All right, welcome everyone, and thank you to everyone for attending this last session of HRS 2025. My name is Carolina Escudero from the University of Alberta in Alberta, Canada, and my co-moderator is Dr. Greg Webster from Lurie Children's Hospital. So I'm going to read out what we are supposed to read out to you, so it's my pleasure to welcome you to San Diego and Heart Rhythm 2025, the 46th annual meeting of the Heart Rhythm Society. If you've not already done so, please download the HRS 2025 mobile app from your preferred app store. This is how you can participate in live Q&A during sessions. Please scan the QR code on the screen to access this session's Q&A, and when using the mobile app, log in with your HRS credentials. And please note that visual reproduction of Heart Rhythm 2025, either by video or still photography, is strictly prohibited. So for today's session, we are looking at new knowledge applied to old problems. We have four speakers. For this session, we are going to have the questions given at the end of everyone's talks, as some people may have to leave early, including our speakers, in order to catch their flights. So it's my pleasure to introduce Dr. Dan Rodin, who's going to be talking to us today about new targets for medications and molecules in EP. Well, thank you, and thank you for putting up with my awkward travel schedule, and thank you for all of you to stick through to the end. Oh, there. Never mind. Somebody already... So I have nothing relevant to disclose. Are we going to do this? So this is the title. This is the talk that I was given. I was looking at the schedule, and I think what I'm going to talk about sort of fits into the rough idea of this session. I gave a talk yesterday on new antiarrhythmic drugs, and I thought to myself, well, maybe I could just give the same talk twice. On the off chance that somebody in the audience was there yesterday, that could happen, I've decided to... It's not working. I've decided to give a second talk, but I'll summarize the first talk really, really quickly. It was called Innovations in Antiarrhythmic Therapy. So I could have given the same talk, don't you think? One of them was previously... So that made two points. One was an unappreciated role for increased calcium current in some forms of the Long QT syndrome. So direct activation of calcium current. Not Timothy syndrome, but other forms of Long QT syndrome. So repurposing calcium channel blockers was the idea there. And then the antiarrhythmic efficacy of RYR2 block and CPVT. That's sort of old news, but new drugs, including a drug called Dantrolene, and Majel Harisis from our group actually won the Cardiac Electrophysiology Society Young Investigator Award for the clinical study that he presented there. If you want to hear more details, you have to come to the EP Society meeting next year where he'll present the data. So I decided for this talk, I was going to... Since Shu was the one who made me give this talk, I decided I'd present a case. The case came to us for Shu from the SADS Foundation. So difficult cases sent to the SADS Foundation, and then there's a bunch of cell-styled experts, I wouldn't say anything more than that, who have a look at them. So this was a neonate with arrhythmias. The initial ejection fraction was 65%, despite the fact that this is a fast and unusual heart rate. So the kid was left alone. He had other surgical procedures which he tolerated, and he had genetic testing which showed three VUSs. The second two were inherited, one from mom, one from dad, who were phenotypically normal. So the thought was that the SCN5A, which was a de novo, was the likely cause of this. I leave it to the pediatric electrophysiologist to figure out what this rhythm is. The bottom one has heart block. I can tell that. And whether the top one is JAT or VT, I'll leave it to somebody else to figure out. So this is pretty typical of – well, his ejection fraction ultimately fell, went on flecainide, and his ejection fraction was restored with some arrhythmias suppression. So this is pretty typical of this unusual entity called MEPPC, multifocal ectopic Purkinje-related premature contractions. And all the other mutations that we had seen are most – the vast majority of the other mutations that we had seen to date involve mutations at a positive charge, usually an arginine, in the voltage-sensing domain. And they generate a so-called poor current, and I'll show you what that means in a second. We made a mouse of the commonest variant, R22Q, and the mouse has this interesting electrophysiology. This is a normal mouse with sodium current. You can't see the bottoms of the sodium current, but this is what inactivation looks like. And then in the R22Q heterozygote mice, you get not only the normal sodium current, but you get this interesting outward current, which probably flows through a different pore than the normal pore through which sodium goes. So I hope this works. This is a cartoon of the cardiac sodium channel generated by the late leader Bill Catterall. And let me just see if I can – so before I do anything, this is the top-down view. So the hole through which sodium goes is here. This is the four domains folded together to make the pore domain. And then there are four other voltage-sensing domains. There's only one shown here. This is the S1 voltage-sensing domain. But you'll be able to see that when this channel opens, it opens because this green thing, which is the S4 voltage sensor, moves and drags the whole protein to do all the things that it's supposed to do. So let me see if I can do this properly. So you can see it opens, and then inactivates, and then closes. Open, inactivated, closes. So you see it's this movement right here that does that. And if you look at it top-down, it opens, and then it inactivates, and the inactivation gate is down here. We're not going to talk about that very much. But I think these cartoons really bring home the idea that it's a big, big protein, but it's this green thing that actually sort of starts the whole process off. So let me see if I can get to the next slide now. So in a static view, this is the top-down view, and there are these four voltage-sensing domains that are the S1, S2, S3, S4 of domain 1, 2, 3, and 4. E171Q, E171, which is the place where Xu's patient had their mutation, is in a helix that's opposite the S4. So it interacts with the S4, we think. And that particular variant, no matter which sodium channel isoform you look at, that's always an E. And when you look in NOMAD, there's no substitutions. So substitutions are very poorly tolerated at that position. One of the people working on this program did a molecular dynamics simulation, and this is the one that counts. This is looking top-down at the voltage-sensing domain, and you can see there's no hole. Am I doing something right? It's not coming through up there, so I might just point for that where you're pointing. I can. Or should I do that? I can probably do this. Yeah, so the voltage-sensing... Yeah. Yeah. So the top-down view is that the voltage-sensing domain doesn't have a hole in it, but the top-down view of the mutant shows that there's a hole. What am I doing wrong? Oh, I have to do it on that screen. I'm looking at four screens, so that's my excuse. So you can see that when you make the mutation, you have a little pore. We think we know what ion goes through the pore. We're not sure, but we think it's probably protons, actually. And then when you turn it on its side, you can demonstrate that the mutants, either this one, which is the old canonical one, and this one, which is Shu's new one, make a water-filled hole. These are water molecules, and the wild type does not do that. So this channel is able to conduct sodium through the sodium hole that everybody has, and it also is able to conduct something through this other pore far, far removed from the pore region of the sodium channel. So we studied that one in great detail. These are data in hex cells showing that there is this outward current, and then we also made the mutation in wild type iPS cells, and again, you see this pretty dramatic gating pore current. We've set up electrophysiologic conditions so that we can study only the gating pore current. You do that by eliminating extracellular