Dr. Joshua Cooper, Director, Cardiac Electrophysiology, Temple University Hospital, takes a deep dive into distinguishing Mobitz 1 from Mobitz 2 block, specifically on if a heart block is occurring in the AV node or in the His-Purkinje system. He also discusses exceptions in these cases.
Please also see Dr. Cooper's other two "telemetry tips" videos, including:
Telemetry Tips: Atrial Flutter and Atrial Tachycardia
Telemetry Tips: Electrical Artifacts
Telemetry Tips: Sinus Node Dysfunction This is Dr Joshua Cooper and this video on telemetry is going to talk about second degree A V block. We're going to review the Mobi one versus Mobi two pattern and get into much greater depth than one typically memorizes so that you can understand the reason we care about Mobi one versus Mobi two. And we're going to focus more on the physiology of whether the block is happening in the A V node or below the A V node. And why that matters? Let's first review what people typically memorize when they learn about Mobi one versus Mobi two A V block. Mobi 1 or winky box block is typically associated with a grouped beating pattern here, three beats, clustered together apr interval that prolongs up until the point where P wave fails to conduct and a narrow QR S morphology. That's altogether the typical findings of Mobi one or Waky Bach second degree block. On the other hand, mots two block is typically characterized by unpredictable AV block, meaning a non repeating pattern. APR interval that is the same throughout and a QR s that is wide. And again, you can memorize those factors and get it right most of the time. But we're going to talk about what each of these patterns means with regards to the heart's anatomy, what the clinical implications are. And that'll help you recognize the exceptions which certainly exist and are pretty common so that you can forecast whether somebody has a problem at the level of the A V node or below. Let's review how conduction occurs from atrium through to ventricle. There are two stages from atrium to ventricle in the normal heart. First signals have to get through the A V node. And second, they go down this rapid transit his Perini tree. These two components of A V conduction have very different electrical properties and they fail in very different ways. The A V node you can think of as the lazy slow part of a V conduction. Whereas the his Perini system is the lightning fast part. And in addition, because the his Perini system forms an electrical tree, the purpose of that tree is to get signals everywhere in the bottom chambers, the ventricles all at once to create a synchronous electrical stimulation of those chambers, which is reflected on the EKG and telemetry as a narrow Qs. Why do we care if the A V node fails? That typically gives us a Mobi one pattern. If the his Perini system fails, that typically gives us a Mobi two pattern. And what is the clinical significance of each if the A V node is failing, it typically does. So in a very gradual, predictable way. And if complete heart block occurs at the level of the A V node, usually the hiss bundle immediately below can jump in and do the work as an escape rhythm. And the person does not have a cysto or brady cardia that is too significant. In contrast, if somebody has disease in the his Perini system that can progress unpredictably and suddenly and there may not be anything more distal to the point of block that will serve as an adequate escape rhythm. The person may have prolonged a cyst or profound brady cardia that could happen at any time. So if we think somebody is having block in the A V node, it typically is not an emergency. And if the patient is asymptomatic, they may not even require treatment. On the other hand, if somebody is having Mobi to block his Perini disease level block, then that is an emergency and that person really should not leave medical care without an intervention, usually a pacemaker of some sort. Let's start by looking at this telemetry strip example. The first thing that strikes you when you look at it is that there's group to beating, there are groups of two ventricular complexes together, followed by little gaps between each little group. The next thing you can do is find the lead where the P waves are the most prominent and identify them all. Notice that in the top and the bottom strips, there are P waves that are hidden inside T waves, which is why I've selected the middle strip in order to highlight the location of each P wave. Next, we look at the QR S complexes here shown with blue lines. And then last, you can figure out which P wave conducts to which QR S I'm using a poorly drawn rope and knott to show that that first dotted P wave conducts to that first blue lined QR s and there's the next pairing between A P wave and a QR s. And then that third P wave does not have a QR s associated with it and so on and so forth down the line. You'll notice of course that there are some P waves that conduct and some P waves that do not conduct and that defines second degree A V block. And as a quick review, first degree A V block, which many people call first degree A V delay because there actually isn't any true block. All the P waves conduct. There is a 1 to 1 relationship between