I'm Dr. James Allred, electrophysiologist and co-founder of CV Remote Solutions. CV Remote Solutions and MedAxiom are excited to present to you the device clinic essentials for the care team module series. Here are my disclosures. Today, we will talk about device and lead function. Our objectives are to discuss the basic principles of pacing and defibrillation, to introduce timing cycles, sensing, pacing thresholds, and additional device features, and explore device functionality and implications for patient care. You'll see here a pacemaker. The portions of the device that we want to focus on today are the device and the leads. Here you'll see a chest x-ray of a patient with an implanted pacemaker. You'll see the pacemaker pulse generator composed of the header and the can. The can has the battery as well as electrical components. And then there are the leads that take this electrical signal to the heart, to the atrium, and to the ventricle. Pacemakers are used in multiple different situations and therefore have to be adaptable to the patient's needs. We use pacemakers for sinus node dysfunction, for atrial fibrillation with a slow ventricular response, for atrial ventricular or AV block, for cardio-inhibitory syncope, and for heart failure management. Pacemakers must function differently to accommodate these different indications. There are a number of electrical characteristics of pacemakers and defibrillators that we will be discussing today. We will discuss battery voltage, current drain, device longevity, lead impedances, lead sensing thresholds, lead pacing thresholds, and other programmable values. Let's begin by talking about the battery. Typically, the battery is a lithium iodide battery. As the battery depletes, its internal resistance goes up. This is helpful for us to be able to track the battery over time. It's very important to check battery status at each device check. Here is an example of evolution of device battery technology over time. You'll see that early on, devices did not last very long. These are single-chamber, dual-chamber, and biventricular devices. Over the last few years, new battery technology has allowed batteries to last much longer, such that CRT devices may last up to nine years, and single- and dual-chamber ICDs may last 10 to 12 years. Let's talk about lead polarity. We refer to lead polarity as unipolar or bipolar. Unipolar means that the anode, the positive, is the implantable pulse generator, and the negative, cathode, is the tip of the lead, such that current typically flows from the device to the lead. A bipolar situation is such that the anode is the lead proximal electrode, and the cathode, which is negative, is the lead tip electrode, such that current flows typically from the proximal to the distal electrode. With a unipolar polarity programming, a large pacing stimulus is usually seen as a pacing artifact, as can be seen here in this example. You'll see the large pacing artifact. There's a greater chance for pacemaker pocket muscle stimulation when programmed unipolar. When looking at bipolar polarity programming, a small pacing spike artifact is seen, as noted here. With bipolar programming, there's a greater chance of non-capture with a lead fracture. It's important to follow the lead impedance in order to test the integrity of the pacing lead system. A high impedance could suggest that there is a concern for a lead fracture. Elevation can also be caused by encapsulation of the tip of the lead, which occurs over time with scarring at the lead muscle interface. At time of implant, if impedance rises abruptly, it may mean that the lead is not well seated within the device, within the header, along the set screw. A low impedance is concerning for damage to the insulation of the lead. At time of implant, this may mean that there is fluid inside of the header, which is disrupting the ability to make contact within the header. Current drain is high, and this may lead to an early battery depletion. Here's an example of a patient whose impedance within their lead has risen dramatically over a short period of time. You'll notice here that the impedance was typically around 450, and that at one point, all of a sudden, this impedance rises fairly abruptly. This would be very concerning in the clinical setting for either lead tip encapsulation or possibly a lead fracture. Now let's turn our attention to measuring pacing thresholds, as well as pacing sensing. The pacing voltage threshold is the minimum pacing voltage at any given pulse width required to consistently achieve myocardial depolarization outside the heart's refractory period. You'll see an example here where a pacing spike leads to a QRS complex. The second pacing spike leads to a pacing QRS complex, as seen here and here. And here you have a pacing spike without a QRS complex, another pacing spike without a QRS complex. And so here we have consistent capture. Here we do not, and we describe this as non-capture. This example is from a pacing threshold being performed in a patient where the pacing output was reduced until loss of capture was seen at 0.5 volts. This is an example of the pacing stimulation threshold strength duration curve. And what this illustrates is a relationship between