sodium and substituting NMDG. So this is just the gating pore current, and the reason we did that was we were interested in drug sensitivity, and this channel is pretty sensitive to flecainide. So this is an experiment looking at this particular setup, and then you ask yourself, well, how sensitive is the peak sodium current to flecainide, and it's one millimolar, which is about where that will block the E171Q current. One millimolar does very little to peak sodium current. So we think it's pretty interesting and selective. We're not sure whether it binds directly or whether that's an allosteric effect. So that's an interesting example of these sodium channel-related mutations that cause heart failure, and Jamie Vandenberg and Diane Fatkin and many others at Sydney have sort of summarized the data, and it's a really, really nice review in CERT Genomics, and you can see that a lot of the mutations that are associated with heart failure are in these voltage-sensing domains, very, very few in here. So we've spent a fair amount of time sort of looking at many of those, and many of them generate a pore current, so generate heart failure by causing very, very frequent ectopy that is suppressible and doesn't have to go to heart transplant, but that's a really nice story, and they looked at the published literature on what drugs people have used, and flecainide is the most commonly used drug, and it often works, but sometimes has adverse events. Amiodarone, there's a whole smattering of very, very small numbers here. So recognizing this is really interesting. I have one other sodium channel story that I want to tell really quickly, and this is a lady who I saw in my clinic. She had left bundle branch block. That was known since she was in her 30s. Her father has left bundle branch block. No other family history. Her ejection fraction was sort of mid-40s, not perfect, not terrible, and you had to talk to her a long time before being convinced that she had some functional defect in exercise. She had a dilated cardiomyopathy panel testing, and she had an SCN5A mutation that was synonymous, G to G, which was labeled pathogenic. And I thought, what's that all about? And obviously, GeneDx or whoever made the assertion was blowing smoke. Until I read the detailed report, it turned out that RNA studies performed in patient-derived lymphocytes using mini-gene blah, blah, blah, results in abnormal splicing, predicted to result in in-frame loss of at least 32 amino acids, and coding exon 26, MJ O'Neill. Well, Matt O'Neill was a graduate student in my lab, so they're actually quoting our paper to designate it pathogenic. I didn't recognize the mutation when I was in clinic, of course. So this is Matt, and what Matt did was study splicing during his MD-PhD experience. So normal splicing occurs because there's an A-G and a G-T at the beginning and end of almost every exon. And so if you think about it, if you looked at mRNA, most mRNA should look like this. That's normal splicing. But occasionally, mRNAs will look like this. They'll truncate. They won't splice correctly. They'll insert things. They'll truncate things, or they'll omit an exon altogether. So you just, you can assess the extent to which abnormal splicing occurs by counting up these guys and counting up the total and then making a little arithmetic. So he did it in a mini-gene way. This is a pretty standard way of studying splicing. And he did it using mini-genes, and you either expect normal splicing or abnormal splicing. The way mini-genes work is that this is a normal exon. This is a normal exon. And you insert your exon and flanking intronic sequence in the middle, and then you put this in cells, and then splicing happens, and splicing will either be normal or abnormal. So no surprise, this is the mutation, and the mutation results in truncation of an exon because the mutation actually generates a new cryptic splice site. So most of the transcripts are abnormal. So that's sort of what I wanted to say about SCN5A. And then the question is, can loss of SCN5A function be targeted? So in the first case, you could say, well, I'm just going to give a drug until the abnormal ectopy goes away, and they'll be fine. In the second case, I think if they have bad heart failure, the debate that we're having with the heart failure docs is whether they should get CRT pacing. And that probably would be helpful. And so the question is whether you could use other ways of targeting therapy. I don't think there's a drug that's going to restore function in the second case except this drug. So I just say that on the horizon is this idea of gene therapy. And how to do gene therapy for sodium channel diseases is very, very complicated, partly because the sodium channel is so big. But there are variants in related channels, not SCN5A but SCN10A, that when put into cells or animal models that have sodium channel defects appear to correct the defects. So I'll just leave it at that. This is a nice paper in the European Heart Journal from late last year. So I gave two talks, the talk yesterday and the talk today. And I think the common theme and the common message is that understanding basic, basic mechanisms of arrhythmogenesis at the level of individual genes or individual variants is where we need to go in order to develop appropriate therapies, whether they're drugs, whether they're gene therapies, whether they're device therapies. All those apply in these situations. But you have to know exactly what the defect is and what the physiologic consequences are. So that's what I'm going to say. Thank you very much. Well, I'll ask a question while people are standing up. As always, there's so much to think about. You said something that was extremely interesting to me that I haven't heard before, which is this idea that the gating pore channel is passing protons, which is a very different size than everything else we talked about. Is that targetable? I don't know. But the debate is exactly what ion, just because there's a hole and it's a sodium channel doesn't mean that sodium is what's going through the hole, the new hole. And we have a little bit of data from one of the other mutations that we've studied where there is no gating pore current until you lower the extracellular pH, and then it's huge. And if you raise the extracellular pH, then it completely goes away. So that would certainly make sense, but more work to be done. And whether that's targetable, I mean, maybe the thing to do is alkalinize all those people and their arrhythmias will go away. Worth thinking about. We have alkaline water up here. Yeah, yeah, yeah, yeah. No, that's actually really fascinating. It's a different method of passing charged current than we talk about in all the rest of electrophysiology. Well, just because it's a sodium current, just because it's a sodium channel doesn't make it a sodium channel, a sodium channel, that hole a sodium channel. Other questions? Late in the day. I'm going to have to run away, but I'll be here for another two minutes. Yeah, yeah. Enjoy. Okay. Okay. Thank you very much. Our next speaker is Dr. Goodyear, who's going to be talking to us about the Cardiac Conduction System at Single Cell Resolution, New Insights in Development and Disease. Thank you very much, Carolina. so, first off, I'd like to thank the organizers for inviting me to speak. Is there anything I do? Just click it. Click on your talk and then start at the time. Perfect, yeah. Let's wait, nothing to disclose. Perfect. Okay, so no disclosures. So I think as this audience well knows, that coordinated contraction of the heart requires a specialized subset of heart cells known as the cardiac conduction system, effectively the wiring system of the heart. So it's made up of multiple distinct components, the SA node, pacemaker, AB node, HISS bundle, model branches, and purgatory fiber network. Importantly, disruption to any one of these components can lead to a host of life-threatening arrhythmias, such as pacemaker dysfunction, heart block, and increasingly, as Dr. Roden even mentioned in the last talk, a source of ventricular tachycardia, ventricular fibrillation. So each one of these components is made up of unique cardiac cell types with their own physiologic and electrochemical properties. And while