0:00 [Music] this is eric from strong medicine and this is the second video in this three video series on pacemakers so what exactly are modes and settings these are the set of instructions that tell a pacemaker what to do the difference between modes and settings for pacemaker is completely analogous to those terms as applied to ventilators modes provide a very general overview of what the device is doing while settings are the small details i'll start with modes the nomenclature for which is universal modes are described using four letters the letter in position one describes which chamber is potentially
0:47 being paced options include a for atrium v for ventricle and d for dual meaning both a and v the second position is for the chamber whose electrical activity is being actively sensed by the device where options include a v d and o if sensing function is temporarily disabled the third position is the tricky one it describes what the pacemaker's response is to sensed events options here include an i for inhibiting a pacemaking pulse in response to a sensed event t for triggering a pulse in response to a sensed event d is for when pulses can either be inhibited or triggered and o if there is no response to sensed events
1:32 or if sensing is disabled notably t that is a triggered only response without any inhibition has no clinical purpose and should never be used in practice when used the fourth position is to indicate whether or not there is rate modulation available in r means that the pace rate changes based on perceived physiologic need the most common way for a pacemaker to determine this is via an accelerometer within the generator which interprets repetitive physical motion of the device as an indicator that the person is walking quickly or jogging or swimming or something active different pacemaker models have alternative means of identifying increased need for situations in which the pacemaker itself might not be moving up and down despite
2:18 heavy exercise such as bicycling if this function is not present or if it's turned off for some reason the fourth position is left blank the nomenclature does allow for a fifth position to indicate the presence of something called multi-site pacing which you will almost never see used in practice so it's typically left off i'll run through examples of the five most representative pacemaker modes and for simplicity we'll assume that none of these are rate responsive modes so the same principles would apply if they were i'll start with the most basic mode voo voo mode uses a right ventricular lead and that lead only paces it has its sensing function turned off
3:03 this is what a rhythm strip might look like for a patient in vo mode there is a mix of different kinds of beats here is a ventricular paste beat which can be identified by the vertical pacing spike followed by a wide and unusual qrs complex although the pacing spike comes right after a p wave the pacemaker either has no atrial lead or the sensing function of the atrial lead is turned off so the timing here is purely coincidental the next beat looks different because the qrs complex is the result of normal intrinsic conduction through the av node and his purkinje system this pacing spike that lands here doesn't do anything because the ventricle is still in its refractory period the next beat looks similar except now the pacing spike occurs right on the t wave which is particularly dangerous as
3:50 that risks triggering ventricular fibrillation luckily in this case it doesn't and is followed by another intrinsic beat and then another paste beat if you look carefully you'll realize that the pacing spikes are separated by an identical duration of time in this case one second or 1000 milliseconds and they are marching right through whatever the heart is doing on its own whether or not a pacing spike triggers a qrs complex and thus ventricular contraction only depends upon whether or not the ventricle is in its refractory period at that precise moment this is called asynchronous pacing meaning the heart's intrinsic conduction system and the artificial pacemaker are firing irrespective of whatever the other one is doing the only typical indication for vo mode
4:37 is as a temporary mode occasionally used in surgery when the electrical noise from an electrocautery device might be misinterpreted by the pacemaker to eliminate the risk of a pacing spike occurring on the t wave this mode should only be used in pacer dependent patients unlike this example which is something you would never want to see in real life next is a much more common mode to see vvi in vvi mode the ventricular lead senses in addition to pacing and when it senses an intrinsic qrs complex the response is to inhibit pacing hence the eye so this narrow qrs complex here has been normally conducted through the heart's own intrinsic conduction system the ventricular lead senses this and therefore doesn't pace right away
5:24 but after a duration of time has passed without the pacemaker sensing another intrinsic complex it does go ahead in pace this mode eliminates the problem of potentially pacing on t waves that was seen in vo but it lacks any synchrony between the atria and ventricles for patients in sinus rhythm which can result in diminished cardiac output and flashes of symptoms whenever the atrial contracts against an already contracting ventricle which is known as pacemaker syndrome therefore this is not a mode to use for patients in sinus rhythm but rather is typically used in the combination of av block and chronic atrial fibrillation the next mode is aai which uses a single lead in the right atrium so instead of sensing and pasting qrs