amplitude and pulse width. The energy required to stimulate the heart is dependent on the duration of the electrical simulation applied. There are two terms that are important when considering the threshold strength duration curve. Rheobase is the lowest voltage resulting in myocardial depolarization at infinitely long pulse duration. Chronaxy is the threshold pulse duration at twice the rheobase voltage. This is important as we consider how to program our device given the patient's threshold to allow for an adequate safety margin to ensure that the patient will consistently pace from the device. Here's another example of the ventricular pacing threshold test. You'll see here that we are running a threshold test on the patient. We are pacing the ventricle and capturing the tissue consistently. And this occurs at 1.75 volts. But when we reduce the current with which we are pacing to 1.5 volts, you'll see that we no longer capture tissue. What this tells us is that our threshold is 1.75 volts for this patient. Now let's look at sensing threshold. Sensing describes the ability of the pacemaker to sense intrinsic electrical signals, and this is typically recorded in millivolts. Sensing is critical to adequate device function. It is important to program the sensing safely so that there is a safety margin to allow for appropriate sensing. Remember, raising the number makes the device less sensitive and causes the device to see less. Oversensing means that the sensing is too high and the device is overseeing what is actually present. The programming sensitivity number is too low. Undersensing means that the sensing is too low and the device is not sensing what is actually present. The program sensitivity number is too high. Here's an example of undersensing. You'll notice a pacing spike here with a QRS. Now here's an intrinsic event. The patient has their intrinsic R-wave here, but this is not sensed by the device, and another pacing spike is sent here as the device does not recognize that the patient had an appropriate heart rhythm here and is trying to generate one here. You'll see a pacing spike here, which captures, followed by another intrinsic event, which is not sensed by the device, and therefore an inappropriate spike is seen here. So when there's undersensing, you are not seeing what you're hoping to see, and the way to improve this is to make the device more sensitive by reducing the sensitivity programmed number. Here's an example of oversensing. The device here is seeing things that actually aren't there. So here you have a pacing spike, pacing spike, pacing spike. The device will sense noise here, but interpret that as an intrinsic heart R-wave and not generate a pulse and then generate one later. And so in this situation, there is oversensing because the device is too sensitive. And so to improve that situation, what you would want to do is to make the device less sensitive by increasing the sensitivity number. When we talk about types of pacing, there's a nomenclature that I would like to introduce to you. Initially, we talk about fixed rate pacing. That means that the device paces consistently at a certain rate, almost like a metronome. There is no changing based on what's happening within the patient. We call fixed rate pacing asynchronous pacing. Demand pacing, on the other hand, means that the device will kick in as needed to augment the patient's heart rhythm according to what the patient's heart rhythm has been doing on its own. We call this synchronous pacing. As we look at the nomenclature for how we describe the way that the device is programmed to pace, we think of this set of information here. So we always describe first the chamber that is paced. And your options are A for atrium, V for ventricle, D for dual, which would include atrial and ventricular pacing, or O, which means that we're not going to pace from either electrode. After we describe the chamber that was paced, then we describe the chamber that has been sensed. Again, this can be A for atrium, V for ventricle, D for dual, which includes both atrial and ventricular sensing, or O for none. Following this, we describe the response to sensing. This can be triggered with a T, inhibited with I, or dual, meaning that we both trigger and inhibit depending what is seen by the sensed electrogram. Or also there can be O for none. The fourth letter in the pacing nomenclature is rate response. If rate response is turned on, then an R would be noted. So here are examples of how we describe a pacing mode. So in this example of AAI, this means that the atrial channel is pacing and sensing, and that if an atrial event is sensed, then that will cause inhibition of pacing. This second example is VVI. This means that we are pacing in the ventricle, we're sensing from the ventricle, and if we sense an intrinsic event, then we will inhibit. The example of DDD means that we are both pacing from the atrium and ventricle and sensing from the atrium and ventricle, and if there's an intrinsic response, then we can both trigger and inhibit pacing depending on the need. Finally, the example of DDDR means that we are pacing and sensing from both the atrium and ventricle, we can trigger and inhibit, and rate response has been turned