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SAMUEL J. ASIRVATHAM: Thanks, everyone, for joining. So today's focus is going to be congenital heart disease, one aspect, micro-reentrant atrial tacycardias. We'll also visit with congenital heart disease as part of a pathology session, and then, subsequently, we can look at device management and congenital heart disease, later in the year. Several of you have anticipated the session, and sent in questions to us.

We'll put this up for you, the questions that have already come in, in the question and answer folder. But please keep your questions from today, what you hear, what you think about, as we go ahead. As many as possible we'll get to during the session. And what we don't, we'll answer and put it as part of the recording for what will be available on the YouTube link.

So today, we have Dr. Bryan Cannon, my colleague, very experienced with ablation and complex congenital heart disease. You've met Bryan commenting as a panelist before. He's going to introduce some of the important ideas to get us started in the conversation. And then we'll all, our regular panelists, myself, and Brian, and anyone else who wants to join in, will tackle the questions as they come up. Bryan?

BRYAN C. CANNON: Well, thank you very much Dr. Asirvatham. It's a pleasure to be here, and it's quite an honor to be with such a distinguished group of people. So we're just going to start out by going through a couple of basic principles about adult congenital. And this is, in my opinion, one of the most important things, because it's one of the most difficult things to see.

So I'm just going to leave this slide up for just one second, and pretty simple question, what is the rhythm. So you can kind of see there's a P-wave before a QRS, looks pretty good. But let's take a little closer look. And the question is, are you sure, were your first thoughts about this rhythm potentially correct?

Was there anything about the ECG that kind of piqued your interest? Was there anything that changed your mind, or your thoughts, or what are some unusual features, because if you look at this, there's a pretty flat baseline here. We see a P-wave and a QRS. But now as we look closer, there's some little funny things that are in here.

So what could potentially that be? Well, I'd like to spend a little bit of time talking about this, because I think this is one of the easiest adult congenital rhythms to overlook. But it's also one of the most important. So this is, as Dr. Asirvatham, said, called scar flutter, intra-atrial reentrant tachycardia, a macro reentered atrial tachycardia, that we see pretty commonly in congenital heart disease.

In fact, we see it in 50% of patients after the Fontan, 30% of patients following the Mustard or Senning, and you can see it in basically any patient that has structural heart disease. It can have thromboembolic complications. And it can be terminated by cardioversion or atrial overdrive pacing. But it's distinct from the flutter seen in normal hearts.

So I'm going to show you a couple of diagrams in a minute. And there can be multiple circuits. But the key is that the rate is typically slower. It's typically between 130 to 220 beats per minute, which is important in the fact that some of these young people that have normal AV nodes may conduct 1 to 1 through the AV node, resulting in hypotension or circulatory collapse. So in standard atrial flutter, we have a circuit that involves the whole atrium.

And it's all healthy tissue that's rotating around in this macro reetrant fashion. So that gives us this pretty characteristic sawtooth pattern on the ECG, with continuous electrical activation. That's distinct from intra-atrial reentrant tachycardia, where there's a zone of poorly conducting or non-conducting tissue. But there's another area of unhealthy, slowly conducting tissue. So there's an area of healthy atrium that makes a good-sized P-wave.

But then there's this area of slow conduction, with very low voltages. So as the circuit goes around, it creates quite a unique pattern on the ECG. And what you'll actually see is this very flat baseline. But if you look closely, you can march through P-waves that are there. So, at times, it's pretty easy to see. You can see that the P-waves march through here, and you notice the variability of the intervals.

So when the atrial signals are big, and there's atrial enlargement, relatively easy to discern. However, in many of these patients, the atrial signals are very small. So it's very difficult to see, unless you look closely. But if you follow it closely, you can see that there are P-waves that are marching through it. Once again, in patients that have other structural heart disease, relatively straightforward to see.

But it's important to look at the end of the QRS or in the T-wave for hidden P-waves, because frequently these patients will have a bundle branch block or other conduction abnormalities. If you look at this ECG, there's a P-wave and a QRS. So sometimes you have to look at the whole ECG. You have to look for areas where it slows down or blocks in order to see those P-waves come through.

