This is the discussion for "A change of Pace: What Happened?" My apologies for the delay!
Pacemakers are amazing pieces of technology. They have evolved continually, and have given patients an increasing quality of life where none existed before.
However, with this amazing technology comes a level of complexity that also has not been seen before. These are remarkable devices, cable of many, many different types of functions. Sensing, pacing, defibrillating, single chamber, dual chamber, atrial paced, ventricular paced, cardiac resynchronization, etc… you get the idea. These are just some of the diverse functions that these devices can perform.
For us, we don't often know what type of pacemaker we are dealing with. And often, our patients do not know either! This created a dilemma for us, when we are confronted with an ill patient, and a pacemaker functioning in a way that we do not often see. Is it performing properly? Or is it performing inappropriately? Is the patient's complaint related to the pacemaker function, or the pacemaker reacting to the illness of the patient? To adequately answer these questions, we have to be able to figure our whether the pacemaker is functioning as it should, and why it is behaving this way.
To this end, I have enlisted the help of Mark Perrin, author of the EP Fellow blog. Peer sourcing is an amazing resource!
We'll go through these strips one at a time.
First, let's revisit the first ECG:
There appears to be a sinus rhythm at a rate of about 75bpm. The PR interval is about 240ms, and the QRS duration is slightly prolonged at about 110ms. There are pacing spikes that appear at the onset of the QRS. There also appears to be a fairly constant interval of 140ms between the P wave and the pacing spike.
So, what is going on here? Are the ventricles being paced by the pacemaker?
The first thing to understand is that the site of the pacing lead is normally in the right ventricle. Most of us already know that. However, what we may not all know is that the pacing lead sees "local activation", not the QRS that we see on the surface ECG.
Mark Perrin: "A pacing spike on the beginning of the QRS is not uncommon. It usually occurs because of a right bundle branch block of a right ventricular conduction delay. The pacing lead is in the RV, therefore if there is delay to the right ventricle, activation of the LV (through a conducted P through the AV node) may occur just before the RV lead paces. The RV lead paces because it has not seen LV activation."
Essentially, there is an atrial sensing window of 140ms in this case. If there is a QRS that occurs within that sensing window, the pacing spike will be inhibited. If there is no QRS impulse sensed, the pacing lead will pace. Because of the conduction delay, the beginning of the QRS (which we see) is not seen by the pacer due to the conduction delay, so it paces, even though the ventricle is already responding to the P wave. What should happen, if this occurs, is that the pacing spike should always be near the beginning of the QRS. If it is, as it is in this case, nothing needs to be done, as this is what you could call a "normal variant".
What about the pause near the end of the strip, followed by the complex that looks different from the rest? You may also notice that the P wave looks a little different as well:
Mark Perrin: "This probably occurs because of a different degree of fusion between the conducted LV activation and RV pacing. The change in the apparent AV interval occurs again because the atrial lead sees "local" activation, not the P wave on the surface ECG. Therefore if the atrial activation occurs closer to the AV node it may start conducting to the ventricle before the wavefront reaches the atrial sensing bipole and triggers ventricular pacing. The opposite also occurs, i.e. atrial activation is further from the node, and reaches the atrial sensing bipole long before getting to the AV node."
Let's take a look at the second strip again:
This strip is a variation on the theme of the first. This strip has a mixture of the complexes seen on the first ECG, some with varying degrees of fusion, plus a fully paced beat near the end of the strip. Why the fully paced beat here?
Mark Perrin: "The fully paced beat occurs (my guess) because the PVC before it invades the AV node thus delaying conduction down the node on the following beat."
Now on to the third ECG:
Here we see a fully paced rhythm at a rate of about 85bpm. Why the change from the prior rhythm to a fully paced rhythm?
To fully understand this, we have to remember a bit of physiology. In older people, or others likely to have diseased conduction systems, the "native" AV interval often increases when the heart rate increases. As the sinus rate increases, as in this case, the native AV interval also increases to the point where it is now longer than the pacemaker AV interval. Anytime the pacemaker AV interval is shorter than the native AV interval we won't see normal conduction anymore as the pacemaker takes over.
For this patient, these strips show normal pacemaker behavior. The main concept to remember is that the pacemaker does not see what wee see on the surface ECG. The pacemaker see local activation which may occur after the onset of the QRS in the ventricles, or after the onset of the P in the atria.
Many thanks again to Mark Perrin. I hope you have learned as much as I have in this case. I find pacemaker rhythms to be challenging but rewarding. They can behave in so many different ways..Normally sometimes, and abnormally others. They really are incredible pieces of technology, and I one day aspire to really master the many ways in which they work. One day!