6:10 complexes it's sensing and pacing p waves here's an example of an atrial sensed beat in which the pacemaker detects the normal intrinsic p wave and thus is inhibited from delivering an immediate atrial pacing spike and for this beat enough time has elapsed without a p wave being sensed that the pacemaker paces the atria if you remember with the vo and vvi examples which intrinsic and paste qrs complexes look different we see the same thing with intrinsic and paste p waves though the difference is much more subtle because p waves are so much smaller now what's going on here that looks like a pacing spike on a t wave again so is that risking v fib well in this case no since that pacing spike is not being
6:56 delivered to the ventricle but rather to the atria which we can only tell for certain because i've told you what mode the pacemaker is in aai mode is typically used for patients with isolated sinus node dysfunction without av block the downside to relying on aai is that the majority of patients with sinus node disease have some degree of av node disease as well and this mode does not protect against brady arrhythmias caused by av block so even though this patient has a pacemaker if they were to suddenly develop complete heart block the pacemaker would be of absolutely no help while in this mode so cardiologists rarely implant a pacemaker with a single atrial lead even if aai is the desired mode it would
7:41 be much more common for the patient to receive a dual chamber pacemaker with the ability to automatically switch modes from aai to something else if the device detects that av block has developed in vdd mode the ventricle is the only chamber paste but the device senses from both the atra and the ventricle in this mode we can see an a-sensed v-sensed beat in which the pacemaker detects normal intrinsic conduction an a-sensed v-paste beat which the patient's sinus node depolarizes normally giving a normal p wave but because of av block it's not followed by a normal qrs complex within a set period of time resulting in a ventricular pacing spike followed by an unusual paste qrs complex this is considered to
8:28 be a triggered response since the timing of the ventricular spike was determined by sensing of the previous p wave and when the ventricular lead senses a premature ventricular contraction the lead output is inhibited resulting in no immediate pacing spike the typical indication for vdd mode would be av block without sinus no dysfunction sort of the opposite of the indication for aai the last node i'll discuss is ddd in ddd mode the atrial and ventricular leads both sense and pace this results in four basic types of beats a sensed v-paste a-paste v-paste a-paste v-sensed
9:13 and a-sensed v-sensed ddd mode is typically used for the combination of sinus node dysfunction and a b block in a patient in sinus rhythm let's move on to pacemaker settings these are the more specific rules that a pacemaker will follow i'll start with those settings that are involved in timing cycles which is how the pacemaker decides the specific moments to pace the atria or ventricle not all timing cycle settings are relevant for all modes but i'll be focusing on ddd since it demonstrates the widest variety of principles there are four basic settings related to the timing cycle the first is the lower rate limit which is the lowest rate at which the device will allow the heart to beat
9:59 typical values are 50 to 60 beats per minute although a heart rate in the 50s might intuitively feel slow since many people consider this to be bradycardic there's generally no harm from rates to slow at least while at rest also the lower the lower rate limit is set the longer the battery of the device will last reducing the interval between generator changes the converse to the lower rate limit is the maximum tracking rate occasionally called the upper rate limit this is the fastest rate at which the device will pace the heart typical values here are 120 to 130 beats per minute though higher rates might be used in middle aged or younger patients with pacemakers the word tracking here refers to the process by which a pacemaker paces the ventricle
10:44 at the intrinsic atrial rate so if the atria naturally beats faster and faster the pacemaker will pace the ventricles faster and faster too in a one-to-one ratio that is up until the maximum tracking rate is reached the av delay is directly programmed in vdd and ddd modes this is the period of time the pacemaker waits after a sensed or paced atrial depolarization before pacing the ventricle if intrinsic ventricular conduction is not sensed it is analogous to the pr interval typical values are 150 to 200 milliseconds though pacemakers can be programmed to have different av delays for sensed versus paced atrial beats since paste atrial beats necessarily have longer intrinsic av
11:32 delays av delay can also be rate adaptive that is shorter at faster heart rates during perceived exertion which is how our av node works naturally and the last major timing setting is something called the post ventricular atrial refractory period or p-varp this is the period of time after ventricular depolarization during which an atrial impulse sensed by the atrial lead will be ignored for timing cycle purposes that might sound complicated and a little esoteric but among other purposes the p-varp prevents sensing and inappropriate tracking of retrograde p waves which could trigger pacemaker mediated tachycardia typical values are 250 to 300 milliseconds
