Thanks, Nishan. So yeah, that's very kind of you. This has been very exciting to see. I mean, there's a lot of different silver linings that we have in this crisis, and one of them has been kind of this push for remote education. So it's my privilege to be a part of this. All right, so the way the format's gonna be for this talk is 10 questions, and then a little bit of a discussion for each question. We're gonna start out with the first one. All right, so on your screen, you'll see a pop-up for a poll, and we've got 115 people viewing, and so this is just a inventory of who's watching. And Nishan, I guess we'll just post. We don't have to wait for every single person. We can just post when we hit a quorum of, let's say, now, 185% yield. We'll then put it out there. Oh, wait, I didn't. That's the second question. Can you post the results, Nishan? There it is. Great, this is exactly what this meeting is intended for. It's for the fellows, and so I'm really happy to see that we have a majority of fellows. And for the attendings who are at home, bored because they're not doing cases, I'm happy that you're on board as well. And I also welcome the allied professionals and other people who are interested in what we're doing. So let's talk about ablation biophysics. I'm gonna start out with this question. Standard catheter ablation creates a lesion by heating a metal tip directly like a branding iron, directly denaturing tissue with electrolysis, or with AC current delivered through an electrode tip to a grounding pad, or through dielectric heating. And for those of you who follow me on Twitter, I posted, this is the first question I posted. 83% got it right. Let's see if we can beat that. All right, it looks like I'm seeing the live progress here. It looks like everyone's getting that right. Good, so the idea here is that it's not a branding iron. In fact, we do a lot to try to keep the catheter tip cool. And what we're doing is we're delivering electricity and electrical current from the catheter tip to a grounding pad. And we used to just call it ablation, but now we call it unipolar ablation. I'll talk to you a little bit about why that is. And if we go through the basics, the basics are this catheter tip is on a piece of tissue, and we're sending electricity from the catheter tip to the grounding pad. It looks like this in your lab probably. And we put this on a patient's thigh or flank. And this is actually ablating as well. This pad is ablating. And if you measure the impedance at the pad, it's very low. And if you measure the impedance here, then it's gonna be higher. What that means is that the resistive heating is gonna be very low on the surface where this pad is, because it's a dispersive electrode. But the heating where all the electrical current is on the tip is gonna be high, and that's this resistive heating concept. Sometimes you can have bad contact. Air comes in between the pad and the skin, and you can develop a skin burn. And in fact, this is kind of like a bipolar ablation, meaning you have the contact with the pad, and it's almost like you have a second catheter right on the skin. And this is a known complication. If you see your impedance very high, that's a good sign that you have air between the pad and the skin. And so it would be important to be able to check that. Okay, that leads to the second question I have here. Increasing the dispersive electrode size or adding a second pad is likely to increase, decrease, or have no effect on the current flow to tissue. All right, I'm watching the live results come in, and it looks like the majority, but not quite the same majority that we had on question one is getting this right. So we increase, just as I explained earlier. If I go back a few slides. We gotta remember Ohm's law, V equals IR. And so if you increase the surface area of the pad, we're decreasing the total resistance in the system. And if we have a fixed voltage, that's our power, then we're gonna have higher current. So when we talk about power, which is current times resistance, and we're able to affect the resistance, then if the resistance goes down on a fixed power, we're gonna have higher current. So let's go back to the basics. And this is gonna be important for all future questions that I have. Again, we have a catheter tip that's touching the tissue. And this electrical current from the catheter tip has to go through the tissue, through the body. The body has bone, fat, skin, air, in the case of the lungs, which is near the heart. And then it goes to the dispersive patch. The generator is what's creating the electrical current. This is very important. We're also surrounded by blood. Now blood has a certain impedance as well. And if you're an electron, you really just go to the path of least resistance, which tends to be the tissue, because the tissue will conduct electricity better than the surrounding blood. But it doesn't conduct it better than say saline or metal that may be near it. And so this whole concept is what you should be thinking in your head when you apply a catheter to a beating heart. And we can alter the impedance. This was done in Westmead in a kind of a phantom model, which is very nice. And they had this model where they adjusted the surface area of the resistor. They adjusted the surface area of the return electrode plates to the ablation medium. And when they did that, when they had a 60 ohm system impedance, the heated phantom created this gel, created a much larger lesion than if you increase the impedance. A lot of answer did this in a beating heart model and proved exactly the same thing when you had a lower impedance and a higher current, then you got a larger lesion. And in fact, it's a little bit of a tradition for us to have our ablation settings in terms of watts, when in reality we should have our ablation settings in terms of current, because that's really what we want. We wanna see, we wanna have, if we're able to control any measure, we wanna measure and control the current. One of the things you can do is you can move the patch. And this was a case report a few years ago where the patch was moved for an AIV or cusp PVC. I think it might've been an AIV PVC that they were approaching from the endocardium. And when you move that patch closer to where your electrode is, and remember the AIV and the LV summit