Types of Pacemaker

00:01 So welcome back to the ECG lecture series.

00:04 We just heard about heart block and some heart blocks are treated with pacemakers and so let’s talk about pacemakers.

00:12 Pacemakers are an amazing development.

00:14 They were developed back in the 1960s and they were developed for, “Patients whose heart was too good to die.” What does that mean? That means people develop complete heart block, they had heart rates in the 20s and 30s.

00:27 The cardiac output went down, the kidneys failed and the patients died.

00:31 At autopsy, the heart looked perfect.

00:33 The muscles was good, the valves were good, the coronary arteries were good - they died because of an abnormality in the electrical system that led to the development of pacemakers, which were quite large at that time, placed under the skin in the abdomen.

00:47 Nowadays, the average pacemaker’s a little bit bigger than the watch that you wear on your wrist and consist most of it consist of the battery, the lithium battery and a little chip, that does all of the thinking, is of course very tiny.

01:02 So let’s talk about pacemakers.

01:04 Pacemakers are used to treat either second degree Mobitz 2 block.

01:10 Remember what second degree Mobitz 2 block is? That’s where there are drop beats without lengthening of the PR interval.

01:17 Lengthening of the PR interval can be type 1 secondary AV block - can occur in athletes and in normal people; but type 2, where you’re suddenly dropping beats do not occur in normal people, that means a lot of injury to the conduction system and you’re very close to third degree AV block which means complete heart block with a very slow heart rate and a reduced blood pressure and a reduced heart rate.

01:42 Patients with high grade AV block are usually symptomatic - often they faint because their blood pressure drops.

01:48 They are hypotensive and hypotension with a very slow heart rate means lowered cardiac output and if prolonged can result in renal failure and eventually in death as occurred in the days before pacemakers.

02:03 Syncope or near syncope is very common in these patients, of course, because of the low blood pressure and the pacemaker therapy is required to restore normal heart rate and a normal blood pressure.

02:14 So the pacemakers have a code that tells you where the pacemakers are being paced and where they are being monitored and so forth and I’m gonna take you through the code so when you see the code somebody says to you, “Oh, this patient has a DDD pacemaker,” you’re not standing there thinking, “I don’t know what that means?” So, let’s see what that means.

02:37 So, the first letter is where the active pacing wire is located.

02:42 So, if the first letter is V that means the pacing wire is in the ventricle and it’s essentially always the right ventricle.

02:49 If there is an A, that means the pacing wire is in the atrium and that’s almost always the right atrium.

02:56 We usually don’t put pacing wires on the left-side of the heart because little clots can form there, break off and go to the brain and cause strokes so almost all the pacing occurs in the right side - right atrium or right ventricle.

03:09 And if there’s a D that means there are pacing wires in both the atrium and the ventricle, D for dual.

03:16 Okay, V means pacing in the ventricle, A mean pacing in the atrium, D means pacing in both the atrium and the ventricle - so that’s the first letter.

03:25 So the example I gave before, DDD, that means, D, the first letter is D it means it’s pacing in both the atrium and the ventricle and I will show you examples of all these as we go along.

03:37 The second letter is where the pacemaker is sensing the underlying rhythm because, you know, these days, the pacemakers don’t just keep firing they have a little chip that reads whether there’s an underlying rhythm so they only fire when there’s a failure of the normal beat to occur, so where do they sense whether normal beats are occurring? Well, if the second letter is a V, they're sensing in the ventricle; if the letter is an A, they're sensing in the atrium and if the letter is a D, they’re sensing in both the atrium and the ventricle.

04:10 So remember the example I said, DDD, okay, the first D says that they are pacing in the atrium and the ventricle.

04:17 The second D says they’re sensing in both the atrium and the ventricle, they're looking for P waves and then they’re looking for QRSs.

04:24 Okay, again, the first two letters are D and D means the pacemaker is pacing in both the atrium and the ventricles and sensing in both the atrium and the ventricles.

04:39 The third letter shows which heart chamber the pacemaker is inhibited that is where does it recognize that there’s a regular beat occurring and therefore it does not pace.

04:50 If it’s inhibited in one ventricle there’s an I and if it’s inhibited in both the atrium and the ventricle there’s a D.

04:59 There’s another characterization called triggered, rarely used and really, we almost never see that, it’s where they're looking to trigger a beat and have it inhibited, that really isn't of any major importance, it’s really, I and D that’s important.

05:14 Let’s see the example. A D inhibited pacemaker watches first for an atrial depolarization wave and doesn’t pace if it sees one, then it waits and it’s looking for a QRS from the ventricle.

05:28 If a QRS occurs, the pacemaker does not pace and if neither a P wave or a QRS occur, the pacemaker puts both in.

05:36 So for example, it might happen, that there’s a failure of the atrial depolarization so the atrium is paced but then there’s a normal ventricular depolarization.

05:46 The opposite can occur, there’s a normal P wave but no QRS so the ventricle is paced or neither occur, you’ll see both atria and ventricle being paced.

05:57 The fourth letter is really used against special pacemakers that can increase the rate when patients exercise or get excited because as we talked about before the pacemakers set at a certain heart rate, they will always code that heart rate but what happens if somebody exercises, they need an increased heart rate so there are pacers that follow certain physiological variables for example, skeletal muscle activity or respiratory rate when they see that increasing, they know the person’s exercising, they increased the heart rate which is what happens in normal people when they are exercising.

