hello good evening good evening who is it nithina Yes, sir. We can hear you, sir. It's all clear. You can share your screen also sir as well now. Shortly.
Video is not visible. Video. Yeah, yeah.
Just give me a little. Online lecture. Hello. Good evening sir.
How are you sir? Good, good. Thank you.
So we'll just wait for a minute and start. Yeah, sure. Yeah, I can share my screen.
Sure, sir. The screen content is visible? Yes, yes, sir. It's visible. You can just put it on full screen and we'll just wait a minute and then we can start, sir.
It's on full screen already. Yeah, so I just introduced her, just a quick intro and then we can kind of start. So good evening everyone for joining in the second session of the program, which is the Essentials of Cardiac Pacing with Dr. Kumar Narayanan.
So we had the first session on the last week on the 6th, which was on basic pacemaker concept pacing sensing. threshold and impedance today dr kumar is going to talk about pacemaker timing cycles we have a couple of more sessions following week and the week after that to conclude on 27th of june and for the radio requested dr kumar to start today's session thank you sir for taking time out of your busy schedule and looking forward to having a great session today thank you sridhar so I hope all of you are able to see my screen and hear me clearly. So last session we covered the including pacing, sensing, threshold etc.
Today we have next important topic which is pacemaker timing cycles. So again here this topic is important for two reasons. One is you need to understand the various annotations which you see on a programmer.
whenever you're interrogating a pacemaker and number two you need to be able to understand certain pacemaker behaviors as to you know why a pacemaker is is kind of behaving in a certain way or why it is pacing at certain times or why it's not pacing at certain times so if you understand the timing cycles then you will be able to understand the way a particular tracing is looking and whether this is normal pacemaker function or abnormal pacemaker functions. That's something you need to troubleshoot. So what exactly are pacemaker timing cycles?
They are a set of rules you can think of which govern the timing of events or you can think of it as a beat behavior of a pacemaker in response to changes which are happening which are either intrinsic events or paced events. Essentially, what can a pacemaker do when we say that it is responding to changes in events? It can either pace or it can inhibit itself from pacing.
Really, these are the only two things it can do. So in other words, if you want to put it simply, you can say that the timing cycle made by the pacemaker has to went to pace based on beat to beat intervals and certain rules which have been given to the pacemaker. So these are essentially what are timing cycles.
Now one important difference which you need to understand right at the outset is that we are accustomed to thinking in terms of heart rate in beats per minute whereas the pacemaker really is seeing events in terms of intervals. So an interval between two events and you know considering that your heart rates are typically in the range of 60 to 100 beats per minute the beat to beat interval is more conveniently measured in milliseconds rather than fractions of a second so this is important the pacemaker has to make a decision from beat to beat it doesn't it's not going to look at the whole picture over one minute and then wait and decide that's not the way things work because we need to be making real-time decisions on a beat to beat basis So that it is going to do based on intervals. And how will you convert from intervals to rates per minute? It's interval in milliseconds because minute contains 60 seconds, which is equal to 60,000 milliseconds.
So this distinction you need to keep in mind. Two periods which we talked about briefly in the previous session, the blanking and the refractory period. These are pretty important in governing timing cycles, so we'll just define it again. So what is a blanking period? After every sensed event, we talked about sensing in the last session, every intrinsic event is going to be sensed by the pacemaker.
And after every sensed event, the pacemaker turns off the sense amplifier. It suspends sensing completely for a very short period of time. So that any event occurring during...
that period of time is completely not seen by the pacemaker that is a blanking period. Okay, the important the function generally of the blanking period is that the same event shouldn't be sensed twice if an R wave has occurred in the ventricle it should not see the same R wave twice because every depolarization has a certain duration to it. senses and then no more after that. So, it basically blanks itself as soon as the event is sensed.
Secondly, pacing spikes which are given by the pacemaker are high voltage compared to the intrinsic events. So, that high voltage should also not cause a spurious sensing by the pacemaker. So, for these two functions, it utilizes the blanking period where the sensing amplifier is completely blank.
The next period is the refractory period which typically immediately follows the blanking period. Here the sense amplifier is enabled, so the pacemaker has its eyes open, but it does not take this to alter the timing cycle. So sensed events do not alter the timing cycles in the refractory period.
So you need to keep these two in mind as these are going to be recurring themes. So let's start with a simple one which is single chamber timing cycles. So You know the NBG codes for pacemaker where you typically use three or five letters to talk about the type of a pacemaker. So the first letter basically talks about the chamber which is spaced, the second letter talks about the chamber being sensed and the third letter talks about the response to sensing. So AAI means it paces the atrium, it senses the atrium and inhibits itself in response to it.
So, most of the time the single chambers will be the inhibitory response. So, similarly VVI which is the most common term which you would have all heard paces the ventricle, senses the ventricle, inhibits itself in response to a sensitive. So, these are pacemakers which have a single lead either in the atrium or in the ventricle and they are timing cycles.
VBI is the most common implanted pacemaker which you will come across. So let's start with that. So what are some definitions which are going to be there across both single and dual chamber pacemakers? The most important, the most basic one is the lower rate interval.
So in other words, this is the basic rate which you are programming. So, the lower rate is what you program as the lowest rate which is allowable before the pacemaker would pace. So, you will be familiar with the lower rate of a pacemaker being defined as 60 beats or 70 beats. But as I said, the way the pacemaker is going to look at this is in terms of the lower rate interval. So, suppose you set a lower rate of, RR does it correspond to?
It basically corresponds to an RR interval of 1000 milliseconds. So this is what the pacemaker is going to be looking at. That is the lower rate interval for the pacemaker, 1000 milliseconds.
And that is the longest time the pacemaker will allow before it delivers a pacing spike. It also will be known as the ventricular escape interval if it's a VVI pacemaker or an atrial escape interval if it's the AAI pacemaker. So any sensed or paced event in the chamber.
will initiate the lower rate interval. Therefore, at the end of the lower rate interval, if nothing happens, if there is no intrinsic activity, the pacemaker will deliver a pacing spot. So, here you have the VBI mode where the pacing is inhibited with intrinsic activity. Vent has occurred, there is a paced ventricular event, the lower rate interval is initiated and end of this lower rate interval if nothing had happened it would have paced here.