is right by the surface. So when you reduce that distance, you're going to have a much lower impedance, which means a higher current at the catheter electrode tip. And that can make the difference between success at 50 watts or failure. And so you should think about these tricks and think about ways you can increase current to your ablation catheter when you really need that deep lesion. This is kind of another model to think of. So we have ablation catheter, we have the tissue, we have the ground or return electrode, which is the patch. And then we have the environment surrounding the catheter. And these are all very important concepts to consider. Okay, question three. A four millimeter catheter produces a smaller, larger or same lesion compared to an eight millimeter catheter. And these are non-irrigated catheters. Okay, we have about 40% who voted, please vote. All right, I think we're going to have to close the voting. So good. Now we're getting into a little bit harder questions. That's what it looks like. Okay, great. So I understand the confusion because a lot of times we will pull out a larger catheter when we want to create a catheter or we'll pull out a larger catheter when we want to create a larger lesion. But it's actually the other way around. Did everyone get those results? It's all about that current density. And actually this was, I don't know, not very well known in 1998 when we started experiment using different catheter tips, but it has to do with that current density. The same reason why the dispersive electrode doesn't have heating at the skin. When we shrink down the electrode size, we're going to have more current density, higher amount of current going into the tissue, less current being lost to the blood pool. But why do we use a larger tip of catheter when we want to have a larger lesion? The main reason is when you use a very small catheter, you're going to be limited in temperature. And once we hit a certain temperature, then we're going to reduce the amount of power because we don't want to have char or clot develop on the catheter tip. So really what we really want is a small catheter tip electrode that remains cool so that we can deliver as much power as we like. But when we can't do that, we increase the electrode size, not to create a larger lesion, but for the ability to deliver more power and not be limited by temperature. This, by the way, ingenious method that established most of ablation biophysics experiments, which is this thigh muscle prep that you're able to pump saline. And then this is what led to a lot of different experiments, preclinical experiments, for understanding ablation biophysics. Okay. Question four. A 4-millimeter irrigated catheter produces a smaller, larger, or the same lesion compared to a 4-millimeter non-irrigated tip. So I'm asking, what does irrigation do? Well, maybe I'll ask you a question since people are voting that came through. Why do we put the patch on the leg traditionally? Well, it's a good question. So that's, we put the patch, we actually put two patches into my lab on the legs because we're able to easily access them. And so if a patch is on the back, which some places still do, because it's closer to the heart, then it's really hard to get to if you're concerned about that patch contact. The difference in impedance between, say, the leg and then closer to the heart, it's small. I mentioned that case report because I thought it was relatively ingenious to kind of do that in a, it allowed for the understanding of how we're creating lesions. But for the most part, it's really about access. Okay, look, okay, larger. It looks like we have larger or the same as winning. Oh, this is great. I love how these questions have gone from really easy to half the people getting it right to most of the people getting it wrong. And you may ask yourself, well, why is it smaller? Why would an irrigated catheter tip be smaller? Well, I just told you that we're trying to keep the catheter tip cool. And the reason we try to keep the catheter tip cool is so we prevent char formation and we allow ourselves to give more power. The reason why everyone thinks an irrigated catheter produces a larger lesion, which is wrong, does not produce a larger lesion. If anything, and I'll tell you why it's not the same, the same would be the more logical reason. But the reason why we think an irrigated catheter produces a larger lesion is because we bring it out to create a larger lesion because when we use a non-irrigated catheter, we hit our temperature limit and we are limited in power. But if you were to give the same exact power, it's a four millimeter electrode and it's the same amount of current that goes there. But why does it give a smaller lesion and not the same? Well, it's because of that cooling aspect. When we cool the catheter, we're actually affecting the temperature of the tissue also at the tissue tip interface. And so we create kind of a teardrop lesion. And I think I have a... So when we're cooling it, we can either cool it internally, this is called the chilly catheter, or externally, and that's the open irrigated, the flexibility, the thermal cool, SF and the thermal cool, there's all these catheters. And this is the most common catheter that we're using. When we cool this catheter tip electrode, you can create this teardrop lesion. And when you don't have that cooling aspect, it's more of a hemisphere. And so there's a very small amount of tissue that is spared because of that cooling. You don't have that convective heating across and that teardrop shape is just a little bit smaller. It's so small that, I mean, it's really, I would have accepted same as the right answer, but smaller or the same is the right answer, theoretically smaller. I don't think it's really been proven to be much of a difference, but it's definitely not larger. So I got everybody on that one. This is kind of a neat concept because there's a few places in the heart. Well, really one that I think of, where we want to have a shallow lesion and we don't want to have a teardrop lesion. And that is on the thin left atrial posterior wall. The anterior esophagus is right up next to it. I mean, you know, if you look on ice, they're like right touching each other. And when you're cooling that electrode, which is very important when we're ablating