So what are some clues to diagnosing IART? These are the things that I look for. So if you have a prolonged PR interval, or a variation in the PR interval in a patient that otherwise has a normal PR interval, always think IART. These patients may have conduction defects and other things that prolong the PR interval, but any time you see an increased PR interval, think IART.

Change in P-wave appearance, if their sinus P-wave always looks one way, and all of a sudden it looks different, if there's no variation in heart rate, and patients whose typical heart rate is in the 40s and 50s, if they come in with a heart rate of 90, that may clue you in. So a patient that's chronically bradycardic that presents with any form of tachycardia, even what we would consider mild, may need that. And the reason it's important, because these atrial arrhythmias can definitely affect ventricular function, and sometimes you need adenosine to see them.

So if you take a look at this monitor tracing, looks pretty good. But if you look closer, you'll see that there are blocked P-waves. So you really have to keep an eye out, because this arrhythmia can certainly sneak up on you. Same thing with this. If you look closely, you can see that there's a variable PR interval.

But when you see a prolonged PR interval, or especially this variability, that tells you that the atrial rate is out of conjunction with the adrenaline effect on the AV node. So if you see this variable conduction, think IART, or some other type of abnormal rhythm. We all have variations in our heart rate. But if you see pretty flat heart rate trends, now you see it won't be perfectly flat. There will be little variations.

But you can see, in general, if the heart rate trend is very flat, and then when they go to sleep, they have further block, this is another clue, if you see no variability in the heart rate, that you may be in an abnormal rhythm. Sometimes you need adenosine to see it, because here you can see the P-waves, but then with adenosine, it makes it much clearer that this is an arrhythmia.

So why do we care? It's just a atrial arrhythmia. Well, if you look at the issue, sudden death can be as high as 6% to 10%, and associated with recurrent IART, and in patients with appropriate ICD shocks and transposition of the great arteries, supraventricular tachycardia procedure co-existed with VT in 50%. So in these adult congenital patients, these atrial arrhythmias have a significant effect and can have a significant effect on ventricular function.

Well, we're going to move on from talking to IART to just talk about a couple of basic principles that you absolutely have to do if you're going to take care of congenital patients. The first is, know the anatomy. You have to know where things are. And just realize that anomalies of the superior vena cava are present in 9% of patients with congenital heart disease, as well as 1% of normal hearts.

So you can see here, there's a left superior vena cava, and, on this image as well, this is a MRI reconstruction of a patient that I did last Friday, and you can see there's a left superior vena cava. So, understanding that, and understanding that the signals that you get and where things meet, maybe in unusual positions, is very important. In addition, if you're putting in a pacemaker and there's not a bridging vein, you'll be in trouble as far as getting that catheter into the right position in the heart.

In addition, it's important to have some type of imaging. For example, this is a patient who, this is their right atrium. It goes from the sternum all the way back to the spine. So you know you're in for a long day of ablation. So it's very important to understand where structures are, how big structures are, and other things, in order to do a successful ablation. In addition, it's important to know where is the AV node, because in patients, particularly with congenital corrected transposition, or AV septal defect or complete AV canal, the conduction system does not run in the normal area.

So if you're not aware of these variations or these changes, you can accidentally damage the AV node and cause AV block. In addition, it's important to know the specifics about the surgery. For example, when they say the patient was on bypass, that may mean a bi-atrial cannulation, where one cannula goes through the SVC, one goes through the IVC. Or it may mean a single cannula through the superior portion of the superior vena cava, or it can mean two cannulas that go through the lateral atrial wall.

So understanding this will show you zones, that where you're expecting scar to be, or zones where you expect diseased tissue to be. In addition, each surgeon does the repair a little bit differently. So it's important to get the operative notes, to know exactly what was done. And that will help guide you for your ablation. So thank you very much for your attention. Now we'll open it up to questions. And hopefully this will be kind of a primer to diagnosing and treating congenital heart disease.

SAMUEL J. ASIRVATHAM: So thanks, Bryan. So that's like an overview for clinicians and for electrophysiologists planning a case, what we need to know. And I think, when we get into the questions, we'll get into some more specific areas, especially for the invasive electrophysiologist. I'd just like to take maybe one, which is a very common question, and a common kind of mistake that can happen when interpreting the ECG in terms of where we want to ablate. You know, you very nicely pointed out that you find P-waves, you look, you see that it is a flutter, that it is, there's more P-waves there than what's obvious in front of the QRS.