on. So as we think about these modes for a single chamber device, that means a device with only one lead, our options are asynchronous pacing, again meaning there's no inhibition or changing in the pacing rate, much like a metronome, and that nomenclature, if this was an atrial single chamber device, would be A, meaning that we're pacing in the atrium, O as we're not sensing anything, and O as we're not triggering or inhibiting pacing. For the example of VOO, this means that we are pacing from the ventricle, but we are neither sensing from the ventricle, we are also not inhibiting or tracking with the device. Again, think of a metronome, we are pacing at a constant rate. Now with a single chamber device, however, we can also program this to be synchronous, meaning that we can, based on the patient's heart rhythm, inhibit or track depending on what is needed. So if we are inhibiting, which is a very common pacing mode, with a single chamber atrial lead device, we would pace from the atrium, we would sense what's happening in the atrium, and if there is an intrinsic event, then we would inhibit, and therefore we would call that AAI. If this single lead device has a lead going to the ventricle instead, then we would pace from the ventricle, we would sense from the ventricle what is happening intrinsically, and we could inhibit or withdraw pacing if needed. A similar nomenclature would be described for trigger pacing, AAT and VVT. For a dual chamber device, if we are asynchronously pacing from both the atrium and the ventricle at the same time, like a metronome, at a fixed rate, then that programming would be considered DOO, meaning that we're pacing from both the atrium and the ventricle. We are not sensing, we are not inhibiting or triggering pacing. One of the most common pacing modes for a dual chamber pacemaker is DDD. Typically, one of the biggest values of having a dual lead device is providing synchrony between the atrium and the ventricle and what's happening across those two chambers. And so with DDD pacing, we would be able to pace both the atrium and the ventricle. We would be able to sense from both the atrium and the ventricle, and we would also be able to inhibit or trigger pacing from either chamber as needed. This is probably the most common pacing mode for dual chamber pacemakers, DDD, though if rate response is added, then we would see DDDR. Other pacing modes that are less common are DDI, where we pace in both the atrium and ventricle if needed. We can sense in both chambers, and if there is sensing of an intrinsic event, then we would inhibit pacing. DDT means a similar mode of pacing in both chambers, being able to sense from both chambers, but rather than inhibiting of pacing with intrinsic events, we can actually trigger pacing for tracking. Here's an example of VVI pacing. This is a patient who actually has an atrial and ventricular intrinsic rhythm, but their device has been programmed to pace at VVI. So what does that mean? It means that it can pace the ventricle, as you see in the last three events, that it can sense in the ventricle as it does. It's basically ignoring any atrial activity, but it can pace in the ventricle if it does not sense any intrinsic ventricular activity, and if it does sense intrinsic ventricular activity is here, then it can inhibit during that time. This is an example of VVI pacing. When we think of timing cycles, single chamber devices and dual chamber devices function to allow for appropriate treatment of patients depending on what their arrhythmia might be. There are lots of things that we have to consider for appropriate timing of when pacing spikes are delivered and when pacing spikes are withheld. Several terms that we will discuss are lower pacing rate, upper pacing rate, blanking period, and refractory period. Blanking period, the first portion of every refractory period. This prevents over-sensing of pacing stimulus and far-field sensing. No sensing can occur during this period. There are no visible marker channels during this time, and the device is effectively blinded from any activity during this time. It is not able to detect any activity that occurs. Refractory period. This is a programmable portion of the patient's refractory period. It prevents far-field sensing. Sensing events are included in the diagnosis of arrhythmias and are present on the marker channels during this time. Events during the refractory period do not reset the synchronization interval. Let's look at AAI pacing as an example. You'll see here a pacing spike followed by atrial activity. There's the AV delay followed by a QRS. And then onto the next pacing of an atrial event followed by an AV interval and a QRS. The lower rate limit is the time from the first pacing spike until, in this example, the next pacing spike. The pacemaker will not let the patient's atrial rate drop below the lower rate limit. If no intrinsic activity is observed during this time, then the patient will receive a pacing spike as demonstrated here. As you'll note, the first portion following the atrial pacing event is referred to as the atrial blanking period. And the second portion is the absolute refractory period. Here's an example of VVI pacing timing. This patient has a lead in