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EP 101 2020: A Virtual Program for Incoming EP Fel ...
Basic Entrainment Concepts
Basic Entrainment Concepts
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job. We're now on to the next session, the afternoon session, and I'm pleased to say that at various times during the morning, we had over 300 participants and some terrific questions coming in. So keep those questions coming in. With that, we're going to go on to Josh, who is actually going to be doing a live talk, a new talk this year on a very important principle, the basic concepts of entrainment. And Josh, at the end of it, if you want me to moderate, I certainly can. If you want to read your own questions, just queue me up and I'm glad to help in any way. Josh. If you don't mind, just because I'm going to be focusing on the slides, as the questions come in, you can sort of formulate a plan and we can banter back and forth and I can answer them. But if you don't mind joining me for the question and answer after this, that would be great. Will do. Thank you. So welcome back, everybody. Sorry for a short break. We wanted to kind of condense as much EP101 information as we could for you. And we're going to proceed with the one live talk today. We'll see how it goes. Hopefully no snafus and you can hear me well. The reason for the live talk is that we decided, based on feedback from all of you last year who attended and those of your colleagues who gave us feedback, that the entrainment talk might have been starting at too high a level, talking about the Waldo criteria, et cetera. So I wanted to redesign this particular talk to get back to some of the foundational principles with regard to reentry. And that's what this talk will do. And then in sort of the 201 or a 301 kind of concept, we can get on to more advanced topics rather than have them presented right at the outset in the 101 version. So let's go. So here we're going to talk about the principles of reentry and what entrainment means. So the most important elements of reentry are a couple fold. One is that there has to be some type of electrical barrier around which signals travel. So here you can see points A and B on the slide. And in order to get a signal from A to B, you have to travel around a barrier in order for reentry to occur. Like this, the arrow signifies way front propagation. The barrier could be a valve, like the tricuspid or mitral valve. It could be the inferior vena cava. It could be scar that spontaneously formed. It could be a surgical incision. It could be a previous ablation area or line. If you didn't have a barrier, then signals traveling from A to B would simply sweep across the chamber in all directions, and you would depolarize all the tissue there simultaneously or sequentially, and there wouldn't be a way for the way front to double back on itself. So that's the reason that you need a barrier for reentry to occur. We always draw circles, in part for historic reasons, but it's just easiest to draw as a cartoon a circle and demonstrate the principles of reentry and entrainment. True circuits really are complex, and not just in two dimensions like this drawing here, but really in three dimensions through the depth of the myocardium, and some groups are demonstrating that just beautifully with endocardial and epicardial simultaneous maps during complex VTs. But forgive the simplicity of a circle, but all of the principles of reentry and entrainment apply to a path of any shape, and in two or three dimensions, but the circle is just easiest to teach principles. The next concept to think about when you're thinking about reentry is that the circuit is made of individual cells that sequentially get depolarized and repolarized, and it's important to really think of that because, you know, a chain is as strong as its weakest link. If you have one area or even one cell, if that's a critical link in a circuit that is refractory at a moment that the wave front is reaching it, then that wave front will not be able to propagate. So think of circuits in terms of the individual components, in this case the myocytes, which are sitting, you know, in bands and sheets oriented in one way or the other, and signals will travel faster lengthwise from cell to cell than widthwise, so the speed around a circuit may actually change and not be constant depending on the orientation of the cells at that part of the circuit. It's important to think of each individual cell, as I just mentioned, as having different phases of the action potential, and that of course includes the depolarization phase and the repolarization phase, and during repolarization, or at least most of it, the cell is refractory. So again, if a wave front were to reach that cell before it has fully recovered, then you're not going to be able to propagate the signal and depolarize that next cell or group of cells. So in order for reentry to occur around that barrier that I just mentioned, you're going to have to have every component of that circuit recover faster than the time it took for the wave front to travel around the circuit. If the wave front travels faster around the circuit than the repolarization time, than the refractory period, then you're going to have a wave front meeting refractory tissue, and that circuit will extinguish. If the cells on