in the left atrium, because it prevents char and we don't want to have a stroke. We don't want to have a thrombus formation. And so it greatly improves the safety. But if we're getting low power and we're cooling that electrode, we get this teardrop lesion and it moves the hotspot deeper. When we don't cool it, the hotspot stays closer to the tip tissue interface and we end up with a more hemisphere lesion. So Greg Michaud actually pioneered this technique where he basically turned off the high flow. So he wrote this paper up in 2017 and it really is a great idea. Essentially, he would use his Thermacool or whatever externally irrigated ablation catheter for the heart in the anterior aspects of the pulmonary veins and all around, except on the posterior wall over the esophagus. He was curious to see whether or not we could create a shallower lesion. And he used an experimental model and sure enough, it does. And you get actually more transmural because of that. And if you look at this, the lesion depth, it's lower on the low flow and the depth of the maximal diameter of the lesion is much, much closer to the surface, 0.04. It's more of a hemisphere, less of a teardrop. And the endocardial surface diameter is much larger. So that's the reason why some people will do this. I think that high power, short duration is a little bit of a different model and we can talk about that. But some people will do this and it's because they understand the ablation biophysics. And at low power, you have low risk for developing char and you don't overheat the catheter anyway. Okay, question five. And Nishant, I like answering questions while people are responding. I think that's reasonable. So if there are any questions, I'm happy to answer them at this time. So delivery of 40 watts for 10 seconds or 400 joules gives a lesion that is deeper, shallower, or the same depth compared to 20 watts for 20 seconds. Same exact amount of energy, just a different formula. Remember, it's power times time is joules. So as they're coming in, out of the 170 votes, we had 30. Any questions, Nishant? Yeah, it's kind of a catheter design question. You know, one of the companies has a 3.5 millimeter tip and others have 4 millimeter tips. Do you have any thoughts on what the optimal size for an electrode is? Well, that's a good question. Yeah, so you may have noticed I use a specific 4 millimeter design. There are actually very subtle differences in some of the different catheters. And if you are, say, using them interchangeably, you should know that the surface area of the electrode that you're using is not the same. And 30 watts with one catheter is not quite exactly the same as 30 watts with another catheter in terms of energy delivery. So you can do the math. And the only thing I would say is that there's enough wiggle room in what we're doing in terms of force, duration, circulation of blood, and current, right? You know, we are only using fixed power, 30 watts. However, a thin young woman will have an impedance of maybe 90 ohms, and a big fat man will have an impedance of 120 ohms. That's also not the same amount of current delivery. So what I would say is I don't actually make corrections for these subtle differences of a 0.5 millimeter difference in catheter tip design. But you have to acknowledge that there is a little bit of wiggle room. And if you ever look at any of these ablation biophysics studies, the amount of variability, you can do 100 lesions, and the spread of lesion depths and lesion volumes is huge because of all these other variables. And I think that having a catheter tip design that's, you know, 10% different doesn't really factor in when you consider all those differences. Okay, so again, no majority. I'm glad I'm asking these questions. I think it reveals a lot about what we know and don't know. But most people are, I said, of the three, shallower one. And that's right. So we were just talking about reducing the irrigation to try to prevent that teardrop lesion. This is kind of an interesting concept where this high power short duration has really taken off, and especially on that left atrial-posterior wall. And a study I like on this is this study where you used, this is out of Bordeaux, where they used different powers. And they can see that the shape of the lesion, the more power you gave, that initial phase of resistive heating leads to a wider and shallower lesion. And so you're taking advantage of that initial few seconds of resistive heating. And when you do that, you have a shallower amount if you control for the total amount of energy that is. And so this has really taken off in terms of our ability to try to safely protect that esophagus because you want the shallow lesion. Throughout the history of electrophysiology and RF ablation, almost all of our attention was devoted to making a deeper lesion. How can we cool the catheter? How can we make it deeper? And in reality, we want a shallower lesion for certain areas. And that has, I think, improved the safety of the procedure. There's a whole bunch of different studies on high power short duration. There's a little bit of controversy in terms of what it does. I'm a believer, I think, that it exploits a biophysical principle. But I do recognize the safety concerns of this sort of, you gotta be really paying attention to every single lesion because the safety concern is that you have a high power, high duration lesion, which is, of course, less safe on that posterior wall. Okay, I'm gonna go back to an easy question. I'm really hoping that everybody gets this right. But we might have a few cheeky wisecrackers out there. We'll see. Any other questions I can answer, Nishant, while we're moving on? Yeah, a couple things have come through a few times. So what's more important? Is it the temperature that the tissue reaches or the power that you deliver with the catheter? Good question. I should have, our whole point of doing this ablation is to heat the tissue. If we could, if we could know for sure the exact temperature of the tissue, and there is some research that's being done on this using microwave thermometry, trying to get there. But the whole idea here is to get the tissue temperature above 50 degrees. David Haynes did a lot of good experiments in the 90s showing that that's where we hit irreversible doses of heat. So we're