the conduction system is essential, obviously, to the function and the normal formation of the heart, the actual basis of the development and function of the conduction cells remains incompletely understood, in large part due to the fact that we still don't have a handle on all of the molecular regulators that mediate the development and function of the conduction system cells. And this is, in turn, really due to the inherent challenges when trying to study the conduction system. Everything from, they represent a very small number of cells relative to the rest of the heart. They take a complex 3D course throughout the heart, making it really tough to isolate them. And alluding to the large cell type heterogeneity that I mentioned before. So to get around all of these problems, several years ago now, we leveraged single-cell RNA sequencing in order to generate the first gene expression profile of the murine conduction system at single-cell resolution. So very briefly, what we did is we just micro-detected out the major regions of the conduction system and then broke that tissue down into single cells. We were able to label all of the, or barcode all of the mRNA for each individual cell prior to sequencing and ultimately quality control analysis. So in doing so, we were able to capture all major cell types of the heart, shown in this UMAP plot. Most importantly, in a very biased sense, the conduction system. So just zooming into, for instance, this zone one or this assay nodal region, we were able to capture, again, bona fide assay nodal cells in blue here. Based on their expression of hallmark genes, such as the funny current, so HN1, HN4, and other crucial transcription factors like ILATE1, SHOX2, and TVX3. But more importantly, we ended up uncovering a host of genes that were specific to the conduction system that had never been found before. Single-cell RNA sequencing also allowed us to really delve into that cell type heterogeneity. So if you zoom into that assay nodal cluster, you can perform sub-cluster analysis and start pulling out the individual components. So in red here, you have the compact assay nodal cells, not shown here, but that actually further divides into the head cells and tail cells based on their unique gene expression profile. We're also able to capture for the first time these transitional cells that had long thought to be an electrical bridge, effectively, between the compact assay node and the surrounding working myocardium. Using whole-mounted immunolabeling, using our new markers, like SMOC2 and RGS6 here, as well as optical clearing, we were also able to finally visualize in 3D these complex structures, such as the assay node shown here. So we were able to do these studies for the AV node and the His, as well as the Purkinje fibers. And in doing so, we were able to set up, again, this gene expression atlas for not only the major components of the conduction system, but all the various cell types that we started seeing within it. So that's cool and all, but what do we do with this, right? So what can we do with all of this information? So as a developmental biologist, I'm particularly interested in all of those genes that we found that we really had no clue what it was doing in the conduction system. And so this is just one example of one of these genes. So COPINE5, or CPNE5, we found it to be, so this is a calcium-dependent protein that's been implicated in membrane trafficking in the CNS, but never studied in the heart. We found it to be enriched in our dataset in the assay node, AV node, and Purkinje fibers. And when we look at a protein level, so this is an intact heart, and what you're seeing in gold is CPNE5 protein expression, we found that to be true, consistent with our data, so it's expressed within the entirety of the conduction system. And interestingly, when we looked at GWAS studies related to heart rate variability, we found that six independent GWAS studies had all implicated variants in CPNE5. And so based on this data, we hypothesized that CPNE5 was necessary for normal function of not just the CCS, but actually the assay node based on that GWAS data. So we can test this now with, now we have human-induced pluripotent stem cells and the ability to differentiate assay nodal-like cells. We can, tough to see on the screen, but you can see these little assay nodal cells beating significantly faster than the traditional ventricular cardiomyocytes that we differentiate. They have all the hallmarks, both electrophysiologic and phenotypic, for a bona fide assay nodal cell. And importantly, just like the mouse data, when we differentiate these assay nodal cells, we see increase in the expression of CPNE5, both at the gene expression and protein level. So that's a nice correlation. But then we moved on and actually knocked down CPNE5 within these iPS cell assay nodal cells. And just true to form, consistent with the GWAS data, we saw that these iPS cells generated lower heart rate, lower beat rates, irregular automaticity, and all in the setting of a prolonged tau decay or delayed calcium reuptake in the SR. We moved on to mice. Similarly, we knocked out CPNE5 as a systemic knockout and again saw the same phenotype. So we saw sinus nodes function on the setting of abnormal calcium transients. Just a quick shout out to Catherine Dang. She's a supremely talented post-bac in the lab who did a lot of these studies and will be heading to Cornell looking for a PhD advisor since she's pursuing her MSTP. So next question that I asked when I started dealing with all of this data is can we start using this data not only for developmental biology but for actual translational research? And I would argue yes. So as a pediatric cardiology fellow, one of the first unmet needs that I came up against was the accidental damage to the conduction system that happens during intracardiac repairs. So what you're looking at right now is the endocardial surface, the inside of a heart with a large ventricular septal defect in which a surgeon will have to suture into place a patch to close it up. But importantly, what you don't see is the conduction system coursing around this defect and other critical structures of the heart, namely the heart valves. So importantly, the conduction system is invisible to the naked eye and current sound of care still remains the use of anatomical landmarks to effectively guess the location of the conduction system. As a result, nearly one to 3% of all congenital heart disease surgeries results in inadvertent damage to the conduction system. And actually, that number goes up quite significantly when you talk about surgeries involving the crux or the center of the heart where the AV node and hyst lie. That includes ventricular septal defects like this, tetralogy of foot low, AV canal, so all very common surgeries. And we see similarly high levels of complication rates in adult heart valve surgeries alone. So taken together, these result in increased costs, longer hospital stay, a mean of 10 extra days in the CVICU across the nation, across the world actually. This represents, this unmet need represents a major source of morbidity. And unfortunately for most, a lifelong pacemaker dependency. So the idea that we had come up with was, could we generate, which we are coining Illuminote, could we generate a technology that would allow for the real-time visualization of the conduction system so that the surgeons could help them avoid hitting it with either sutures or incision? And so the idea was really simple. It was, could we generate an antibody directed against the conduction system and covalently conjugate that to a near-infrared dye that could be used for visualization? Where in principle, the idea here is that every patient would receive a single IV injection of Illuminote prior to their surgery, thereby allowing the surgeons to visualize the conduction system in real-time with the help of a camera and thereby help to minimize iatrogenic damage. So how the heck were we gonna do this? So one idea that we had was to leverage this single-cell data that we had in order to specifically uncover novel conduction markers, but that genes that would express proteins on the cell surface of each one of these conduction system components, such that they could act