the other side of the equation take a long time, at least in one place, to recover, their refractory period is long, then when a wave front comes back to that point, then the cells may not be ready to further propagate the wave front, and again, the circuit will not sustain. So that is the circuit. So in order to have that circuit perpetuate, you have to have a zone, which may be narrow, it may be wide, of myocytes that are recovered. I'm going to use for the remainder of this presentation a yellow arrow to denote the wave front and the refractory period. So everywhere, the head of the arrow is the oncoming wave front, and the body of the arrow, or anything that's yellow in this picture, tells us that the tissue there is refractory. It's in the middle of its recovery time. The tail of the arrow shows where cells have finally fully recovered enough to be activated again, and the zone between the head and the tail is the area that has recovered. Those myocytes there are in there, they're excitable once again. And so in order for this arrow to be able to perpetuate, the head of the arrow can never catch the tail of the arrow, because again, if that happened, then the wave front would not be able to propagate forward, and the rhythm would extinguish. That zone between the head and the tail is known as the excitable gap. Those are the myocytes that are recovered, and there has to be, again, some excitable gap for reentry to occur, which could be small, but it has to be large, and it has to be which could be small, could be big, depends on the properties of the tissue, and the size of the barrier, and therefore the circuit. It's important to think not only of the arrow rotating around the circuit, but the excitable gap is rotating around the circuit. Just as the wave front is moving forward and depolarizing cells, so too are cells at the tail of the arrow further recovering, and that excitable gap, green zone, is rotating and revolving around along with the oncoming wave front. Very important principle when we're talking about trying to jump into the circuit with pacing, trying to entrain it or take over the circuit with faster pacing. So if we wanted to modify the circuit, meaning take it over by pacing in the same chamber, then we have to have each beat be able to depolarize tissue in the circuit, meaning where we pace, that wave front has to get into that green zone, into the excitable gap. If we were to pace and our wave front reaches the body of the arrow, it reaches refractory tissue, where those cells have not yet fully recovered from the wave front having passed, then we're not going to engage the circuit with that paced beat. And that is why when we try to entrain, we typically need more than one paced beat to do so, because the chances of a single beat being able to get into the circuit are directly related to the relationship geometrically, I should say anatomically, between the pacing site and the closest part of the circuit, how large the excitable gap is, and what is the location of that excitable gap at the moment you decide to deliver that pacing stimulus. And if things are not perfect, then that one beat will not get into the circuit. So if you wanted to capture the circuit, you'd need more than one beat. So let's say that this circuit is traveling at a cycle length of 300 milliseconds, and that we give a paced beat that meets refractory tissue. And let's say we want to compete with the circuit and we pace at exactly the same fast rate that the circuit is going. So here's the circuit. It traveled at 300 milliseconds. And let's say we pace at a second beat at 300 milliseconds. Well, notice what happened there is that the circuit revolved completely exactly 360 degrees. Everywhere that was refractory remains refractory. The excitable gap that could have been depolarized remains in the same location relative to the pacing site. And on that second beat, we were still unable to get into the circuit because the circuit was at the same unfavorable orientation, and the excitable gap was at an inaccessible location a second time. And if we keep pacing at exactly that same rate that matches the tachycardia, every single time we pace, we're going to continue to meet refractory tissue, kind of like a strobe light firing in a dark room, and you're watching a wheel that is spinning at exactly the same pace that the strobe light is firing, giving you brief glimpses. The wheel will look stationary because you're only seeing the wheel at exactly the same location if the light is flashing at the same pace precisely as the wheel is turning. So in order for you to pace and get your signal into the circuit, you have to pace at a different rate than the circuit is going for this reason. And that could be slower or faster if you're talking simply about changing the location of the excitable gap. So let's do that experiment and pace slower than the tachycardia. So here's beat one that comes in and reaches refractory tissue. And we say, okay, well, let's pace slower. And here we paced a little slower. We paced at 330 milliseconds compared to the tachycardia at 300. And no doubt the excitable gap is in a new location. But because the circuit is going faster than our pacing rate, we were unable to compete with the circuit. In fact, the wavefront that the circuit generated coming back toward our pacing site