Let's move on and talk about second-degree AV block.
Second-degree AV block is more serious in that there are dropped beats.
There are two types of second-degree AV block. There's type I and type II.
Type I is often called Wenckebach block or Mobitz type I AV block.
And you'll see there's a Mobitz type II AV block, both of these are second-degree AV block.
And Wenckebach and Mobitz were people -
physicians who studied electrical activity in the early days of the electrocardiogram.
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.
The interval gets so long that the impulse doesn't get through the ventricle
and you drop a beat and the sequence repeats itself.
Now usually it's 3 to 5 beats and a dropped beat.
For example; three beats and the fourth beat is missing and again, three beats and the fourth beat is missing and so forth.
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.
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.
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.
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.
So, the block of the electrical transmission occurs just above or in the upper portion of the AV node.
And you can see the little green box showing where that is.
Now, here's an example; notice, this is a Wenckebach block.
Three impulses get through with lengthening PR interval and the fourth one is blocked.
Let's take it one sequence at a time.
The first the one with the first green box shows you a normal P-wave with a normal PR interval.
Then there's a QRS and a T.
The next P is normal-looking, but look, the PR interval has lengthened; the second green box followed by a QRS and T.
Now look at the third. Oh, the PR interval is even longer QRS and then T, then comes a P-wave.
Uh-oh, we're missing the QRS and then eventually it resets itself and starts again.
So, this is 3 to 4, that is three impulses get through to four atrial impulses.
And you can see the increasing PR intervals.
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.
And the next P-wave starts the whole sequence over again.
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.
But the bottom line shows you the P wave nicely. You see four beats and a dropped beat.
Four beats and a dropped beat and so forth.
And so, when you hold this out at arm’s length, you see this groups with pauses.
Groups, abnormal beats and a pause, groups, abnormal beats and a pause.
And here of course we show you where the missing QRS is.
The second type of type II AV block is a serious type.
It's also called high-grade AV block or Mobitz type II, AV block.
It is lower down in the conduction system, not usually below the AV node.
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.
It can be 3:1 in which you have three normal beats followed by a blocked beat after the fourth P-wave.
Or it could be a single blocked beat that is repeated multiple times.
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.
With 2:1 AV block, it may be difficult to tell whether you're seeing type I or type II.
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.
So, let's look at an example. Here is an example of type II second-degree AV block.
You'll notice the first complex PQRS and T: normal. Second complex, PQRS and T: normal.
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.
So, it - the P-wave is completely blocked in or below the AV node.
It can be frequent with many dropped beats and usually is.
So, when you get a long rhythm strip, you see what's going on.
And again, it implies serious disease in the conduction system, usually requiring pacemaker therapy.
Here's another example. You'll notice here, we see it twice.
There's a normal beat, the first beat is a PQRS and T.
Then there's a P-wave with the green arrow, no QRS.
Then we have a normal beat then another P-wave; green arrow, no QRS.
So, here's two complexes that are lacking conduction to the ventricle.
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.
Just a little information about it.
Type I AV block - second-degree AV block can occur in normal, healthy individuals.
It's common in well trained athletes and in young people.
It's due to increased vagus nerve activity which goes with training and usually it's benign.
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.
And pacing is definitely not needed. There - we reassure them, this is normal.
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.
Now, type II second-degree AV block is the nasty one.
The block of electrical transmission occurs just below or in a lower portion of the AV node.
Here, we see the system again.
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.
And type II is more serious because it often can lead on to 3rd degree or complete heart block as we'll talk about.
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.