essentially doing a controlled heating of the tissue. It doesn't matter what the power is. It could be 20 watts, 100 watts, as long as we're getting the tissue temperature above 50 degrees. I should say really about 55 degrees. There's a little bit of a spectrum. And then keep in mind, a lot of the studies were using healthy tissue. The healthier tissue may actually be more sensitive to heat compared to say fibrotic tissue. It's a whole different area. But our goal is to heat the tissue, not the catheter tip. And if we could magically understand what the tissue temperature is, that's when we would control the duration and power and try to titrate it to the internal deep tissue so we can get a durable lesion. OK, I'm proud to say that 74% of you guys got this right. And I'm happy to hear that. So yes, that's where our phase change is at 100 degrees Celsius. And so it wasn't a trick question. Here's a steam pop in my lab. And you can watch this. And I encourage the fellows that every lesion that you create, watch this impedance. Watch the tip temperature, OK? And here's a ultrasound video. I'm heating up chicken in my lab. And you can see that's the steam pop. So what happened right here? That bright spot is steam. And it's echogenic. And when you do that, thankfully, most of the time, the pop goes to the interior. You're delivering an endocardial lesion. And the pop goes into the chamber. When it goes into the chamber, you avoid cardiac perforation. When it goes to the external, when it goes to the epicardium and outside the chamber, that's when you have this emergency that can really lead to a serious complication. So we really want to do what we can to avoid steam pops, not just for that complication, but when you do have the pop, endothelium is denuded. We're going to increase our chances of stroke if you're on the left side. And this is really what we're trying to avoid. Even though we're keeping our catheter tip cool, the catheter tip is at 30 degrees, 39 degrees. However, deep into the tissue, and notice it's not at the surface, right? The steam pop generated deep into the tissue, almost like when you're microwaving leftover chicken. You can hear those pops. It's deeper in the tissue, and then the steam goes out. OK, so let's go ahead and follow a lesion along. And if you have PRUCA, you can look at each one of these lesions. I think the function is called plot when you highlight an ablation lesion. And here's the impedance, and you can see it's going down. That's normal. Whenever we're delivering RF current, we're taking tissue. We're desiccating it. There's less water content. It's also the tissue's hotter, and so the impedance will be reduced. It takes less. There's less resistance for the electrons to go through that tissue. And here's our power. Here's our temperature. And this is a cooled catheter. This is, I believe, a thermocool. And we're going along. You can see that the impedance comes down. Here, the temperature goes up very subtly, OK? I think that it's hard, you know, because there's a lot of variation. There's a beating heart. There's different circulation. But there's a little bit of variation, and it's being externally cooled. And tissue goes up slightly. What do I do? I go up on the power. Now, I'm going to tell you, in retrospect, bad idea. The temperature continues to go up, and then we have a steam pop. And that's what this is. By the time you see this, it's already happened. This represents a gradient in impedance, a higher impedance, because steam doesn't conduct electricity quite as well as a tissue. So you see this little, and every now and then, you know, someone yells off. It's too late. You already had the pop. It's pretty scary. Sometimes you can hear it. And it is a terrifying feeling to have that. And if you have eyes, you're looking. You're making sure that the steam pop didn't go external. And so you really want to prevent this complication from occurring. OK, here's another example. I'm going to have a question here, a two-part question. Here's the impedance. This is the temperature. This is the power, OK? And this is a 4 millimeter non-irrigated cathode. OK, so the question is going to be on these spikes, OK? And the question will be, what are these spikes? The impedance rises marked by the arrows are from an impending steam pop, micro bubble formation, impedance changes with respiration, or the tissue temperature changes. And then while those votes are coming in, I can answer a question. Yeah, many questions about steam pops. So I guess one was, why does the ablation lesion actually lower impedance? There's the thought that maybe the impedance should go up with ablation. And then sometimes you see steam pops, as you said, you get a steam pop and there may not be an impedance rise. So what else can you do to try and prevent them? OK, good questions. So we want the impedance to go down. In fact, we use impedance reduction as a measure of the quality of a lesion. If you're delivering energy and the impedance doesn't change, it's very likely that you're not heating tissue. And maybe you're not in contact with tissue. So impedance goes down with ablation for two reasons. One, if you just increase the temperature of something, impedance will go down. And that's just the physics of entropy. And so if you were to heat tissue, and then before you move, if you just keep the catheter tip as the tissue cools down, you may see a slight increase in impedance. So just a temperature increase of tissue will reduce impedance. The other thing is you're changing the properties of the tissue. You're taking a tissue with intact cell membranes, a phospholipid bilayer on every cell that's containing water, and if you look at the lesion, if you look at what's happening, you're actually compressing the tissue, you're breaking down those cell membranes, you're changing the proteins, denaturing them, and you're creating a medium that is much better for conducting electricity. So scar has a lower impedance than healthy tissue, plump tissue that has water and has intact cell membranes. So the tissue characteristics also, by changing them, by ablating them and cooking it, you're changing the impedance. Okay, how do you prevent steampops? I'm gonna delay that question because I have a few slides talking about how we can prevent steampops without the use of looking