beat out our pacing rate. So in fact, the tissue is refractory in between our pacing site and the circuit. So we made less progress. And in fact, the circuit itself is going to dominate the chamber if we try to pace slower. It may be once in a blue moon that we'll even capture the myocardium, but we're going to be swamped out by the faster pace and the oncoming wavefronts from the circuit. And we will be unable to approach the excitable gap. So really, if you wanted to entrain or capture tissue from a circuit, you have to pace at a different rate that has to be faster than the tachycardia. And here's how that plays out. Here's that first beat again that we paced and meet refractory tissue. And now we're going to pace at, let's say, 280 milliseconds, faster than the tachycardia cycle length of 300. And notice that we still met refractory tissue, but the excitable gap is at a little bit different location compared to where it had started. It's moving toward us because we didn't give the circuit enough time for that excitable gap to get right back to 12 o'clock here on this diagram. It didn't make it all the way around, but it migrated backward slightly. Well, let's give a third beat. Still refractory tissue, but the excitable gap is closer to where we're pacing from. And on beat four, same thing, but really close. And then finally, on beat five, the excitable gap is accessible to us in location. We were able to depolarize tissue that was excitable. And we can launch a new wavefront from that pacing stimulus ahead of when the wavefront from the circuit itself would have reached that point. So let's think in two ways for a moment about the excitable gap and its location compared to our pacing site. If we were to pace and get into the excitable gap near the front edge of it, meaning near the tail of the arrow, what's going to happen? Whenever we depolarize tissue that is excitable, then we're of course going to send wavefronts in both directions, the retrograde direction, and that will collide with the oncoming tachycardia wavefront and extinguish, and also in the anterograde direction. And if we're really close to the tail of the arrow, we're going to catch it. So we're going to meet refractory tissue backwards because of the oncoming wavefront and forwards because that tissue that we rapidly approached hadn't yet recovered. If that's the case, then we will terminate reentry and the circuit will be done. This is how anti-tachycardia pacing works in a defibrillator is that you pace many beats fast enough and you hope that one of those beats gets into the excitable gap near the tail of the arrow and terminate that rhythm in precisely the fashion I just drew in cartoon form. But if you pace further back and you capture the excitable gap nearer to the head of the arrow, and note when we talked about pacing faster than the tachycardia, as it slowly migrates backward, as long as we're not pacing too fast, then we're always going to catch the excitable gap right near the head of the arrow, further back in the excitable gap, and not near the leading, the trailing edge of the arrow, the leading edge of the excitable gap, and terminate it. So if you pace just barely faster than the tachycardia, chances are very low, less than 1%, that you're actually going to terminate the tachycardia. But instead, what will happen? Your retrograde wavefront will still, it's less tissue there, but you're still going to meet the oncoming wavefront. That little part will extinguish. But as you move forward, you're going to be able to propagate forward. And in fact, what I just drew is not entirely accurate as to what's going on because just as the tachycardia has a migrating excitable gap, so too will that happen if we pace further back in the excitable gap. And it'll look more like this. We capture tissue, and we're depolarizing tissue in the forward direction. But of course, the tissue that had been depolarized from the circuit itself is also continuing to recover. So that slightly smaller excitable gap will continue to migrate forward as we pace and capture tissue. So what is going on when you think about the interplay between pacing and the tachycardia itself? Let's start with the simplest scenario where we, in fact, are pacing in the circuit itself. So this sun-shaped icon signifies a pacing site. And let's say the tachycardia still has a cycle length of 300, and we pace at 280. At the moment we pace in the excitable gap, we're leapfrogging the signal from where the wavefront had reached up to this new location. And when we do so, we send the next wavefront forward 20 milliseconds earlier than would have happened if we had just waited for the wavefront of the tachycardia to reach that point. So we, in a sense, have accelerated the circuit. Now, we have not actually sped up conduction velocity around the circuit. But what we've done is we've skipped one segment of the circuit and just teleported that arrowhead from where it was to 20 milliseconds, in this case, forward than where it had been. So we're sort of leaping forward in time. We're not changing the conduction properties of the circuit so much. So you can keep doing that. We just showed with that diagram 1B. But let's say wavefront's coming round, and you pace in that excitable gap, and you leapfrog that arrowhead forward and launch the next wavefront at 280 