their right ventricle and you'll see a pacing spike here followed by a QRS. And then through the lower rate limit period, there is no intrinsic activity observed and so another pacing spike is delivered followed by a QRS. The initial portion of time following the pacing spike is the ventricular blanking period and following the ventricular blanking period is the ventricular refractory period. Here's an example of AAI pacing. You'll see a pacing spike with atrial activity followed by an AV interval and then a intrinsic QRS and this is repeated. Now there is what is known as the upper sensing rate. That means that anything shorter than this will not be tracked. This defines the shortest interval that the pacemaker can pace during sensor-driven pacing such as AAIR or VVIR modes. Now let's look at DDD pacing cycles. So pacing and sensing in both the atrium and ventricle can occur. Both triggering and inhibition are observed. Atrial sensing inhibits the next scheduled APACE event and triggers an AV interval which we call the sensed AV or SAV. An atrial paced event triggers an AV interval called the paced AV or PAV. Ventricular sensing inhibits the next ventricular paced event. Here's an example. You'll see atrial events followed by ventricular events here. Atrial events followed by ventricular events. With DDD pacing, what is allowed is that the patient's atrial and ventricular contractions are in sync with each other. This promotes adequate blood flow for that patient. When we look at dual chamber pacing timing cycles, terms that we should consider are lower rate, upper rate, AV and VA intervals, blanking period, and refractory period. Let's look at AV intervals. So as we said, the paced AV event and the sensed AV event are abbreviated PAV and SAV respectively. These determine the AV timing of the device and they begin after a paced event or a PAV and a sensed event or an SAV. Once intervals begin, either a sensed ventricular event will occur or the AV delay will time out and a ventricular paced event will be triggered. Here's an example. A paced atrial event triggers the paced AV interval, which is programmed at 200 milliseconds. Because there is no intrinsic ventricular activity during this time, a ventricular paced event is triggered by the device, which leads to a QRS. The AV time interval is determined by the paced AV delay, which for this particular example is programmed at 200 milliseconds. The lower pacing rate is not observed as the patient has an intrinsic atrial sensed event. And therefore the SAV period is triggered. And so once you have the SAV period, which is programmed at 170 milliseconds from the time of the initiation of the atrial sensed event at 170 milliseconds, if there is no intrinsic ventricular event, then a ventricular pacing event is triggered by the device, which leads to the next QRS. Let's talk about postventricular atrial blanking. The first portion of every refractory period after the ventricular paced event is known as postventricular atrial blanking or PVAP. This prevents far field sensing. No sensing can occur during this period on the atrial channel. This is not visible on the marker channels and the device is effectively blind during this time. This PVAP is followed by the postventricular atrial refractory period, also known as PVARP. During this period, atrial events are seen, but the pacemaker does not respond to them. Let's talk more about PVARP. PVARP is the programmable interval and the dual chamber pacing modes initiated after the sensed or paced ventricular events. If an atrial event occurs during PVARP, the ventricular atrial interval and the lower rate limit are not reset. Atrial sensing during PVARP allows for atrial arrhythmia detection, but prevents inappropriate tracking of retrograde P waves, premature atrial contractions, and far field ventricular activity. PVARP is initiated by the ventricular channel, but makes the atrial channel refractory. Let's look at PVAB and PVARP more closely. Here you see an atrial event or an atrial pacing spike followed by a P wave. There's the AV delay, which is a paced AV because it followed a paced atrial event. After the paced AV timing cycle times out, there is a ventricular spike delivered as the patient has ventricular pacing here, which leads to a QRS. Immediately following the pacing spike is the interval known as PVAB, or postventricular atrial blanking, followed by PVARP, postventricular atrial refractory period. As the patient does not have intrinsic atrial activity during this time, the lower pacing rate causes the patient to have a triggered atrial spike here, followed by a P wave. The paced AV delay times out. The patient has a ventricular spike here, followed by a QRS. And then this ventricular pacing spike sets up the PVAB followed by the PVARP. And this repeats again. So what is the total atrial refractory period, also known as TARP? TARP is the timing cycle on the atrial channel during which the pacemaker will not respond to incoming signals. TARP is equal to the AV delay plus the PVARP. When the atrial rate exceeds the maximum tracking rate, it results in pacemaker winky block. And if the atrial rate exceeds TARP, it will result in pacemaker two to one AV block. TARP itself is not