But another piece that we have to translate from normal heart ablations is looking at the P-wave axis to try and figure out where we need to ablate. So early in our EP training, in our EP career, we learned these diagnostic algorithms based mostly in normal hearts, and automatic tachycardia. So if a P-wave is positive in V1, and V1 is the right-sided lead, we think that this means that this is originating on the left side.

If a P-wave is positive in two, three AVF, we think origin high in the heart. But I think it's important for us to explain that the analysis of the P-wave in re-entrant tachycardia itself is not straightforward, but especially when you're dealing with highly diseased hearts. And the reason for that is that, and maybe I'll just draw something here. The reason for that is, in re-entrant tachycardia, there's really always some electrical activity somewhere in that atrium or ventricle for VT.

So there's no real focus that will determine what the P-wave axis is. That P-wave, part of it is going to be one part of the circuit, another part, the heart that's getting activated late, early, it's really almost impossible to make sense of the P-wave in diseased heart re-entrant VT. But one of the uses that there can be is, because it's so diseased, that diseased portion doesn't contribute to the P-wave. It's electrically silent.

And what really makes the P-wave, however small it is, is from where that circuit exits to the rest of the heart, that's relatively normal. And this can be an especially useful clue in congenital heart re-entrant tachycardia, because the circuits are in such diseased portions, low voltage can't generate enough of a voltage stream to be part of the EKG. Where you do start seeing it gives you an idea of that transition between the normal heart and the diseased portion that's housing the circuit.

So kind of a way of interpreting is to say, let me do my usual electrocardiogram analysis, but then let me interpret it differently here by trying to back in from that exit site, to where I'm anticipating the type of signals and abnormalities that's going to help me find the circuit. So that's one frequent question that comes up. There are a few others as well, that if we have time, we'll get to.

But we have some that I'd like to get started with as questions, and anyone here, please do add in anything that you'd like to look through. So one of them is a very important question. And that is how do we figure out how to check for block when doing linear ablation in patients who are post-op congenital heart disease. So we've gotten used to saying, how do we check for block. When we have normal heart bi-directional block, we're putting a catheter multi-electrode wavefront two ways, and see if there's a reversal of conduction.

But how do we figure that out in patients who've got congenital heart disease, difficult to access the anatomy, and is there bidirectional block, after the ablation that we've done. So maybe I'll just open that up a little bit for comments. I see we've got Malini, who's Malini Madhavan, who's also very experienced in congenital heart ablation. And maybe I'll just start with you, Bryan.

Comments about how do we look for block when we can't get easily to those structures, that we've done a linear ablation on. And then we'll get some comments from Dr. Madhavan.

BRYAN C. CANNON: Yeah, I think that's an excellent point. And I think that's one of the most challenging things about these ablations, is there are multiple lines of block and multiple-- can be circuits going through there. So I think it's important, at least when I look at it, is to spend your time and your due diligence in mapping, before you do an ablation. See what the activation patterns are, see if you can find areas of scar or areas of low conduction, and that may help you plan your ablation lines, as well as check for block.

But it can be very difficult to figure things out. Another important thing is having some type of imaging, whether that be ICE imaging or a CT or MRI before, to make sure that you're mapping the entire circuit, because one of the biggest problems that we run into is there's an area of the circuit that's critical, that either you can't get to easily with your catheter. So you miss it with the mapping system, or it's completely blocked off from access, based on the surgical results. So Dr. Madhavan, I don't know if you want to add anything to that, or--

MALINI MADHAVAN: It's great, in terms of checking block, I would just add a couple of other things. Checking block across lines after atrial flutter ablation is, the concepts are very similar, in ACHD versus compared to someone who doesn't have congenital heart disease. But there may be challenges, right? So because sometimes pacing from certain portions of the heart, such as axis to the proximal CS, may not be present, but the concepts are very similar. So if I can share a couple of slides, Dr. Asirvatham, that would be OK.

SAMUEL J. ASIRVATHAM: Sure, sure. Yes, please.

MALINI MADHAVAN: And I'll just show a couple of examples of checking block. Just a second here. Is that coming across?