P waves and QR S complexes in first degree A V delay or first degree A V block and in third degree AV block, no P waves get through. The atria are doing one thing and the P waves march through on their own rhythm and the QR S complexes separately march through at a totally different and slower rhythm having absolutely nothing to do with the atrial activity. That's third degree AV block. So if we further look at the relationship between P waves and QR S complexes here, you can see that there is in fact, a prolonging pr interval from one beat to the next on the two adjacent beats that conduct, that defines this as we reviewed earlier as Mobi one or Waky block A V block. And as we said, that is block at the level of the A V node, not an emergency, not something that requires immediate attention, especially if the patient is asymptomatic. If you want to get fancy, you can call this 3 to 2 Waky box. And that refers to the ratio of P waves to Q RS complexes. There are three P waves for every two Qs complexes. Let's look at this example, which is a little bit less straightforward here, we don't have obvious grouped beating in a repeating pattern. There are groups of 12 or three at a time if we look at the pr interval. However, from one conducted P to QR S to the next, you'll see that the pr intervals are indeed prolonging as they were in the previous strip that actually defines this again as Mobi one or Waky block A V block. Also at the level of the A V node. The difference here is is that you're seeing different ratios at different times probably because of changing autonomic tone, which we'll review in a moment. Um And here you see, there's 4 to 3 waky box, 3 to 2 wank box and even 2 to 1 waky back on this same strip, but it's all Mobi one because the pr interval on adjacent beats is clearly prolonging again, that, that suggests that the block is happening in the A V node, not an emergency, not something that you would expect to progress suddenly and unpredictably here is yet another strip looking a little different from the previous two because you have fewer P waves that are blocked and again, not an obvious repeating pattern. The challenge on this strip is that if you start looking at adjacent pr intervals, you're going to see that some of them look identical. If you compare sequential beats, that's what starts to get confusing when you see this uh type of pattern of what's going to turn out to be waky block or Mobi one block as well. When you see longer runs of 1 to 1 conduction before a beat is dropped, the extension of the PR interval can get very subtly different from one beat to the next. But here is where you can find the waky box pattern becoming obvious. Look for the blocked P wave and compare the pr interval immediately before and immediately after. And you can see that you must have prolonged to get from that 200 millisecond pr all the way to the 366 millisecond pr interval. Um In each sort of grouping, obviously, the numbers may be a little bit different from one group to the next. But clearly, you had a shorter pr that progressed to a longer pr and it's much more obvious if you look at the longest and the shortest rather than sequential beats, which may not be as obvious in terms of their difference. If you also here, look at the Q RS complexes, you can see that they are narrow and that suggests that the his Perini tree is actually intact and working well. So when you put all these pieces together in this less common version of second degree block, it is also Mobi one. And the key here again is to compare the longest and the shortest pr intervals to show that in fact, it is changing. And the way to do that is to find the blocked P wave and look for the pr immediately before and immediately after and compare the two again. This is Mobi one block at the level of the A V node, not an emergency here is that same patient again, but with ambulation with more cats, and you can see that the second degree block has resolved. And now there is a 1 to 1 pattern between P waves and QR S complexes with no dropped beats whatsoever. This is yet another clue that we're dealing with a V node level problem because the A V node is under the influence of the autonomic nervous system, sympathetic tone increase, such as during ambulation, physical activity or giving intravenous catacholamines will improve a V node function even in the setting of second degree AV block. And if you decrease parasympathetic tone, of course, the parasympathetic nervous system, the vagus nerve has a suppressive or slowing effect on the AV node. If you eliminate that with intravenous atropine, then again, you will improve the AV node function. Conversely, if you increase the parasympathetic tone to the A V node, such as doing vagal maneuvers of valsalva or a carotid sinus massage maneuver, then you should worsen A V node function. If you decrease sympathetic tone, again, you will worsen A V node functions such as during rest or sleep or if beta blockers are given which block sympathetic tone to the A V node, there are maneuvers. Therefore, that one can do in the context of second degree block that can further help you clarify whether the block is happening in the A V node, which is under the influence