Now what happened here? The patient had an own intrinsic sensed event a Vs and therefore the lower rate interval is cut short here and a new lower rate interval is initiated. At the end of this lower rate interval nothing happened and therefore the triple paced.
Now note here that you have the blanking and the refractory periods which we talked about. As soon as the ventricular event happens, there is this box within the box, which is the ventricular blanking. So that the pacing spike or the same QRS is not sensed again.
If it does so, then it will keep initiating lower rate intervals within a short period of time. And then following that, there is a ventricular refractory period. Now, what is the function of the ventricular refractory period? It's typically meant to avoid T-wave oversensing. Remember sometimes the T waves can be pretty large and last time we talked about different issues like the sensitivity threshold and the frequency content which help avoid basements.
But despite all that it can happen. So here you have one more protection in the form of the ventricular refractory period. So there is a prominent P wave here and it is enough to reach the sensing threshold but if it falls within the ventricular refractory period it will not initiate a lower rate of trouble. Then after the V-pace, you have the black, which is the ventricular blanking, the red, which is the ventricular refraction.
Look how the ventricular refractory is kind of covering the location of the T-wave. And then the low rate interval expires, nothing happens. So the pacemaker paced again, again initiates a low blanking refractory period, low rate interval.
Then at the expiration of the low rate interval paces again. And then here, this low rate interval is cut short. by a ventricular sensitive event. So that is annotated as Vs and from here again after Vs event you have the initiation of a blanking and a refractory period and then you have the lower rate interval initiated again at the expiration of the lower rate interval it faces again and so forth. So this is essentially your Vbi timing.
Now let us look at another step. So this is the same you have v-pace v-pace and you have an intrinsic event v-sense and then here after v-pace let us say you have some kind of a noise or you have a prominent t-wave now what's going to happen here it's falling within the ventricular refractory period and it says vr so that is how it designates a sensed event with the refractory periods going to use the term r so there is a vr so will it alter the lower rate interval will it start a new lower rate interval no not The lower rate interval continues uninterrupted. But note what happens here. This is the reason why during the refractory period, the sensor is still enabled.
It wants to see. Unlike the blanking period, it's not completely turned off because it has two functions. Number one, an event sensed within the refractory period, VR, will initiate another blanking and refractory period.
So the pacemaker says, I saw something in my refractory period. it is potentially undesirable, maybe it is noise or something, I am going to again initiate, I am not going to change my basic timing cycle, but I am going to initiate another blanking and refractory so that if there is further noise, then again that is going to fall within a refractory period. So that is one function of why the sensor is still enabled during a refractory period.
The other is for the purpose of sensing tachyarrhythmias which we will see at a later point of. time. So again it does not alter the lower rate interval.
At the end of the lower rate interval it faces. If this had not fallen within the refractory period then here it would have sensed an inappropriately initiated a lower rate interval which is what. Now look at this patient the VP and then the blanking in the refractory is initiated.
Unfortunately a very prominent T wave falls just outside the ventricular refractory period. Therefore it ends up sensing it and inappropriately re-initiating the lower rate interval at that point of time and then it has to wait till that expires before it paces. So if you take this VV interval, it will be longer than the lower rate interval and you'll find the pacemaker is pacing even slower than the set lower rate and then when you see such a tracing and you're thinking what's going on, one of the things you must suspect is some form of oversensing. If you interrogate the pacemaker and then you look at this marker designation which every pacemaker will always provide you, you will understand that a Vs is happening corresponding to the T wave consistently and you can understand that this is a case of T wave oversensing.
Look at the importance of every refractory event reinitiating another refractory period. Here let's see there is some kind of a cyclical noise which is going on. It falls within the ventricular refractory period. Then it initiates one more refractory period.
Then the noise repeats but due to the extension of the refractory period it again falls within the refractory period. And then all of these noise events couldn't affect the lower rate interval and the next ventricular pacing comes on time. Similarly even though there is cyclical noise going on it doesn't affect the pacing. The only problem with this is sometimes here you have a noise but here if you had a genuine r wave or a genuine depolarization that will not be sensed at all. So repetitive noise such as this the extension of the ventricular refractory period with each of these can be potentially life-saving in a person who is pacing dependent and has no intrinsic activity so the pacing will not have this essentially becoming asynchronous spacing that the pacemaker is absolutely not responding to any sensed events because it is keeping on extending its refractory periods and that can be disadvantageous when the noise is intermittent and there are some genuine sensed events at some points of time which the pacemaker will not be able to look at.
So essentially asynchronous spacing means VOO, it paces the ventricle, doesn't see, doesn't sense, And obviously, when it does not sense, there is no question of a response to sensing. So, this is the VOO mode where sensing is suspended. So, this is a programmable mode. Here, essentially become asynchronous because of repetitive noise. This is a case where it is programmed in a VOO mode where there is no sensing going on.
And this VOO mode, you can see what is happening is that there is ventricular pacing. lower rate interval ventricular pacing initiates the lower rate interval it doesn't care if any sensitive event happens there is no sensing at the expiration of the lower rate interval it paced here the spacing spike did not capture because the ventricular just depolarized it is the myocardial refractory period and then again a lower rate interval did not take the sensitive event into account then kept pacing now this can sometimes the VO can be sometimes risky in a patient with own activity like that because here the pacing spike did not manage to capture. If this does manage to capture, sometimes you can have an R on T and you can trigger an arrhythmia. So a VOO mode is used mainly in situations where some kind of an interference is expected and the patient is completely patient independent.
For example, if the patient is going for a surgery where someone is going to use long cautery very close to the device. you know the patient has no or very poor intrinsic rhythm, you would then want to program the pacemaker in an asynchronous or a VOO mode. On the other hand, if the patient has a very good escape, then you do not really need to use this mode and you can sometimes end up having a little bit of risk and the patient has his own intrinsic. Now, what about AAI? It is essentially the same.