programmable. However, the AV delay and PVARP can be programmed to adjust TARP value. So here's an example of PVARP and TARP. Very similar example. We have a pacing atrial event followed by a P wave, a ventricular pacing spike followed by the R wave. Once we have the ventricular pacing spike, we have our postventricular atrial refractory period. And the entire period, which includes the paced AV delay plus PVARP, that entire duration is known as TARP. And so you'll see the TARP here and here. But TARP, again, is a period of time during which we cannot receive any additional atrial activity or input. And this will determine tracking of atrial arrhythmias. Here's an example of the entire ventricular refractory period here. From the pacing spike through the end of PVARP. So now in review, we have lots of dual chamber pacing timing cycle events. We have talked about AV delay, ventricular blanking, PVARP, atrial blanking, which is the first part after the ventricular pacing spike. The entire ventricular or VA interval, which is from the ventricular pacing spike up until the next atrial event is signaled. You'll see the ventricular refractory period, which includes both the PVAB as well as PVARP intervals. And then we'll also notice the maximum tracking rate here, as well as the lower rate limit, which is from this atrial spike to this one and from this ventricular spike to this one. What is a pacemaker-mediated tachycardia or PMT? Tachycardia can be initiated from the device with a reentrant loop often initiated by a retrograde PVC or at the end of a ventricular pacing threshold test. This can cause rapid heartbeats for the patient for which they can be quite symptomatic. We want to program their device to prevent PMT events by extending the PVARP to longer than the retrograde VA time. Suspect PMT when you see paced tachycardia at the maximum tracking rate. This can only occur in DDD and VDD pacing modes. Here's an example of PMT. The patient has a ventricular paced event, which leads to a QRS here. The ventricular paced event would go retrograde through the AV node to the atrium, causing an atrial event, which would be detected by the atrial lead, which would then trigger the next pacing event and creating a loop on and on of tachycardia. Let's talk about the maximum sensor rate versus the maximum tracking rate. So the maximum sensor rate or MSR is the maximum rate at which the pacemaker is allowed to respond to the activity sensor within the device. The maximum tracking rate or MTR is the maximum rate at which the pacemaker is allowed to track sensed intrinsic atrial activity. Pacemakers also have accelerometers, which help determine the pacing rate. You can see the slide here includes an overview of that. And we did discuss this further in our second module earlier, which was device technology. Ventricular safety pacing occurs if activity is sensed on the ventricular sensing channel during the initial part of the AVI. This is designed to prevent ventricular inhibition. The device delivers a ventricular paste event 110 milliseconds after an atrial paste event. This is a safety feature designed for the device to prevent ventricular inhibition, which could cause problems in patients who are device dependent. Automatic mode switching, or AMS, is very important for patient care. Mode switch occurs when the atrial rate exceeds the programmed mode switch rate. The device switches from its current pacing mode to either a VVI or a DDI pacing mode when the automatic mode switch is determined. This is designed to prevent rapid ventricular tracking of atrial arrhythmias such as atrial fibrillation, atrial tachycardia, or atrial flutter. It's also used to detect atrial arrhythmias such as atrial fibrillation. Noise reversion is very important to prevent inhibition of pacing, which can occur in the setting of external noise. So this occurs when there's continuous refractory sensing on the lead. It's designed, again, to prevent inhibition of pacing, and pacing will occur at the lower pacing rate during prolonged periods of noise. In this example, you will see that the patient is pacing along the ventricular channel. They seem to have a P-wave prior to each paced event, which suggests that they are sensing intrinsic atrial activity and pacing the ventricular channel. And so this is a DDD pacing mode. But you'll see here, all of a sudden, we only see the P-waves, and we no longer see pacing spikes. And this tells us that the patient has inhibition of ventricular pacing. This patient only has atrial activity and is in ventricular standstill, which could lead to a catastrophic event for this patient. And you'll see here an example of noise, which can cause inhibition of pacing. Therefore, a safety feature of the device is, if there is continuous refractory sensing on a channel, the device will pace to prevent this type of situation. Electromagnetic interference, or EMI, are examples of how noise can impact a device. External stimuli that produce noise causing inhibition or trigger pacing, noise reversion, damage to circuitry, or an inappropriate ICD shock are often called EMI. You can see here examples of EMI in the hospital setting, as well as other locations. Magnet mode. So a magnet can be placed over a device to