Why ICDs malfunction

One of the biggest complaints by patients with ICDs is that they sometimes receive "shocks" inappropriately. This can affect their lives quite profoundly. In this lesson, you'll discover how devices can sometimes deliver therapy when it is not required, and why it happens.

Want to learn how to recognize and treat ventricular arrhythmias with ICDs? Take our ICD Essentials course and confidently deal with ICDs in a clinical setting. Your instructor, Dr Kristian Webb– a board-certified devices specialist, author, and cardiology education enthusiast based in London–will guide you through the fundamentals of ICD use and troubleshooting.

[00:00:00] Inappropriate therapy—so, what are we talking about when we mention inappropriate therapy? Well, really it's something that holds a lot of fear for us and especially our patients. And it's when the device confuses either normal rhythm or a non-benign arrhythmia, for something like VT or VF and tries to treat it. The one that really worries the patients is receiving a high energy shock inappropriately,

[00:00:30] as this can really affect their life. Well, inappropriate shocks happen and these are the main causes. Atrial fibrillation is the most common cause and then, in order, we have supraventricular tachycardias, lead fractures, T-wave oversensing, and electromagnetic interference. And in this presentation, we're just going to cover those and show exactly how they lead to an inappropriate shock. What I want to make really clear is that inappropriate shocks happen. And

[00:01:00] we can't always look into a crystal ball, see how the patient's heart rhythm is going to behave and set the device perfectly. Instead, it is how we respond to these inappropriate shocks, which will really set you apart as a skilled physician. So, we'll look as well in this chapter, at how we can adjust our discriminators to try and troubleshoot this. But for now, let's have a look at exactly how they cause an inappropriate shock. So, atrial fibrillation,

[00:01:30] well, we know with atrial fibrillation, we can have a varied response in the ventricles. And in some patients, the ventricles contract very quickly, as a direct result of the atrial fibrillation. This can confuse the device into thinking that a ventricular arrhythmia is taking place, either VT here or VF markers, depending how fast it's conducted through. And ultimately, the device may end up delivering therapy. We know we have discriminators to help identify these rhythms, but sometimes they don't work

[00:02:00] quite as we had hoped or they might not be active. Superventricular tachycardias—well, again, these are just very fast pace but they're driven from the atria but again, the device can get confused as to why exactly the ventricles going quickly. It may assume the worst and deliver therapy. Now, a lead fracture is even harder to equate for because, really, it shouldn't be happening and it's not a programming fault, it's a mechanical fault. Now, there are some things

[00:02:30] built into the device to help prevent inappropriate therapy for lead fracture, but essentially, the interface, that where the fractures occurred, can generate noise on the lead. Now, the noise looks pretty similar to a ventricular event to the device. It can't tell the difference, so it starts to mark them as true ventricular events. And you can see here that this lead noise is being marked as VF events and if that continues, the device would deliver inappropriate therapy.

[00:03:00] T-wave oversensing—this is something that we try and troubleshoot at implant. T-wave oversensing is quite a simple concept. Essentially, the T waves that we can see here are also being picked up on the ventricular electrogram and being marked as a genuine ventricular event. Now, obviously, if this patient's heart rate was 100 beats per minute and we were T-wave oversensing, you're essentially doubling their heart rate. And the device might have their rate down as 200

[00:03:30] beats per minute and delivering inappropriate therapy. Electromagnetic interference—there are many devices that our patients may come in contact with or be in close proximity to, that generate a huge amount of electromagnetic interference. Some of the most common ones are induction hob cookers and arc welding. Now, if a patient is close to a source with electromagnetic interference, this can be detected on the lead. Now, each one of these deflections on the

[00:04:00] lead is also assumed to be a ventricular event by the device. And we can see here, that this electromagnetic interference is being categorized as VF events, but in reality, in the background, the rhythm has remained stable, but the device can't see that, and therefore, inappropriate therapy may be delivered.