milliseconds. You can keep doing that over and over and over from that site. And we've essentially commandeered the circuit with our pacing. That's known as entrainment. Some people also use the term continuous resetting, because basically you are taking over the circuit by putting in a new wavefront ahead of when the wavefront would have normally reached that same point. And you can do that indefinitely. You can do the same thing if you're not pacing right on the circuit tissue from a remote site, as long as the wavefront reaches the circuit at a location where the excitable gap is present, and you're further back in the excitable gap so you don't terminate the circuit. So again, you're leapfrogging forward, just doing it from a remote site. And you're skipping, in this case, if you pace at 280 milliseconds, you're skipping 20 milliseconds. It might have taken, by the way, a few more beats for that remote site to reach the excitable gap, because you're going to have a collision in between your pacing site and the oncoming wavefront from the tachycardia. But as long as you're pacing faster, that collision point will get closer and closer to the circuit, and then your excitable gap will migrate back, back, back to that location where you interact, and you will take it over. So again, if you're remote from the circuit, you can take it over, you can entrain, but it just might take a few more initial beats to get to the point where you're reproducibly able to accelerate and advance the tachycardia from that remote site. And here's a diagram demonstrating entrainment. Let's switch gears for a moment to recording electrograms within the circuit. No pacing for the moment. I'm going to use the same icon to demonstrate a bipolar that you're recording from. Two little gray squares signify the bipolar. The green oval signifies the field of view of that bipolar. And here I'm going to show electrograms in this black recording box, similar to your electrogram recording system. So if you have a wavefront that's traveling round and round the circuit and you're recording a bipolar from one spot, you're going to see these bipolar electrograms every time the wavefront passes that spot. Now, we usually during an arrhythmia have more than one site that we're recording from. Oh, sorry. Of course, the spacing between the electrograms is, of course, going to be the same as the tachycardia cycle length because that's how long it took the wavefront to revolve 360 degrees around the circuit. And so you'll get a new bipolar electrogram every time that wavefront reaches that point. My point was going to be we often have multiple electrodes recording at different locations, let's say within the circuit. And let's say we move our recording site from one set of bipolar to another. And let's do the same thing. Well, we're again going to record every time the arrowhead, the wavefront that's oncoming, reaches that bipolar. We're going to record an electrogram. And the spacing, of course, between them again will be the tachycardia cycle length. But if you were to compare two different sites, even though the spacing between the electrograms is the same, the timing will vary, of course, because the wavefront is reaching those two sites at different times. What if we record at a remote site that's not in the circuit? What's going to happen then? Well, let's say we put a mapping catheter there. And when the wavefront comes around the circuit and reaches the closest points to where we're recording from this remote bipolar, the wavefront is going to approach that bipolar. And we're going to record an electrogram. And then as the circuit continues to revolve, every time it reaches that closest point, it's going to throw off another wavefront toward our recording bipolar. And even though we're remote, we're getting signals at exactly the same pace that we were when we were recording from within the circuit, still at 300 milliseconds. This is a very important principle as we synthesize in the next two slides what happens when we are pacing and recording sequentially, what's going to happen when we're in a circuit versus away from a circuit. So everything you just remembered, keep in mind and file away, because it's all going to come to bear on the next two slides. So here, we're going to start pacing within the circuit, which we've already reviewed. And then we're going to stop pacing and see what happens to the electrical signals at the timing between our pacing and our recording. Again, I'm using the starburst, the sun-shaped icon to signify pacing. And the bipole is that green oval and the little gray squares. And it's important to remember that we are going to be pacing and recording from the same bipole in these examples. This is what entrainment and looking at the post-pacing interval is all about. So we are pacing. And when we pace, by the way, from the same channel that we're recording from, we're going to have a big electrical artifact, a big deflection that goes off the screen because we're basically putting current right through that bipolar channel that we're recording from. So this is a stimulus artifact, artifact associated with pacing from that same channel. So as we now entrain from that site, which we've already reviewed, if we pace in this example at 280 milliseconds in a tachycardia that was going at 300 milliseconds, we're