activate a magnetic read switch within the device. This will cause most pacemakers to convert to asynchronous pacing, such as AOO, VOO, or DOO pacing modes. The pacing rate is manufacturer-specific and may also decrease once ERI has been reached. This may be used during surgery to prevent interference from cautery. In contrast to pacemakers, defibrillators will not convert to an asynchronous pacing mode, but will inhibit detection of ventricular arrhythmias, thus disabling therapy while the magnet is over the device. Now let's move our attention to defibrillators. As was discussed in module two, defibrillators have multiple components, including the header, the battery, the capacitor, and the high and low voltage circuits, as well as the defibrillator lead. Here's a chest X-ray showing a patient with their implantable pulse generator with the header and CAN components. You can see here the large battery, and you can see also the electrical circuits. The leads take activity to the atrium, to the right ventricle, and this is also a CRT device. And so there's a third lead, which communicates to the lateral wall of the left ventricle. ICDs are used in different situations. They are used primarily to prevent sudden cardiac death. They can be used to treat ventricular arrhythmias, to monitor for arrhythmias, to treat sinus node dysfunction, atrial fibrillation with slow ventricular rates, AV block, and also for heart failure management. Basically, a defibrillator is primarily there to treat the ventricular arrhythmias and to prevent sudden cardiac death, but they can also function much like a pacemaker to treat slower heart rhythms should they occur in a patient with a defibrillator. What this means is that a patient should never need both a pacemaker and a defibrillator. If a patient just needs a pacemaker and they are at low risk for sudden death, then they would just have a pacemaker implanted. If, however, the patient is felt to be at high risk for sudden cardiac death or requires treatment for ventricular arrhythmias, then they would have a defibrillator placed and that defibrillator would also function to treat bradyarrhythmias, as we have discussed up until this point in this module. All of the timing cycle features, all of the other features that we have described for pacemakers are applicable to defibrillators. There's always a balance with an implanted cardiac defibrillator. And so the concern is that we absolutely want to treat sudden cardiac death, but the trade-off is fear of inappropriate ICD shocks or procedure-related risk or device infection. So ICDs have two main functions. They sense and recognize arrhythmias, such as tachycardias, primarily ventricular tachycardia or ventricular fibrillation. They also will detect atrial arrhythmias, such as atrial fibrillation, and they will detect and treat bradycardias, as we've already described. And then the other function is to treat arrhythmias. Through pacing, the device can perform anti-tachycardia pacing to treat ventricular arrhythmias, anti-bradycardia pacing, as we've described for pacemakers already, or resynchronization therapy for treatment of heart failure. In addition, in treating arrhythmias, the device can deliver cardioversion or defibrillation. Anti-tachycardia pacing allows painless treatment of ventricular arrhythmias via overdrive suppression of reentrant ventricular tachycardia. As you can see, a pro of this is often termination of ventricular tachycardia without ICD shock required. A con is that this may delay the time to an ICD shock, which could lead to syncope. Programmable algorithms, including burst, ramp, and adaptive therapies are allowed with ATP pacing. So let's talk about sensing. We have true bipole, integrated bipole leads. Oversensing may occur that would lead to an inappropriate ICD shock. Undersensing could occur, which could cause withholding ICD therapy during ventricular arrhythmias. ICDs have very sophisticated algorithms to ensure arrhythmia detection. Adequate detection of arrhythmias is incredibly important to avoid inappropriate therapies for the patient. Here's an example of sensing of ventricular fibrillation. You can see the patient's ventricular channel shows ventricular fibrillation. These F markers detect fibrillation and a 25 joule shock is delivered from the device, converting the patient to sinus rhythm. Here's a second example. Now this is an inappropriate sensing. This patient has atrial fibrillation in the atrial marker channel, as you can see here, leading to rapid ventricular rates or RVR in the ventricular marker here. And we know that this is not ventricular fibrillation because it's not very fine and frequent as we see here. It's irregular and not consistent with ventricular tachycardia. This patient, however, does receive a 25 joule shock, converting them to sinus rhythm. We would prefer typically to not shock patients for atrial fibrillation with rapid ventricular rates. And so for this patient, we could adjust their detection algorithm to allow for the top successful termination of ventricular fibrillation, but hopefully avoiding shocks for atrial fibrillation with rapid ventricular rates. Here's another episode. This is a patient who has inappropriate detection leading to