Here the only difference is we are talking about the atrial lower rate interval and the atrial escape interval. So you have any atrial event which is going to initiate the lower rate interval. Let us say for example it is 1000 milliseconds here.
At the expiration of the 1000 milliseconds it is going to pace and then there is no ventricular lead here. It is AAI. So the patient has his own conduction. Here again the lower rate atrial interval is initiated. It is cut short by a sensed P wave at 800 milliseconds and therefore another low rate interval is initiated and so forth.
ventricle, the atrium is also going to have its blanking and refractory periods immediately after an atrial sensed or atrial paced event. The issues which can happen with an atrial bead is that the ventricle is a big chamber with a lot of myocardium. So obviously, ventricular depolarization signals are much bigger and bulkier.
On the other hand, the atrial depolarization is of a smaller magnitude. Therefore, in order to ensure that atrial events are sensed appropriately, you need to keep the, program the atrial sensitivity. The atrial lead is typically programmed in a more sensitive fashion than the ventricle.
And due to this, one of the issues which can happen is that sometimes it can end up sensing the big ventricular depolarization, which happens in the ventricle. And here you can see, wherever there is the star. The AA interval is longer. Look at this AA interval. It's shorter and corresponding to the appropriately programmed lower rate interval.
Here the AA interval is a little longer and if you looked at the pacemaker annotation, it would probably say AP and then here it would say AS corresponding to this R wave. So there is an inappropriate sensing of the far-field R wave due to which the lower rate interval is getting reset here and then it it is facing only after the expiration of the low rate interval. So, you should remember that the AEI timing cycle depends upon the A and not the ventricular events. Now why does this oversensing happen? We said one reason that the R wave is a much bigger depolarization.
But that the atrial lead in our mind we would think that it is very remote from the ventricle, right? You have put it high up in the atrium, so why should it face the ventricle? Actually if you look at it anatomically, it is not that remote because if you recall the anatomy, the atrial appendage often over lies the RVOT.
So we will be thinking of in terms of you know HL being somewhere here and eventually below but if you look at it anatomically the HL appendage where the lead is typically placed often is directly lying over the RV. So it's not very surprising and you will in fact if you look at many of the HL leaves you will find a little bit of an R wave almost seen. many cases and this is the anatomical reason for that.
Alright, so we finished this slightly easy part. Let us go to dual chamber timing. So here you have two leads, one in the atrium and one in the ventricle.
So we said already that in AAI or BVI, we have to pace the chamber, sense the particular event and the response is generally only one. In response to a sensed event, the pacemaker has to innovate itself. there is little complexity added to it.
There is pacing going on and sensing going on in both the atrium and the ventricle. But what is the response to sensing? What does DDD mean?
D is dual. So both chambers are paced, both chambers are sensed. But we are using the third D to say that there is a dual response. So what exactly is this dual response?
The first response is that there should be inhibition of pacing. by sensed events in the same chamber. Atrium when it senses an atrial event should inhibit, ventricle when it senses a ventricular event should inhibit itself.
So, pacing in the same chamber is inhibited by sensed events in the same chamber. But a dual response is that a sensed event in the atrium should then lead to pacing in the ventricle if required. This is what is AV coordinated pacing. This is the main advantage of a dual chamber pacemaker as opposed to a VVI single chamber pacemaker. You're going to achieve AV coordination or AV synchronicity.
A sensed event, P wave should inhibit atrial pacing, R wave should inhibit ventricular pacing. But an atrial paced or sensed event should be tracked. into the ventricle which means it should be followed by a ventricular paced output in a particular defined time interval to achieve AV track AV synchrony.
So there are four possible situations in DDD pacing. You can have atrial and ventricular pace where basically there is no activity in the atrium, the pacemaker is having to pace, there is no intrinsic ventricular activity so after a certain time delay the ventricle has to be paced. You can have an atrial pace but so there is a ventricular sense this is AP and VS. You can and ventricular pace where the sinus is doing good, the atrium is depolarizing at a regular rate the atrium does not need to pace but AV conduction is not good and therefore the rate the ventricle has to be paced after a certain delay.
This is tracking the sensed atrial event or the sinus driven rate is being tracked into the ventricle. And then lastly, situation where you are having both the atrium and the ventricle having their own activity, then you are having the AS and BS. So, in a patient with intermittent AV block, you can have a proportion of time where the own intrinsic activity is going on and the pacemaker has to is appropriately inhibiting itself.
Alright, so there are more timing parameters for DDD as compared to single chamber. So the lower rate and lower rate interval is fundamental. It applies to the dual chamber situation as well.
You need a particular lower rate below which the pacemaker will not allow the rate to fall. Alright, then you have two other intervals. You have an AV interval which is the basically from the A to the V and then you have a VA interval the other way around from the V to the A.
So we'll see how that works. Then you have something other than the lower rate. per rate interval, we will see what that is. Then we have already seen blanking and refractory periods, but there are a few more in a dual chamber pacemaker compared to.
So let us take these one by one. So what is the AV delay? The AV delay is basically the maximum time after an atrial event which the pacemaker will allow before delivery of ventricular pacing.
So this is a programmable value and there is, there is This is a pretty important value where you have to program it taking into account various issues. So you can have the AV interval being initiated after either a sensed atrial event, which is called sensed AV delay, or after a paced atrial event, which is called the paced AV delay. AV interval is an important thing so you can achieve optimal AV coordination. If your AV delay is too long, then what's going to happen is that you're... next P wave is going to come pretty soon in the ventricular filling cycle.
So if you keep the ECG in your mind, you have your P and your QRS. If your AV delay is too long, then after that QRS, the next P wave is going to come relatively sooner in diastole and this is going to cause impaired ventricular filling. On the other hand, if you keep too short an AV delay, you will find that you do not give any opportunity for intrinsic conduction.
to come through. And this we will see in later sessions when we talk about some pacemaker algorithms is important because you do not want to unnecessarily pace the ventricle. Unnecessarily over pacing the ventricle is deleterious. You really want to pace, you really do not want to pace in case intrinsic conduction is there.
So you have to achieve the balance. So then we talked about paced versus sensed AB delay. Are they going to be the same value? No. The sensed AV delay is typically programmed shorter than the paced AV delay.