going to first see, while we're pacing, the pacing artifact separated from each other by 280 milliseconds. And here's where the big question comes. What happens when we stop pacing? We have that last beat. And then we say, let's just let the tachycardia keep doing its thing after I stop pacing. The question comes up, and this is a thought experiment. I want everybody who's logged on now to think about their answer. We're not going to push out a multiple choice question, but think in your mind, what is going to be the spacing here between that last paced beat and the first sensed bipolar electrogram when we came off pacing? Is it going to be 280 milliseconds, 300 milliseconds, more than 300 milliseconds, less than 280 milliseconds? Think in your mind and take a moment to answer that question. And then I'll review the way I think about it. When I myself think about this question, I imagine myself shrinking down to the size of an electron that is emitted from that last. It isn't as simple as this, but it works mathematically. That electron I am emitted from the bipolar on that last paced beat, and I have to go on a journey. And that journey is going to be around the circuit, and then I'm going to get back to my recording site. Here I am in electron, paced and released, and then getting back to be recorded. Well, I went basically 360 degrees around that circuit. And if the tachycardia cycle length remains 300 milliseconds, which it is during this example, the correct answer here is going to be 300 milliseconds. I paced. I let that wavefront travel all the way around. I didn't shortcut it. I didn't leapfrog anything. This is known as the post-pacing interval. And then all of the other intervals, now that I'm no longer pacing, are going to be, just as we discussed before, 300 milliseconds, because that circuit will continue to revolve at the same pace. And this we've already reviewed. And right now, maybe you're thinking, well, this sounds really simple and silly. Why didn't we just go through this exercise? But this next slide is the full reason why this whole presentation exists and the way you should formulate your thinking about entrainment pacing and the post-pacing interval, depending on the location of your pacing site and the location of the circuit. We're going to repeat what we just did from a remote site. And let's just say, for this case, that it took 40 milliseconds for that wavefront to leave your pacing site and get to the closest place on the circuit. And we know from before that we can, from this remote site, take over the circuit and entrain, or do continuous resetting. And here we go. We're in training from this remote site, pacing at 280 milliseconds. The cycle length would have been 300 milliseconds if we weren't pacing. We see on the electrogram recording system that our signals, our pacing artifact, is separated by 280 milliseconds. Fabulous. Here's the question. Now we come off pacing. What, in your mind, is going to be the timing from that last paced signal to the first recorded electrogram with the numbers that I mentioned? Is it going to be 260 milliseconds, 300 milliseconds, 340 milliseconds, or 380 milliseconds? Take a moment to think in your head what you think might be going on. And let's imagine ourselves shrinking down to be an electron that's released from that pacing site. And here's the journey we're going to go on toward the circuit, all the way around the circuit, and back. And if we think about what time that took, it took 40 to get to the circuit, 300 milliseconds to get around, and then another 40 coming back. Many people will actually forget that last 40 milliseconds. They'll remember, ah, it took 40 milliseconds to get to the circuit. But there's also a getting back part. So the correct answer in this case, with the entrance and the exit from that big circuit being at the same location, is going to be 380 milliseconds. So the post-pacing interval is going to be long, longer than the tachycardia cycle length, as a function of being remote from the site. And then if you continue to not pace, then the same principle applies of recording remote from a circuit. All of the subsequent signals will be at the same distance apart as the tachycardia itself. It's just that one interval, the post-pacing interval, from the last pacing to the first return electrogram, is going to tell you whether you were pacing in the circuit or away from the circuit, as a function of two times, more or less, the amount of time it took you to get to the circuit and to travel away from it. And that will be true, even if the entrance and the exit sites are at different locations within the circuit, which can happen in VT, not uncommonly. But whether or not they're in the same place or different places, there's still, if you're not in the circuit, going to be an extra time it took to get to the circuit, and then extra time it took to get back from the circuit back to your recording site. So the PPI, the post-pacing interval, will always be long. And if the PPI is the same as the tachycardia, you are in the circuit. Let's look at a quick case with the last couple of minutes, and then we'll take some questions. Here is a patient who had a history of coronary disease, had cardiac surgery, and had atrial flutter. And we looked at the flutter waves, and we said, OK, it looks kind of like typical