SAMUEL J. ASIRVATHAM: Yes.

MALINI MADHAVAN: OK, so the most common way we check for block across a line is, probably the best way I would say, is to place a closely spaced multi-electrode catheter across the line. So that's the most common way. And we want to phase on either side of the line, and show that the activation is occurring on the other side of the line in a reverse direction. So that's the concept we apply for checking for CTI block.

And the same concept would apply for any line that we perform in an ACHD flutter ablation, or, for that matter, any VT ablation, like in a ventricle if there is a problem. Some of the other ways this can be done is, this is for example, a common type of flutter. This is a right atrial map. And the second most common flutters we see in these patients after CTI flutter is insertional flutter occurring on the lateral right atrium.

So on this map, you are seeing the lateral aspect of the right atrium we have ablated, through a previous surgical scar, and anchored this line to the IVC. And this previous surgical scar started at the base of the right atrial appendage that you can see here. So it really wasn't anchored to the SVC, but was anchored to the IVC and that terminated flutter.

So then, one of the questions is, is there block across this particular line. So the way we checked for this, is, instead of putting the catheter across the line, the multi-electrode catheter was placed on one side of the line, straddling that line in an up to down fashion. And then there was pacing from the ablation from the other side of the line.

So in this tracing here, you are seeing pacing from the ablation catheter. And the isthmus catheter, IS catheter here, that's labeled here, is getting activated from top to bottom here. So IS-1 is on the top, and IS-1920 is on the bottom. So there is block across this line. So the activation is going around to the right atrial appendage, and then coming back.

So there is block across this line. Similar concept if we do an intra-caval line. We can either place a catheter across the line, or we can place it parallel and show that there is block across the line. The other way to do this, if there isn't some way of placing a catheter across the line, is to pace one side of the line, and then map on the other side. So this is another example.

And this is a map from a patient with a Mustard procedure who had a CTI ablation. So what we are seeing here is we have done retrograde access into the pulmonary venous atrial side, and then through the IVC, we have gone into the systemic venous atrium. I don't know if my-- is my arrow coming across?

SAMUEL J. ASIRVATHAM: Yes. Yes.

MALINI MADHAVAN: OK, so this is a map of the systemic venous atrium. And this is the map of the pulmonary venous atrium. So, although this was CTI flutter in this particular situation, the large part of the cavotricuspid isthmus is now in the pulmonary venous atrium, which is the opposite side of the catheter that the surgeon had put in. And the coronary sinus was also located on the other side.

So what we did for this patient to check for block, was we paced from the median side of the CTI line that we had done, in the pulmonary venous atrium. And then we took the ablation catheter and, using the mapping system, mapped the activation sequence. So you can see the activation sequence is earlier, away from the line, which is the red dot. And then, as we come closer and closer to the line, it becomes later and later, which is the purple dot, right?

So that's another way you can check for block across lines in patients who have difficult access to certain chambers. So but the concepts are similar. You're pacing on one side of the line and mapping on the other side, whether that's using a mapping system or using a multi-electrode catheter. This depends on the situation and where we are ablating.

And then we can also talk, Dr. Asirvatham, if we can also talk about how to interpret doubles and all that later, if that's OK.

SAMUEL J. ASIRVATHAM: Sure thing, thanks, thanks a lot. So just to kind of explain this, two approaches to be sure everyone's on the same page. So the problem with our congenital heart disease patients is we draw lines in places we are not accustomed to doing. So we have to have a generic game plan for how can I check for block anywhere.

The second is, it's not easy where we drew the line to place more than one catheter. If we had to go through a baffle puncture, a retrograde approach, it can be difficult. So Dr. Madhavan here has pointed out two things. Let's say this is where we've done the ablation, somewhere in the atrium, could be in the ventricle. And we want to know that there's no gap here, because the premise is, this has been anchored.

We haven't anchored to both sides, two anchors, because we didn't want to isolate or delay conduction entirely to one portion of the atrium. But we don't want to have a way for a circuit to make its way back. So, in that example that Dr. Madhavan showed, there was anchoring to the IVC. So how to know that this line is blocked? So simple one there, there's you pace on one side of your line. And you have at least two, preferably a multiple electrode, or even just a proximal and distal electrode, from a single catheter.

