EP 101 2020: A Virtual Program for Incoming EP Fellows-Introduction to EP

Introduction to EP
Video Transcription
Video Summary
Sorry for the camera work. Take two. So, without further ado, let's queue up the first video. That's Introduction to EP, as presented by me last year. And I will see you on the other side. So let's start by talking about intracardiac electrograms. I know there's probably a range of exposure here in the audience. How many people have been to the EP lab and have... I'm trying to use the slide advancer, there it goes. Different people have different exposure in terms of intracardiac electrograms, understanding what they are, how to interpret them. So I wanted everyone to be on the same page, which is why we introduced this talk. And the first point that I want to make is the difference between an EKG and an electrogram. In both cases, you have a bipole that's recording small electrical signals. The difference is, of course, that with an EKG, the bipole, this is requiring multiple clicks. I don't know if the battery is dead or if I need to aim it in a certain way. There we go. I don't know if there's an alternate that I could use. Thank you so much. With a surface EKG, the bipole is from one arm to the other, or arm to the leg, thanks. And you're therefore spanning the entire heart within that bipole. And as a consequence, of course, you're recording the field of view is large and incorporates the entire heart. And therefore, of course, you see a P wave, you see a QRS, you see the repolarization T wave. However, if you were to take that bipole and shrink it down and put it inside the heart, such as here, here's an intracardiac catheter, maybe it's just not a direct line. There we go. The field of view of a bipole inside the heart is very small so that you're not seeing multiple chambers. You're not even seeing the electrical activity of the entire chamber, but just the immediate vicinity around that bipole. And we call it an electrogram. So notice here, instead of seeing the P wave and the QRS, you see just a signal that times with the QRS. And think of it as a slice of the QRS. It's a piece of the ventricular electrical recording. You don't see in the RV apex, for example, any atrial signals, and you typically don't see a T wave. Think about bipoles when you're thinking about recordings. There we go. So I wanted to start about unipolar recordings, and of course the first question is, well, we just talked about a bipole having a cathode and an anode. How is it that you can take a unipolar recording? It's like having a magnet that has only a north pole and not a south pole. The answer, of course, is there is no such thing. Unipolar recordings are bipolar recordings, but where the cathode is not in the heart, it's remote. So there are two ways that we can record what we refer to as a unipolar recording, but it's really a bipolar recording. One is to have the catheter inside the heart, and we usually, by convention, have that be a positive electrode, be an anode. And the negative electrode, you can actually have it be the surface EKG leads. And this is known as Wilson Central Terminal, it's just the name it's given. And if we use the arm and the leg electrodes, and specifically, actually, the left leg, those surface EKG leads sort of form somewhat of an equilateral triangle, and each of the forces, if you combine the three, kind of cancel each other out. So you end up with a negative pole that really doesn't have much of a deflection to it whatsoever. So what you end up recording, really, is a reflection of the single electrode inside the heart, and not contributing to that is the negative electrode, which is the sum of the limb leads. Again, known as Wilson Central Terminal. Because there is some artifact that can be introduced at the level of the sticky, of the electrode skin interface, in order to try to reduce that artifact that has to do with impedance at the level of the skin. In addition to just using these EKG leads, some resistors are actually added to the system. It used to be 5,000 ohms, it's now 50,000 ohms in most current systems. And that sort of negates the impedance step up at the level of the skin, and it just helps clean up the signal. The alternative way that you can create a unipolar recording is to, again, have a catheter in the heart. But instead of using the EKG leads, you can have an electrode in the inferior vena cava, inside the body but away from the heart. And there are catheters that are designed that have well down the shaft of the catheter a separate electrode that can be used for this purpose, if you purchase catheters that have that electrode down on the shaft for this reason. Or you can put any catheter, a quadrupolar catheter, and park it in the inferior vena cava, and use any of the four poles as the negative electrode in your unipolar recording, and you configure your recording system accordingly. And this is known as the indifferent electrode or a remote electrode, and again, you can generate a unipolar recording. So let's go back to the inside of the heart and look at the difference between a unipolar and a bipolar recording, because it's important to understand. So