.

1. In the preceding chapter, you learned about four arrhythmias that originate in the

AV junction. You will next learn about four different arrhythmias that don’t actually

originate in the AV junction but are the result of conduction disturbances at the AV

node (or sometimes just below it, within or below the Bundle of His). Each of these

arrhythmias is caused when an impulse that originates above the AV node, usually

in the sinus node, has trouble getting through the AV node to the ventricles. This category of arrhythmias is most commonly called heart block because conduction fails

to make it through the AV node to depolarize the ventricles normally. Heart blocks

Heart Blocks

7

Heart Blocks 175

are arrhythmias caused when a supraventricular impulse is unable to be conducted

normally through to the ventricles because of a conduction disturbance at or below

the . Because this is a basic book, we will not explore the different pathology sites around the AV node. Instead, we are going to cluster these areas

of pathology by calling them all conduction problems at the AV node. You will know

that when we say the block is “at the node,” we include the entire area of the node.

This means that the block might be in the node, below the node, and even lower in the

Bundle of .

Note: You may hear people refer to another type of block, called bundle branch block. This

is a condition caused by a conduction defect below the area of the node, within one of

the branches of the ventricular conduction system. Bundle branch block is not a rhythm

itself, but it does cause a rhythm to have an abnormally wide QRS. Because it is not an

arrhythmia, and because it is analyzed using a 12-lead EKG rather than the single-lead

rhythm strips we are learning about here, we will not learn about bundle branch block

now. It is mentioned here only so you will realize that it is different from the AV heart

blocks discussed in this chapter.

Heart Blocks

2. The different types of AV heart blocks are categorized according to the severity

of obstruction at the AV node. A First-Degree Block indicates that the obstruction at

the is not complete; all impulses are conducted but each undergoes a delay before being transmitted to the . A Second-Degree

Heart Block means that there is an actual block, but it is intermittent; some of

the impulses will be conducted through to the ventricles, but others will not.

A Third-Degree Block means that the block is complete; that is, none of the impulses

will be conducted through to the ventricles. First-Degree Block is the mildest because it

is a delay rather than an actual block. In a First-Degree Block, each impulse is delayed

but all are conducted through to the . A Second-Degree Block

is more serious because some impulses are actually , whereas

others are allowed to be conducted through to the ventricles. Third-Degree is the

most serious because of the impulses reach the ventricles;

Third-Degree Block is also called a Complete Heart Block (CHB).

3. This is an easy way to think of the blocks:

When all beats are conducted, it’s -Degree Heart Block.

When some beats are conducted, it’s -Degree Heart Block.

And if no beats are conducted, it’s -Degree Heart Block.

Both First- and Third-Degree Block categories have one arrhythmia apiece to learn.

Second-Degree Block has two: Type I Wencheback (pronounced wink’-ee-bok), and

Type II, which technically includes several different pathology types; however, detailing them goes beyond basic arrhythmias, so they’re all clustered together as Type II.

So, you’ll be learning four types of heart block in this chapter:

1. First Degree

2. Second Degree Type I (Wenckebach)

3. Second Degree Type II

4. Third Degree (Complete Heart Block)

Each of these is considered a heart block because there is a disturbance in conduction

through the AV .

AV node

His

AV node

ventricles

ventricles

blocked

none

First

Second

Third

node

176 Chapter 7

4. A First-Degree Heart Block is not really a true block at all, because each impulse is

conducted through to the ventricles. But it is included with the blocks because a partial

block exists, which causes a in transmission of each impulse

to the ventricles.

5. Both types of Second-Degree Heart Block exhibit some type of intermittent block at

the AV node. Both types allow some impulses through to the ventricles, whereas others

are .

6. There are two types of Second-Degree Heart Block: Type I and Type II. Both types

of Second-Degree Heart Block allow some impulses through to the ventricles while

intermittently others.

7. Third-Degree Block is called Heart Block (CHB) because

all impulses are completely at the AV node; no impulses are

allowed through to the ventricles.

First-Degree Heart Block

8. Now that you have a general idea of the types of heart blocks and the mechanisms of each, let’s take each one individually and examine it in more detail.

We’ll start with First-Degree Heart Block (Figure 47) because it is the least serious.

First-Degree Heart Block is the least of all the heart blocks

because, even though it does cause a in conduction, it still

allows impulses through to the ventricles.