You pace here and see how is the activation to these catheters. If it's coming like this, chances are this is blocked, and the activation is coming down. If on the other hand, you are leaking through, then this will come earlier. But very difficult to use this technique, with just single catheter or a few points, because anybody can distinguish conduction from block. The trick, the talent, the skill, is distinguishing block from almost block, so extremely slow conduction.

So if we are using this technique, we have to make sure that the catheters are close to where the conduction might be happening, and try to take closely spaced catheters to show the entire wavefront, even close to where this line was created, is reverse activity. And to exaggerate that further, you want to pace close to where that line is, as well. So in other words, you're trying to make it best case scenario for conduction to occur, if it's possible to occur.

Well, the second technique that Dr. Madhavan pointed out was, you don't even need to have a multi-electrode catheter on the other side of a line, as long as you pace on one side, and you can map on the other side. So just take several points and show that, even though you're pacing, all the conduction is moving towards where you did that line. So here are also a few caveats, if you use this technique, is you have to make sure that you're pacing close to, but not too close, to the line because, if you're too close, you might inadvertently capture tissue on the other side.

Second is your map should be strictly only on the side of the line that you're checking for block. If by mistake, you interpret a signal from this side, or you take a point on this side, it will become an entirely meaningless map. So you can do it, but with those caveats. And the nice thing about that technique is you can do that, even in the ventricle. So this is an example of a patient with post-tetralogy of fallow VT.

And a line was done connecting a patch, a non-transannular patch, to the pulmonary annulus, and want to know, do you have blocked it. Good luck with trying to get a multi-electrode catheter there. So one way to do it is simply take a pacing site, and that pacing site might be in the other ventricle. It could be a second catheter. Keep pacing from that same location, and then map on the other side.

Here's your line. You're pacing close to the line. And the whole wavefront is coming towards the light. And if that's happening, you can say you're blocked in this direction.

And then what you can do is just reverse this pace here, do the map on the other side, to show that you have bi-directional block. So that's some of the things we would think about. There's an allied question that also comes up some time, is what can you anchor to in patients who have congenital heart disease. So this is actually a very important question.

So you can anchor to anything that will not conduct. That's our premise for when we say anchor, so not possible to conduct. And that usually is valves in the normal heart. But in congenital heart disease, a potential pitfall is when we have a scar.

And a scar, we say let's anchor to the scar. This is a very frequent cause of recurrence of arrhythmia, because what we call scar is not really scar. So in other words, we might have defined scar as this is something with lower voltage, with some arbitrary cutoff.

But little living tissue could be there. And then, when we draw a line here, you can force conduction through that little living tissue. And this can become an incessant slow flutter. So if you call something scar, while doing a map, thinking that I might do a line to connect to that, you have to be 100% sure there's no viable tissue there.

That's why many electrophysiologists, even if they think it is scar, they'll actually ablate all across that to make sure or homogenize the scar, to be sure that that really has no viable tissue into it. The other kind of unique thing that comes up, in congenital heart disease, is sometimes, as a result of the surgery, there may be isolated portions of the atria, isolated portions. This has its own rhythm.

It's already been isolated from the rest, even though this is entirely viable. This is a potential anchor for a line. Because this is isolated, it's not going to go anywhere. It can be treated like scar, even though you have viable tissue. And there's also another unique situation, where the isolated portion might be in a rapid rhythm, like fibrillation, even while the rest of the heart is in flutter. I don't know if any of the panelists here have a quick example to share. Otherwise, I can find one as well.

But if anyone does, let me know, otherwise.

MALINI MADHAVAN: I have one here, Dr. Asirvatham, I'd really like to show.

SAMUEL J. ASIRVATHAM: Oh, you do? OK, great. Why don't you show that to us.

MALINI MADHAVAN: Let me get rid of this so you can actually see.

SAMUEL J. ASIRVATHAM: There, no, that's OK. We can see it. That's beautiful. It's a beautiful example. So maybe I'll just talk through this one as you point, Malini. So if we look at that inset, we see that, even though there's an organized arrhythmia, in one portion of the atrium there is fibrillation. So how can this fibrillation be an anchor point?

