Detecting arrhythmias with ICDs

Discover how ICDs detect arrhythmias and prevent cardiac arrest.

Kristian Webb
Kristian Webb
6th Mar 2018 • 3m read

How do ICD's work to detect arrhythmias and prevent cardiac arrest? In this video, you'll find out why having more leads in the heart provides a more accurate picture of the type of rhythm your patient is experiencing and how to decipher the results.

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Video Transcript

[00:00:00] So, it's all well and good seeing that these devices are able to see the heart's rhythm and recognize when the patient's having a cardiac arrest. But I really want to drill down into how these devices are able to do this. So, let's start by having a look at the S-ICD. It's able to see what the heart is doing by creating an electrical circuit, which is actually comprised of a shocking coil,

[00:00:30] the cathode in this example, the box itself, the anode but most importantly, some of the heart tissue itself. So, in actual fact, between the cathode and the anodes, any electrical variations are picked up by the device. So, this probably comes from the heart but obviously, you might have some other muscle, subcutaneous muscle working. They could be picked up as well but the majority of information is coming from the heart muscle itself. So, the fluctuations between

[00:01:00] these two electrodes are then depicted as pretty much a surface ECG. So, in an S-ICD, this is the information you're given. But I really want to emphasize what's great about a transvenous ICD device. Now, these devices give you localized electrograms. As opposed to a complete surface ECG, it's giving you a localized electrogram. So, let's have a look. The circuit in this single-chamber ICD

[00:01:30] is comprised of the tip of the lead, which is playing the part of the cathode and the coil itself, which is playing the part of the anode. You can see the heart muscle, this is slightly exaggerated, just so you can see it clearly, but the heart muscle itself is involved in that circuit once more. But it's only the ventricle that's really involved, so that means that the signal, the information we're picking up, is only from the ventricle. Now, what if we have another lead in the atria as well. What happens if we had a second lead

[00:02:00] and we had a dual-chamber ICD? Well, in actual fact, as opposed to a generalized ECG, we get two bits of information, we have the electrical activity from the atria. So, that's picked up this P wave, but that's not showing on the ventricular channel. But we do have the ventricular depolarization on that channel and the T wave, whereas, we don't see that on the atrial lead. Now, it doesn't look as beautiful as this. In most

[00:02:30] examples, we just see deflections. If we have a look here, at this example of what we might get during a VT, you can see the atrial deflections representing atrial activity, most likely intrinsic sinus beats. But here, interestingly, on the ventricular lead, we're picking up multiple depolarizations, some very fast activity coming from the ventricle itself. So, we're getting two bits of information to help us ascertain what that rhythm is.

[00:03:00] Now, because there's more ventricular events than atrial events, we can be really confident in saying that this is a ventricular arrhythmia and that's key, that essentially, the more leads you have in the heart, the more information you're able to gain, and that's really useful in determining exactly what rhythm a patient is in. I'm just going to end with a real-life example of a VT from an electrogram and this is more like you'll see if you print out information from a device

[00:03:30] or you're looking at information from a device, using the monitor itself. Here, we have the atrial marker, we can see, okay, they're not regular, so maybe there's a few ectopics going through or something else happening but they're not regular. But if we look at the ventricular channel, we can see that they're more numerous. And again, more ventricular events than atrial events, suggestive of a VT. So, the fact that we have leads in different chambers

[00:04:00] and the fact that we get a localized electrogram, is incredibly useful in determining what rhythm a patient is in.