9. As you recall, atrial depolarization is depicted on the EKG by the

, and the delay in the AV node is shown by

the segment. Together, these make up the PR

. Thus, if a heart block causes an increased delay in the AV

node, you would expect the PRI to become prolonged. This is one of the foremost

delay

blocked

blocking

Complete

blocked

serious

delay

all

P wave

PR

interval

MECHANISM OF BLOCK

FIRST-DEGREE

• Not a true block

• Delay at the AV node

• Each impulse is eventually conducted

SECOND-DEGREE

TYPE I (Wenckebach)

TYPE II

• Intermittent block: some beats are conducted, others are blocked

• Pathology can be within the AV node or below it in the Bundle

of His

THIRD-DEGREE

(CHB)

• Atria and ventricles are completely dissociated

• There is a total block at the AV node

Figure 46 AV Heart Blocks

Heart Blocks 177

clues to a First-Degree Heart Block. In First-Degree Heart Block, the PRI is

 than usual.

10. It’s important to keep in mind with First-Degree Heart Block that each sinus

impulse, even though delayed in the AV node, does eventually reach the ventricles to

depolarize them. The PRI will be but will be the same duration from one beat to the next. This is because each pacemaker impulse is coming

from the same site (usually the SA node) and is being conducted in the same manner

through the AV node. So each impulse takes the same amount of time to pass through

the atria and is delayed the same amount of time in the AV node. Even though the PRI

is in First-Degree Heart Block, all of the PRIs will be the same

length because they all come from the same site and are conducted in the same manner.

11. Thus, by definition, the PRI in First-Degree Heart Block must be longer than 0.20

second and must be constant from one beat to the next. This is the only abnormality in this arrhythmia. Ventricular conduction is normal, producing a QRS complex

of less than second. Because the SA node is the usual pacemaker, the rhythm is usually regular, although this can change if the underlying

rhythm is something other than NSR. First-Degree Heart Block is usually regular, has

a PRI greater than second, and each PRI is the same as all

other across the strip. Because ventricular conduction is normal, the QRS will be less than second.

12. At this point, it should be apparent that First-Degree Heart Block is not really a

rhythm itself but is actually a condition that is superimposed on another arrhythmia. This is an important distinction to keep in mind because you will also need to

identify the underlying arrhythmia. For example, if you have a rhythm that fits all

of the rules for Sinus Tachycardia except that the PRI is prolonged, you would call

the rhythm Sinus Tachycardia with -Degree Heart Block. In

the same way, if the underlying rhythm fits the definition for NSR except that the

PRI was greater than 0.20 second, you would call the arrhythmia a Sinus Rhythm

with Block.

13. First-Degree Heart Block is not a rhythm in itself. It is a prolonged PRI in an otherwise normal rhythm. Thus, in addition to recognizing that a First-Degree Heart Block

exists, you must also identify the rhythm.

longer

prolonged

prolonged

0.12

0.20

PRIs

0.12

First

First-Degree

underlying

Figure 47 Mechanism of First-Degree Heart Block

Pacemaker: SA node Delay

Rate: depends on

underlying rhythm.

Regularity: depends on

underlying rhythm

Conduction: AV node

holds each sinus impulse

longer than normal, but

eventually allows each

impulse to proceed

normally through to the

ventricles

The AV node holds each impulse longer than normal before conducting it through to the ventricles.

Each impulse is eventually conducted. Once into the ventricles, conduction proceeds normally. This

is not a rhythm itself, but a conduction problem affecting an underlying rhythm, which also must be

identified.

178 Chapter 7

14. Here are the rules for First-Degree Heart Block (Figure 48):

Regularity: depends on underlying rhythm

Rate: depends on underlying rhythm

P Waves: upright and uniform; each P wave will be followed by a QRS complex

PRI: greater than 0.20 second; constant across the strip

QRS: less than 0.12 second

Second-Degree Heart Blocks

15. There are two types of Second-Degree Heart Block. Both occur when the AV node

begins selectively blocking impulses that are being initiated in the SA node. On the

EKG this will be seen as normal P waves, but not every one will be followed by a

QRS complex. This indicates that the atria are being depolarized normally but that not

every impulse is being conducted through to the ventricles. Hence, you will see more

 depolarizations than depolarizations. In Second-Degree Heart Block, you will always see more

waves than complexes.

16. With all heart blocks, the problem is the way pacemaker impulses are conducted

through to the ventricles. Because there is no pathology in the sinus node itself, you

would expect the P waves to be in all the blocks, and the

P–P interval should be regular. Where you will see evidence of block is in the EKG features that show the relationship between atrial and ventricular activity, namely, the

 intervals and the ratio of P waves to

complexes. The PRIs might change, and there can be more P waves than

 complexes, but you would expect the P–P intervals to be

 across the strip.

atrial; ventricular

P

QRS

normal

PR; QRS

QRS

regular

Figure 48 Rules for First-Degree Heart Block

First-Degree Heart Block

Regularity: This will depend on the regularity of the underlying rhythm.

Rate: The rate will depend on the rate of the underlying rhythm.

P Waves: The P waves will be upright and uniform. Each P wave will be followed by a QRS complex.

PRI: The PRI will be constant across the entire strip, but it will always be greater than 0.20 second.