Well, how come the fibrillation hasn't spread? So effectively, this could be an isolated portion. Now, to know for sure, this is only valid if the rest of the atrium is in a paced rhythm, a stable flutter, or in sinus rhythm. If it's a flutter that's changing, then it's iffy, if whether this is a good anchor, because there may be conduction. It's just slow and delayed, and that's what's changing the organization of the flutter. Thanks, thanks, Malini, that's an exact example of what we had in mind.

Now we have a question, and I believe that--

BRYAN C. CANNON: Samuel, if I can just make just a couple of quick comments.

SAMUEL J. ASIRVATHAM: Yes, please.

BRYAN C. CANNON: First of all, I think sometimes it can be challenging, figuring out which arrhythmia is the clinical arrhythmia, and if there's a connection in between the two. So sometimes you have to spend time doing that. The second point is sometimes the most obvious area to connect involves a very large portion of atria. You have to look at several different options, including going through old areas of scar, to potentially look for ways to not damage the other atrium, because you can cause atrial dyssynchrony by putting too many ablation lesions in there.

And sometimes these patients are very dependent upon their atrial kick for their cardiac output. So I think your points are excellent, about trying to figure out that, and then, but you have to map and figure out what's the clinical arrhythmia? What's the best route to get rid of it, so that we can leave the limited amount of healthy tissue that's in place intact.

SAMUEL J. ASIRVATHAM: Great, thanks, thanks, Bryan. So we have another question, kind of generic. It's like, it's difficult to interpret signals in congenital heart disease, annotate the signals, et cetera. So difficulty with signal interpretation, I think this is also a very important point. And we'll get some multiple opinions here. But I see Dr. Del Carpioi has joined as a panelist. And I know he's given a lot of thought to signal interpretation and how we include it in a map. Freddy, do you want to share some thoughts with us?

FREDDY DEL-CARPIOI MUNOZ: Well, when mapping the atrial flutters, typically, we want to deal with complex signals. We want to make sure that we annotate right the activation sequence of the local atrial electrical program. So the difficulties are typically dealing with double potentials. Or so there are different strategies.

But we want to make sure, you know, that we annotate the near-field signal all the time. But sometimes that's not possible. So, for example, we have a fractionated electrogram.

SAMUEL J. ASIRVATHAM: Yes, Freddy, I think you're cutting out. I think I know what you mean. And then, let me share an example of-- Freddy, you're cutting out. I'll just share an example, with the thing of what I think you're referring to. So you know, sometimes what we'll get is, something we're used to now a little bit more, because we're doing structural heart VT. But it's a very important principle for especially new electrophysiologists.

The smallest signals are sometimes the most important to have an understanding of and recognize, in a tachycardia. So, especially with congenital heart arrhythmias, you have to be really cautious about saying, is this scar or not, and recognizing that there may be a big signal on our catheter, because surrounding tissue might be what's contaminating the signal, and the real critical signal might be something in between. So these kind of fragmented, fractionated signals, I'll just give a few tips from my own practice, and then we'll hear from Bryan and Malini as well.

But one of the things that's useful is understanding where is your catheter. So in a congenital heart VT, for example, if the catheter you know is in a position where multiple structures are papillary muscle, overlapping chambers near a baffle, in those circumstances, anticipate that there could be two sets of signals. The second thing that comes into it is ask yourself, which of these signals is more likely to be relevant to this circuit.

So in other words, you look at the ECG. You look at the cycle length of the tachycardia, and then look at the signals that are present in the catheter. You have the surrounding tissues already mapped. Which of these signals fits best, if this is really completing a circuit in that location?

But beyond that, I personally find the most important maneuver is pacing. So, if you pace from that site, when you have that luxury, you're not worried about the arrhythmia changing, when you pace, see what it is that you capture. And if what you capture is what is allowing perfect entrainment of that circuit, then that signal is relevant, and that's the one that you need to include.

Now sometimes, we just can't tell. It's like, you know, which one do you want to go with, this one or this one, which component of this one. It's good to just step back and say, I can't tell, and just annotate on the map that there is a location where I couldn't tell, and I'm just going to say this is an abnormal signal. And then, when you have the rest of the map, you can come back to that location and see, well, now, can I tell, looking at what else is around that.