of these autonomic nervous system components or in the his Perini system, which is not at all under the influence of sympathetic tone or parasympathetic tone. Here's another patient and another strip with second degree AV block. And let's review this strip in detail. First, we can see that there is grouped beating but not in a consistent pattern. And that may not be so helpful in distinguishing a V node level block from his Perini level block. But if we compare the pr intervals, we can see that when you find the block P wave as we did before and look at the pr interval immediately before and the pr interval immediately after there is now no difference between the two. This is different from what we saw before with Mobi one with Avio block. This is instead a Mobi two pattern. In addition, here, the QR S is wide suggesting that there's a problem when beets do conduct with the his Perini tree. So this is Mobi to a block. This is block in the his Perini system and it is something we worry about. It is something that is important. Even if the patient is not symptomatic, this is expected to progress at an unpredictable time frame and could leave you with a dangerously slow or absent heartbeat. So this is a patient who should not leave the medical setting without an intervention. Again, usually a pacemaker, let's think about the his Perini tree in a little bit more detail so that you can think about it the way an E P doc thinks about the his Perini tree I mentioned before that a normal QR S a narrow QR S results because the entire tree is functioning well and signals get down the entirety of the tree lightning fast to give electrical stimulation to the various parts of the ventricular muscle all at once. And the normal tree consists of the hiss bundle which is the trunk of the tree. The right and left bundle branches and the left bundle branch divides into two main sub branches, the left posterior facile and the left anterior facile. Many people therefore think of the his Perini tree as a trunk and two main branches. But E P docs actually think more about the sub branches of the left bundle. We instead think of the three branches, the right bundle branch being one, the posterior facile on the left side being the second and the anterior facile being the third. So we think about three fales rather than two bundle branches. When we're thinking about his Perini disease, you can have two out of three branches be faulty giving you in almost all circumstances, a wide QR S complex and that leaves you with only one remaining branch. So E P docs get a little nervous when we see an EKG or telemetry that has evidence of two out of the three fass not working and and second degree block that suggests that maybe that last branch is also not working intermittently suggestive of Mobi two or his Perini disease putting you into that dangerous, unpredictable category. Here are some examples of how that might look. So a left bundle branch block, which again, you might initially think, oh, that's just one branch that actually comprises two of the three fales. So just by having a left bundle branch block. Two out of three parts of the his Perini tree are not working. And that will give you, of course a wide QR s consistent with his Perini disease. You can have bifascicular block and there are two flavors. Here's one where you have the right bundle branch and the left anterior facile, both being blocked again, giving you a wide QR S worrisome again to the E P doc. And here's the other pattern where you have a right bundle branch block and a left posterior fascicular block. Again, two out of three fales not working giving you a wide QR s. And there's something called a non specific interventricular conduction delay. That's just a way of saying the curious is wide, but it's not in a perfect left bundle branch block or right bundle branch block pattern. And it probably means that there's various twigs in the his Perini tree that are not working well, giving you poor conduction down to the ventricles and poor synchrony, but not in an exact uh territory distribution that is seen in a typical bundle branch block. But all of these patterns signify his Perini disease and all of them create a wide QR S. That's why looking at the QR S with can be helpful when you're look, deciding whether the level of block is at the E V node or in the his Perini tree. Let's move on to 2 to 1 A V block because this can be very confusing. The reason that 2 to 1 A V block is not initially described as Mobi one or Mobi two is that you can find the P waves again, there are two P waves for each QR s but there's no real comparison to be made. You only have one pr interval and then the next beat is blocked and the preceding beat is blocked. So there's no way to compare that pr interval to its next door neighbor because there isn't one. So that really impedes our ability to determine whether somebody is having a V node level block or infra nodal block in the his Perini system. Let's look at some tricks to figure out how we can sort this out. One of them is to go on telemetry and find another place where the heart rate is different, hopefully faster because if the heart rate is faster, that suggests that you may have a pattern