Second-degree AV Blocks

00:01 Let's move on and talk about second-degree AV block.

00:03 Second-degree AV block is more serious in that there are dropped beats.

00:08 There are two types of second-degree AV block. There's type I and type II.

00:13 Type I is often called Wenckebach block or Mobitz type I AV block.

00:18 And you'll see there's a Mobitz type II AV block, both of these are second-degree AV block.

00:23 And Wenckebach and Mobitz were people - physicians who studied electrical activity in the early days of the electrocardiogram.

00:31 What happens with type I second-degree AV block or Wenckebach or Mobitz one, it's characterized by a progressive increase in the PR interval and eventually there's a missing QRS.

00:45 The interval gets so long that the impulse doesn't get through the ventricle and you drop a beat and the sequence repeats itself.

00:54 Now usually it's 3 to 5 beats and a dropped beat.

00:57 For example; three beats and the fourth beat is missing and again, three beats and the fourth beat is missing and so forth.

01:02 Could be up to five beats and the sixth one is missing and you'll see increasing PR interval's, I'm gonna show you that. And of course, the last P-wave is not followed by a QRS.

01:12 So, type I second-degree AV block occurs when the impulse travels down from the sinus node through the atrium and arrives at the AV node.

01:23 And there's a pause before it gets through into the main His-Purkinje system and enter the ventricle, and of course the left bundle and the right bundle, and the Purkinje fibers and the ventricle.

01:36 So, the block occurs higher up in the AV node and therefore the impulse doesn't get through to the ventricles, to the right and left bundle and so forth.

01:45 So, the block of the electrical transmission occurs just above or in the upper portion of the AV node.

01:52 And you can see the little green box showing where that is.

01:58 Now, here's an example; notice, this is a Wenckebach block.

02:02 Three impulses get through with lengthening PR interval and the fourth one is blocked.

02:08 Let's take it one sequence at a time.

02:12 The first the one with the first green box shows you a normal P-wave with a normal PR interval.

02:18 Then there's a QRS and a T.

02:20 The next P is normal-looking, but look, the PR interval has lengthened; the second green box followed by a QRS and T.

02:28 Now look at the third. Oh, the PR interval is even longer QRS and then T, then comes a P-wave.

02:35 Uh-oh, we're missing the QRS and then eventually it resets itself and starts again.

02:41 So, this is 3 to 4, that is three impulses get through to four atrial impulses.

02:50 And you can see the increasing PR intervals.

02:53 So, this is type I second-degree AV block and as it says here, the fourth PR interval is too prolonged to capture the ventricle and therefore it's missing.

03:02 And the next P-wave starts the whole sequence over again.

03:07 Now, here is another example of it and if you stand back and look at this from a distance, you see what's known as group beating. Notice, look at the bottom line or at the top line.

03:23 But the bottom line shows you the P wave nicely. You see four beats and a dropped beat.

03:28 Four beats and a dropped beat and so forth.

03:32 And so, when you hold this out at arm’s length, you see this groups with pauses.

03:36 Groups, abnormal beats and a pause, groups, abnormal beats and a pause.

03:40 And here of course we show you where the missing QRS is.

03:44 The second type of type II AV block is a serious type.

03:49 It's also called high-grade AV block or Mobitz type II, AV block.

03:55 It is lower down in the conduction system, not usually below the AV node.

04:03 And it consists of P-wave that are repeatedly not followed by a QRS telling you that there's higher grade block below the AV node.

04:14 It can be 3:1 in which you have three normal beats followed by a blocked beat after the fourth P-wave.

04:23 Or it could be a single blocked beat that is repeated multiple times.

04:32 And the PR intervals of course on the normal beats are normal and then all of a sudden, there's a P-wave with no QRS and that's an example where the fourth beat was blocked, so you had a P-wave with no QRS.

04:46 With 2:1 AV block, it may be difficult to tell whether you're seeing type I or type II.

04:52 And there the electrophysiologist can make measurements often in the electrophysiology laboratory and if it's type II AV block, they often follow that with pacemaker insertion.

05:04 So, let's look at an example. Here is an example of type II second-degree AV block.

05:10 You'll notice the first complex PQRS and T: normal. Second complex, PQRS and T: normal.

05:18 Third complex, PQRS and T: fourth P-wave -- no QRS. And then it takes over normally again; PQRS and T.0 So, this could have been a very - a 2:1 type I block, but it's very worrisome and we would have to make sure that it's not a type II. Why? Because this kind rapidly goes into complete heart block, 3rd degree heart block that could be life-threatening.

05:48 So, it - the P-wave is completely blocked in or below the AV node.

05:54 It can be frequent with many dropped beats and usually is.

05:57 So, when you get a long rhythm strip, you see what's going on.

06:00 And again, it implies serious disease in the conduction system, usually requiring pacemaker therapy.

06:07 Here's another example. You'll notice here, we see it twice.

06:11 There's a normal beat, the first beat is a PQRS and T.

06:16 Then there's a P-wave with the green arrow, no QRS.

06:20 Then we have a normal beat then another P-wave; green arrow, no QRS.

06:25 So, here's two complexes that are lacking conduction to the ventricle.

06:29 And when you take a longer rhythm strip and you see multiples of these, the patient gets almost always a pacemaker because they're very close to 3rd degree heart block.

06:39 Just a little information about it.

06:43 Type I AV block - second-degree AV block can occur in normal, healthy individuals.

06:48 It's common in well trained athletes and in young people.

06:52 It's due to increased vagus nerve activity which goes with training and usually it's benign.

06:58 And when I'm at the University, every year I see some of our super athletes who are training for the Olympics or for a national competition? And they're sent over, "Oh, this person has an irregular heartbeat." And often they have type I second-degree AV block and sinus bradycardia because of the training effect.

07:14 And pacing is definitely not needed. There - we reassure them, this is normal.

07:19 You put a patient like that on the treadmill, their heart rate accelerates normally, so it's not really disease, it's a training effect.

07:26 Now, type II second-degree AV block is the nasty one.

07:30 The block of electrical transmission occurs just below or in a lower portion of the AV node.

07:36 Here, we see the system again.

07:38 So again, both type II's are occurring in and around the AV node but type I in the upper portion or just above. Type II, in the lower portion or just below.

07:48 And type II is more serious because it often can lead on to 3rd degree or complete heart block as we'll talk about.

07:54 And of course, you see the impulse once it gets through the His-Purkinje system, it gets transmitted throughout the ventricle through the left and right bundles and into the ventricular muscle.