at impedance. Okay, so let's go back to our question here. And good, most people got it right. And this makes things a little bit hard. It is a change with respiration. And what we have is this person is just breathing. And as you breathe, you bring in more air into your lungs. And air is an electrical insulator. And so it's a very subtle amount. We're talking about a different, sometimes it can be high, it can be like 10 or 20 ohms. But that air acts as an effective insulator. And that's how you get this rise in impedance. Okay, let's go back to this lesion. So patient's breathing, that's always a good thing. And then we see this over here. And the question is, is that a breath? Is that impedance? And I want you to look at this whole picture here where we have titrated our power up to 50 watts. And the temperature is now 55 degrees here. So it's gone up a little bit. And then we have this impedance change. And I'm gonna tell you that this person had a steam pop. And the question is, why did they have a steam pop? Is it because there's a temperature rise? There's an unexplained impedance rise? There is a temperature rise at the same time as the impedance rise? Or no, there won't be a steam pop, I'm wrong. A couple more questions came through, Will. One was, is the spike in impedance due to catheter movement from the pop? And if you look at your impedance curves, can you gain any information about the transition from resistive to conductive heating? Okay, so the impedance rise at the time of the steam pop could either be that you are in contact and that you now have a bubble underneath your, a bubble of steam, a gas underneath your catheter, and so you have the relative insulation. The pop will cause your catheter to move. And so it's also possible that you're now floating there. But really, it's most likely because of alterations in the tissue architecture and the tissue composition of gas. Even when there's a steam pop, there's still small, a little, if I were to play that video again, you'd still see that there's a bright spot. Not all the steam escapes. And in fact, you can sometimes see that when you do ablation leaches, you can see a bright spot and that's microbubbles that are trapped. In terms of the transition of resistive to conductive heating, it's a good question. I haven't really thought of that because impedance by definition is only resistive, right? So it's the electrical part. I guess I would say that if you see, so I would say the initial drop in impedance is from resistive changes and continued drops that are likely to be explained by conductive changes. I don't have a great measure of that, but I will say that you can predict steam pops upon the initial drop. I'll show you a graph of that. Okay, everybody voted. So we have 70% got this right. And I would say that if you see a temperature rise and an unexplained impedance rise, this is the breathing here. This is the temperature rise, the impedance rise, and it occurs the same exact time as the temperature rise. And if we follow that out, we see this here. And you can see that the temperature continued to rise and the power falls, but even despite that fall in power, this spike here is that steam pop. Okay, all right, next one. So here we have changes in impedance with each heartbeat. So this is actually represents, atrial beating is probably fibrillating. And you can see that the catheter contact is intermittent here. And then we have the respirations here. Same thing, four millimeter. And this is the drop in impedance. This is the temperature rise that we have. And here's the power delivery. Okay, everything looks good. Here's our drop in impedance. We have a stable temperature here. Everything is looking good to me. And then a slight temperature rise, unclear. If this is maybe, this is not a force sensing catheter. It could be that just at this moment, we put a little bit more force on it. And then we have a steam pop. Now this gets at that question of, hey, sometimes it's not so obvious that you have a steam pop. And I put this up here because I know, what I'd like you to do is to, every time you have a steam pop is to see whether or not you can look at these graphs and see if you can predict it. But what I would say is that this initial drop is a good sign that you've got a good lesion. And when I look back at this, I would say maybe we wouldn't do 50 watts. Look at this drop. We have a nice drop of impedance. Maybe we should stop it right here at 35 or 40. And so I think that while it's difficult to tell whether or not you're gonna have a steam pop, lower power is associated with fewer steam pops, of course. And if you're achieving your goals of impedance reduction, then consider, even though your plan was to give 50 watts because you wanted to have a good lesion, well, maybe when you see this drop in impedance like that, limit the power because that drop of impedance happened by 40 watts. And when we looked at this, the average impedance over the initial five seconds when we saw the drop of impedance, the steam pops had a much steeper decline. We also looked at the change in impedance in those five seconds. And like I said, tons of variability. That is the main weakness of this type of research is we have lots of variability. We've become amateur statisticians to try to get over that. But the percentage in drop in impedance appears to be important. And once you get to, for when we looked at our steam pop history, 12%, plus or minus 5%. So we actually use a 10% impedance drop as kind of a way of saying, okay, maybe we don't titrate the power up on this one. We've already had a 10%. Let's leave it at wherever it was when we achieved that. Okay, and this is that example. This is a more than a 10% drop, still went up on power, and we had a steam pop. Here's less than a 10% drop. We went up in power. We have continued drop of impedance, went up in power. No, no steam pop. This is what you're aiming for. You want this controlled drop in impedance. Okay, question eight. Changing the external irrigant from normal saline to D5W will result in a deeper, unchanged, or shallower lesion because the osmolarity is higher, the ionic tonicity is higher or lower. I guess now that I look at this, this question's a little bit complicated for me to read out aloud. I'll let you digest the question, and I'll answer