And why is that? There are two reasons for this. Number one, if you look at the fact of a sensed AV delay, the depolarization is coming from the sinus node. The atrial lead is maybe sitting in the right atrial appendage or it's sitting in the right atrial lateral wall or somewhere.
It will be a short duration before the intrinsic depolarization. reaches the atrial lead. So that will cause the sensing to be a little delayed. The other reason why the sensed AV delay is kept shorter, on the other hand if you take the right from the atrial lead of the pacemaker. So the pacemaker straight away knows as soon as there is a paced event that the paced event has happened.
So it's very efficient and immediately recognizable. Secondly, when an intrinsic atrial depolarization happens, just like the His-Burkinje system is the best and the quickest way of depolarizing the ventricle, the intrinsic atrial activity from the sinus node is also the quickest and the most efficient way to depolarize the atrium. And this is especially important with respect to left atrial depolarization. So if you think about it, the sinoatrial node is sitting in the supra lateral right atrium. It's in one corner.
The usual sinus activity, would there be a big delay between right atrial and left atrial activation? It turns out that that is not the case because there are specific specialized intra interatrial conduction fibers, which, which depolarize conduct. extremely rapidly to the left atrium.
So these on the roof of the atrium which is known as the Buckman's bundle and the presence of this rapidly conducting track ensures that the left atrium is depolarized with a short delay after the right atrium or the delay is minimal and this is important otherwise there will be a discrepancy between the right-sided AV synchrony and the left-sided AV synchrony. So that kind of an efficient activation is taken care of when there is intrinsic atrial depolarization. So you need to have only a short AV delay by which time the atrium is nicely, all the atrium is depolarized. On the other hand, if you are pacing from an alternate site in the atrium, it has to go and slowly recruit these tracks which will take a little more time.
So for both of these reasons, you need to give a little longer paced AV delay as compared to a shorter sensed AV delay. It's just like in the normal physiologic situation of the conduction system. The AV delay cannot stay fixed at all rates. Now you yourself have seen many ECGs will find that if the heart rate is low, the AV delay will be a little longer.
The heart rate is faster as when you're exercising or for whatever physiologic reasons, the PR interval is going to progressively correspondingly shorten. And you need to move this physiology and therefore most modern pacemakers incorporate what is known as a rate adaptive or dynamic AV delay. Now again all of these are programmable and we will see what are the various parameters which programming of the AV delay can affect. We talk about is the VA interval, this is also known as the atrial escape interval. So here we said that after an atrial or a paced, sensed or a paced event a certain interval is initiated the AV interval at the expiry of that AV interval the ventricle will be paced.
As soon as a ventricular paced or sensed event happens, another interval called the VA interval is initiated. And if nothing happens by the expiry of the VA interval, the next atrial pacing is the atrial escape interval. So how does the pacemaker calculate the VA interval?
It basically takes the lower rate interval, for example, if it's 1000 milliseconds, and it's sub the Av delay from the lower rate interval. So, here if you take 1000, you subtract Av interval 200 milliseconds and what is remaining 800 milliseconds is the Va interval. So, Av interval plus Va interval equals the lower rate interval. So, basically here we already said that Anand is basically going to start a new Va interval. Here in a dual chamber pacemaker when we are talking about the VA interval, you can have two different types of timing.
But you can count the lower rate interval either of the ventricle or of the atrium. So it is ventricular based timing or atrial based timing. So in a dual chamber, you have this option.
So what is the difference? So without making it too complicated, if you are having ventricular the V-V interval can never fall below the lower rate interval. If it is 1000 milliseconds, the V-2V cannot be longer than 1000 milliseconds.
It is counting the lower rate of the V. If you are having an atrial based timing, then the AA interval is what is going to be sacred. And depending on the AV delay, the V-V interval can be slightly longer than the AA interval. So, how does the pacemaker achieve this? In a ventricular base timing, the VA is fixed and if a V occurs interval, the VA interval gets reset and another VA interval gets started.
So, the V event during the VA interval is important. But on the other hand, if I hit the VA interval, then the AV interval is triggered and at the end of the AV interval, the ventricle will have to be paced. So, therefore, the ventricular rate can not become slower than the lower rate interval. Whereas on the atrial base timing, the AA lower rate interval is fixed. Therefore, if an R occurs during the AV interval, the AA will still not be affected.
The AA will continue to run and at the expiration of the lower rate interval, the atrium will also be faced. Example of a V based timing. You can see that the RR is pretty much maintained at 1000 and occasionally based on the occurrence of an intrinsic R ahead of time, you can have a slight shortening of the RR interval. So it can be a little shorter but not longer than the lower region. Whereas in an atrial based timing, what it is looking at is the A to A.
That's not and here for example, when the PR interval is pretty much fixed at 160, it So fortuitously becomes 1000. But here for example, if you have lack of a R wave and then there is a little longer AB milliseconds and the ventricle has to be paced, the RR can be slightly longer because what the thing is the AA interval and because of a slight delay in the here the AB delay, you can have a slightly longer RR. So just by looking at the RR interval, in a dual chamber phase sneaker you can make out whether the pacemaker is following an A based or a B based. So let us talk about the blanking and refraction.
Why do you need more periods in a dual chamber pacemaker? This is basically to prevent something known as crosstalk. So about a small example of crosstalk when we talked about the AAI pacemaker in a problem.
sensing the par-fit are. In a dual chamber pacemaker, both the issues are possible. You can have an atrial lead which is inappropriately sensing events from the opposite chamber, that is the ventricular chamber.
Less commonly, but sometimes possible, you can have the ventricle sensing inappropriately, for example, the atrial facing spike or atrial depolarization. Now if such an oversensing happens, it leads to inappropriate pacing inhibition in that particular chamber and therefore in addition to same chamber blanking and refractory periods, you also need to have cross chamber blanking and refractory periods in a dual chamber pacemaker. So let's look at the ventricular channel in a dual chamber pacemaker, right.
So this bar is totally talking only about the refractory periods in the ventricular channel. Now, let us focus on this part first. We already know that a ventricular paced or sensitive event will initiate a ventricular refractory period.