other than 2-1. So go click on that part of the telemetry and see if you can find two adjacent pr intervals that exist two beats in a row that conduct so that you can now compare pr interval before and after the blocked beat. Here's an example of a patient with 2-1 block. There are the P waves with red dots and we looked at a different part of telemetry and aha, we found a place where there are two beats in a row that conduct. So let's compare pr intervals. Here is the blocked P wave and the pr interval before and after the blocked P wave is identical. That is suggestive of a Mobi two type pattern. His Perini disease pattern, something that's worrisome. In addition, the QS here is wide suggesting that there is disease in the his Perini tree. Another clue and consistent with the Mobi two pattern to tell us that in fact, we're having block in the hyper kgi system, something that requires attention. Here's another patient example. Number two, where on telemetry, you have 2 to 1 A V block, there are the P waves, we go to a different part of the telemetry and we find somewhere where there is 3-2 conduction. So we have a basis for comparing different pr intervals. Here's the blocked P wave and now we look at the pr before and after and there is a significant difference. This is a Mobi one pattern. In addition, we look at the QR S width and it is narrow suggesting that the his Perini tree is working well. Another clue consistent with the fact that this is a V node level disease. The his Perini tree looks fine. The pattern is consistent with Mobi one A V node level block, not something that's immediately worrisome. Obviously, if the patient has symptoms related to bradycardia, then certainly we would treat that if there's no reversible cause such as a medication that's blocking the AV node. Then we treat this with a pacemaker for symptom relief. But this is not an emergency. Here is another third example of a patient with 2 - 1 AV block. There are the P waves and let's show something different. In this example, you look at telemetry and the patient's been lying in bed and this 2-1 block and you can't find anywhere to compare. So you can have that patient, you can manipulate the autonomic nervous system by having that patient get up and out of bed. Or if they're not able to emulate, have them do activities in bed by doing arm exercises or leg lifts or something of the sort to get their cats up. And if we do that in this patient, you're going to see much better A V conduction. In fact, if you go back to telemetry while you were in the room doing the leg lifts or the ambulation with the patient, you'll see that as the sinus rate was accelerating, signifying increased catacholamines to the sinus node simultaneously, those Cata Coles were improving AV conduction by affecting the AV node. So as the sinus rate increased, the AV conduction improved, that is a clue to suggest that we're dealing with a V node level block because it did improve with increased sympathetic tone. And if we on top of that, go back and look at a blocked P wave and compare the pr interval before and after we'll see that we had in fact, a Mobi one pattern because now we're no longer 2 to 1, we have 3 to 2 or 5 to 4. So we easily can compare pr intervals through the sequence or before and after the blocked P wave clearly a Mobi one pattern. But interestingly look at the Q here, it's wide. And so now you have a bit of a conflict or a discrepancy before the examples we looked at were all consistent. And the point here is you can have his Perini disease and simultaneous AV node problem with second degree AV block occurring at the level of the A V node, which we've demonstrated is true because of the pr interval comparison and the improvement with increased cate cola means. So this person would be expected to have a chronic wide QR S some type of a vesicular block or bundle branch block that is stable. But this person developed a V node problems and this pattern of second degree AV block. So you can have Mobi one with a wide QS. So I would put more stock in the pr interval comparison and the response to autonomic changes more than the QS width. Here's 1/4 example of a patient with 2-1 block. Again, here are the P waves and in this patient at, at rest in bed, the sinus rate is 86 beats per minute and there's 2 to 1 conduction. So the ventricular rate is 43 beats per minute. And in this patient, we also decided to take the patient for a walk again, you can exercise their limbs or get them out of bed. And here the sinus rate clearly increased. However, the ventricular rate slowed because now we have 3 to 1 A V conduction rather than 2 to 1. The conduction got worse in the context of cats. That is not suggestive of an A V node problem. You would have expected a V conduction to improve. So we're dealing instead with his Perini disease because increased Cics made conduction worse. But the question that you have in your mind is why is the QR S narrow? I thought you said that if you have hyper disease, then you almost always have a wide QR S because on the beats that do conduct some of the branches are not functioning well. The key word, there is almost there are exceptions to every rule. See if you can think where in the