another one. Yeah, I guess a couple things came through. Paul said I wanted to just mention that the unpredictability of steam pops might be due to local tissue characteristics like coronary flow and fibrosis that can change current density. And then the other question was when you were talking about respiratory variation in temperature and impedance, how much is due to the lung air and how much is just catheter movement? Okay, well first, my colleague, Dr. Zhai, is absolutely correct. You know, I'm presenting a sort of a in vitro kind of analysis, but oh, it gets really complicated when we start talking about heat sinks like coronary flow or getting buried in tissue, blading in a vein like the AIV or the middle cardiac vein. It gets to be challenging to try to understand all of these different components, and it can be challenging. The second question, sorry, repeat the second question again. I just want to make sure I get that right. The changes in temperature and impedance with respiratory variation? Yes. How much is catheter movement versus the air? Yeah, same thing, lots of variability. So, you know, not only have force sensing catheters, I think that we're a little bit better at sort of doing adjustments of our catheter, especially with respiration. But yeah, if you compress the tissue, the more that we go into the tissue, like bury it, the lower the impedance is, right? So if we just lightly touch the surface and we'll get an impedance measurement, if we put more force in there and we surround that tissue with tissue, surround the electrode with tissue, then the impedance will change. In fact, there's like a rate responsive thing called CLS, closed loop stimulation on biotronics, which is based on that principle, how much tissue is surrounding the electrode. And it's pretty neat to think of it like that. So contact will affect impedance and respiration can affect contact. And so you may also see that. I explained it by having more air in the circuit, but it could also be from differences in contact. Okay, so everybody got this right. 68% got it right. D5W, arrogant, will make the lesion deeper. Actually, now that I think about it, I think I gave you the answer. I will have to look back. If I gave you the answer, I didn't challenge you enough. So let's look at this in vitro model that I put up. We have the catheter, we have the tissue, we have the ground. Okay, so if we don't change the catheter, if we don't change the tissue, if we don't change the ground, we can change the environment surrounding the catheter. When we use normal saline, normal saline has a low impedance, right? It has a ionic tenacity. And so you're gonna lose RF to the surrounding medium as you start diluting the blood with normal saline, especially if you're in a closed environment like the epicardium, or if you're in a vein, an epicardial vein, or if you're up in the RVOT, if you're in like a little corner or in the cusp, you can change the environment that way. If we change this number, if we make it a higher impedance, let's say 180 ohms, that's what you might achieve if you use D5W which has no ionic tenacity, it's essentially an insulation, or half normal saline which also has a lower ionic tenacity then you're gonna drive more RF current into the tissue. Now remember what I was saying earlier, all of this is about the current density that is at the catheter tip, and how much electrical current is going into the tissue to create that resistive heating. And so by changing the environment you can increase, it's one way to increase the amount of current going to tissue. Can we influence it? I think actually you really see it in the epicardium. I have to say in the endocardium it's a little bit more challenging because you have circulating blood, it's going around the catheter, it's less predictable. But nonetheless, I'm gonna skip through, well, I'm gonna skip through this, I just basically described all of this. Nonetheless we can measure it. And so in this experiment I used a fish tank, a piece of chicken, and this kind of a plastic tube that would kind of recreate a chamber. And then I used different irrigants through the catheter tip. And what I want to show you is that... Yeah, so we have a, are we recording? So what I want to show you is that when we place this cylinder around the catheter and try to seal it with the tissue, notice what happens, I'm just gonna put it here because I'm not sure how the volume's coming across with this. But as I move this catheter, the impedance goes down. Let me just try. The impedance goes up. So you can see the impedance goes up as we put this cylinder. Now I'm not touching the catheter, I'm just surrounding it with something that doesn't conduct electricity. And so once we have this impedance we are then able to measure how much impedance, we measure the impedance in this chamber when the catheter is touching the tissue and when the catheter is not touching the tissue. Yeah, so. And when we do that. Yeah. Sorry. When we do that, we find that most of the energy is delivered to the surrounding blood. So this blue bar represents the blood. And we can measure how much electricity is going into the tissue. And less than 20% of the energy that we're delivering actually goes into the tissue. Most of it is wasted into the blood pool and we're not delivering it to the tissue. But if we use half normal saline, we increase the amount of energy going into the tissue and decrease the amount going into the blood. So that's the principle of it. When we did the experiments with this, we used half normal saline, D5W. And you can see that we did get deeper lesions when we used D5W and over half normal saline over normal saline. Unfortunately, we also had a lot more steam pups. And so we originally did this with slabs and we decided that we were gonna move over to animal studies using half normal saline, where there was a non-significant trend in steam pups when we used the half normal saline. But we did get deeper lesions. It's a nice story of bench to bedside. So in 2012, Matt Olson, who was a resident at the time when I was at University of Colorado, helped me and did all of these experiments. And originally, we were just kind of looking at temperature of the irrigants as well as the ionic tonicity. And we noticed this phenomenon. And this was the original description of