as a homing beacon for these antibodies, antibody dye conjugates. So that's what we did. So we took our data and effectively took all significantly upregulated genes throughout the conduction system, but selected only those that encoded for a protein that would be expressed on the outside of the cells. So in doing so, we were able to come up with novel markers throughout all major components and validated these guys, but also we found several that were expressed throughout the entirety of the conduction system. Sorry for some of the wonky formatting on this computer. So long story short, it worked. So Illuminode allows for highly sensitive, specific, and safe labeling of the entire conduction system. And so here I'm just showing two examples, two cell surface markers that we went after, so Contactin-2 and Neuroplastin, which are expressed throughout the entirety of the conduction system. We generated antibodies against these, conjugated to this dye, injected mice with a single IV injection, and could show lighting, we could label the entirety of the conduction system. So what you're seeing here is an intact heart that was from a mouse that was injected 24 hours prior with Illuminode, and everything that you see in gold is indeed the conduction system. We're very happy with the resolution that we're getting too, because the size of this heart is less than the size of a dime. So, so far I've talked to you guys about single cell multiomics, what we can do with that data of the conduction system. But really, I've only talked to you about mice so far. What about humans? So recently, Sarah Teichman's group came out with a paper in Nature showing us our first glimpse into the adult human cardiac conduction system. And so they were, one, not only able to show the global transcriptional profile of some of the major structures, but started using tools, bioinformatic tools, to start looking at how these conduction cells interact through ligand receptor interactomes with the surrounding various cell types in the heart. True to this session, they were also able to show how we can, in silico, use things, use bioinformatic tools to be able to do, for instance, a drug screen, an in silico drug screen, looking to see what on-market drugs are able to actually affect, or suspected to affect, your cell type of interest. So here, the acinetal cell having chronotropic effects. But the nice thing about the single cell data as well is that you not only get your cell type of interest, but all other major cell types in the heart. So as a developmental biologist, I'm, again, particularly interested in what are the molecules, what are the regulators that dictate the differentiation and the function of these conduction cells in human development. So in collaboration with Jesse Eingratz at Stanford, we were able to perform a comprehensive fetal human conduction atlas at single cell resolution. And so what we did is we took over 40 hearts, fetal human hearts ranging from development six weeks to post-conception to 22 weeks, and we performed a combined single nuc RNA-seq to look at the transcriptomic profile, as well as ATAC-seq to look at the non-coding genome, so to look at enhancers and silencers. We were able to acquire on the order of about three quarters of a million unique single cells, and unearth over 90 unique cardiac cell types. Again, very biased. I was very much looking forward to the conduction system and very pleased to see that we acquired all major cell types. We're able to use fancy bioinformatics to be able to look at things like gene programs, where you can weight, in a non-biased fashion, you can find out those that were differentially expressed, genes that were differentially expressed within the human developing conduction system, but also signaling pathways that are prioritized to start getting a sense of what is dictating their development. We were happy about the resolution and the depth of our analysis, so instead of just tens to hundreds of cells, we were getting thousands of cells for the various components. And for the first time, we're starting to see all of that cell type heterogeneity that we had previously shown in the mouse in terms of these transitional cells. They seem to exist in humans as well, which is exciting. And then finally, looking at the non-coding genome, we're able to start unearthing, with the depth of our app, the less bona fide enhancers for all of the major genes within this conduction system. So, looking forward, we're trying to take all this data, both human and mouse, and cross-reference this data to find those both genes and enhancers, regulatory regions that are conserved across evolution. We're intersecting that data with established GWAS data relating to human heart rhythm. And then finally, using both our in vitro and in vivo platforms to start really delving into what is necessary, what is sufficient to making a conduction cell. So with that, a lot of thanks to a lot of people, and I'm free to take questions. Thank you. either use the QR code or come up to one of the microphones. That's an incredible talk and really exciting stuff that you're working on. I was curious, and this may be a bit down the road, but have you thought about some of the clinical things that we encounter, apart from the acquired AV block, but sinus node dysfunction, for example? Our kids with trisomy 21 seem to have a predilection to it post-op. Some of the other, even the concept of dual AV node physiology, are you able to genetically target some of those things that we're seeing, or are we getting ahead of things? It's a brilliant question. I think targeting these things, one, just given the rarity, certainly like twin AV nodes, it's super, super rare and it's going to happen early in development. I think that that's probably unlikely to happen. Having said that, I think part of the battle in our jobs is to even just diagnosis these things and understand how the heck the conduction system develops, not just in the variety that we see in just adults with structurally normal hearts, but in congenital heart disease. One of the things, and I don't have time to talk about this today, but one of the things that we're trying to do to help in just even understanding the landscape of how much variation we see in congenital heart disease is we're expanding on this tech, so Illumina I had mentioned where we put on that near-infrared dye so that surgeons can see it in real time. What we're doing right now is we have a grant from the AHA to be able to just swap out that dye with contrast agents to be able to actually visualize the conduction system using MRI and CT. The idea is that ideally I would like to be able to start understanding what the conduction system looks like across the board, non-invasively in kids before they even walk into the OR. I think it'll help a lot with preoperative planning. I think from an EP perspective it'll help a lot. I'm also very biased. I would love to be able to take this technology in, and there's no reason I can't, to be able to swap out that cargo again with something that would be more amenable to our catheters so that when we're in the EP lab and creating our maps, that the actual AV node, sinus node, as we're walking around, we can pick it up as well. So I think both from a diagnostic and potentially therapeutic of avoiding that region or taking out one of the two AV nodes, in the case of twin AV nodes, I think is not far off. But actual gene therapy, I think, just given the rarity, probably not going to happen. You just said something super important. This idea that you could conjugate to something that's visible on CT scanner means that you've got a tool that as long as you can identify by RNA-seq a unique surface protein on a target of interest, the tool is ubiquitous across your target. So if we don't want to ablate the coronary arteries in the middle of an ablation, as long as you've used RNA-seq to identify an endothelial marker of coronary arteries and you've got a conjugate kit that can be seen with fluoroscopy, you could outline the coronary arteries. You could outline the esophagus if you were doing adult care. Like, if you could find a conjugate that was visible by fluoroscopy, that represents a major step forward because the process of identifying