his Perini tree, you could have a problem with block occurring, causing a P wave with no QR s but conduction occurring down all the branches. And the answer of course is in the common trunk of the tree if you have his bundle disease alone. And this is by the way, something that can be seen in the context of an aortic valve surgery or a tar percutaneous aortic valve replacement, those procedures are done adjacent and near the hiss bundle. You can have only hiss bundle disease and intact conduction down all of the branches below the hiss bundle. And you can have a pattern that is consistent with Mobi two with his Perini disease. There's a reason that we say his Perini disease and not just Perini disease. You can see Mobi two block at the level of the his bundle, which is what this is. And we know that's true because Cics made the problem worse. And that's the opposite of what would have happened if this was a problem in the AV node. This is called intra his disease to E P docs and is also worrisome. This patient should not leave the hospital without a pacemaker or a very close observation because just like other Perini disease intra his disease can progress at an unpredictable pace and can leave the patient radic card or asystolic at an unpredictable period of time. Here's another mechanism of a V block actually in a hospital setting, commonly seen in an IC U. And let me highlight, first of all that, the top strip continues down to the bottom strip. It was just too long to kind of show it in one in one strip across the page. And let me highlight the P waves here because they're pretty low amplitude they're better seen on the lower of the two strips. Um There are two things that you should notice on this strip that are occurring simultaneously. Number one notice that the sinus rate is slowing the distance between the red dots. With each P wave, the distance is increasing little by little. And at the same time, notice the pr interval is prolonging. The sinus rate suggests something is happening to the sinus node which is up near the superior viva in the high lateral part of the right atrium. The pr interval suggests that there's a problem happening at the level of AV conduction. And with the pr prolonging the way it is, that's a typical Mobi one pattern, you're talking about the level of the AV node and that's down at the bottom of the septum at a very distinct, geographically different location. How can it be that at exactly the same time, the sinus node is having a problem and the A V node is having a problem. And the answer there is the common innervation of the two by the vagus nerve. This is a vasovagal event where you see the vagus nerve feeding into the sinus node causing it to slow down. And at the very same time, the pr interval is prolonging and then you get uh P waves that don't conduct at all second degree block. And so the hallmark of vagas a high vagal tone, a vasovagal event is a simultaneous sinus slowing and apr prolongation. Before you see block beats, you don't always see this. Sometimes you can see just sinus slowing or sinus pause or just A B block with very little change in the sinus node. That can be a little bit harder to diagnose. But when you see both happening simultaneously, you should very strongly consider a vasovagal etiology. These are usually self limited, usually do not require long term treatment. Although in patients are intubated or have had a neck injury or there's or an abdominal process, something that's causing repeated episodes, sometimes temporary pacing, or sometimes even permanent pacing is required. But again, each individual event is usually self limited and not life threatening. Again, it's important to remember that the vagus nerve autonomic tone affects both the sinus node and the A V node, but not the his Perini system. Let's do a few more prac practice examples to solidify our understanding of Avio level block and his Perini level block. Here's example, number one, take a look at the pr relationship, the group beating the QR S morphology and I'm going to create little checklist so that we can review the principles that we've discussed here. Let's talk more about a V node block versus his Perini block rather than Mobi one or Mobi two because really this is what we're getting at and here's a checklist at the bottom, right? With some of the principles that we have reviewed. So is there a grouped beating in this tracing? Not really, there's 326. So no repeating pattern. What about the pr interval pattern? If you look at a blocked P wave in the Pr before and after there's a dramatic difference, this is a Mobi one type pattern. And what about the QR S width that is narrow suggesting that the hyper Kinji tree appears to be intact. So here we have in total a couple clues that point to a V node level block and that's what's going on here. This is wey block with varying ratios between P wave and QR s here is practice example, two, let's go through our checklist. Is there grouped beating or a repeating pattern here? No, there is not. Um what is the pr interval pattern? Let's find a blocked P wave and compare the PR interval before and after they are the same that suggests Mobi two pattern. And what about the QR S width? It's