Hi, this is Eric Prostowski and welcome to another segment of EP on EP. It's always a pleasure for me to interview my next guest, Dr. Roderick Tung, who is currently the chief of cardiology at the University of Arizona in Phoenix. And today we're going to discuss conduction system pacing. Rod, welcome to the show. Good to see you. So let's start with basics. You know, this whole area of his bundle pacing and now left bundle pacing you've gotten into early on, but let's go back to the basics. For example, what's a left bundle? That was the million dollar question that I think that we thought we understood what left bundle branch block was. We thought that it might be more of a distal disease. It might be in many areas and it's diffuse. And that's when we really sought to try to understand it better with high density mapping, which is something that we were familiar with and use those techniques to get us to understand what was happening on the proximal left septum of the conduction system. And as you're aware, we've discussed this as well, right? We found that a lot of the disease was high, proximal, proximal to the left bundle, actually in the left intra-his fibers and really focal. And that's why if you pace the his, it can leapfrog and capture a latent Purkinje that just is waiting there to be recruited and change the paradigm that this is all due to diffuse disease. Right. In that situation, and by the way, don't be so modest. You should quote your article. I mean, I read it and it was actually part of a journal club for us. It's just a wonderful piece of physiology. I don't remember the exact source. So take a second to tell them because everyone should read this paper. It was in circulation in 2019. Yeah. It's a great paper. Upadhyay et al. What did you say? Upadhyay. Gaurav Upadhyay. It's a wonderful paper. Having read your paper and talked to you about this, we don't know until we get in the lab what's going to happen, but there must be clues you've figured out now if you're going to pace. Which are the kind of patients that you would think would be the highest success if you're going to do it? So now that we have some correlation between the mapping findings and then you've got to come back to the surface ECG, because at the end of the day, if we can't discern something in clinic, then it doesn't really help the patient at hand and you can't mandate mapping for all of these left-sided EP studies, although we love them. So it does appear that you've got all these definitions of left bundle, right? You've got the AHA criteria, the ACC criteria, you've got different criteria, but the Strauss criteria do appear to be emerging as the most specific and sensitive, which requires 130 in women, 140 in men, mid-QRS notching. And that notching is really important because it tells you that there's a difference between ARV and LV activation. And that's a true left bundle branch block pattern. And I agree. People need to understand what the real left bundle is. Having said that though, and having done so much of this, you must have gone back and taken a look and seen the correlation. So how good was it if you were in clinic and you said, aha, this is one that's going to be a great success. How good was that actually from ECG only? Right. That's a great question. I think it's going to depend on the patient subgroup that you're analyzing. Unfortunately, you can't erase the underlying structural heart disease. So there's always a myopathic picture that will complicate the QRS. And many patients will have left bundle, what we call left bundle plus, which means they have a legit left bundle, but the QRS is 230. How can you get to 230 unless you have a myopathy as well with intramyocardial conduction delay? So these Strauss criteria, as we found it, were somewhere in the high 80s and low 90s, so nothing's ever going to get you to 95, a hundred, but it also depends on the population you're studying. Yeah, that's not bad. So let's now say, okay, now we're going to do it. Somebody's been adequately trained, you get in the lab, do you do his bundle pacing or do you just skip that and go right to left bundle branch block pacing? Well, when this all first started and we really had to thank Pugal Vijayaraman, Weijin Wang for taking the lead and, you know, everything that's new was previously old. Nothing's truly that original. You've got to look back at El-Sharif, Nabi El-Sharif, and look at Jagat Narula and thinking about longitudinal association and getting us to think about the fact that this might be intrahistian disease. But now, again, we understand that there's a complete disruption of left bundle. But I think that the way we've moved here is that the his has problems because of either lead stability, some evidence of microdislodgements, increasing thresholds, and there's a number needed to harm of around 10 or 15 for early generator replacement whenever there's high outputs. Right. Therefore, you get Weijin Wang coming in to save the day doing this intraceptal fixation and that intraceptal fixation is yielding very low thresholds and then able to conduct conduction to be