flutter in lead II and V1. But when we look at lead III, I'm not sure. So we go into the case with an open mind, wondering where is this atrial flutter located. We put up a 20-pole catheter that has 10 bipoles. This is an LAO view. And here, more or less, are where the right and the left atrium are located. And notice that electrode pairs 1 and 2 are in the coronary sinus, and the others are wrapping around the roof and down the lateral wall and floor across the isthmus of the right atrium. And here is the recording sequence in those electrograms. I know, Mark, I'm going a minute or two over into our question and answer session, but I want to show this one example, and then we'll take some questions. The electrograms show a cycle length of 260 milliseconds. Thank God, not 300 milliseconds anymore. We're sick of that. This is a real case, obviously. And the sequence is from 10 down to 1 in the activation sequence. So back on this map, we can see that the wavefront is traveling counterclockwise around the right atrium. And you might say, good, the question is answered. It's counterclockwise right atrial flutter. But in fact, there are different mechanisms that could result in that activation sequence in the 10 electrodes where we're sampling. You could have typical flutter. You could have a flutter on the roof of the right atrium that travels around the right atrium and out the coronary sinus. Or you even could have something just on the other side of the roof, the left atrium, coming over across Bachman's bundle quickly, activating the right atrium all the way around in that same sequence. So those would be obviously different in terms of your ablation strategy and important to distinguish from one another. So let's do some entrainment. Let's pace from electrode 9. You can see the pacing artifact there. And you can see we're pacing at 240 milliseconds. We can see that we didn't terminate the tachycardia, which remains ongoing at 260 milliseconds. And we can demonstrate that we, in fact, accelerated the atrium to that 240-millisecond pacing rate by looking at a different pair of electrograms, not the pacing site, but a different one. And we can see our post-pacing interval here. The last paced beat from the channel that we're pacing from to the first recording site is 260 milliseconds. So site 9 is in the circuit. If we go to another site, site 5, electrode pair 5, same thing, pacing at 240 milliseconds. We didn't terminate the tachycardia. We verified that we've accelerated the whole chamber to our pacing rate. And here we are at a post-pacing interval at this site, also 260 milliseconds, the same as the tachycardia. We're in the circuit. Let's pace from the first pair of electrodes, which is at the floor of the left atrium in the coronary sinus. We pace 240 milliseconds. We didn't terminate the tachycardia. We accelerated the atrium to the pacing rate. And here the post-pacing interval is long. We are not in the circuit at this location. All right, let's do one last site, pacing from the isthmus. We put up now a mapping catheter, an ablation catheter that's in aqua, paced at 240. We didn't terminate the tachycardia. We accelerated the chamber to 240 milliseconds. And this post-pacing interval is also in the circuit. So in total, we have shown that site 9, site 5, and the isthmus, where the ablation catheter is, also are all in, but the left atrium is out of the example possibilities I showed. All of them are in, sorry, 9, 5, and the ablation catheter are in the circuit. The only possible circuit that could be explained by these entrainment findings is counterclockwise right atrial flutter, where the circuit occupies the entirety of the right atrium. And of course, the left atrium is not part of it. And this has answered the question of whether or not this is typical flutter. The answer is yes. And if we go on and ablate on the isthmus, the tachycardia terminates, just as you would have hoped. If you had found a different circuit, then ablation at the wrong place obviously would not have worked. I'm going to leave it there. And actually, let me just look at the last slide and just show you the summary we've discussed. I don't need to read through this. But basically, if you pace faster than the tachycardia, you can take it over. The post-pacing interval tells you that if you are the same as the tachycardia cycle length, you're in the circuit. If it's greater than the tachycardia cycle length, you're remote from the circuit. And there are lots of other things that entrainment can show you, which I didn't include in this talk, particularly in SCAR-VT, but that'll be the subject of a more advanced session. So why don't we go back to Mark and to me with the cameras. And Mark, you can tell me what questions have cropped up during the presentation, please. Thanks, Josh, very much. And we've got about 10 minutes, 11 minutes here for questions and answers. And there are a number of good questions that have come in. And one of the first questions that came in is acceleration of the chamber of the tachycardia to the pacing cycle length sufficient by itself to say that you have entrainment? And it might be a good opportunity to comment on the ways in which you can get fooled by pacing and not capturing. Yeah, so I went through it a little quickly just for time reasons. But that's a wonderful question