Pacemaker sensing and fixed sensitivity
Learn all about the problems associated with pacemaker sensing and how to resolve them.
Understand the nuts and bolts of pacemakers and how they utilize electrodes to visualize what the heart is doing. By the end of this video from our Pacemaker Essentials course, you'll know all about the problems associated with pacemaker sensing and how to resolve them.
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[00:00:00] In the previous lesson, we learned how pacemakers use two electrodes to see what the heart is doing, to sense the cardiac activity. Hopefully, you'll be able to recognize this circuit as a unipartner circuit. Now, the pacemaker will actually give you a nice graph of what it's seeing. These graphs are called electrograms but they're commonly referred to as e-grams. In the example here,
[00:00:30] we have a ventricular e-gram, so just involved in the ventricular lead, and actually separate leads will have separate e-grams. Just make sure you know which e-gram you're looking at. In this case, it's a ventricular e-gram. Now, we can see we have deflections and these represent electrical activity, measured in millivolts. The larger the deflection, the larger the signal. Again, like an ECG,
[00:01:00] it's against time, so as we move along we can see that this signal here occurred after this signal here. So, what we're looking at here is the depolarization of the ventricles, which is great because the heart is seeing that the ventricles are contracting. Also, the pacemaker tells you what it thinks is happening and it marks the events. So, we can see here this VS marker underneath
[00:01:30] each of the deflections and that's just saying a ventricular sensed event. Now, you can get different size deflections on your e-gram, so different amplitudes and millivolts. This can be down to the amounts of tissue depolarizing, the position of the leads, the contact of the leads with the myocardium, and even the direction of the current. I think of this as how loud the signal is. You can have a small signal
[00:02:00] just quite quiet or a nice large signal. When we implant a pacemaker, we want to ensure we have a nice loud signal because that makes life easier in the long run and now we'll see why. Unfortunately, your pacemaker circuit will pick up more than just the heart's depolarization. In actual fact, there's two main types of artifact: muscular artifact, which
[00:02:30] generally looks ununiform and slightly irregular. And all that is, is the electrical activity generated by your other muscles in the body, contracting, maybe your pectoral muscles or your biceps. You can see on this electrogram that the pacemaker has become confused and started to label this artifact as ventricular sensed events. To paraphrase, the device thinks that the heart is beating at
[00:03:00] every single one of these markers and that can cause problems that we'll come onto later. The other main type of artifact is electromagnetic artifact. This is essentially electrical interference given off by every electrical appliance particularly if a patient is very close to an electrical appliance or the electrical appliance is poorly earthed. It's generally much more uniform, the muscular artifact, and can appear as a slightly thick,
[00:03:30] dark bar across the electrogram. You can see though it has the same effect. It is confusing the pacemaker into thinking that the heart is beating. So, we use a very simple tool to remove this artifact and this confusion from the pacemaker's decision-making process. We use something called fixed pacemaker sensitivity. Now, it's really quite simple. All we do is set a fence for
[00:04:00] the pacemaker and we say, you are to ignore all signals that are less than, for example, 2.8 millivolts in amplitude. You can see here, that is meant that all these artifacts are ignored but all the genuine heartbeats or the ventricular activity that is larger than 2.8 mV is still being seen and still being registered by the pacemaker, and that is enough to ensure that the pacemaker sees things much clearer.
[00:04:30] Now, to make the device more or less sensitive, we can actually change the sensitivity. I just want you to be a little bit cautious about the terminology and I'll explain why. By increasing the sensitivity, we make the pacemaker less sensitive. Let's have a look at an extreme example so we understand that concept. If I risk to change the pacemaker's sensitivity to 12 mV, it wouldn't detect anything
[00:05:00] that we've seen so far, therefore, we've increased the sensitivity and made the device less sensitive. Alternatively, if you reduce sensitivity, you make the device more sensitive. You can see that actually now that some of these muscular artifact’s being picked up and all of this electromagnetic interference is being registered once more because we have reduced the sensitivity to 1 mV. So, the takeaway message—sensitivity is used to
[00:05:30] ensure pacemakers only detect the genuine cardiac activity.