And actually the first part of this should be black, which is going to be the ventricular blanking period. So, a ventricular blanking period and a ventricular refractory period, this is the same as a single chamber pacemaker. In addition to that, here after an atrial spike having a post atrial ventricular blanking, PAVB.
Because it does not want to falsely see this big pacing spike in the atrium, the ventricular channel is also closing its eyes briefly when an atrial pacing spike is delivered. So, this is the post atrial ventricular blanking. And here there is something called a crosstalk sensing window.
We will not talk about this today. We will talk about it at a later session. And then you have the here in this part you have the sensing window or the ventricular to be able to detect a intrinsic R wave so that then the ventricular pacing inhibited.
So, here you have ventricular blanking, ventricular refractory period, but you also have a post atrial ventricular blanking. It is a little one more period is there in the A. So, here we know from the single chamber after an atrial pacing or a sensed event you are going to have an atrial blanking period that is there. But in addition to that after a ventriculars postventricular atrial blanking, EV, AV. So, there you had post atrial ventricular.
Here you have postventricular atrial blanking. And here, following that, you also have something known as postventricular atrial refractory period or PVAR. Now, the function of the postventricular atrial blanking is fairly straightforward.
The ventricular facing spike, the function of the PVAR for the postventricular atrial refractory period will be. We need one which is to prevent what is known as retrograde P waves. We will talk about this. No interval to finish off with the dual chamber intervals is the upper rate or the max or the low rate interval and if it falls if the low rate interval is exceeded pacemaker is going to peace. The distinction we said in a dual chamber pacemaker is that you need AV tracking.
Sensed events of paced events in the atrium should be tracked with the appropriate AV delay to the ventricle. But do you want to do this indefinitely? If the atrial rates keep keeps going higher and higher and higher, do you want to keep on tracking it to the ventricle?
You need to draw a limit. The most obvious example is if an atrial arrhythmia has happened, the atrium is inappropriately going at an arrhythmia of say 160, 170 beats per minute, you do not want to then pays the atrium at such an inappropriately fast rate. So you need to set a limit, which is basically the upper rate. or the maximum tracking rate. The fastest rate at which the atrial activity will be tracked one is to one the ventricle that is the upper tracking rate or the maximum tracking rate.
Corresponding to that you will have an upper rate interval. The upper tracking rate is 120 beats per minute just as an example so 60,000 by 120, 500 milliseconds will be your upper tracking. operate interval. So, this is the additional interval dual chamber pacemaker.
So, we talked about P-VARC now. So, what is the point of a P-VARC? After a sensed or a paced ventricular event, there is a certain period during which the atrial channel is not going to take sensed events into account. What does it mean exactly? If there is an atrial sensed event, it will not initiate a heavy dealing and this is to prevent sensing of retrograde P waves.
So what are these retrograde P waves? So this is a unique phenomenon where even if you have complete AV block in the anti-grade direction that is A to V, the system is rather unique in the sense in the opposite direction it can still have intact conduction. It sounds funny but this is very well known in the conduction system.
And this property of different opposite directions is known as anisotropy. So the A to V can be completely blocked. You have third degree AV block, but the V to A conduction can tank.
Therefore after QRS or a paced ventricular event, you can have a retrograde P wave. And if this retrograde P wave gets sensed, it can lead to an undesirable response called pacemaker mediated tachycardia. I'll show that later. period which we need to talk about is the TAR or the total atrial refractory period.
Now this is nothing but the sum of the AV delay and the PVAR. Now following an atrial event you are going to have a AV delay initiated and again the atrium is not going to take into account any of the sensed atrial events. So essentially atrial channel is refractory during the AV period. After that, when a ventricular depolarization has happened, you have the P-VARP which is initiated. During that time also, the atrial channel is refractory.
Therefore, putting these two together, the AV delay plus the P-VARP, you get the total period during which the atrial channel is refractory, the total atrial refractory period or the TARP. The TARP is important as we will see later in governing what is known as the upper rate behavior of the pacemaker. And corresponding to the TARP interval, you will also have a TARP corresponds to a, you can convert every interval to a rate.
For example, take this example, pVARP of 300, TARP is 500 milliseconds. So, the TARP rate is corresponding to 120 beats per minute. We will see how.
So, let us look at some examples now of the we have learnt all the periods. So let us look at some examples. Here we are starting with a V-pace.
We have a ventricular blanking, ventricular refractory period. We initiated a VA interval. End of the VA interval, if nothing happens, atrium gets spaced. Now along with these periods, there is a cross chamber blanking and refraction.
Postventricular atrial blanking, postventricular atrial refractory period, the P-bar. At the end of the VA interval, A-spaced. After an A-pace, you have this tiny thing, the post-atrial ventricular blanking. This typically happens after only an A-paced event, not after a A-sensed event because the big pacing spike is what could potentially be sensed in the opposite channel. So A-pace, post-atrial ventricular blanking in the ventricular channel initiates what delay?
Initiates the AV delay. Here in the atrial Atrial blanking, atrial refractory period, the entire AV delay is refractory for the atrium. At the end of the AV delay, nothing happens.
Ventricle gets spaced. Again, same blanking refractory period in the V, blanking and refractory period in the A, and then so on it continues. V-pace, V-pace, dual chamber mode. What if you look at intrinsic activity of patient is there? So, atrial sense.
you have an atrial blanking and an atrial refractory period, AV delay is initiated, the AV delay is cut short by a ventricular sensitive event. The VA interval is cut short after ventricular sensitive event, of course, we are going to have the ventricular blanking and refractory, we are going to have post ventricular atrial blanking and post ventricular atrial refractory period, initiation of the VA interval, the VA interval is cut short by an atrial sensitive event. Atrial sensitive event will initiate an AV delay again that is cut short by a ventricular sensitivity and so forth.
Essentially the pacemaker is following in an inhibited fashion due to the patient's over intrinsic activity. The third one it is APVS, atrial pace, atrial blanking and refractory period. AV delay is initiated but cut short by ventricular sensing. Then after ventricular sensing, you are going to have the ventricular blanking refractory periods and the cross chamber refractory periods in the atrium.