and an important component of interpreting an entrainment or a resetting pacing maneuver. You have to obviously be capturing tissue and advancing the chamber and therefore the tachycardia in order to interpret the maneuver. And so if you are pacing and you measure the intervals between the pacing artifacts, that is insufficient to tell you that you've in fact accelerated the tachycardia because the artifact is going to show up whether or not you're capturing tissue, whether or not you're advancing the chamber. And so you need to use a different electrode pair that isn't the pair that you're pacing from and see if in fact the electrograms from elsewhere in the chamber have accelerated, have shortened to your pacing cycle length. It is possible for at least a beat or two sometimes that you've accelerated locally near your pacing site, but that wavefront hasn't reached a remote site. So you want to pace long enough that all electrograms in the chambers of interest, atria or ventricles, have all shortened and accelerated to your pacing site so that you're not just capturing local tissue but didn't actually get to the circuit itself. So the answer is yes, you need to ensure that that has happened. Terrific. Thanks, Josh. Another question that came in from Dr. Anderson is with regards to the post-pacing interval, what range of post-pacing intervals, commonly called the PPI, are acceptable to represent that you're within the circuit? What's the range of PPIs to represent being in or out of the circuit? Yeah, that's a great question. And the purest answer is they have to be identical. Now, the slightly longer-winded answer is that when you pace faster, sometimes the conduction properties of the tissue between your pacing site and the circuit or within the circuit itself could change. You could have a little bit of decrement. Usually ventricular muscle and atrial muscle doesn't decrement, but it can, a little bit. So sometimes by pacing faster, you alter those properties, in which case we leave a little bit of wiggle room and usually people will say, all right, if my post-pacing interval is within 10 or 20 milliseconds of the tachycardia cycling, that's good enough for me. Now, really, it may not be. And pay attention when you're pacing at different sites. What's the variation in the post-pacing interval that you're seeing? And if you're seeing some that are zero, then use zero. But if the shortest difference that you get is 10 or 20 milliseconds, maybe you've altered the tissue a little bit. And that leads to another point, which is it isn't simply whether you are in or out of the circuit, but how close you are. So look at the absolute number, the difference between the two. If your PPI minus TCL is 40 milliseconds, then you may be out of the circuit but close to it. And if you pace at a different site and it's 90 milliseconds, you're out but further away from the circuit. So you can play a game of hot and cold, not just am I in or out, but am I getting closer to the circuit or further away from the circuit based on how different the PPI is from the TCL. Terrific. A couple of questions came in from Dr. Raul from Mexico. An important one that I think merits emphasis here is the post-pacing interval must be measured from which electrode. And this is an opportunity to clarify that it needs to be from the same electrode you're pacing from. Not uncommon for people to get confused by seeing an electrode that is not the right one and measuring that. Yeah, it's a wonderful point. And if I misspoke, forgive me. And it's worth re-emphasizing that this maneuver is valid if you are only measuring the time from the last paced beat to the first return electrogram in that same channel from which you were pacing. Because otherwise, it's not a valid interpretation of the maneuver. It can be difficult sometimes if you're pacing with high output or even unipolar pacing, which sometimes people do. You may have a longer period of time, especially with a faster tachycardia, before the signal returns to baseline. The artifact is so big it can obscure the first return electrogram, in which case you may have missed it. So maybe turn down, and this is always a good point, to turn down the output to the lowest output that will still capture. And bipolar tends to be better than unipolar in terms of reducing artifact. Terrific. A question from Dr. Hedley, who looks like based on his email. He's at the Cleveland Clinic. Does accelerating tachycardia with entrainment imply that you've changed to a different physical circuit? It's certainly possible that when you're pacing faster than a circuit, that you have engaged a new circuit that has taken over. That should be pretty evident, that when you come off pacing, that the cycle length that remains is different and or the activation sequence in the electrograms that you're recording is different. But if it remains the same, meaning what you're seeing before pacing and after pacing has the same tachycardia cycle length and the same activation sequence, then while you're pacing, the assumption is that you are engaging that same circuit, not in its entirety, because you're coming on pacing a little bit before the circuit has had a chance to complete each time. But on that last pace beat, you pace, you go around that same circuit, and it is able to complete for the first time. So a couple