12:17 and as with av delay it can be rate adaptive so let's take a closer look at how these settings determine the timing of cardiac events so here's an example of what an ecg might record for a patient in ddd mode we have a mix of a sensed vpaste beats a paste v paste beats a paste v-sensed beats and a-sensed v-sense beats the four timing settings i've already discussed are directly programmed into the pacemaker while a few other values are subsequently determined indirectly let's start with this p wave here this is an intrinsic natural p wave that is sensed by the pacemaker the pacemaker then waits to see if intrinsic a b conduction occurs and it
13:03 waits for the length of the programmed av delay in this case intrinsic conduction did not occur so a ventricular pacing spike is delivered after the pacing spike the p var begins which extends some time into the t wave based on the setting when taken together the a b delay and the p-varp are known as the total atrial refractory period now let's look at the next beat in this case because there was no intrinsic p-wave sensed the pacemaker delivers an atrial pacing spike followed by the av delay and then a ventricular pacing spike the interval between these two successive pace ventricular beats is defined by the lower rate limit although we commonly think about the lower rate limit in terms of beats per minute
13:48 from a pacemaker's perspective it's more logical to think of it in terms of milliseconds which in this case is about 1040. the timing of this atrial spike that is how long it waits for an intrinsic p wave is based on this time interval known as the va interval the va interval is indirectly determined as the difference between the lower rate limit in milliseconds and the av delay sometimes the timing intervals initially appear to be different than they've been programmed for example with this a paste vsense beat the pr interval is less than the av delay this happens because a b conduction occurred naturally and naturally happened to be shorter than the a b delay likewise the interval between two successive ventricular depolarizations
14:34 can be less than the lower rate limit interval for similar reasons here the interval is less than the lower rate limit because the naturally occurring p wave happens sooner than the va interval expires in this case it's the shorter intrinsic a b conduction that leads to the shorter interval between ventricular depolarizations and with this final a-sensed v-sensed beat here it's the combination of both an intrinsic p wave and intrinsic a b conduction that leads to the shorter interval between ventricular depolarizations with modern pacemakers and newer algorithms timing cycles can be much more complex than this but this is the basic idea of how the settings impact the timing of cardiac events in addition to those settings that are
15:20 related to timing cycles there are three additional important ones to discuss first is the sensitivity this is the voltage a lead must measure in order to respond to an intrinsic depolarization that is to either inhibit or to trigger a pacing spike the lower the sensitivity setting the more sensitive the pacemaker is and vice versa sensitivity should have a safety margin of about two so if an atrial depolarization has an amplitude of 1.0 millivolts the sensitivity should be no higher than 0.5 millivolts typical sensitivity settings for the device out of the box are 0.5 millivolts for an atrial lead and 2 millivolts for a ventricular lead in contrast to all the other settings
16:06 i'm discussing in this video lead sensitivities are usually not changed once the pacemaker has been implanted the output is the voltage the pacemaker delivers to the myocardium in order to pace the heart the ability of a pacemaker's output to initiate depolarization is dependent upon both the voltage and the pulse width of the pacemaker spike the higher the voltage the shorter the pulse width needs to be and vice versa the output should be high enough to pace 100 of the time that the pacemaker intends to but not so high as to unnecessarily waste battery life typical output is 1.5 to 2 times the threshold at which the pacemaker can initiate depolarization known as capturing the
16:51 heart contemporary pacemakers auto-adjust the output daily based on self-measured thresholds to ensure capture the last additional setting of notes is something called node switching which is really a collection of settings that determine when the pacing mode should automatically switch from one to another with changes in the heart's intrinsic rhythm for example in a particularly common mode switch a pacemaker in ddd mode will switch to either vvi or ddi if sinus rhythm were to convert into atrial fibrillation atrial flutter or another superventricular tachycardia in order to prevent extreme ventricular rates there are also settings regarding rate modulation which i'm not listing here but they include the range of heart
17:37 rates the speed of acceleration and deceleration of heart rates and the degree of activity required to trigger a change in rate with all these modes and settings one might wonder how a cardiologist chooses them for a specific patient i mentioned common indications for some of the modes but it's obviously a lot more complicated than that it would take an electrophysiology fellowship to fully understand it but some of the considerations include the patient's underlying conduction problem the underlying rhythm the desire to avoid rv pacing which helps to preserve cardiac output and decreases the risk of pacing induced cardiomyopathy we want to preserve ab synchrony whenever possible since this
18:23 also helps to preserve the cardiac output and also decreases the risk of pacemaker syndrome and last we want to preserve battery life as much as possible by minimizing unnecessary pacing and optimizing pacing output that's it for this video on modes and settings and the final one in the pacemaker series will cover icd's biventricular pacemakers and the effects of magnets and mri scanners on these devices you
