All About Junctional Beats

If your experience was like mine, when you were first learning to read ECGs (not really at the level where you could actually interpret them yet), you were told that junctional beats had QRS complexes that were identical to the sinus beats. You recognized the junctional beats by the fact that they had no P waves. Later, you learned that they could have P waves but they followed the QRS. Then you learned that the P waves were inverted. Eventually you learned that the P waves could appear in front of the QRS. But, given all this, you were also probably left with the impression that all P waves on the ECG should be inverted during a junctional rhythm. This monograph is about junctional beats – from P´ waves to T waves. This paper is NOT, however, about complicated junctional rhythms – just the junctional beats themselves. Let’s start…

Junctional beats generally originate from the bundle of His or very near to it. There are several things that you should understand about the bundle of His before we go any further.

  1. The bundle of His is located mostly in the right atrium. A small amount of the bundle crosses into the interventricular septum where it almost immediately divides into the right and left bundle branches.
  2. The fibers that will become the right bundle branch and the fibers that will become the left bundle branch have already separated and arranged themselves within the bundle of His well before the division.
  3. When an impulse arrives from the atrium – in most cases from the sinus node – it enters the bundle of His at its  very origin, i.e., at the “top” of the bundle. All the fibers in the bundle are activated simultaneously.
  4. If an impulse arises in the bundle of His itself – or very near to it – it will NOT be entering the bundle “from the top.” It will NOT activate all the fibers simultaneously. This is an eccentric entry point.
  5. For the record, MOST QRS complexes of junctional ectopic beats resemble the normal, sinus-conducted QRS complexes. The similarity ranges from “completely indistinguishable” to “bearing no resemblance at all!” That’s right… a junctional QRS does not always look like the sinus-conducted QRS!

All throughout the bundle of His and the Purkinje fibers are cells (or collections of cells) that have the capability of spontaneous depolarization. Remember that most myocytes depend on an arriving impulse to depolarize them and stimulate them to fire and then pass along the depolarization wave. But some cells outside the SA node are capable of spontaneously depolarizing and thus initiate their own impulse that can spread through the heart (just like the sinus node). In the ventricles, these cells are located in the conducting tissue of the bundle of His and the Purkinje fibers. We call them accessory (or subsidiary) pacemakers. They are able to assume control should the sinus node fail.

Let’s say the sinus node has suddenly started to depolarize very slowly. The next accessory pacemaker is typically the junctional site. The atria also have accessory pacemaker sites, but it is rare for them to intervene. Why? They are often affected by the same force(s) that cause the sinus node to slow down – usually autonomic nervous system input. By the time we reach the bundle of His, there is very little autonomic neural influence and so the junctional accessory pacemakers are usually the next to intervene.

The diagram on the left represents the bundle of His.

All junctional beats are somewhat eccentric (red arrow in diagram). Even if the impulse were to arise from a cell that is exactly in the middle of the bundle of His, that is still not quite the same as entering the His bundle “from the top” (blue 3-D arrow).  In the first case, the fibers are excited outward in all directions  from the middle and not all are excited at exactly the same time or with the same strength. When the fibers are excited “from the top,” as by a sinus-conducted impulse (blue 3-D arrow), they are all stimulated simultaneously and with the same strength. Already we have the rationale for a slightly different QRS complex. Granted, the difference in the QRS may be so slight that not even a very experienced electrocardiographer would be able to detect it – but something else on the ECG will most likely be able to detect it and “tell” you about it – if you will only “listen!” More on this later…

Let’s suppose that a junctional ectopic pacemaker lies just outside the bundle of His but very, very close to it. When it discharges, the impulse will enter the bundle of His from just one side. Those fibers will be stimulated very strongly, but as the impulse moves further into the bundle of His it will have less and less effect*. In fact, it may lose so much strength that it doesn’t pass all the way through the bundle. Therefore, the fibers on the same (ipsilateral) side as the ectopic junctional pacemaker will be strongly stimulated while others will be stimulated with less force and some fibers may not be stimulated at all. This will result in a QRS that will most likely be very visibly different than the sinus-conducted QRS – yet it is still a junctional beat!

*When an impulse travels along the length of the muscle fiber longitudinally, it travels very efficiently and maintains its strength for greater distances. When an impulse travels in a direction that is perpendicular to the fibers, it has less strength and ends sooner. Transmission of an impulse in a direction that is perpendicular to the muscle fibers is called anisotropic conduction.

If an ectopic junctional  impulse enters the bundle of His from the left side after the fibers have already arranged themselves for the division into left and right bundle branches, it is quite possible that the fibers destined for the left bundle may be the only ones that receive any significant stimulus and could, theoretically, result in a QRS with right bundle branch block morphology.