using half normal saline to create deeper lesions back in 2012. Then Dewey Wynn carried on a lot of this research and we showed that in a number of papers over at JCE and Jack EP, where we basically showed that by using half normal saline, we created larger lesions. And it culminated in this multicenter study where we took difficult ablations, PVCs or VT ablations that would have failed. And just from me talking about this concept with a couple of other electrophysiologists, we pooled together the experience that we had on 94 cases. And as you might expect, those 94 cases were the pap muscle, the septum, the LV summit or intramyocardial tissue that's hard to get to. And every one of these patients had failed with normal saline, every single one. And when we converted it over to half normal saline, we got an 83% acute success rate. So that really, I think, helped us to use this phenomenon to try to improve lesion depth and success for patients. We also saw higher steam pop rates, especially in low flow catheters. So that ThermoCool SF for the flexibility, those catheters had a higher rate of steam pops. And I can tell you from my experience that using half normal saline, you'll see a higher rate of steam pops even without. So it almost was 20% in the low flow. It's less, it was like five or 10% or maybe less than 5% in the high flow catheters. Okay, what else can we use? Well, we can also use bipolar ablation. So this is the ultimate in manipulating impedance. Remember I showed you how you can adjust impedance by moving the dispersive patch up anteriorly. Well, you can also, the ultimate adjustment impedance is to shrink down that dispersive electrode and put it into a different catheter, essentially making a bipolar ablation. And so you can see as you move the catheters around, as you get closer and closer to the tissue, your impedance goes lower and lower. And there's a lot of different ways. The other advantage is that as you're ablating from this catheter with bipolar ablation, you're heating the other side. And so you remove a heat sink from there by using bipolar ablation. So you reduce the trans tissue impedance and you take away the heat sink by adding heat to the back end and getting a deeper lesion that way. And you can see there's a big difference. You get a cylinder type lesion when you do bipolar ablation. This effect of the heat sink can be seen in simultaneous unipolar ablation. But this is pretty effective to get those deeper lesions. And this is sort of an advanced concept for using RF. And you have to kind of modify the system. This is a little bit off label. Well, I would say a lot off label in terms of what you're doing. But desperate times call for desperate measures sometimes. Sometimes you have the arrhythmia focus right in the middle of the tissue and you have to get there. And this is one way to do it. Okay, question nine. Bipolar ablation will lead to deeper lesions if the intercatheter distance is less than 15, greater than 15, greater than 20, or there's no difference compared to unipolar ablation. Okay, well, a number of questions here. So is there a benefit to half normal saline if your power is less than 50 watts? Like should you go up to max power before you switch? Yeah, so when we first did this, Wendy Zou was the first person to do this actually. I think, you know, we were doing this. When we first did this, it was exactly as you described, where we would go to 50 watts. And then if it didn't work at 50 watts, we said, okay, well, let's try 50 watts with half normal saline. Now, there are more people who are just sort of reaching for the half normal saline because there's an added benefit I didn't talk about. And that is sometimes we are ablating people with a very low ejection fraction. And so the salt load is much lower. And so that's where we're seeing lower powers being used with the half normal saline. So my current practice, I don't start with half normal saline. I do everything with normal saline. And it's only when I get where I feel like I need a deeper lesion and I need more power, I've already maxed out 50 watts that I reach for the half normal saline at that point. A couple more. Do you have any thoughts on local impedance drop versus the generator imported impedance drop? Very sophisticated question. So we only get the generator impedance measurement. We don't get the impedance measurements of local impedance, which would be say to the bipolar electrode. But that is an area of active research. There is a company that's looking at local impedance changes to use for catheter navigation. But the only impedance value that we get, we only get one impedance value during an ablation, and that is the generator impedance. So that's what I'm looking at. Okay, question nine. So maybe evenly split between less than or greater than 15 millimeters. So it's less than 15 millimeters. And so what we did is an experiment where we looked at catheter tip distances. And this is a bipolar ablation that's from the LVOT to RVOT. Every now and then this is needed, not necessarily because of depth of lesion, but more because of fibrotics. Sometimes people have a redo ablation and it's almost like they have a compartmentalized protected circuit or focus in between these two sites. And then when we looked at the distances, it's really after 20 millimeters, after 15 millimeters, we lose that. So if you look, it's just the same. So if you were to do a bipolar ablation and you're greater than 15, it's really not making much of a difference. You really need to be 15 or less. If you're at 20, it's no difference. We showed that in a laboratory model. Okay, last part, last question. Okay, when a metallic device is near an ablation catheter, delivery of RF energy will cause heating of the exposed metal, cooling of the exposed metal, or unchanged temperature of the exposed metal. There's a question on bipolar ablation. Does it matter if the ground catheter is irrigated or non-irrigated? It's the same thing as, yes, it matters. It's the same thing as when you give unipolar ablation. You want to keep the catheter tip cool. One of the problems with bipolar ablation is that it's not commercially available, so we don't have active temperature readings. There are a couple of case series where there's ways to sort of work around that, but my suggestion is to use an