an inappropriate shock. As you can see, this patient is in atrial fibrillation with an irregular RR interval. This is a subcutaneous ICD. You can see the sinus markers here. But what you'll notice all of a sudden is lots of tachycardia markers here that do not correspond to the patient's R wave. And so this patient is having inappropriate over-sensing from their device. So the device doesn't recognize that the rate is much slower than the tachycardia markers are suggesting. And so this patient received an inappropriate shock due to over-sensing from the device. Let's now move on to chronic resynchronization therapy, or CRT. CRT is allowable in patients with either a pacemaker or a defibrillator if they have two ventricular leads. So you can see here a patient has a right ventricular lead. They also have a lead in the coronary sinus with this lead going out to the lateral wall of the left ventricle to allow for resynchronization. Lead technology has advanced over time in the space of CRT. Initially, leads were unipolar. The CAN of the device and the tip electrode were the anode-cathode configuration, leaving a very large current circuit. This would often allow for things such as diaphragmatic stimulation for the patient. Over time, bipolar leads were developed to allow for a much smaller area of pacing. And then with time, we now have quadrupolar leads, which have four poles, which allow for multiple sites of pacing, which can deliver a much more therapeutic response to patients with a reduction in concerns for things like diaphragmatic stimulation or areas over potential cardiac fat, which might have a very high threshold in certain locations. Ventricular-to-ventricular timing can promote synchronization of the lower chambers of the heart. The optimal VV delay is the V-to-V contraction sequence that provides the largest stroke volume for the patient. Here is an EKG of a patient with a left bundle branch block and a QRS of greater than 150 milliseconds. As we discussed in module two, this is a patient who would be a great candidate for CRT therapy. This patient had a biventricular ICD placed, and you can see here the result. You'll notice now that the patient's pacing spike is followed by a nice narrow QRS. This patient should have a very nice response to CRT pacing. As we discussed in module two, defibrillators also have heart failure monitoring capacities with thoracic impedance, heart logic, and interstitial impedance monitoring allow. Here's an example that we discussed in module two. As we said then, a high impedance represents a dry patient. A low impedance suggests that they could be accumulating chest wall water and could be having symptoms of heart failure. Let's look at a couple of questions. A 60-year-old female has sinus bradycardia and symptomatic pauses. She reports dizziness and fatigue. No reversible causes are observed. AV conduction is noted to be intact. The following pacing modes would be appropriate. What would you recommend? A, AAIR, B, VVI, C, DDDR, or D, both A and C? C, DDDR, or D, both A and C? The correct answer is D, both A and C. Why is that? Well, the patient's arrhythmia generates from their sinus node and that they're having sick sinus syndrome with symptomatic pauses. Being able to pace the atrium, sense the atrium, is very important for this patient. That could be performed with AAIR pacing or with DDDR pacing. Remember that VVI would be pacing of the ventricle and sensing from the ventricle alone, and so that would not be the best recommendation for this patient. Here's an example of A, a premature atrial contraction, B, atrial undersensing, C, ventricular over-sensing, or D, normal device function. The correct answer is B, atrial undersensing. You'll notice here that we have appropriate sensing of an atrial P-wave, followed by a pacing spike and a QRS after the AV timeout. The next P-wave appears to be sensed correctly. There's the AV timeout and then a pacing spike with a QRS. Now we have a P-wave, we should have the same AV timeout period and we do not see a pacing spike here, but what we do see is an atrial pacing spike here. And so what happened was this P-wave was not sensed by the device, and so there was no ventricular spike after the AV timeout, but rather because this P-wave was not seen after the lower limit, this pacing spike here was attempted to try to capture the atrial tissue, which was at that time refractory. This is an example of atrial undersensing and it can be seen again here in this example. This is an example of, A, appropriate ICD function, B, lead noise, C, T-wave oversensing, or D, undersensing. The correct answer is C, T-wave oversensing. As you'll see here, the QRS is sensed appropriately. Here we have a PVC that's a little early, which is sensed, but you'll also see that there is sensing of a second event, which is actually part of the T-wave. This is an example of T-wave oversensing. Please see the attached references for your convenience. Thank you so much for attending this module on device and lead function. We look forward to seeing you further on additional modules.

Dr. James Allred

electrophysiologist

CV Remote Solutions

device clinic essentials

pacing and defibrillation

pacemakers

defibrillators

CRT devices