! – will be conducted normally while the second half of the QRS will be wider and “aberrant.” Ectopic beats are typically wide from the very beginning of the QRS.
There is a caveat about the Ashman phenomenon, however: although a long-short R-R interval is a great “set-up” for aberrant conduction – it is also a great set-up for reentrant ectopy. Do not automatically assume that a wide QRS complex that ends a long-short R-R interval during atrial fibrillation is always an aberrantly-conducted supraventricular beat.