VA interval is initiated, VA interval everything happens, atrium is spaced and then a ventricular sensitive and so on. Lastly, you have the atrial tracking mode where you have atrial activity, but there is no intrinsic AV conduction. Atrial refractory periods nothing happens ventricular phase, ventricular refractory periods corresponding cross chamber atrial refractory periods VA interval is initiated but VA interval is cut short by an atrial sensitive and goes on ASVP, ASVP.
The sinus rate is being tracked into the ventricle. Another mode which is used sometimes by the pacemaker again you can use it also as a programmed option if you don't want to track the atrium. This is called the DDI mode.
So we have removed the last D in the sense we are going to only have an inhibitory response but an atrium is going to sense and track into the ventricle by an AV delay. That is not going to happen. So here you use this whenever you don't want atrial tracking.
And what is the situation where you typically do not want atrial tracking, you have an atrial arrhythmia going on and this is a mode which is used in a programming option called a mode switch. Again, we will talk about this in a future session. Therefore, here in a DDI mode, the ventricular paced rate, remember, we are talking not about intrinsic rate, intrinsic rate is under the control of the control of the pacemaker. The ventricular paced rate in a DDI mode can never increase beyond the lower rate ddi mode a pace you know the refractory periods ab delay and then you are going to have ventricular pace but if an atrial sensed event happens watch here the va interval is cut short by this particular p wave here it's not going to take that into account it says okay there is an essence but i'm not going to initiate an AV delay.
I will simply wait for expiration of the lower weight interval and I will face the ventricle. The V-pace again there is one more in the pacemaker sees that is going to not initiate an AV delay. Nevertheless the patient conducted on his own and you had a ventricular sensitive event. After the ventricular sensitive event the pacemaker is going to reinitiate the lower rate instrument.
So here essentially the sensed events do not initiate an AV delay. Only a paced event can initiate an AV delay. That is a DDI timing. So let's come back to the P-VAR.
We already talked that we do not want to sense the retrograde P-vays. And this is an example of a situation, a common situation where a retrograde P-way can happen. So if you follow this, we have intrinsic activity ASVS, then you have APVS, then you have a PVC. Now a PVC is occurring remote from this atrial depolarization, therefore the atrium is receptive to get depolarized again and this PVC conducts retrograde atrium and here this falls within the atrial refractory period. therefore does not initiate an AV delay.
This is the problem which we can end up with which is known as PMT or pacemaker mediated tachycardia or endless loop tachycardia. So let's look at the same PVC here which had a retrograde conduction and then you have a retrograde P wave and this fellow therefore was sensed by the atrium initiated an AV delay at the end of it the pacemaker paced the ventricle This paced beat again had a retrograde P wave which fell outside the P var was sensed by the atrium initiated an AV delay paced again retrograde P wave again initiation of an AV delay and therefore this can repetitively start pacing the ventricle at a fast rate and how fast will this rate be this will be at the upward track pacemaker mediated tachycardia. So you can see this tachycardia which is running and you will be able to diagnose this. Number one, the pace rate will be pretty much equal to the upper rate interval or the upper tracking rate of the pacemaker. Secondly, if you take a 12 lead ECG, you will find that the P waves will be inverted in the inferior leaves.
Why? Because the P waves are retrograde. They are coming from ventricle to atrium, from below to above.
You will typically find inverted. P waves in the inferior loop. So, then if you see this combination, you must suspect a PMT.
So, that is why you keep a P warp in order to prevent the occurrence of this pacemaker mediated tachycardia. But as we just saw in this example, sometimes the retrograde P wave can fall outside the P warp. So, you need to keep the P warp long, but how long?
Can you keep prolonging it indefinitely so that you never sense the retrograde P wave? You cannot do that because of the issue what we talked about the total atrial refractory period. We remember that the AV delay plus the PVAR is equal to TARC. If you prolong the PVAR or if you prolong the AV delay, you also prolong the TARC.
Why is this important? Let us have a, this is important because we are going to now discuss something called the upper rate behavior of the pacemaker. So, we already talked about the basic function of dual chamber pacemaker which is basically AV tracking. You do not want to be it to be too fast especially if there is going to be an atrial.
You need to set a limit beyond that limit the pacemaker is not going to track one-to-one atrium to the ventricle. What happens therefore if the atrial rate for example the sinus rate itself suppose it reaches the upper rate limit, what is going to happen? If your upper tracking rate is 120, will it be that the ventricular paced rate as the sinus rate increases, the ventricular paced rate will reach 120 and then keep staying at 120?
Is this what will happen? It turns out that it does not behave like that. There is a specific way the pacemaker behaves when the rates start reaching the upper rate and this particular pattern is known as as the upper rate behavior. There are essentially two types of response.
The first one which should happen ideally in a properly programmed pacemaker is known as the pseudo AV vancubal. It looks pretty much similar to a physiologic vancubal. So let us see what starts happening as the sinus rate starts reaching the upper rate interval.
So you have the, follow this graph from left to right. So you have the atrial sensed event, the AV delay, you have the ventricular paste, no problem. The VA interval is cut short by a next atrial sensed event which is coming pretty quickly because there is a sinus tachycardia going.
It initiates an AV delay. Now the AV delay has expired here, but there is a problem here because there is a one interval right below which is the upper rate interval. Remember we said that the pacemaker will not allow the ventricular rate to exceed the upper tracking rate.
In other words, the VV interval cannot be shorter than the upper rate interval. So the AV delay has expired but not the upper rate interval. So what the pacemaker does till the upper rate interval is reached then it paces.
So, the AB delay gets prolonged a little bit. Now, thanks to that the atrium which is running at a very steady rate, the next P wave is going to come even earlier relative to this QRS. AV delay is initiated, but the upper rate interval is not satisfied, little more stretching of the AV delay. So, the AV delay gets stretched a little more here compared to here, then ventricular phase.
Due to this the next atrial event comes even earlier corresponding to this QRS to satisfy the upper rate interval, the AV delay has to be stretched even more. So little more stretching of the AV delay before the ventricular paste event happens. And then here, this particular A comes even earlier, even more stretching of the AV delay, ventricular paste.