wide suggesting there's a problem with the his Perini tree. So here we have two out of three features very strongly suggesting that we're having a problem in the his Perini system. This is somebody that you want to pay attention to because you're worried about progression, you're worried about asystole. In fact, if we follow up this patient who we predicted had Mobi two block, you will see that they subsequently progressed to higher level block when we see two or more P waves in a row that don't conduct the term that most people will use to describe that is high grade AV block. That really means two P waves in a row or more that fail to conduct. So, this person clearly was in trouble as we had predicted because we thought this patient had his Perini disease and it looks like we were right. If you look at the telemetry over time, you will see that there are these pauses which sort of increased over time and then you'll look at the heart rate trend. This is where the pauses are occurring, where you're having a heart rate of zero temporarily and coming back up and then you plateau at this rate in the low thirties. What happened there? Let's review by clicking on that part of the telemetry and looking at what is the heart rhythm during that time? Here are the P waves in red dots. Here are the QR S complexes with blue lines. The R R intervals are no longer varying. They are absolutely bang on regular and they have nothing to do with the atrial activity. We have progressed to complete heart block. The atria are doing one thing, the ventricles are doing another thing slower. And the hallmark of this is that when you put calipers on the QR S complexes, they are exactly the same. That's because there is no conduction occurring. There is a new, thankfully a new pacemaker spot that has woken up and is uh causing the QR S beats to happen and they don't know anything about what's going on up top because AV conduction is completely blocked. So this person had Mobi to block. It progressed to high grade block and then progressed to complete heart block. That's exactly what our fear was. Thankfully, this patient was in a hospital setting. And thankfully, this patient did not have asystole for a prolonged period of time, but a junction rhythm kicked in. And of course, this patient in short order got a pacemaker. Here's another practice example, let's see in our checklist. If there's grouped beating at first, it may not look like there is. But if you look very carefully and especially if you use calipers, you're going to notice that there's an alternating pattern of shorter and longer QR S in intervals. So there actually is grouped beating groups of two. It's just that the space between the groups and the groups themselves are actually not too different in their cycle length. So it may not have been immediately obvious to the eye. But if you measure it with the calipers, it becomes clear. So yes, there's grouped beating. Let's look at the pr interval pattern. We found a blocked P wave and the pr interval before and after is extraordinarily different. This is a Mobi one type pattern and let's look at the QR S width. It is narrow on the wide side of narrow, but nonetheless, it's less than 120 milliseconds. It's narrow. So actually, we have all three checklist items consistent with a V node level block which is what's going on here. Not a dangerous pattern, not something we're worried about. You would assess symptoms, see if there are any reversible causes, but not somebody that immediately needs attention or pacing. Let's follow up on this patient uh in the hospital setting, this patient uh got out of bed at times and here you can see that the heart rate actually picked up at those times. And if you put your cursor, you click on the telemetry at those times, you can see that you have actually improved from second degree A V block to first degree A V block where there's a 1 to 1 relationship and no blocked P waves. The pr interval is actually quite long. Uh But in fact, you have a 1 to 1 conduction pattern. We've switched or improved uh from Mobi uh one second degree block to first degree A V block or absence of block whatsoever. This is consistent also with a V node level block. So on top of our checklist that we had three out of three items pointing to the A V node, we actually now have 1/4 checklist item, which is that Cata Cola means improved conduction further solidifying our assessment that this is block at the level of the A V node, not something that we worry about in terms of safety. So let's review the points from this telemetry talk on second degree AV block so that you can solidify in your mind how to differentiate block at the level of the AV node from block. In the His Perini system. We discuss that the Mobi one pattern usually suggests that block is at the A V node level. And Mobi two pattern usually means there's block in the His Perini system. We discussed the 2 to 1 AV block could be either in the A V node or in the His Perini system. And let's flesh each of these out in a little more detail. Remember that Mobi one, the Hallmark is that if you find the P wave that's blocked and you compare the PR interval before versus after, you'll see a significant difference. Now I put