the RNA-seq is not unique to the conduction system. So, absolute nail on the head. I can't really talk too much about it in this context, but we are very much interested in not just, I mean, I think dyes and contrast agents, absolutely, but we're also thinking about also ADCs in terms of antibody drug conjugates to actually start having targeted therapeutic opportunities. And a lot of those therapeutic opportunities, actually, right, in the EP world, a lot of arrhythmias that we see are not actually derived from the conduction system. It's actually derived from other parts of the heart, the atria, the ventricles. And so, yes, and yes, we are very interested in targeting not just the conduction system, but all components of the heart, both for diagnostic and therapeutic purposes, for sure. It's a cool system. Thanks very much, everybody. All right. I'd like to welcome our next speaker, Dr. Mary New, who will be talking to us about When the Wolf is Not the Villain, Atypical and Pseudopreexcitation. Thank you to the organizers for the opportunity to talk about atypical and pseudopreexcitation. I have no relevant disclosures. Did not come up with the title, but I'm going to borrow from the title and take a look at some friends from the canine family, starting with this one. This is Ella. She is a 100-pound black German shepherd and the best walking companion ever. Couldn't tell you how safe I felt walking with her, particularly because anytime I encountered individuals, they would create a wide space for me because not everybody could tell the difference between Ella and this guy, who you would definitely want to avoid, even though he probably weighs the same as Ella. Not to be confused with this next creature, I do not have audience response, but does anybody know what this is? Yell it out if you do. That's a coyote. Average weight about 35 to 40 pounds smaller, but deceptively subtle and dangerous when overlooked. So I think these guys aptly illustrate the idea that not all threats look the same, but the consequences of this recognition are real. So our learning objectives today by the end of our short 15 minutes together are hopefully to be able to, one, differentiate Wolff-Parkinson-White syndrome from atypical and pseudopreexcitation syndromes. Describe the anatomical and EP characteristics of Maheim-like pathways, including those listed afterwards. Review arrhythmia mechanisms associated with atypical preexcitation, including which ones can participate in SVT or preexcited AFib. Explain key ECG and EP features that distinguish true preexcitation from pseudopreexcitation. And then apply a couple of diagnostic strategies to classify these accessory pathways and guide their management. So going back to basics, I wanted to define preexcitation, which refers to early activation of the ventricles due to conduction over an accessory pathway that bypasses the AV node. On surface ECG, this generates a short PR interval, a delta wave, and a bundle branch block-like morphology. Because the ventricle is being activated early, the EP definition is an HV interval less than 35 milliseconds. On the ECG on the right side, the vertical line corresponds to the earliest ventricular activation. You can see that the HV interval is five milliseconds, which is short. The prototypical preexcitation syndrome was described by doctors Wolff, Parkinson, and White here in 1930s. And they described an abnormally short PR interval, bundle branch block-like morphology with a wide QRS, and episodes of paroxysmal tachycardia. Over time and investigation, we have furthered our understanding of WPW syndrome. And if doctors Wolff, Parkinson, and White were with us today, with the help of AI, imagine to look something like this. And hopefully they would agree with our current understanding that WPW syndrome is a reentry circuit involving an accessory pathway with both antegrade and retrograde conduction. Also, there is usually non-decremental conduction over the accessory pathway. Why do we care? Why don't we want to miss this? This is because an inherent feature of WPW is the presence of a bypass track that can conduct retrograde and cause reentrant SVT that looks like this. But more concerning is that the antegrade conduction during atrial fibrillation can lead to rapid ventricular rates and degeneration into VF. And so indeed proving that proper response depends entirely on proper recognition. So what about atypical pre-excitation? From here, I'm going to refer to these as MAHIM or MAHIM-like fibers. Are they more like this guy, someone you don't want to trifle with? Or more like Ella, who might look intimidating, but is for the most part harmless? Or are they more like this coyote? Deceptively subtle and dangerous when overlooked. Before we answer that, I'd like to briefly review the evolution of our understanding of MAHIM fibers, starting with what did MAHIM really describe? So here we start in the 1930s and 40s when Dr. MAHIM described what he called paraspecific connections between the AV conduction system and the ventricles. And then in 1941, he and Dr. Winston histologically confirmed that these notoventricular fibers were in human autopsies. Then by 1947, Dr. MAHIM speculates broadly about Kent fibers, which we know to be involved in WPW syndrome. Fast forwarding to the 1970s and 80s, we move from anatomy to electrophysiology. And in 1971, Dr. Wellens publishes the first EP evidence demonstrating decremental conduction, long conduction times, and the classic left bundle branch block morphology during arrhythmias. By 1980s, surgical and catheter mapping studies reveal that most of these so-called MAHIM pathways weren't actually notoventricular after all. They were atrial fascicular connecting the right atrium and the right bundle branch. 1988 marks a turning point. Dr. Klein and colleagues formally characterized these pathways with AV node-like properties inserting into the right bundle. So again, this is very distinct from MAHIM's original concept. Then we move into the 1990s and present, and our understanding has deepened with genetic and molecular insights. We now know MAHIM-like pathways are sometimes associated with PRKAG2 mutations and broader cardiac conduction abnormalities. Additionally, we also refine our anatomical understanding by ablation and now know that there are left MAHIMs that are recognized, and the true notovascicular pathways are confirmed to be rare. So looking beyond 2025 and onward, maybe one day AIECG is going to be able to help us discriminate between WPW MAHIM-like fibers and fascicular ventricular fibers. The key takeaway from this slide is that what MAHIM originally described has evolved dramatically across anatomy, electrophysiology, and now genetics. But if you were here, maybe he would look like this, and hopefully he would agree that currently our conception is that the unifying feature of these pathways is that they display decremental conduction. Other key ECG and EP features include the following. Predominantly occurring on the right side and having a close connection to the right bundle branch, that programmed atrial pacing leads to obvious manifest pre-excitation following an increase in the AV interval along with shortening of HV interval. And I will show examples, so it's okay if you can't read all the words out there. And finally, there are some key features during antigrade pre-excitation and SVT. So we'll look quickly at where they are located. The nomenclature of these fibers depends on the proximal insertion and distal insertion sites. So on the left side of this line, you can see that the proximal insertion sites are on the atrial side, and depending on where they insert, they are called atrial fascicular or short and long atrial ventricular fibers. On the right side, sorry, on the right side, the proximal insertion sites are within the node and they connect either to the ventricle or fascicle. And although these are the OG Mahimes, I'm actually not going to spend some time on these because they're so rare. But collectively, they do demonstrate antigrade and decremental conduction, and they lack retrograde conduction, which is a key feature distinguishing them from typical accessory pathways, like in WPW syndrome, which often conduct in both directions. So again, mostly on