here is your anode, the one electrode in the heart, and the red bar is meant to show a sheet of myocardium. And let's say we have a wave front that passes from left to right toward and then underneath and then away from that electrode. You can see the signal that's inscribed in the recording initially is flat, because the wave front is out of the range of view of that electrode. As the wave front approaches the electrode, you have a positive deflection indicating movement toward that electrode. As the wave front passes the electrode, you have a rapid shift from positive to negative. And then as you recede from the electrode, it's a negative deflection. And then as you get out of the field of view, it's flat again. So this is a simple unipolar recording of a simple wave front moving in a line toward, underneath, and past an electrode. If you had the same strip of tissue in that same signal and you repositioned your electrode at the very end such that there's nowhere to go once you reach that electrode, here is the recording you'd get. A longer flat portion, because there's more myocardium out of the field of view, and only a positive deflection, because you're approaching the electrode, and then you stop at that electrode in this artificial construct. And there's no negative component, because at no point are you moving away. And conversely, if you positioned the electrode at the very start, you're going to see the opposite. You'll see a negative deflection as you start moving away, and then it will even out as you get out of the field of view of that electrode. Similarly, if you position the electrode in the middle portion of this strip of tissue, and let's say you're able to create a beat that didn't start at the end, but started right underneath the electrode and moved away in opposite directions like this, again, you'll only have a negative deflection, because at no point is there a wave front moving toward your electrode. Why do we care about this? We care about this in particular when we're mapping a focal arrhythmia, a PAC, a PVC, a focal ATAC, or a focal VT. Because if you can position your mapping electrode at the tip of your ablation catheter right over the origin, you're going to see a recording that looks like this. A wave front will only be able to move away from your electrode, because it's positioned at the origin. Here's an example. Here's a PVC. You can see the top six lines here are EKG recordings. And then we have unipolar at the distal electrode labeled "Unid", and then the second electrode, "Unip", the proximal electrode in that distal pair. And you can see here that there is an initial positive deflection in this unipolar recording. What does that tell us? It tells us that we're not positioned over the origin. Because at some point the signal is coming toward that electrode. Well, if it's coming toward it, then it must have started remote from it. Maybe close, but not right underneath. In contrast, if we move this catheter from site A to site B, and we see this recording, we are very excited and happy because we can see that there's only, what we call, a QS. People will use EKG terminology to refer to the morphology of electrograms. Maybe that's not quite right, but that's what people use. So I will use that language. A QS, initial downstroke with no R component. There's no positive deflection here, reflecting the fact that this wavefront is only moving away from the electrode as one piece of evidence that maybe the electrode is positioned right at the origin. This is useful for focal arrhythmias, focal tachycardias. Yes? Josh, why don't we see local ventricular repolarization on, you know, intracardiacs? That's a great question. And I'll answer it now, and then I'll reinforce it in a few slides. So the repolarization signal, the T wave, is a much lower frequency event as opposed to the sharper electrogram that is from depolarization. And we have filters that we apply on our recordings that filter out low and higher frequency information that is beyond the range of what is of interest to us. So the T wave signal, the repolarization signal, tends to be filtered out because it's not information we find useful and we don't include it in the range of frequencies that we display on our recording system. That answer that? What if we took a unipolar recording and we used the opposite polarity? We ended up using a negative electrode, a cathode, instead of an anode. Here's that same wavefront moving toward, underneath, and away. And you're going to get an upside-down version of what we saw before. As the wavefront moves toward the electrode, instead of a positive deflection, you're going to see a negative deflection. And vice versa, when it's receding, you'll see a positive deflection. Who cares? This is of critical importance. To understand bipolar recordings, because what is a bipolar recording? It is two unipolar recordings recorded from electrodes that are close together, but not occupying the same location, and they are of opposite polarities. If you take these two unipolar recordings and display them as this simple wavefront moves from left to right, you're going to see in the anode recording