Understanding markers and annotations on an ECG

Electrograms aren't always easy to interpret. Thankfully, they come with markers so you don't have to guess what the device was picking up on. In this lesson, you'll learn about the markers and annotations that accompany electrograms.

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[00:00:00] So, ICD is very complex in how they work and actually they're always processing information, so they're seeing they're making decisions and often they're acting upon it. So, luckily, the devices don't really leave us with blank electrograms, for us to guess what the device is thinking. Devices actually supply us with markers and annotations, that really help us understand the decision-making process.

[00:00:30] So, here, in its most basic form, we can see a leadless ECG. So, the device has generated an ECG using the can itself and the right ventricular lead. We can see some markers, some AS, A sensed events, and V sensed events. And then we can see our electrograms themselves, so our atrial electrogram and our ventricular electrogram. So, in this basic example, we have an A sensed event, that means that an atrial event has been sensed on the atrial lead.

[00:01:00] And again, a V sensed event, which corresponds to this ventricular depolarization. This ventricular event, seen by the device. As we know, these devices also have pacemaker functionality. So, you might see atrial pace markers or ventricular pace markers, and they again, can be seen here as AP and VP. And again, these correspond to an atrial depolarization and a ventricular depolarization, which suggests

[00:01:30] we've got good A-pace, V-pace capture. So, this device is dual-chamber pacing quite nicely. And this can get very cluttered and apparently quite complicated but don't worry. One thing the device does, is keeps its markers separate from the electrograms so it doesn't become unclear what's going on. So, let me take you through this example. And just to begin with, I want you to focus on the ventricular channel. So, in particular, these markers along the bottom.

[00:02:00] Now, we can see from the ventricular electrogram, we have some fast events occurring, and these are marked here as FS. This actually stands for Fib-sense. So, the device is considering this a ventricular fibrillation event. We can see as well that the time between two events is very fast 190 milliseconds, so it makes a good sense. And so, the events are marked as FS, Fib-sense. This continues until we get FD,

[00:02:30] which stands for Fib-detect. The device is now convinced that we have a ventricular arrhythmia of note and it's decided to go ahead and prescribe defibrillation therapy. So, it's going to defibrillate the arrhythmia. At this point, it's interesting to note that the markers change, which could be confusing, but this is an anomaly of this particular manufacturer, that when the device starts to charge its capacitors, ready to shock, that it changes how it labels

[00:03:00] the events. And it just drops them back to V sensed events, even though it still knows that a dangerous arrhythmia is occurring. This continues towards the end of the event, where we get the CE marker. That means charge end. So, the device is now charged and it's ready to give a shock, in as much the way that an external defibrillator needs time to charge. It has a quick look at the arrhythmia, to make sure that it's still occurring and we have one

[00:03:30] and two sensed events. It doesn't mark this one but as soon as its convinced, it delivers, CD, charge delivered. And we can see that it also annotates the energy given, so 33.9 joules. Looking at the electrogram, it’s good news. A normal rhythm has been returned. We can actually see an A-pace marker and that's because maybe the rhythm is a little bit slow, as a result of the heart having just been shocked.

[00:04:00] Those of you who are paying particular attention, you might have noticed that at the beginning of the event, we have some unusual atrial markers as well, AB and AR. What this means is atrial blanking and atrial refractory. What happens is, after a ventricular sensed event, different timing cycles are started in atrial channel. These are called the atrial blanking period and the atrial refractory period. The very fast

[00:04:30] ventricular rhythm has affected how the atrial events are marked, another way that this can all become confusing. But again, don't worry, the point in this lesson is not so you comprehensively understand every single marker immediately, it's to really drive home that to understand what the device is doing, you have to understand what the markers mean. Now, the problem is they're hugely diverse, so I've just put a few more in the screen, here and really annoyingly, is

[00:05:00] that actually, they vary from manufacturer to manufacturer. So, different markers can mean the same thing, depending on which device you're looking at. But that's the take-home message, if you're really struggling to understand what the device was doing or the device was thinking, then maybe have a step back, double check what the markers mean, and hopefully, it will all become perfectly clear.