the right side, you can see from the image that they're usually found between seven and 11 o'clock on the tricuspid valve. And in tachycardia, they display a left funnel branch pattern, as you can see from this EKG, looking at V1 and V6. So now moving on to some programmed atrial pacing maneuvers that can bring out their characteristics. This EGM is taken from a patient with a right-sided fiber, and you can see with S1 how long the AV interval is. When you apply S2, bring in B at a shorter coupling interval, there is AV lengthening, and sometimes the HV shortens, and sometimes reverses. You can't really see where the His bundle is on this electrogram, but it's somewhere in there. You can see it's definitely shorter than the prior interval. And finally, bringing in that PAC will augment pre-excitation, widening the QRS. And now moving on to EGM and ECG characteristics during anterode conduction and SVT. So again, this is also taken from a patient with a right-sided fiber, which wound up localizing to 830 on the tricuspid valve. You can see that there is a left bundle branch block-like morphology as illustrated in V1. And then in tachycardia, there is a long AV interval. And retrograde activation, or the retrograde limb of SVT is through the AV node. And so the His, sorry, the right bundle electrogram precedes the His bundle. So that would give you a VH interval. Left-sided pathways, though rare, are similar, have similar characteristics, except that in SVT, you have a right bundle branch-like morphology. And then looking at the EGM characteristics from this patient, you can see in the drivetrain what the AV interval look like. And then with S2 here, the AV interval lengthens. Once tachycardia is induced, there's a right bundle branch-like morphology. And again, retrograde activation is through the AV node. Whoops, and the earliest A is on the His channel. So what about sudden cardiac death risk? The thought is that Maheim pathways are less arrhythmogenic than WPW. There are some things that are protective about it, one being that there is decremental conduction, which limits rapid one-to-one conduction. And antigrade-only pathways do reduce reentry risk. However, there are potentially high-risk scenarios, including the coexistence of multiple Maheim pathways or Maheim plus fast AV nodal pathways. And there are individuals with genetic predispositions. So there's a thought that a gene like PRKAG2 could enhance conduction system abnormality. So the key takeaway is that although the baseline risk is likely lower than WPW, risk stratification should be employed to decide what the AP refractory periods are and whether or not there is inducible arrhythmia. Moving on to pseudopreexcitation, these syndromes look like preexcitation, but they're not. And they can kind of be put into two buckets. One is a situation where there's no actual bypass track. So that would include LGL, which is where the AV node has enhanced conduction, and hypertrophic cardiomyopathy. In both cases, there's not an actual substrate for reentry. Obviously, there is sudden cardiac death risk with hypertrophic cardiomyopathy, but not from an accessory pathway. And then for the remainder of the talk, we're going to talk about this benign bystander bypass tracks. And these are fasciculoventricular pathways. The characteristics include fixed minimal delta waves, preexcitation persisting until the AV block occurs, no retrograde conduction, and they don't participate in tachycardia and usually are not a substrate for sudden cardiac death. So I said no arrhythmogenic risk, but there are two stars because there have been one or two case reports where these pathways insert, they're long and they insert into part of the HERS Purkinje system and there's bundle branch reentry. But going back to these ECG characteristics, how do we make sure that this, which is taken from a patient with a fasciculoventricular pathway, is different from this, which is taken from a patient with WPW? They look pretty awfully similar. So there are some ECG clues or clues from ambulatory monitoring that could be helpful. So first, the preexcitation pattern is fixed and usually the QRS frontal plane axis is normal. And then the degree of preexcitation does not change, even with variable PR intervals. So you might be able to see this on a Holter rather than an ECG. They also have a noticeable, notable response to adenosine where there's no change in the degree of preexcitation, even as the PR lengthens before you then proceed to AV block. So some EGM characteristics, you can see from the schematic that the fasciculoventricular pathways on the right side, circled in blue. And because the base of the ventricle is activated early, there is a short HV interval. So on this EGM, the His bundle is labeled in red. You can see that the HV interval is indeed short at 30 milliseconds. However, when you bring in a shorter HV extra stimulus, what you'll find is that the PR lengthens or the AH interval lengthens without a change in the HV. So you can see that on this, the PR interval lengthens to 156 and the HV stays at 30. So key summary and takeaway points, atypical preexcitation involves non-WPW accessory pathways with distinct anatomy, ECG, and EGM features. Pseudo-preexcitation requires exclusion of true accessory pathways through non-invasive and invasive investigation. And then here's a table just illustrating some of the characteristics discussed previously. These are my references and I'm happy to take any questions. Thank you. All right, thank you for that excellent talk, and again, I'll remind the audience to either come up to a microphone or to please submit a question using the online system. I was trained by Dr. Klein, and he's a bit of a strickler for terminology, and he would advise us not to use the term mahaim, because mahaim has various different descriptions, and better to use the term atriofascicular fibers, decremental or non-decremental. Is that something, I mean, in your reading that we are moving towards, or are we going to continue calling it mahaim? I would personally advocate with just describing what you see, because when we use specific words, we communicate exactly what we mean, but I do think that it is in the vernacular, and it's going to take a long time to wipe it out from the vernacular. When you have a patient who you're wondering whether or not there's a fasciculoventricular pathway, what is your usual approach to trying to sort that out? Do you do a Holter monitor? Do you do an adenosine challenge before EP study? Definitely an ambulatory monitor, looking for some clues where the PR prolongs, or if there's wanky buck at night, or that can be really helpful. I used to do adenosine challenges, and what I found was that sometimes the patients are so anxious and revved up that I don't get true AV block, and maybe there's a little bit of PR lengthening, but I can't tell for sure. So now, typically, I'll take a patient to the lab and say, I will do the adenosine challenge first without putting catheters in, but with some sedation on board, and if I can figure out that way, great, then we don't have to proceed with the EP study, but if not, then we proceed with an EP study. There's a question from the audience. Do you think AI will reliably be able to diagnose fasciculoventricular fibers? I'm hopeful that it can with signal average ECG and looking at enough. If we put good data into AI models, then I think we can get good information out. So I'm hopeful that in the future we can. And our last talk for the session is Cytoskeletons in the Closet, Tachycardia-Induced Cardiomyopathy by Suketha Vijayashanker. Thank you, thanks to the organizers for the invitation. I click and it should play? Oh, start. Okay, so we made it. Last talk of the day. All the wuzzers in EP are out of here, only the hardcore people are remaining. We have 42 slides to go through in 15 minutes, so that's about 20 seconds per slide. Let's get on with it. First patient comes, seven-year-old, quite sick, quite unwell. I'll let you guess how symptomatic she is, looking at the chest X-ray, that's her X-ray, and this is the ECG at presentation. The cardiology fellows in the emerge, this is at 2 a.m. in the night, decides to give adenosine to the patient. This is the response to adenosine. You