the positive-negative, in the cathode, the negative-positive, and they're going to be offset by a little bit of time because of the distance between them. The time it takes to get to the first electrode is sooner than the time it takes to get to the second electrode. If you combine the electrical information from both of these unipolar recordings into one channel and superimpose them, you get a bipolar recording. That's what bipolar recordings are. Now I'm going to show only in one image those two waveforms superimposed. For example, at the start where the positive electrode is recording a positive deflection, but the negative electrode is still flat, in summation you're going to get an initial positive deflection in the bipolar recording, and the same with every other component. If you take these two electrodes and let's say you superimpose them and put them on exactly the same spot, if that were physically possible, then the net fusion between the two would give you a flat line because there's no offset in timing. They are exactly seeing things at the same time, but in opposite polarities, and they would completely cancel each other out. So we don't take a bipolar recording with the two electrodes at exactly the same place. However, if you move them closer together, like here, you will come up with a different quality of the signal. And what do I mean by that? Most of the signal is going to be negated by the other polarity electrode, but the space in between the two is still going to be a difference between them, because that wavefront will be moving away from the first electrode and toward the second electrode, and so you're going to take an electrical recording in that little space. So what I'm trying to say is that the closer space you get the two electrodes, you're going to be more precise in terms of identifying the local activation between the two. If you have wide space electrodes, you have a larger field of view. If you have closely spaced electrodes, you're going to see exactly what's going on right there in between the two. It's very precise, but the amplitude gets smaller, because you start canceling each other out. So there is a limit to what our recording system will be able to detect, because of external noise, our ability to amplify the signal. So generally, closely spaced electrodes are, you know, two millimeters, three millimeters apart. There are studies being done looking at even closer spaced electrodes. So the resolution improves as you get them closer together, but the signal shrinks, and you need to have a system that's able to accommodate that. That make sense? I wanted to just interrupt my train of thought for a second in terms of, ah, okay, there's my timer, I couldn't see it, I just want to make sure I know my timeframe here. There are exceptions to ignoring the shape of a bipolar signal. There are scenarios where the shape is important, not telling you about wavefront coming toward or away, but other features. So for example, if you have in this simple myocardial sheet a wavefront that moves in this direction, and you put your mapping catheter right on top of it, you'll record a bipolar signal like we discussed. But what if you have an area of scar? And now that you're going into EP, you're going to think about scar in a more sophisticated way than just dead tissue, not alive, not contracting. You're going to think of scar as fibrosis intermingled with live but sick heart cells where you can have little electrical routes meandering through this area of scar because it's not all dead. This is true whether it's from myocardial infarction, from sarcoid, from non-ischemic cardiomyopathies. If you have scar, you have live cells in there that can generate little signals in various directions and in various routes. If you put a mapping catheter on this scar, you're going to see a reflection of all of these roots traveling through this region of scar wherever there's live myocardium. And the signal you record is going to look very different from the one I just showed you. You're going to have what's known as a fractionated signal or it will usually be of low amplitude. And the reason is, of course, that in this small physical space, you're going to have wave fronts traveling in different directions at different times and your bipole is going to record all of them over a wider period of time and lots of deflections reflecting the different directions that the wave fronts are moving. Why do we get excited when we see this type of signal when we're mapping for a reentrant rhythm? This same scenario in sinus rhythm that generates all these little signals over time has exactly the machinery, has exactly the potential to create a reentry circuit. Usually we think about this in ventricular tachycardia, but it's also true when you have atrial scar as well. You can get reentry circuits in an area of fibrosis. But if you have an electrical barrier so that a wave front reaches this barrier and it can't go through it, in order to get to the other side, it has to travel around the barrier and then it gets to the other side after a delay. If you happen to have a mapping catheter positioned right on that barrier