QRS: The QRS complex measurement will be less than 0.12 second.

Heart Blocks 179

17. A key feature of Second-Degree Heart Blocks is that not every P wave is followed

by a QRS complex. Sometimes you will see P waves without an associated ventricular

depolarization (Figure 49). The appearance of P waves without a subsequent QRS complex indicates that the atria were depolarized by a pacemaker impulse, but that impulse

was not conducted through to the because it was blocked at

the .

18. An important distinction is that some of the impulses are being conducted to

the ventricles. Therefore, the QRS complexes you do see were conducted from

the same impulse that produced the immediately preceding P wave. When a sinus

impulse passes through the AV node and depolarizes the ventricles normally, the

QRS measurement will be . On those complexes that are preceded by a P wave with a normal PRI, you would expect the QRS to measure less

than second. However, we know that this isn’t always the case

with Second-Degree Block. Conduction is often delayed below the node, causing the

QRS to be greater than second. When this happens, you can

simply identify the rhythm and note that it has a “wide QRS.”

19. Regardless of the QRS measurement, both types of Second-Degree Block will

have some P waves that are followed by QRS complexes and some that are not. Even

though some pacemaker impulses are blocked at the AV node, some do get through to

depolarize the .

20. There are two categories of Second-Degree Heart Block. One is called Wenckebach

(Type I), and the other is called Type II. In both types, the impulse originates in the

sinus node but is conducted through the AV node in an intermittent fashion. That is,

not every P wave will be followed by a QRS complex. These arrhythmias are classified

as Second-Degree Heart Block because some of the impulses are conducted through

the AV node, but others are not. In Second-Degree Heart Block, the AV node is unreliable in conducting impulses. Conduction to the ventricles is accomplished only on

an basis.

21. The difference between Wenckebach and Type II Second-Degree Block is the pattern in which the P waves are blocked. Because the activity of the AV node is depicted

ventricles

AV node

normal

0.12

0.12

ventricles

intermittent

Figure 49 Blocked P Waves

These are non-conducted P waves

(also called “blocked” P waves or

“dropped QRS complexes”)

This P wave is conducted through to the

ventricles to produce this QRS complex

180 Chapter 7

by the interval, the PRI is the most important clue to distinguishing between these two arrhythmias. When attempting to distinguish between

Wenckebach and a Type II Second-Degree Heart Block, you should concentrate on the

 intervals.

Type II Second-Degree Heart Block

22. Let’s skip Wenckebach for the moment and look first at Type II Second-Degree Heart

Block. Type II Heart Block is really a grouping of several types of blocks with similar

mechanisms. We’re not going to learn each of them separately; we’ll just look at them as

a single rhythm. In Type II Second-Degree Heart Block (Figure 50), the AV node selectively chooses either to conduct or to block individual impulses from the SA node. This

results in a pattern of P waves than QRS complexes. Sometimes

the AV node will allow every other P wave to be conducted, resulting in a ratio of two

P waves for one QRS, called a 2:1 conduction ratio. When every third P is conducted,

the pattern is 3:1. You might also see 4:3, 5:4, or other ratios, but there will always be

more waves than complexes.

23. Sometimes the ratio will vary within one strip. That is, rather than maintaining a

constant ratio across the strip, the ratio will change: for example, 4:3, 3:2, 4:3, 3:2, all

within one strip (Figure 51). This is called variable conduction. Conduction ratios refer

to the number of P waves to QRS complexes and can be constant across the strip or

can within a single strip.

24. Regardless of the conduction ratio, there will always be more

 waves than QRS complexes. However, when you do see

QRS complexes, the PR intervals preceding them will all have the same measurement

because conduction through the node proceeds uniformly on conducted beats. Hence,

the PRI in a Type II Second-Degree Block will always be constant from one complex

to the next across the entire strip. This is probably the most important feature about a

Type II Second-Degree Heart Block. The PRI will always be

on those complexes that were conducted.

25. It is also possible for a Type II Second-Degree Heart Block to have a prolonged

PRI. That is, the PRI will be constant across the strip, there will be more than one

P wave for every QRS complex, and the PRI will be greater than 0.20 second on the

conducted beats. However, even though this fits the rules for calling the arrhythmia

PR

PR

more

P; QRS

vary

P

constant

Figure 50 Mechanism of Type II Second-Degree Heart Block

Intermittent

Block

Regularity: depends on

conduction ratio; regular

if ratio is consistent,

irregular if ratio varies

Conduction: AV node

selectively allows some

impulses through while

blocking others. Those

that get through the AV

node proceed normally

through the ventricles

Rate: atrial rate normal;

ventricular rate slow,

usually 1/2 to 1/3 atrial

rate

Pacemaker: SA node

The AV node selectively conducts some beats while blocking others. Those that are not blocked are

conducted through to the ventricles, although they may encounter a slight delay in the node. Once in

the ventricles, conduction proceeds normally.