As I said previously, even when the sinus and junctional QRS complexes look exactly the same, the ECG may still alert  you regarding which is sinus and which is junctional (ectopic). How does it do that? Look closely at the T waves. Look at the morphology of the T waves in all those beats that you know to be sinus-conducted and then compare them to the T wave of the possible junctional beat. Even when the QRS complexes look exactly the same, the T waves will often show some dissimilarity. Note the difference (blue arrow) in the rhythm strip on the top. In the bottom rhythm strip, the junctional QRS is identical to the sinus-conducted QRS complexes, but notice the difference in the T waves: the junctional T wave is much more peaked and rejoins the baseline with much less slurring.  

How close should the P´ wave (we use the designation P´ to indicate any P wave that did not originate in the sinus node) be to the QRS in a junctional beat?

To understand the answer to this question you must learn a new concept of the PR interval. Aside from the delay in the AV node, normally the PR interval will be affected by the time it takes for the impulse to arrive at the AV node from its origin in the atrium. If it comes from the sinus node, it will take “X” msec. If it comes from an ectopic pacemaker in the left atrium, it will take “Y” msec and if it comes from a low right atrial ectopic pacemaker, it will take “Z” msec. All these values are based on the distance of the origin of the impulse from the AV node.

That is NOT the case with a junctional beat. There are actually four options for the P´ wave of a junctional beat:

  1. Appear BEFORE the QRS
  2. Appear AFTER the QRS
  3. Appear SIMULTANEOUSLY with the QRS (and thus be hidden)
  4. Never develop due to a retrograde block in either the bundle of His or the AV node

What determines whether the P´ wave appears before, during or after the QRS as well as how far in front of or how far after the QRS is the DIFFERENCE in conduction time from the site of the ectopic junctional pacemaker to the atria and to the ventricles.

If conduction to the atria is faster than conduction to the ventricles, then the P´ wave will appear in front of the QRS. If conduction to the ventricles is faster than conduction to the atria, then the P´ wave will appear after the QRS. The majority of the time, transmission times are almost equal in both directions, therefore the P´ wave appears simultaneously with the QRS and is thus hidden within it and not detectable on the ECG.

There is no rule regarding the duration of the P´R interval of a junctional beat, but in my experience, it has always been less than 0.12 seconds and usually 0.08 seconds or less. The RP´ interval is usually about the same duration though, again, there is no fixed value for the duration.

If someone asked you to show them the junctional beat in the illustration to the left, what would your response be? Would you indicate the large QRS complex or would you respond, “I can’t do that.” If you haven’t already surmised, “I can’t do that” is the correct answer. The actual firing of the junctional ectopic pacemaker is not visible on the ECG for the same reason the firing of the SA node is not visible.

The QRS and, if present, the inverted P´ wave are not the junctional beat – they are the products of the junctional beat. Think of them as fraternal twins – they don’t look the same but they came from the same mother at the same time.

This illustration of the junctional beat and its analysis by laddergram on the left demonstrates the relationship of the inverted P´ wave and the QRS. In this case, the junctional impulse experienced a bit of delay arriving in the ventricles, so the inverted P´ wave appears first. As you can see, the inverted P´R interval has nothing really to do with the distance the impulse travels. It depends on the difference in the conduction time from the junction to the atrium (JA) and the junction to the ventricles (JV).

 

In the second laddergram of a junctional beat, we see that the ventricle is activated first, but the retrograde atrial impulse follows it so closely that it is buried in the QRS complex and thus not visible on the ECG tracing.

The first arrow in the tracing to the left indicates the presumed location of the firing of the junctional ectopic pacemaker. How did we know where to put the arrow? Very simple: we guessed! When we want to laddergram the depolarization of a pacemaker that does not appear normally on the ECG, we arbitrarily place it 0.08 msec or 0.12 msec (the choice is yours) in front of the first deflection created by the pacemaker discharge. That would be 2 or 3 small squares, respectively. It really doesn’t matter exactly where the pacemaker fired. This approach will work just fine as long as we keep using the same distance (or duration).

Now, let’s clear up a bit of confusion regarding the orientation of the P´ waves. The P´ waves will be traveling upward through the AV node. This will result in inverted P´ waves in the inferior leads and upright P´ waves in the superior leads. Leads II, III and aVF are the inferior leads that will have inverted P´ waves and Leads aVR and aVL are the superior leads that will have upright P´ waves. Lead I runs perpendicular to the direction of propagation of the P´ waves so you generally don’t see them very well in Lead I; if you see them at all they will be minimally upright (but again, they are generally very difficult to see in Lead I). V1 and usually V2 – V4 will have upright P´ waves, but Leads V5 and V6 can be variable – sometimes upright and sometimes inverted. Why is that? Remember that the electrodes for Leads V5 and V6 are very often placed lower than they should be on the chest wall. In such cases Leads V5 and V6 are able to see not only left-right forces but also superior-inferior forces (just like the inferior leads). If the electrodes for V5 and V6 have been placed too low, they may demonstrate inverted P´ waves; if they are placed correctly, they will likely show upright P´ waves.

If you would like to learn more about interpreting ECGs at an advanced level, come join us in one of our presentations of the Advanced ECG Interpretation Boot Camp. Go to our home page to see what we have to offer! At Medicus of Houston, you are always a participant…  never just an audience!

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