able to circumvent conduction blocks that are a little bit more distal than the his. So whenever we had problems that were more in the left bundle, you know, after the branching bundle, then that's when you start having a lower rate of corrective his pacing. Right. That's what you show in your paper. And then left bundle correct really saves the day there. Right. I think that implant-wise, left bundle may be easier once people learn how to pierce the RV endocardium and get into the septum. It's actually quite easy to get to the LV endocardial layers. So I think that you're seeing a movement now towards left bundle pacing, but we need to understand left bundle pacing physiology better because when you think about it, that should yield a wide right bunduloid QRS. It's not as beautiful as the tight, narrow QRS that you get from his. And then therefore, the resynchronization that you get is predicated upon synchronizing with intrinsic AV delay or non-selectivity because you're actually getting fusion with the left bundle and then the septum. So let's go. It's kind of like WPW. Yeah. You're getting partial fusion. Let's go into what's going to happen. You and I both know what's going to happen there, regardless of what the rules are now in the studies, people are going to probably use this as a substitute for CRT, whether they should or shouldn't, we won't go into that part. But there are patients we both have that we see that have a QRS duration of 180 milliseconds. There's no way there's not disease in that myocardium. And even if you could pace a specialized conduction system, maybe that's not, if you're looking at CRT possibilities, maybe that's a person who would do better with a late area on the left ventricle. I'd like to get your thoughts on that because it's probably not one size fits all. So how do you look to the future? What you're thinking is you're going to subdivide the population. I think that we are going to now find the sweet spot for conduction system pacing. Can't be too narrow because the IVCDs have intact Purkinje activation. Can't help those patients. Can't be too wide because they're so myopathic, like even correction. So there probably will be this window that we find that's ideal for conduction system pacing. This needs a lot of collaboration, a lot of prospective registries, a lot of comparative randomized trials. And this is an area that we're kind of lagging behind. There's a lot of single center. There's a lot of multi-center retro. We really need to forge forward. And I'm pleased to announce the conspire registry that we're going to get off the ground. Conspire? Conspire. Oh, inspire. Conspire is to come together. You've actually named the study conspire. Conspire. We're going to conspire. Sort of like the root word for conspirators. Correct. Okay. Not in a negative way, but in a positive way. And this is going to be an international registry of a thousand patients prospectively at whatever discretionary indications that they're currently being used. It could be AV block, infranodal, intranodal, intrahistian, or for CRT indications as bailout. Yeah. But we're going to be studying a thousand prospectively to understand where the value may or may not be. That's a good start. That's a good start. It's not a substitute for what you and I both would want as a head to head, but at least it's some collection of data. Let me ask you just two quick things before we talk about the future. What do you anticipate the average life, the average generator life for left bundle pacing, for example? Well, in an ideal situation, we're not accepting anything over one and a half anymore. It was before it was, oh, we'll take two, we'll take three. We'll accept that for the synchronization. But now with left bundle, and I think that's why it's really swept and won a lot of hearts. I think that these thresholds that we're seeing are quite low. The durability needs to be shown. The future. Always end with the future, right? So where do you think we're headed as far as technology and things we'll be doing? I'm going to be bold. You're a bold person. It gives me permission to do so. Right bundle branch block is likely going to be best served with conduction system pacing with HISS pacing. Okay. But we need to show that. Right. IVCDs, tough population, no matter what. Yeah. Probably best served with standard by IV. And maybe new endocardial JAG scenes is going to present some YCRT. True left bundles will have to have a complex decision tree based on physiology, right? The ones that are true left bundle plus, I think are going to be better served with by V or what Pugal Vijayaraman calls hot CRT, which is HISS optimized by V. So the sweet spot is going to probably be conduction system pacing or standard by V. Congratulations all your wonderful work. I look forward to seeing your new stuff and I'm sure you'll get it done. Whether you conspire or not, I'm sure you'll get it done. Thanks for joining the show. Thank you so much. Always a pleasure to be on your show.

conduction system pacing

left bundle pacing

left bundle branch block

ECG findings

his bundle pacing