06:35 So when one of those bodily variables, these are skeletal muscle activity or respiratory rate goes up, the pacemaker increases the rate of pacing and the amount that the heart rate will increase is programmed during implementation but it really can’t increase above 25 beats; but again, if you have patients being paced at 72 and then suddenly, they are exercising, the other heart rate goes up to 97 so that increases the cardiac output so it makes their exercise more physiologic.

07:08 The fifth letter is rarely used, it’s mostly for experimental pacemakers where a multiple site are being used within the atrium or ventricle and these are sometimes special arrhythmia pacemakers for example to control atrial fibrillation, you almost never see it in routine daily clinical work.

07:25 As noted, many of the pacemakers are experimental not used routinely and clinicians almost always deal with a three to four letter code describing the pacemaker implanted and we're gonna take some examples.

07:38 So, here is a VVI pacemaker, first V, where is it sensing? In the ventricle; second V, where is it pacing? In the ventricle; third letter I, it’s inhibited when there is a normal QRS.

07:54 So the pacemaker does not pace or is inhibited when it senses a QRS in the right ventricle.

08:01 What do the four initials stand for in D-D-D-R? The first two stand for sensing and pacing.

08:11 So what does the D stand for? It stands for dual.

08:15 That means the pacemaker senses in both the atria and the ventricle.

08:21 And it can pace in either the atria or the ventricle.

08:25 So, dual sensing, dual pacing.

08:28 The third D stands for inhibition.

08:32 Again, inhibition is dual.

08:34 So, for example, if the sinus node is abnormal, and there's no beat in the atrium, of course, the atrial lead will fire and cause an atrial impulse to pass down into the ventricle.

08:50 However, if there is a normal sinus node beat that passes down through the atria, that pacing is inhibited.

08:59 And the same thing happens in the ventricle.

09:01 If no beat occurs in the ventricle after there is an impulse from above from the atria, then the pacemaker supplies that stimulus and causes a ventricular depolarization.

09:13 But if there is already a ventricular depolarization, the pacing is inhibited in the ventricle.

09:20 And so you can see the pacemaker could put in a beat in the atrium, could put a beat in the ventricle, could put beats in both the atrium and the ventricle, and be inhibited in both the atria and the ventricle.

09:32 And then finally, the R.

09:34 This is a pacemaker that's often put into younger or very active older people who want to exercise.

09:41 And what happens is the pacemaker speeds up when the patient is active.

09:46 It reads increased respiration, or increased muscle activity, and it increases the heart rate, thereby increasing the cardiac output for exercise.

09:56 So that's a clarification of the D, D, D, R pacemaker.

10:03 In this slide, we see a little diagram, a little cartoon, of how the pacemaker is inserted? And where it's inserted? As you know, noted from earlier, it's almost always on the right side of chambers.

10:15 A wire is usually inserted through a catheter that's placed in one of the central veins, often the subclavian vein percutaneously, that is with a needle.

10:27 And the wire is then positioned, either if it's a pacemaker that's in both the atria and the ventricle, you have two wires, one putting the atria, right atrium, one put in the ventricle, and that's what this little diagram shows, or shows a wire in that right atrium and a wire on the right ventricle.

10:43 Then a little Novocaine is put in the skin.

10:46 And then a little incision is made, and a little pocket is made under the skin.

10:50 And that's where the actual pacemaker a box itself is inserted, and the wire is hooked up to it, and the incision is closed.

10:59 And I'll show you a picture of that.

11:00 So, you actually see it.

11:02 You'll notice the pacemakers not in the left atrium, it's not in the left ventricle, it's in the right atrium, and right ventricle.

11:11 And as I said before, it's because little clots can form on the pacemaker, and we don't want them breaking off, and going to the brain, and causing strokes.

11:19 And here's you see the pulse generator, that's the actual box itself.

11:23 And you see the two pacing leads: one in the right atrium, and one in the right ventricle.

11:29 And so, that's where the wires are, that are connected to the box that results in the generation of the impulses when the heart itself does not create an impulse.

11:42 And you can see here, what I said before, the pacemaker is inserted under the skin with the electrodes in the right atrium and the right ventricle.

11:50 And here you can see the tip of the pacemaker in the apex of the right ventricle.

11:56 And here's a chest X-ray showing a patient with a VVI pacemaker.

12:01 That is a single lead in the ventricle.

12:04 So, it's sensing and pacing in the ventricle, and it's inhibited when there's a normal QRS.

12:10 Look, first of all, at the right side of the diagram, which is the patient's left chest.

12:15 You can see the pacemaker there, that's under the skin, and you'll see a wire coming from it, all the way down and into the right ventricle.

12:24 And so this is a typical pattern of so called Posterior/anterior or PA chest X-ray showing a single lead of VVI pacemaker in this patient.

12:36 And here we see the pacer and the wire that's going to go in.

12:39 You see how small the pacer is in this person's normal size palm.

12:43 It's a little bigger than a pocket watch or a wrist watch, but not very much bigger.

12:48 And you'll see the wires quite small and thin.

12:52 And here's one that's been implanted.

12:54 You'll notice the green arrow, there a little clips there closing the incision that was used to make the pocket.

13:00 And you'll see a little bulge just below those clips that bulges is the pacemaker, and you see where there's a little patch there, a little band aid, that's where the wire went in through a jugular correction, through a subclavian vein insertion.

13:19 Sometimes, we'll use the jugular vein but usually, rarely, it's usually the subclavian vein.