of other questions, Josh, in the remaining four minutes or so is, how do you know if the tachycardia is entrained or you're not just suppressing an automatic tachycardia? For example, in the instance of an atrial tachycardia. It's a great point. And it's a little bit beyond the scope of what I intended to talk about here. But depending on how the atrial tachycardia behaves, meaning you're going to suppress it if you pace faster than an focal atrial tachycardia. But different tachycardias may return, may resume with different delays or absence of a delay after you come off pacing. So the different atrial tachycardias will behave differently. And there are, in fact, criteria if you pace for different lengths of time and at different cycle lengths to see what happens. Atrial tachycardias, in short, will vary to a greater degree than a fixed circuit, which typically behaves in a very predictable way. So doing entrainment from the same place at different cycle lengths and at different durations will distinguish often between reentry and focal. Before you ask perhaps your last question, if we have time for one or two more, make sure everyone please answers the survey questions, the feedback for this particular presentation, which I think was put up. So thanks, everyone, for your responses to that. Again, helping us hone our talks for next year. Sorry, go ahead, Mark. Well, not at all, Josh. And it looks like we've got about three minutes before Brad Knight joins us. And I can see that Brad's joined us online. So welcome, Brad. We'll introduce you in a second and go through your video. But one of these is this important concept of resetting. And the question that came up is, tell us about the concept of resetting. Does it help you to identify a large macro-entrant circuit versus a small reentrant circuit? It might be worth reemphasizing this point about a single stimulus can reset a tachycardia, a concept that Sam introduced as well. Yeah, so there are a number of principles there. So a circuit, no matter how small, should behave in the same way in response to overdrive pacing, to constant resetting, except that the post-pacing interval for a larger area within the chamber is going to be longer than the tachycardia cycle length. So you really have to be right on top of a micro-reentrant circuit in order for your post-pacing interval to match the tachycardia cycle length. So if you find that you are doing entrainment pacing at different places in a chamber and it seems out everywhere, maybe it's a little bit closer here than there, you may be dealing with a small circuit. On the other hand, like for example, right atrial flutter, if you pace from lots of places throughout the right atrium, they're all going to be in. That suggests that you have a large circuit. So looking at different pacing sites and comparing the tachycardia cycle length and the post-pacing interval at each of those sites is going to tell you a lot about the size of the circuit. And again, depending on whether it's focal or reentrant, that difference is going to determine the response to pacing at different rates and for different lengths of time. OK, Josh, one quick question before we introduce Brad, which is, does the PPI work the same for orthodromic and antidromic entrainment? Yeah, if you're traveling around the circuit and coming back, as long as usually when you entrain, just by sheer fact that if you're going to get into the excitable gap, you're never going to switch the direction of the circuit without having terminated it first. So you need to make sure that the tachycardia doesn't skip a beat when you come off pacing and you didn't terminate it and reinitiate it. But because the oncoming wavefront is going to meet the retrograde component of capturing the excitable gap, you're always going to end up activating the circuit in the same orientation that was ongoing before and after you did your pacing. Excellent. Josh, thank you very much for the efforts putting that terrific talk together.
Video Summary
The speaker discusses the principles of entrainment and how it is used to identify and understand reentrant circuits in cardiac arrhythmias. Entrainment involves pacing the heart at a faster rate than the ongoing tachycardia and observing the response of the circuit. The post-pacing interval (PPI), which is the time between the last paced beat and the first return electrogram, can provide valuable information about the location of the circuit. If the PPI is the same as the tachycardia cycle length (TCL), it suggests that the pacing has accelerated the tachycardia and the pacing site is within the circuit. If the PPI is longer than the TCL, it indicates that the pacing site is remote from the circuit. The speaker also discusses how the PPI can help differentiate between different types of circuits, such as reentrant circuits and focal tachycardias. Additionally, the speaker explains the concept of resetting, where a single stimulus can temporarily interrupt a tachycardia. Overall, entrainment and the analysis of the PPI can provide valuable insights into the nature and location of cardiac arrhythmias.
Asset Subtitle
Josh Cooper, MD
Keywords
entrainment
reentrant circuits
cardiac arrhythmias
pacing
post-pacing interval
tachycardia cycle length
differentiation
resetting
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