and it's able to see in its field of view both sides, then you're going to get a two-component signal that we refer to as a split potential or a double potential. And what this signal is telling you is that you have a line of electrical block so that your signal is reaching one side of the line and then after a delay reaching the other side. That's important to recognize. For example, here's a patient who has a scar and was undergoing an ablation. And you can see in white there are, for every paced QRS complex, this is during ventricular pacing, there are two electrical signals separated by a distance. And at first you might say, well, this maybe that's a P wave and this is retrograde VA conduction and the second signal is an atrial signal. And it's not. You can just see barely at the bottom of the screen the coronary sinus catheter in green is showing the atrial sinus activity marching through independent. So each of those two signals is generated by ventricular activity from each of those beats. And the reason that they're separated is there was a line of ablation performed and it was pretty complete so that there was activation on one side of the line. The signal had to travel all the way around to get to the other side. So this tells you that there was a line of block at the location where this catheter is currently located. Another way that the morphology of a bipolar signal can be helpful is during ablation. So look here at the channel I've boxed in red. This is during a flutter ablation. Look at the difference between the very first aqua signal at the left and the very last signal on the right. This is only after a few seconds of radio frequency energy application. As you're killing off tissue, the sharp nature of the signal starts to disappear and you start to see a more blunt signal that we call far field. Far field meaning further away. Sharp signals are usually right beneath or adjacent to the bipole. Blunt signals that are more smooth looking are further away. As you kill off the near tissue, you start to see the far tissue. So you might, if you see this change, say I don't need to stay on ablation for the full 60 seconds or 40 seconds or whatever that I had planned, it looks already like I've killed off this tissue and I don't want to ablate into adjacent structures like the esophagus or whatever. And so looking at the quality of the electrograms, the shape of the bipolar electrogram can tell you something about tissue destruction during RF. Well, you don't know from the shape of an electrogram if a wavefront is coming toward or away, if you see a change without moving the catheter, a change in the polarity of a recording, it tells you that something changed about the wavefront approaching that bipole. So for example, this was a flutter case where the flutter terminated but there was still slow conduction through the isthmus, through the line of lesions that was created to try to create block on the cavatricuspid isthmus. But it was challenging because there seemed to be an electrical leak through the line and we didn't see it. It got more and more subtle when we got block but then it recovered in terms of the timing of the signal on the lateral side of the line during medial pacing at the coronary sinus. And here you can see circled in red on the left beat we had a leak through the line and on the right beat we got block in the line. The timing of that signal normally would be dramatic. It would have been earlier on the left and later on the right as it took longer to get to the lateral side of the line over the roof of the right atrium all the way around instead of through the line. But this line had had so many lesions, I think this was even a redo procedure, there was such slow conduction through to the line that even when the signal was traveling through, there was still a delay. And it was only really through seeing the change in the morphology of the bipolar electrogram that we said, "Aha, we created a meaningful difference in the way the wavefront approached that bipolar on the lateral side of the line." So now, instead of going through the line slowly, it was traveling over the roof and hitting that bipolar from the opposite direction. And it was because of the polarity change in the bipolar electrogram that was a clue that block had been achieved at this point. Last point about morphology of bipolar signals is to reiterate the point about near field versus far field, and I'll do it with a case. Here's another PVC case where we see the ablation catheter positioned on the endocardial surface at a spot that we thought was probably near the site of origin. And I'm going to blow up the signal. You can see there are multiple components to this signal. There is an initial domey-looking thing that is sort of smooth and rounded and was present on every single beat. So it was a meaningful reproducible signal. And then there was a sharp component. That domey signal was very early compared to the PVC onset. But because it wasn't sharp, we said, "Hmm, maybe this is not reflecting a wavefront that is immediately underneath this bipolar." We moved the ablation catheter around on the endocardial surface trying