Block

Pacemaker: SA node

Conduction: each sinus

impulse is held at the AV

node for progressively

longer times until one is

blocked entirely; then the

cycle starts over

Rate: atrial rate is normal;

ventricular rate slightly

slower than atrial rate

Regularity: atria regular;

ventricles always irregular

in a pattern of grouped

beating

As the sinus node initiates impulses, each one is delayed in the AV node a little longer than the

preceding one, until one impulse is eventually blocked completely. Those impulses that are

conducted travel normally through the ventricles.

184 Chapter 7

However, it will consistently follow a pattern of increasing PRIs until one P wave is

not followed by a QRS complex. Regardless of the conduction ratio, a Wenckebach

will always have progressively longer intervals with blocked

P waves. (Figure 55)

32. As with Type II Second-Degree Heart Block, those P waves that are conducted are

expected to produce normal QRS complexes, meaning that the QRS measurements in

Wenckebach should be less than second. (Of course, we know

that all the blocks frequently have wide QRS complexes because they so often have associated

conduction disturbances lower in the conduction system. However, Wenckebach alone would

have a normal QRS measurement.)

33. Because the PR intervals are changing in Wenckebach and some of the QRS complexes are being dropped, the R–R intervals will be irregular. The changing PRI creates a cyclic pattern to the irregularity. Wenckebach has an

R–R interval that reflects the changes in the PR intervals.

34. However, Wenckebach does not usually block out as many P waves as does Type II

Second-Degree Heart Block. Therefore, the rate of a Wenckebach is generally faster

than Type II block but probably in the low/normal range. Because Wenckebach usually conducts two out of three, or three out of four, impulses, the ventricular rate will

be somewhat slower than normal but still than a Type II

Second-Degree Heart Block.

PR

0.12

irregular

faster

Figure 55 Examples of Conduction Patterns in Wenckebach

P P 0.16 0.20 0.16 0.20 P 0.16 0.20

P P 0.16 0.18 0.24 0.16 0.18 0.24

Heart Blocks 185

35. Here are the rules for Wenckebach Type I Second-Degree Heart Block (Figure 56):

Regularity: irregular in a pattern of grouped beating

Rate: usually slightly slower than normal

P Waves: upright and uniform; some P waves are not followed by QRS

complexes

PRI: progressively lengthens until one P wave is blocked

QRS: less than 0.12 second

Third-Degree Heart Block

(Complete Heart Block)

36. You now know that First-Degree Heart Block is simply a in

conduction of impulses from the SA node through the AV node, but each of the impulses

is conducted. Both types of Second-Degree Heart Block have intermittent AV conduction, where some impulses are conducted but others are . We’ll

now look at Third-Degree Heart Block, where none of the impulses is conducted because

of a total block at the AV node. Third-Degree Heart Block is also called Complete Heart

Block (CHB) (Figure 57) because the block at the is complete.

37. The pathology of CHB is at the AV node; the higher pacemaker in the SA node is

not affected. Therefore, the P waves will be normal, and atrial activity will be within a

normal rate range. However, all of the P waves are blocked at the node. This means that

the ventricles won’t be , and, unless one of the heart’s fail-safe

mechanisms comes into play, they won’t be able to to pump

blood. Because CHB involves a total block at the AV node, a lower escape mechanism

will have to take over to the ventricles.

delay

blocked

AV node

depolarized

contract

depolarize

Figure 56 Rules for Wenckebach

Wenckebach

Regularity: The R–R interval is irregular in a pattern of grouped beating.

Rate: Since some beats are not conducted, the ventricular rate is usually slightly slower than normal

(< 100bpm). The atrial rate is normal (60–100 bpm).

P Waves: The P waves are upright and uniform. Some P waves are not followed by QRS complexes.

PRI: The PR intervals get progressively longer, until one P wave is not followed by a QRS complex. After the

blocked beat, the cycle starts again.

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

186 Chapter 7

38. If possible, a junctional focus below the block site will take over pacemaking

responsibilities by initiating a junctional rhythm to depolarize the ventricles. However, if damage to the node extends into the junction, a

ventricular focus may have to assume pacemaking responsibility. In either case, the

ventricles are controlled by a lower escape focus. In CHB, the SA node functions

normally but cannot get past the block at the AV node, so a lower escape focus in

either the AV junction or the takes over to control ventricular

activity.

39. This means that ventricular activity will fall into one of two categories. If the

escape focus originated in the AV junction, the rate will be in the range of

 bpm, and the QRS complex will measure less than

0.12 second. But if a ventricular focus initiates the escape rhythm, the rate will be

 bpm, and the QRS will be wider than 0.12 second because of

longer conduction time within the ventricles. This information can help you determine

the source of the escape pacemaker. If the rate is 20–40 bpm and the QRS complex

is greater than 0.12 second, you assume that the impulse is

in origin. But if the junction initiated the rhythm, the QRS complex is usually less than

0.12 second and the rate will be bpm. Remember, though,

that because lower sites are less reliable than higher sites, and because the blocks often

involve more than one type of conduction pathology, both the rate and QRS ranges

are guidelines rather than concrete rules.