And then here, the P wave comes very early and it says unsensed. And why is this unsensed? because you have the P-Var because immediately after the ventricular event you are going to have a post ventricular atrial refractory period.
If a normal P-wave comes soon enough it will fall within the P-Var, it will not be tracked, there is no ventricular pacing here, then you have the next sinus vect coming which is tracked with a normal AV delay with a ventricular pace. This cycle will be itself. from a normal AV delay, little prolongation, more prolongation, more prolongation, more prolongation and block.
So, resembles the usual AV Venky bag which you see in a Mobitz type 1 block. It is pretty much similar looking phenomenon to that. But the reason why this is happening is a problem the AV delay in order to satisfy the upper rate interval with finally one P wave falling within the P warp which is not going to be tracked. What happens if it even exceeds beyond that, right?
If it exceeds even beyond that what is going to end up happening is that every other P wave, one P wave will get tracked but the next P wave will end up falling within the P warp and it will not get You have ascensed v-pace. The next ascense is coming so fast that it's falling within the p-bar. It's not tracked. The next ascense is tracked as usual.
Again, this falls within the p-bar and is not tracked. And then this phenomenon keeps repeating. This is 2 is to 1 AV block where the ventricular rate will be exactly half of the atrial rate. And you can understand here that when is this going to happen?
If your A to A rate, the A to A rate exceeds the TARP rate or your AA interval is shorter than the TARP interval that is when your next A will fall within the P-VARP. You can understand that this is the AV delay, this is the P-VARP, this is the total atrial refractory period. So any A wave if it falls within the TARP is not going to get tracked.
So what is the 2 is to 1 block rate of a pacemaker? It is basically corresponding to the TARP rate. Okay. So you can calculate the 2 is to 1 block rate of a pacemaker as 60,000 divided by the TARP in milliseconds.
For example, if you have an AV delay of 150 milliseconds and a PVAR of 250 milliseconds, what's going to be the 2 is to 1 block rate? You add the AV delay and PVAR, that's 400, 250 and 150. 400 milliseconds and your tarp rate works out to be 60,000 by 400 which is 150 beats per minute. So the moment the sinus rate hits 150, what's going to be the ventricular rate?
The ventricular rate is going to fall to 75. What will be the rate when the rate go atrial rate goes to 160? It will be 80 beats per minute. So sinus rates or atrial rates exceeding the tarp rate will result in 2 is to 1 AV block with the ventricular rate being half of the atrial rate.
So this is a graph which shows the upper rate behavior. Now below the lower rate, so here is the atrial rate on x-axis, ventricular rate on y-axis. This is the upper tracking rate. Below the lower rate interval there is no ventricular pacing. There is going to be 1 to 1 atrial tracking as the atrial rate increases till it starts coming close to the upper tracking rate.
As it starts reaching the upper tracking rate, you start finding first the pseudo AV. Further progressive increases in atrial rate, once the TARP rate is reached, you see how the ventricular rate falls sharply to half the atrial rate. And then as the atrial rate further increases, it will be tracked in a 2 is to 1 fashion with every other P wave falling within that. So here is where the issue comes.
We talked about you cannot prolong the P-VAR indefinite retrograde atrial tracking because EVAR is part of the task. So that is one issue. Second issue is you should have a healthy upper rate interval in the sense it should be short enough to permit atrial tracking at physiologic rates.
Now why do we need a atrial to some extent? Because you know a person exercises, a person is anxious or whatever. Just like in a normal situation atrial rate increases, the ventricular rate increases.
You want to maintain that synchrony to a physiologic You should permit atrial tracking at physiologic levels. But at the same time, you cannot be too generous. You should ensure that the upper rate interval should be longer than the TARP interval.
In other words, the upper tracking rate should be shorter than the TARP rate. I hope you are able to understand the interplay between intervals and rates. Now, why is this? If you keep, for example, an upper tracking rate at one, but your TARP rate is actually only 140. What is the point of that?
Because by the rules governing the pacemaker, the moment the sinus rate hits 140, what's going to be your ventricular rate? It's going to be 70, right? That's a good situation because the patient in some kind of an exertion or some abrupt twist one, maybe block.
So you need to keep your upper rate interval longer than the top rate. This will ensure that the usual pattern of a pseudo AV Wencke-Bach phenomenon is first followed and that will already kind of give a warning that you know one-to-one tracking is not happening and then this is important for you to understand the concept but modern because we already have safeguards written. where it will not allow you to program the upper rate interval shorter than the TARP interval.
But this is important for you to know the concept. So, you want to keep the TARP rate, TARP interval I said shorter. So, achieve that. You know that TARP is responding to AV delay plus P bar.
So, we can shorten the TARP by a shortening the PVAR. So here you see we got the two opposing issues. On one hand we said we want to keep the PVAR long enough to prevent retrograde sensing clock.
Don't have it too long PVAR because in your torque interval and your corresponding torque rate will fall. Kind of these two things. How you shorten the TARP in such a way that you can allow a healthy upper tracking rate.
One, you can shorten the PVAR, but then you are risking tachycardia. Or you can have rate adaptive intervals, which is what most modern pacemakers do. You have rate adaptive delay and rate adaptive PVAR pulse. Which means that at slower rates, your AV delay and your PVAR will be longer.
But at fast... AVR is going to come into play, you can have dynamic shortening of AV delay, dynamic shortening of PVAR at faster rates and therefore you will not have the interference with a good upper tracking rate. And this is also, it actually makes sense because if you are going to have retrograde tracking, the retrograde VA interval will also shorten at such a long PVAR at faster heart rates.
So dynamic TARP and a dynamic upper tracking rate will be some of these issues. And again, most modern pacemakers automatically incorporate this. Lastly, the VDD timing cycle, so in a VDD, you have only sensing in atrium, so you are able to track sensed HAL events to the ventricle, but there is nothing the pacemaker can do.
the pacemaker can only pace the ventricle at the lower rate in trouble. So, this is as far as the VDD timing cycle is concerned. So, we already talked VDI mode where there is going to be no atrial tracking which is basically useful in an atrial tachyarrhythmia and then asynchronous modes also we have talked about will absolutely suspense and expecting the situation of some extrinsic noise in a pacemaker dependent patients.