irrigated catheter because you know you're given high power. By the time you get to bipolar ablation, it's like half normal saline. You're not gonna be messing around at 20, 30 watts, and so you're gonna have high power. The tip temperature of the electrode is gonna be hot if you don't irrigate it or you don't cool it. Okay, great. Most of you got this right. We heat metal. This was sort of an unknown thing, and it's kind of a neat thing. So if you look here, this is an experiment I did in my lab where I just took an ablation catheter, and this is just a cutting of a copper wire. It's just floating on the medium, and when I deliver the ablation, when I deliver the power, if you look closely, it's the copper that heats up first. It's almost like by magic I'm ablating from this free-floating copper wire, when in reality what's happening is I'm delivering RF energy and it takes time for the catheter tip to heat up, of course, and the medium below it to heat up, but the current density is much higher at this very small little clip, and so it's like the reasons why we don't have metal when we wanna get an MRI is because the metal will heat in the MRI environment. Why is that? It's because of the RF field that's generated. We're having like a little microcosm of an RF field, and this little tiny piece of metal heats up. Now, you'd think that this is not a big problem or it may not be an issue, but it actually comes up. Some people use mapping catheters, and when you use a mapping catheter, you can see that this electrode heats up here, and this is kind of a nice study that I think is taking advantage of this concept kind of serendipitously, meaning that this is a study looking at AIV and LE-SUMMIT mapping where my friend Fermin Garcia placed a wire in a vein to map an intramural focus, and the whole point of the paper was to show that if you ablate right next to the intramural focus, this wire helps you do that, but I actually think that you have an added advantage of not only having the wire steer you to the right focus from the endocardium, but the wire itself will get hot, and so I think that this technique could be useful in certain arrhythmias that it may be difficult to get to from the endo or epicardium, but you can get to with a coronary vein. Okay, that concludes my talk. In summary, larger tip and irrigated ablation results in larger RF lesion development because it removes the temperature limitation. That's a very important concept. You can avoid steam pops if you pay close attention to the impedance and tip temperature changes, and then finally, we can modify the impedance of the environment surrounding an ablation catheter, and a lot of the little tricks that we use are all about modifying impedance, including bipolar ablation, half-normal saline, and using metal to heat in the presence of RF energy. Thanks very much, and I can take some more questions. I hope I finished on time. That was fantastic. Thanks for that. There are a couple other questions. A lot of the temperature graphs are often shown for non-irrigated catheters. Is it different when you're using an irrigated catheter and the temperature doesn't really reflect tissue temperature? Yeah, so that's very observant. So, yeah, what I would say is that irrigation makes catheter tip temperature monitoring almost impossible. In fact, I think it is impossible with some of the low-flow catheters. I think that it is marginally possible with some of the higher-flow catheters because the electrode does heat a little bit. Two reasons why that it's difficult. One is the temperature sensor is actually located approximately instead of in the actual tip, as opposed to the non-irrigated catheter. And the second reason is that the tip temperature really is cool. It does a very good job of cooling the catheter tip. And so if you have a very cool catheter tip, it's very difficult to measure the temperature at the tip-tissue interface. There are new catheters that are designed with microelectrodes that will potentially get at that problem. But tip-tissue interface monitoring is almost impossible with irrigated catheters. And then a few questions about esophageal temperatures. So do you have concerns about temperature probes in the esophagus when you're ablating on the posterior wall or using a quad the way some people are doing for trying to avoid using fluoro? Yeah, I think that that's reasonable concerns. I just showed you that metal can heat. And so I'd probably try to move away from ablating directly over metal in the esophagus. And actually in one of the papers, we showed that there is a metal temperature probe that it'll actually heat. And you can actually ablate all the way to the posterior wall of the esophagus because that heating conducts the RF, that metal conducts the RF all the way to the posterior wall. And the whole esophageal temperature, is it a paradox that we're somehow amplifying risk by looking? Should we not look? I don't know if I know the answer to that. I know that putting something in the esophagus probably does affect subtly the current flow, but I guess it helps to know if you're heating the esophagus and it will alter things. When we've looked at it, it's a marginal difference and we don't think it's a clinically relevant difference. And so we monitor, in our lab, we monitor esophageal temperatures for safety. And is there the same concern with leads? I think you addressed that in that paper. Yeah, so every now and then, people are astonished that they lose by the pacing, for example, if they're by an LV lead or the threshold goes up in a pacemaker lead. And you can heat the pacemaker lead. Let's say you're on the LV portion of the septum and on the RV portion of the septum, you have a defibrillator lead. You can heat the metal of that defibrillator lead and you can actually see the RF interference happen because you'll see that it goes into noise reversion mode and you can cause an increase in the threshold that way. So I would just be careful about delivering RF next to leads. And it might be worth checking a threshold before and after lesions to make sure that you're not gonna need a lead reversion.

ablation biophysics

remote education

standard catheter ablation

unipolar ablation

lesion size

irrigated catheter

power and duration

lesion depth

distance between catheter tips

metallic devices