So just some things to share about one type of difficult signal, small, but maybe high frequency and fragmented signals. Bryan, any comments about signal interpretation, and then Malini?

BRYAN C. CANNON: Yeah, I think that this can be one of the most difficult things, and one of the most surefire way to derail your case is to have a map that doesn't make any sense. And so, I think you have to be very careful when you annotate or use auto-annotation programs, just because it may not be annotating the right thing. And especially the lines of block and other things can really throw your map off quickly.

So I agree with you, any point you have questions about or double potentials, you should probably just take it as a location, and then come back and look at that in the scheme of things, because you can get a completely different map, depending upon where you take the points individually. And trying to figure out what small signals are relevant and not, is something that can be difficult. But I agree. I think pacing and looking to see how those signals respond is very important.

SAMUEL J. ASIRVATHAM: Malini, before I get a question or comment from you, I just want to share quickly another situation where this is important, is when we're ablating, I'm sorry, when we're ablating accessory pathways. And we have in congenital heart disease sometimes a situation where we have to try and figure out for our map just the simple concept of early site or early activation location. So there, some very simple maneuvers can be very helpful, also related to pacing.

So you have, this is a patient with Epstein anomaly and accessory pathway ablation. And what you wind up is, we're trying to map for a pathway potential early V, and you also know the atrial signal can be complex, and multi-component. The ventricular signal can be complex and multi-component, so sometimes, while we have all the signals of interest in line, we pace to just change something, like block in the pathway.

And then everything that was related to the pathway is going to move. So then, you just backtrack from what moved back to your original beat, in whatever electrodes you have, even if it's a very small signal, in one electrode. Like this one here, you'll say that's related to the pathway. Its V or its pathway potential is related to the pathway.

So another unique challenge in congenital disease, where you sometimes, even a so-called simple arrhythmia, because the signals are complex from surgery, maybe prior ablation. Good to have a few maneuvers up your sleeve that you can use. Malini, you have an example to share with us? Oh, you're still muted I think.

MALINI MADHAVAN: Sorry about that. Yes, sure, I can show an example, maybe to highlight what you and Dr. Cannon said about pacing, to help distinguish which signal is relevant. And I think it's worth, again, emphasizing what Dr. Cannon said about these automatic annotation programs. I think they are good, but they're not good enough to always know exactly which component of the multi-component signal is of relevance.

So it's really important to pay attention. I always tag some of these multi-component signals as location only, come back, pace, and figure out what is of relevance to the circuit. The other thing to keep in mind is, when we try to pace, it's really important not to pace too fast, much faster than the cycle length. Going about maybe 10 to 20 milliseconds shorter than the cycle length rarely leads to degeneration or changes in the flutter, or termination of the flutter.

So pacing maneuvers, entrainment maneuvers, can be extremely helpful, but just need to be used very carefully, making sure that we have good sensing when we start pacing, and we are not pacing too much faster than the underlying flutter, or any other VT we are mapping. So this is one example where there is a multi-component signal. We can see there are three different components on this electrode.

And when we pace, we see that two of those components actually are not getting captured. And it is this third component here, with the red arrow on it, that is actually getting captured. Now we know exactly what is the relevant signal at that site, and we can annotate to that.

And it also tells us that this is probably a site that is relevant to the circuit, and likely within the circuit, based on the entrainment we have just done. So I do find it very helpful. Entrainment is very helpful to sort out which signal is relevant.

SAMUEL J. ASIRVATHAM: Great, thanks. Now maybe, while I'll ask someone to just check with the doctor who's asked a question, if they'd like to just ask the question online, about atrial fibrillation making arrhythmia difficult. And we'll come to that in a bit. And we'll maybe go to another question here. It has to do with the coronary sinus position in congenital heart disease. Any principles, anything where we look at, to say of where it's important to know before going into the procedure, how to anticipate where the coronary sinus is. Maybe Malini, I'll ask you this question.

MALINI MADHAVAN: Planning before the procedure is really critical, with any kind of repaired congenital heart disease. And part of the planning is knowing where the coronary sinus is. So I routinely either obtain a CT or MRI, or if there is already one in the system, I would review it in detail with the radiologist, to really figure out, depending on the complexity, things like, where is the coronary sinus opening into. If it is something very complex, like Fontan, which part of the atrium has been excluded, how to get to that portion, is there a fenestration.