to find a spot nearby because it clearly was nearby. We're recording it from the bipolar here just a little bit further in the field of view. We could never get a sharp signal at that early timing in the endocardial plane. So we said, "Maybe we need to think three-dimensionally, and maybe the signal is coming from deeper in the myocardium or even on the epicardial surface so that we're able to see it from the inside of the heart but we're not right on it." So in this case, it was an outflow-track PVC. We said, "Well, let's position the catheter into the coronary sinus and out to the great cardiac vein. Opposite where we had been on the endocardium. And now we see this signal with a very sharp near-field early signal at that same early timing as the domey thing that we recorded from the inside of the heart. So the point to be made here is that sharp signals on a bipolar recording are called near-field. They're right adjacent to the bipolar, but you shouldn't ignore the far-field signals. It tells you something's happening. It's just a little further away. I need to figure out how to get there if, in fact, your goal is to find the point of origin and ablate. Josh, two questions. The first one, what's the frequency of a cardiac electrogram that obviously varies by QRS versus T-wave, but it's come up a couple different ways. Yeah. So you can sort of surmise by looking at the filters, the high-pass and low-pass filters, 30 hertz to 500 hertz is typically the range that we view intracardiac recordings. And things that are outside of that are typically not physiologic signals that we want to see, and the range is therefore going to be between those two. And the other one, on the fractionated potentials, what's the difference between systolic and diastolic fractionated potentials? Good questions. Very good question. So a fractionated signal is of the morphology I showed with multiple components over time. And if a signal of that nature is appearing during a QRS, whether it's during sinus rhythm, during pacing, during a tachycardia, then we say that it's occurring during electrical systole. If, however, that fractionated signal is late, it's displaced until after the QRS is complete, during sinus rhythm or pacing, we would call it a late potential<br /><br />the activation of that fractionated within the scar tissue is happening even after the rest of the heart has depolarized, so it's late. During a tachycardia, where there is a circuit happening, there's always, because the definition of a circuit is the same thing keeps happening over and over, at any point in time, something is being depolarized. And if you happen to be in the spot where there's slow conduction within scar, between QRS complexes, because you're within the scar, a small mass of muscle, we call that a diastolic fractionated signal because it's happening during electrical diastole, not during the QRS, but between QRSs. And that is a clue that maybe that's what's happening within the scar that we can only record from inside the heart. We can't see it on the surface CKG because it's such a small amount of myocardium. And it may be part of the mechanism of that tachycardia. Now, you could have other late things that are not part of a circuit, but are also happening late. So it's not sufficient to say, this is a diastolic fractionated signal during V-tach, and therefore, if I ablated, the tach will stop. It may be, but there are other what's called bystanders, other areas in the scar that could be activated late that are not part of that particular circuit. So systolic versus diastolic refers to timing with the QRS. Fractionated refers to the multi-component morphology. So one final question here. I think that I've been asked before, I just want to keep the most current answers in my own mind, is if tissue stunning, not death, during ablation can cause a QS signal only? Yeah, and the answer is yes. So if you have cells that are currently not electrically being stimulated, then they won't generate an electrical recording. And so you can see a change in the electrical bipolar recording that could simulate cell death, but there could be some reversibility to it. That's seen actually more with cryoablation, where there's a longer delay between disabling cells and cell death, compared to RF, where there is a much shorter timeframe between injuring a cell and killing it. So generally... So you might say, "Okay, well, I'm not gonna come off RF the moment I start to see that change. Maybe wait a few more seconds or whatever, but the point being is... that most of the change in the morphology is resulting from cell death, but you're right, there can be a rim of surviving tissue. And it's clinical judgment with each lesion how long you stay on, incorporating the bipolar electrogram information. I may transition to the next video, please. Thank you. Thank you again for participating in the evaluation. I appreciate that I have a lot to learn every year as well in order to bring the best quality conference to you. So please continue with submitting your questions, and I'm addressing them right now. Thank you.
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Josh Cooper, MD