have a break in the tachycardia, but the tachycardia breaks with a V. The atrial activation looks like a low to high, and this is how the tachycardia breaks, and this is the echo on admission, so you can imagine how sick this child is, and this is basically cardiomyopathy, and we now have a nice term called arrhythmia-induced cardiomyopathy, which is an encompassing term which combines multiple different etiologies. This terminology has been there for a few decades now and was initially described in atrial fibrillation patients in the 1930s, and also Wolf Parkinson White, in their initial description, do denote that one of their patients had significant respiratory distress and would get tired very easily when walking, when he was in sinus rhythm as well, so this kind of entity involves one rapid depolarization of your His-Purkinje system, like you see here, or an irregular depolarization of your His-Purkinje system, like you see in atrial fibrillation. Frequent premature ventricular contractions. These were known for many decades, but only became well-defined around the 1999-2000 period, and now has been well-established into literature. Pacing-induced cardiomyopathy is also coming into this blanket, and also left pundal branch area block-induced cardiomyopathy so there's also another entity mostly seen in pediatrics, which is pre-excitation-induced cardiomyopathy, which kind of is similar, but it's not been classically described along with the other arrhythmia-induced cardiomyopathies. So why does this happen? You follow a basic common pathway which goes down if you have tachycardia, frequent PVCs, or atrial fibrillation, and what happens is you have AV dissociation, you have a lot of post-extrasystolic potentiations when you have PVCs, and then you end up having high LV end-diastolic pressures, high LA pressure, high pulmonary capillary wedge pressures, which take about a week to set in, and about, at four weeks time, you have heart failure setting in with reduced cardiac output. This leads to significant calcium overload within myocardial cells, as well as a lot of mishandling of the calcium, especially at the dyad complexes where the T-tubules are talking to your actin myosin segments. Overall leads to an increase in your sympathetic tone, so what used to be PVC now ends up sometimes becoming VT, and then you have the entire gambit of heart failure setting in with heart failure begetting more heart failure. So this patient goes to the lab, fairly typical case of permanent junctional reciprocating tachycardia. We localize it to the posterior aspect of the tricuspid valve, ablated, and me and Shu and everybody else is very happy. We are scratching each other's backs, high-fiving each other, yes, well done. We did note some PVCs in this patient in the lab, but we discounted it. We thought it's in the kind of smoke and dust of battle, and these are gonna go down with time, but she never completely recovers her function, and this is her post-ablation halter. A lot of non-sustained ventricular rhythms as well as some couplets, triplets, and frequent premature ventricular contractions. The PVCs had your usual pattern of kind of a left bundle branch-ish block with an inferior axis, and this made us think, oh my god, does this child have dual tachycardia? She has PGRT and she has PVC-induced cardiomyopathy. We waited for a few months. Function didn't really completely recover. Then we started reading papers about PVC-induced cardiomyopathy. So the dog that you see there is Jimmy the dog. Augustus Waller, who initially described the ECG well before Eindhoven, he had this dog that would place its limbs in saline, and which would connect to a kind of transducer, and he would do ECGs of this, and he would present in various different conferences around the world, and this dog apparently died due to a sudden cardiac arrest, and on review of the ECGs of this dog, a lot of PVCs were noted. So there is a theory that this dog had some kind of an arrhythmogenic syndrome, but PVCs are very common. If you do halters, almost 70% of patients less than 50 years old, and in about 5% of all ECGs done will show a PVC. It's been known for a long time, and it's now becoming more and more well-defined, and it's a cardiomyopathy associated with significant ventricular dysfunction, and when you take these myocytes and put them under electron microscopes, this is what you see. The PVC myocyte is much longer, much narrower, has a higher kind of length-to-width ratio, and also the action potentials of these myocytes are much longer when compared to control. We know that if you have more PVCs, you're more likely to have it. If you have certain types of PVCs, especially originating from the left side of the heart, more likely to have it, and also the QRS duration in itself seems to be a factor. The wider the QRS duration, the more the likelihood that you have PVC-induced cardiomyopathy. So we took this child back to the lab, and we found this very narrow sleeve just around the interface of the right ventricle to the pulmonary artery, and we were able to successfully ablate it in first go, and then because of PTSD, we gave a few extra lesions around it to make sure it doesn't come back. This is her echo before, and this is her echo after, and this is serial echoes. Unfortunately, it goes from left to right, but you see that initially very poor, and then with the PVCs, has some reduction in function, but better, and then with complete elimination of the PVCs, the function is much snappier. Patient became asymptomatic, but she's still dealing with a lot of PTSD that came with that initial PICU admission. Next patient, nine-year-old girl. Again, you can guess how symptomatic she is by that X-ray, and this is what she came with. A slower rate, but still narrow complex, and if you look at the P wave carefully, abnormal activation of the atrium. The axis itself is normal, positive in one and AVF, but the V1, V2 look quite wrong, and we thought that this might be an ectopic atrial arrhythmia and we decided to treat it as such unless proven otherwise, and this is the echocardiogram at admission. Again, a very sick child. Initial ECGs actually were quite deceptive, and they looked like sinus rhythm, and the child was kind of heading down a kind of transplant route, so this is an ideal segue to talk about Peppa Pig, and here you have Peppa Pig, but this is no ordinary Peppa Pig. This Peppa Pig lives in South Carolina in Charlottesville and meets two cardiothoracic surgeons, a cardiothoracic surgeon and a scientist, Embel and Spinell, who did this landmark work in the early 2000s where they took 20 pigs and put pacemakers in them and paced their hearts at 240 beats per minute, and a lot of this work kind of forms the prototype for a lot of pacing-induced cardiomyopathy that we study and all other arrhythmogenic cardiomyopathy work that we do, and to understand the changes that they found, we kind of need to understand a little bit about the protein structure within the heart. All cells have highly conserved proteins. The actin microtubules and intermediate filaments are quite conserved, but in the sacromeres, what happens is the actin, myosin, the troponins, they kind of form this specialized sacromeric unit, and those are the famous proteins that we know of, but there are also many cytoskeletal proteins which are much smaller, formed of things like actinin, tubulin, which also are very important for the cells. This is a nice micrograph picture of cardiac myocyte in active contraction, and you can see the various cytoskeletal proteins which have been stained. So these two wonderful scientists took the 20 pigs, sacrificed them, and did comparative studies on the two. You can see the picture on the left is a control, and whose heart function is normal, but on the right, you have significant LV dilation induced after, I think after six weeks of pacing at 240 beats per minute. Very significant change in cardiac size as well. The picture on the left is control, and when you look at what the intracellular kind of slides look like, you can see, again, these very long, slender cells which are much thinner than what they used to be, very similar to the PVC-induced cardiomyopathy electron micrographs that we saw earlier. The scientists also