There are so many aspects to this planning. But coronary sinus catheter, of knowing where the CS is opening into, is really important, ahead of the case, I think. During the case, ICE imaging is also really important.

So that also helps us really know which part of the chamber the CS is opening into. And that becomes more relevant in patients, for example, with the Mustard procedure, where, at least in my limited experience, I find that it's about 50-50, 50% of patients that I have seen have the coronary sinus open into the systemic venous side, and 50% on the pulmonary venous side.

Fontan flutters, it would also be very important, knowing where the CS opens into. And another reason to know that is if somebody has had tricuspid valve replacement surgery. Knowing if the CS is atrial or ventricular to the replaced tricuspid valve can also be very helpful, ahead of time, so it helps us plan how to place the CS catheter. Most of my cases, I place the CS catheter from the IJ.

I find that to be the most stable location for a CS catheter. But if it says coronary sinus, that is ventricular to a replaced tricuspid valve, I tend to do that more often from the leg. I just find it more helpful that way. But it is really critical, because, whenever we are mapping something in the atrium, the CS is often our reference electrode.

The other trick to some of these cases is if we have a patient where the CS is not accessible, and we are unable to place a reference electrode in the CS, one of the things I do is use a screw and atrial lead. So I take a regular pacing lead, place it through, typically, the IJ or the subclavian vein, and then screw it into some part of the atrium. And then that is connected through alligator clips to the Carto system, or whichever mapping system we are using.

And that would serve as a very stable reference for us. Having a stable reference is really important, before we start mapping and figuring out ahead of time what our reference is going to be, as part of the planning process.

SAMUEL J. ASIRVATHAM: So Malini, I'll just share this image here, while we go to the next question. This is what Dr. Madhavan was mentioning. Are you--

MALINI MADHAVAN: Oh.

SAMUEL J. ASIRVATHAM: No, it's OK.

MALINI MADHAVAN: No, I think it's yours.

SAMUEL J. ASIRVATHAM: Yeah, I'll get that back here. So this question of where is the CS to identify, I think it's important for a couple of different things. One issue that comes up is CS is our landmark for AVNRT ablation. So slow pathway, so it's not always just access to the CS, just the anatomic location of the CS can be important.

And that's useful to know from imaging. One useful thing is, if the patient has had a coronary arteriogram, sometimes, if we can look at the venous phase, when they had the arteriogram, that will give you a nice clue. If there's been a CT, that's useful. But this is what Dr. Madhavan was mentioning about CS positioning in patients with DTGA and an atrial level baffle. So this is really kind of surgical preference.

So it's not so much that the CS is misplaced. It's just where the suture line is. So if it includes the CS, then you can only access from the left sided circulation. If not, you can even place a BiV device through the CS in patients whose CS is not included in the baffle. You also have these like kind of in between situations, where you may have a part of the CS on one side, and part of the CS on another side.

Few surgeons do that, especially in the middle period, for the Mustard and Senning procedures. And so sometimes, you might be able to get in. But there's still part of the CS on the other side of the circulation. But just getting that orientation, for the anatomy, can be important.

So allied question also is which situations, where we have to worry about AV node re-entry, I'll just make a quick point here. Relationship to the CS and the re-conduction system, in most normal hearts, and in most congenital heart disease, is fairly consistent. You'll see the CS is posterior, close and inferior, compared to the compact AV node.

And the CS starts out relatively atrial to the compact AV node, and then winds up being more ventricular to the compact AV node. It's a good rule of thumb to keep even in congenital heart disease. But, sometimes, there's a disjoint, and what that disjoint comes down to is when there's a defect in the normal location of the compact AV node.

So a defect in the triangle of Koch, and this is our primum ASD complex with endocardial cushion defects, that this is punched out. This is the defect, part of the defect, all or part of the defect. But yet, these persons don't have AV block.

So what that tells you is the AV node has gone somewhere, and that's very difficult to define. But pattern-wise, most of the time, the AV node is displaced posteriorly and will be located, not juxtaposed to the coronary sinus, but in the coronary sinus, in the same region, where either the coronary sinus is, or where you anticipate it to be. So this is one rule to keep in mind.