Typically use asynchronous modes for short durations. such as when a patient is undergoing an MRI, coterie, and then we put the patient back in a neutral mode. So summarizing timing cycles are important to understand what the pacemaker is doing and to be able to sort out normal from malfunction. And then you have to look at what is the physiologic situation of the patient?
What is the underlying condition and appropriately program the clinical scenario? I'll stop my talk here. I think we have slightly exceeded the time. so if you have any questions that so let me um questions here the qt is i think so this is in reference to the t wave over sensing somebody has asked so remember the the ventricular refractory of the wave which the pacemaker is able to avoid T wave over sensing.
As we discussed in the last class there are also other things like the sensitivity threshold, T wave is typically smaller than an R wave. So you can program your ventricular sensitivity. There is also a different frequency content in the T wave as compared to the R wave which also helps the pacemaker generally avoid sensing T waves.
But finally nothing is perfect. So yes, If a patient has a prolonged QT with a very prominent T wave, you can end up with T wave coescence. What are the advantages and disadvantages of atrial and ventricular based timing cycles?
I cannot think of any specific advantages. Even in an atrial based timing with some of the dual chamber pacemakers used, you pretty much have safety as far as it is concerned. So I think it's just... a couple of ways in which the pacemaker looks at it.
But other than that, I don't think there are any specific advantages or disadvantages of that. Atrial blanking and postventricular atrial blanking. So atrial blanking is the same chamber blanking. So after an atrial paced or a sensed event, you suspend its sensing for some time.
The postventricular atrial blanking is after a ventricular event, not after an atrial event. So after a ventricular paste or a sensed event, the atrium is going to blank itself for some time, right? Because it does not want to see that number one, that R wave and also the big ventricular facing spike. Is there any situation where the pacemaker exceeds the maximum tracking?
So the ventricular paced rate in a dual chamber pacemaker cannot exceed the maximum tracking rate. Just like the lower rate interval, the upper rate interval is an inviolable rule. So the paced rate can never exceed. Remember, we are not talking about sensed rates. If a patient has an atrial arrhythmia, which is being conducted by his own AV node, he's having a very fast ventricular sensed rate, his own heart rate, the pacemaker can do nothing about that, right?
So that can be much faster than the upper tracking rate of the pacemaker. But as far as the ventricular paced events are concerned, it cannot exceed the MTR. Before it reaches the MTR itself, you will start finding the pseudo-wenckeback response and if it hits the TARP rate, the ventricular rate will fall to half the atrial rate. Can PMT happen less than MTR? Usually not.
the PMT is usually at or just below the MTR rate. Is it possible to have pacing below the lower rate? Means if the lower rate is set at 60, patient is having ASVP at 50 beats per.
So this really shouldn't be possible. If you have a VDD, the only advantage is that If you have atrial sensing, which is faster than, so if your low rate is set at 60 beats per minute, but you have sinus activity going on at 75 beats per minute, the advantage of the VDD is that with an appropriate AV delay, you can have ventricular pacing at 75 beats per minute. So you maintain AV synchrony. On the other hand, the lower rate interval is inviolable.
So if your sinus rate is slower than your lower rate limit, your ventricle will get paced at the lower rate limit. So the way that will happen is you might have one atrial sensed event which is tracked and you have a ventricular pacing. The ventricular pacing will initiate the lower rate interval. At the end of the lower rate interval, if you do not have a ventricular event, it will pace the V. So the lower rate interval is a very fundamental interval which is inviolable.
So if your sinus rate is slower than the lower rate interval you could potentially lose the AV synchrony in a VDD pacemaker. This is we will probably talk about it in the next session where we talk about some common pacemaker algorithms. These are the questions. So I think there is nothing else. We will conclude this session.
So we are going to have two more sessions that in the next week we are going to talk about some of the common pacemaker algorithms. Again that is an important session which I will encourage everybody to attend because many of these algorithms are misunderstood and sometimes mistaken for pacemaker malfunction. And then the week after that we will try to have a demonstration of programming by you know showing your programming screen online so that you can get familiar with it how to play with the different buttons and so forth so hopefully today's session was um useful uh there's one more last uh question which says please explain ddi so i'll quickly do that so ddi as i said the nomenclature the first chamber the first letter is chamber paste d both chambers are being paced dual d both chambers are being sensed that is the second letter and then lastly instead of DDD you are saying DDI which means that the only response possible in the DDI mode is inhibition.
So we already know that if an event is sensed in the same chamber it is going to inhibit itself but in tracking if it senses an atrial event it is going to track it to the ventricle. that is the D part in a DDD. But if you put it on DDI, this tracking is essentially what you're not doing. You're not going to track atrial sensed events to the ventricle. So here the AV delay will not be initiated by an atrial sensed event.
It will ignore all atrial sensed events and simply pace the ventricle at the lower age. If an atrial paced event happens, yes, that will get tracked to the ventricle. And remember the atrial paced event is also going to follow the lower atrial.
So DDI is used when you want to suspend atrial tracking which is typically in the situation of an atrial arrhythmia. The atrium is going very fast, you do not want it tracked at a fast ventricular rate, use the DDI mode. Please explain reverse arterial pacing used in post-op.
junctional tachycardia this reverse arterial pacing now look at this in a future session because i think we are kind of a little bit over time so bring the session to a close so Just want to thank both Abbott as well as Vijay Bhaskar for being instrumental in organizing these sessions. So hopefully these are donor remedies and then we have also Abbott, both of them are taking the initiative. So hopefully these sessions will benefit you to be able to be more confident in handling the pacemaker programming.
so thank you anything any any other comments sridhar or kashif Thank you. Thank you for the wonderful talk, sir. So definitely, I would like to request everyone to join next Saturday.
That is on 20th June. And the agenda will be common pacemaker algorithms. So definitely, sir.
Thank you for the wonderful talk, sir. We'll see you next week. Okay, sir.
Thank you, sir. Thank you.