ran mRNA levels of different proteins within these cells, and what they found was that your contraction proteins, like the myosin heavy chain and the alpha cardiac actin, had similar protein content before the cardiomyopathy, I mean, in the controls as well as the ones with cardiomyopathy. However, the microskeletal structure, such as the beta actin and the gamma actin and the alpha tubulin, which formed the kind of the skeletal structural units, there was a significant increase in the total amount of these proteins, and what they concluded was that since your active protein content totally remained the same, it's only the microskeletal structure which changed, hence you don't have the hypertrophic response that you often note in some cardiomyopathies, and that's why these cells tend to thin out. They also did some immunofluorescent staining in these patients, and you can see that there is disarray in the way the proteins are maintained. We took this patient to the lab. I must say this is one of the toughest transeptal punches I've done ever. Usually I'm worried about an ASD septal occluder. I don't sleep the night before. I think about all the things I can do, but this one was even tougher. It took about four or five cardiologists to get across. The basic problem was the LA end-diastolic pressure was 25, so it was very hard to push across, and you see there there's an actual GERBOD defect which happened during one of the septal punches, and this is a nice carto image where we were able to isolate an ectopic atrial focus just at the carinal level of the left superior pulmonary vein. Successfully ablated, and you have a remarkable increase in function, so these stories are often heard, and we know of these things. This is another similar story of a 10-year-old boy post total anomalous pulmonary venous repair who came with a macro-reentrant atrial flutter and had reduction in function which we were able to treat. A slightly different story here. This is an older teenager, post-bental procedure. Unfortunately, during the bental procedure, he lost his conduction, and an artificial, or a transvenous pacemaker had to be implanted. The transvenous pacemaker was placed in a bit of duress, and we weren't really in a very good RV apical position when we placed it. It went a bit more anterior, and there was inability to capture via the temporary pacemaker, so we had to push, and we took whatever we got, and you can see it's not a very nice position. Looks RV anterior and a very wide QRS complex, and patient ended up having signs and symptoms of heart failure, so we went and spoke with the patient's mother, told her that we can maybe optimize this pacemaker position for you. We can try putting in a left bundle branch area pacemaker, or we can just go with the evidence and put a CRT in this patient and see how things go. Patient's mom said, try the left bundle. If you can get it, good. If not, do the CRT. So we put the left bundle in, and there was a remarkable improvement in function that we noted on the transesophageal echo itself during the implantation, so we let it be, and this is not a perfect left bundle capture. If you see, you see that delta wave, which shouldn't really be there, so it's kind of a pseudo left bundle with a non-selective capture, as we call it, but the patient improved remarkably, and he's doing well, and his heart failure symptoms went away. Similarly, another case of a 11-year-old boy who had an arterial switch procedure for TGA and a very large VSD as an infant, and unfortunately had complete heart block and pacemaker-induced cardiomyopathy after that, and for that, he had a CRT put in. This CRT's RV lead, which is the sensing lead, started to fail, and he had a lot of inappropriate non-pacing happening, and we had to change this pacemaker, and we told the mom that we have a newer technology. We can either put this, because now he's 11 years old, we can put a transvenous. We can try the left bundle, or we can give you a CRT. She said, try the left bundle, and this is the left bundle that we tried, and remarkable decrease in the QRS duration, nice left ventricular activation, and this is the chest x-ray. So you can see both the CRT leads are still in place. I was a bit scared to take it out. I just left it in as a backup, but the left bundle lead is not in its usual position. I'm almost done. So treat. Remove the offending agent, if you can find it. Oftentimes, you have myocarditis causing it. Manage the heart failure, manage the arrhythmia. If curative, otherwise palliative ablation, like an AV node ablation, and resynchronize when able. The last paper on this in pediatrics was in 2014, so we looked at it, and we did a review ourselves from Vancouver last year, and basically, we just wanted to see how things have changed since then. We found 52 pediatric cases in total. Atrial tachycardia was still very common, followed by PGRT, but VT seems to have become more common over time in the pediatric age group, maybe because we are doing more complicated procedures. Direct ablation was less common now compared to before. Looks like people are waiting to treat first, see how things go, and then want to ablate, and sometimes, patients were getting complete success with antiarrhythmic medication and were not even taking to the lab. Mean time to recovery was 80 days. Cytoskeletal-targeted pharmacotherapy is in the process, but these are not human trials as of now. People are trying this on mouse models in tachycardia-induced, as well as kind of norepinephrine-induced cardiomyopathies, where they are looking at microtubule target. The most studied is colchicine, which has been tried in humans as well, but there are many post-translational modifications which occur in the microskeletal structure, which are also being targeted, especially kind of medications to target the diet structure to see if they can change it, and hopefully, with more understanding of the isoforms, as in some patients have PVC-induced cardiomyopathy, some of us don't. We might get some targeted gene therapies as well. So conclusion, incessant tachycardia and dysrhythmia can give you a heartache. Remove the offender, medicate, ablate, and make sure that your arsenal is kept nice and shiny. Thank you very much. Thank you. Thank you. Are there any questions from the audience? Fantastic talk. So one question is, in terms of the cytoskeletal changes, how much do we know about the actual causality as opposed to it just being a reflection of what's going on, to the point of the kind of emerging data in terms of the therapeutic potential targets? And secondly, if you can talk about those changes in the context of also the variability in presentation that we see. So a kid that has RVOVT and has 50% PVC burden has no problem for years and years and years versus that, another kiddo with similar presentation, even a lower degree of burden, and yet has severe heart failure. Nice question. I'll do the second one first because it's easier a little bit. So there is a significant difference in the kind of isoform of protein that is expressed in different human beings. So you might have microtubules, but the microtubule isoforms are actually different from person to person. So what they're doing right now is trying to see if you can identify by just DNA sequencing who has what isoform and seeing if the ones who have PVC-induced cardiomyopathy have a specific isoform compared to the ones who don't. And that's the thought process. There are some early signals that you can actually force certain isoforms in mice to undergo cardiomyopathy if you have that differentiated. As to the other question, it's hard to know what the causation is, but most of the epinephrine-induced models of heart failure have produced hypertrophy while the pacing-induced models have not. So we think maybe there's microcytoskeleton that's more involved. All right, well thank you all for coming to the very end of the sessions and have a great trip home. Thank you.

Heart Rhythm Society 2025

cardiac electrophysiology

sodium channel mutations

single-cell RNA sequencing

Illuminote technology

arrhythmia-induced cardiomyopathy

Mahaim-like pathways

tachycardia-induced cardiomyopathy

cardiac conduction system

electrophysiological conditions

therapeutic strategies