here by at least 60 milliseconds because another exception that we didn't talk about is that you can see very subtle difference in the longest versus the shortest pr interval of 60 milliseconds or less, which I guess officially is a waky box pattern, but very, very subtle waky box. You can see that in the his Perini system, that's one little box or 1.5 little box is on an EKG or telemetry difference between the longest and the shortest. If you see that, then actually it may be his Perini disease and it would fall into the Mobi two category if something worrisome. But if you see a longer, bigger difference between the longest and the shortest pr interval, that pretty much solidifies that you're dealing with a V node level block and usually, but not always, the QR S is narrow because usually you have a problem in the AV node and a normal his Perini tree. Although of course, we did review examples where there was a problem with one or more fales and simultaneous A V node level block Mobi two is blocking the his Perini tree. And again, the reason that we worry is that that can be a sudden unpredictable progression to complete heart block and you may not have an escape rhythm. You may have syncopy, you may have a life threatening situation with asystole. So you need to identify this reliably and make sure the patient gets the proper attention from the E P team. Usually in Mobi two, you have a wide QR S except for that one example where you can see intra his disease only where you can see a Mobi two pattern with a narrow QR S. It's important to recognize it's rare but it happens. And one of the most common scenarios where, where we see this is an aortic valve disease with aortic valve procedure done either per cutaneous tar or aortic valve, uh surgery with regards to 2 to 1 A V block. Look at other parts of the telemetry for clues areas where there is not 2 to 1 A V block where there is 3 to 2 or areas where you can compare pr intervals in sequence or before and after the blocked P wave and you can do autonomic manipulation, either with a medication or withdrawing a medication or most easily by emulating the patient or having them do exercise to temporarily increase CICS. And see if the block improves if conduction improves, that would suggest that it's an AV node level problem, not worrisome or if it worsens the AV conduction which suggests hyper canI disease, which is worrisome and should require immediate attention. One point that I didn't really make, but I'll make now is that we can always confirm the diagnosis of A V node level block or his Perini level block. In the E P lab. We can do an E P study and put catheters up inside the heart very easily and we can record atrial signals, ventricular signals, his bundle signals Perini signals. And we can absolutely determine with certainty whether we're dealing with a V node problem or his Perini problem. And of course, implement the appropriate treatment. So don't hesitate to ask us even if it doesn't involve an E P study, we'd be more than happy on the E P end to help you figure out if somebody is having second degree block, whether we think it's at the level of the A V node, and we don't really need to worry right now or if it's in the hyper system where this is a safety issue. And in fact, we do need to intervene or at least confirm in fact that we're dealing with hyper canI disease so that we can do something about it, monitor the patient closely and implant a pacemaker if that's appropriate for that particular patient. Remember that in a hospital setting, patients can have various reasons for high vagal tone, including at night during sleep and including in the IC U where throat stimulation occurs very frequently and patients are intubated. So patients who have sleep apnea events uh very frequently will have vasovagal events, even high grade block. But the key there is to look for sinus node, slowing and pr prolongation and A V block occurring simultaneously in the IC U setting. If the patient is turned or suctioned, that is another common time that you can see vasovagal events. But the A V block pattern almost always here is pretty obvious you have pr prolongation, which is suggestive of block at the level of the A V node because the vagus nerve innervates the A V node but not the hyper system. These events are usually self limited, do not uh individually require attention. Although if they're very frequent and they're interfering with the patient's care. Uh then sometimes uh a temporary pacing system is placed or even permanent pacing. It depends on the specific patient situation. What the impact of these events are, how frequent they are, uh et cetera. But it is important to obviously make the correct diagnosis so that you can figure out what is the urgency and what might be the appropriate treatment. I hope overall, this presentation was helpful in uh distinguishing Mobi one from Mobi two, understanding what the implications of each are so that you can think more about the anatomy, the A V node versus the hyper system and what the implications would be clinically. Get E P involved. Happy to educate and teach and certainly happy to take care of the patient if you think they require our attention. Thank you so much.