Heart Blocks 181

a Second-Degree Heart Block with a First-Degree Heart Block, this is a redundant

label and should not be used. Such an arrhythmia should simply be called a Type II

Second-Degree Heart Block, and the PRI duration should be noted separately

(Figure 52). A Type II Second-Degree Heart Block must have a

PRI on conducted beats, and the PRI may even be .

Regardless, it is not called a First-Degree Heart Block but simply a Type II

-Degree Heart Block.

constant

prolonged

Second

Figure 51 Examples of Conduction Ratios in Type II Second-Degree Heart Block

PPPPPPPPP

Atrial Rate: 90

Ventricular Rate: 45

2: 1

PPPPPPPPP

Atrial Rate: 90

Ventricular Rate: 30

3: 1

PPPPPPPPP

Atrial Rate: 90

Ventricular Rate: 22

4:1

PPPPPPPPP

Atrial Rate: 90

Ventricular Varies

Variable

182 Chapter 7

26. Because the normal rate for the sinus node is bpm, and

a Type II Second-Degree Heart Block conducts only some of the sinus impulses, the

ventricular rate for Type II Second-Degree Heart Block will generally be in the bradycardia range. Often the rate will be one-half to one-third the normal rate, depending on

the ratio of conduction. In Type II Second-Degree Heart Block, the ventricular rate will

be than normal because many of the impulses are blocked at

the AV node.

27. The regularity of the R–R intervals will depend on the manner in which the AV

node is blocking the impulses. If it is a regular ratio of block (e.g., always 2:1, or

always 4:3), the ventricular rhythm will be regular. However, if the ratio is

variable (e.g.,  3:2, 4:3, 3:2, 5:4), the ventricular rhythm will be irregular. Type II

Second-Degree Heart Block can be regular or irregular, depending on the

conduction .

28. Here are the rules for Type II Second-Degree Heart Block (Figure 53):

Regularity: R–R interval can be regular or irregular; P–P interval is regular

Rate: usually in the bradycardia range ( 6 < 0 bpm); can be one-half to onethird the normal rate

P Waves: upright and uniform; more than one P wave for every QRS complex

PRI: always constant across the strip; can be greater than 0.20 second

QRS: less than 0.12 second

Wenckebach (Type I Second-Degree

Heart Block)

29. Now that we’ve looked at Type II Second-Degree Heart Block, let’s go back to

Type I. This rhythm is called Wenckebach (Figure 54), after the man who first defined it.

Wenckebach is a -Degree Heart Block, but its conduction pattern is distinctly different than Type II Second-Degree Heart Block, even though they

both result in some “blocked” (non-conducted) beats.

60–100

slower

ratio

Second

Figure 52 Type II Second-Degree Heart Block with a Prolonged PRI

PPP P P P P

PRI 0.28 PRI 0.28 PRI 0.28 PRI 0.28

When a Type II Second-Degree Heart Block has a prolonged PRI on the conducted beats, it is still called a

Second-Degree Heart Block, not a Second-Degree Heart Block with a First-Degree Heart Block.

Heart Blocks 183

30. As with Type II Second-Degree Heart Block, the key to recognizing a Wenckebach

is in the PR intervals. Each PRI will get progressively longer until you see a P wave

without a resultant QRS complex. Then the cycle starts again with the shortest PRI. As

you measure the PRIs across the strip, you will notice a pattern of “long PRI, longer

PRI, longer PRI, longer PRI, blocked P wave.” This conduction cycle runs continuously

across the strip. Wenckebach is characterized by increasingly long PRIs followed by

a P wave.

31. The classic cycle seen with Wenckebach does not have to adhere to the 5:4 conduction ratio described in the previous frame. It can have any variety of conduction

ratios: 4:3 (long, longer, longer, blocked), 3:2 (long, longer, blocked), or even variable.

blocked

Figure 53 Rules for Type II Second-Degree Heart Block

Type II Second-Degree Heart Block

Regularity: If the conduction ratio is consistent, the R–R interval will be constant, and the rhythm will be regular. If

the conduction ratio varies, the R–R will be irregular.

Rate: The atrial rate is usually normal (60–100 bpm). Since many of the atrial impulses are blocked, the

ventricular rate will usually be in the bradycardia range ( 6 < 0bpm), often one-half, one-third, or one-fourth

of the atrial rate.

P Waves: P waves are upright and uniform. There are always more P waves than QRS complexes.

PRI: The PRI on conducted beats will be constant across the strip, although it might be longer than a normal

PRI measurement.

QRS: The QRS complex measurement will be less than 0.12 second.

Figure 54 Mechanism of Wenckebach (Type I Second-Degree Heart Block)

Intermittent