40. While all this takes place, the SA node continues to control the atria. When you

have two pacemakers controlling the upper and lower chambers of the heart without

regard to each other, the situation is called atrioventricular (A–V) dissociation—the

atria and the ventricles are dissociated (Figure 58). A–V dissociation is not a rhythm in

itself. It is a description of the condition that exists in CHB (and some other arrhythmias) when the atria and ventricles function totally of each

other. On the EKG you will see normal P waves marching regularly across the strip.

You will also see QRS complexes at regular intervals. But the two wave forms will not

have any to each other. The PRIs will be totally inconsistent,

and you may even see P waves superimposed in the middle of QRS complexes. There

will be more P waves than QRS complexes because the intrinsic rate of the sinus node

is than either the junctional or ventricular escape pacemaker.

In CHB, the waves will have absolutely no relation to the

QRS complexes, and you may even see P waves superimposed on QRS complexes.

escape

ventricles

40–60

20–40

ventricular

40–60

independently

relation

faster

P

Figure 57 Mechanism of Complete Heart Block (CHB)

Complete

Block

Pacemaker: the SA node

is firing, but an escape

pacemaker (junctional or

ventricular) below the

block is controlling the heart

Conduction: no sinus

impulse gets through

the AV node, if a

junctional focus is in

control, conduction

through the ventricles is

normal; if a ventricular

pacemaker is in control,

conduction will be

delayed

Rate: atrial rate is normal,

but ventricular rate is

slower than normal

Regularity: atria regular,

ventricles regular

The block at the AV node is complete. The sinus beats cannot penetrate the node and thus are not

conducted through to the ventricles. An escape mechanism from either the junction or the ventricles

will take over to pace the ventricles. The atria and ventricles function in a totally dissociated fashion.

Heart Blocks 187

41. As with other forms of AV block, the PRI is one of your most important clues

to interpreting CHB. In CHB, the PRIs are totally inconsistent across the strip. The

P waves have no relation to the QRS complexes; thus, the PR intervals will not

be .

42. Another important feature about CHB is that the R–R interval is regular. This is an

important item to remember because the PRIs can occasionally appear to be progressively lengthening and can be confused with Wenckebach. This is purely coincidental,

however, because the atria and ventricles are completely in

Third-Degree Heart Block. If you are trying to distinguish a Wenckebach from a CHB,

you should recall that the R–R interval in CHB is , whereas in

Wenckebach the R–R interval is .

constant

dissociated

regular

irregular

Figure 58 A–V Dissociation in CHB

Atrial Activity (Rate 75)

(a)

(b)

Ventricular Activity (Rate 47)

(c)

Combined Atrial and Ventricular Activity Showing A–V Dissociation

188 Chapter 7

43. Here are the rules for Third-Degree Heart Block (CHB) (Figure 59):

Regularity: regular

Rate: AR—usually normal (60–100 bpm);

VR—40–60 if focus is junctional; 20–40 if focus is ventricular

P Waves: upright and uniform; more P waves than QRS complexes

PRI: no relationship between P waves and QRS complexes; P waves can

occasionally be found superimposed on the QRS complexes

QRS: less than 0.12 second if focus is junctional; 0.12 second or greater if

focus is ventricular

44. Third-Degree Heart Block (CHB) is a total block at the AV node, resulting in

A–V dissociation. On the EKG, this is seen as P waves and QRS complexes that have

no to each other.

45. In both types of Second-Degree Heart Block, some P waves will initiate QRS complexes, whereas others will be at the AV node. There will be

some P waves that are not followed by QRS complexes, but the QRS complexes that do

exist were initiated by the preceding P waves (Figure 60).

46. In First-Degree Heart Block, there is no real block. Instead, there is a delay in conduction at the AV , resulting in a PR

interval. But all P waves are conducted through to the ventricles.

47. In Wenckebach, the delay at the AV node gets increasingly longer, resulting in progressively longer intervals. Those impulses that are conducted

through produce normal QRS complexes.

relation

blocked

node; prolonged

PR

Figure 59 Rules for Complete Heart Block

Complete Heart Block

Regularity: Both the atrial and the ventricular foci are firing regularly; thus, the P–P intervals and the R–R intervals

are regular.

Rate: The atrial rate will usually be in a normal range. The ventricular rate will be slower. If a junctional focus

is controlling the ventricles, the rate will be 40–60 bpm. If the focus is ventricular, the rate will be

20–40 bpm.

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

PRI: Since the block at the AV node is complete, none of the atrial impulses is conducted through to the

ventricles. There is no PRI. The P waves have no relationship to the QRS complexes. You may

occasionally see a P wave superimposed on the QRS complex.

QRS: If the ventricles are being controlled by a junctional focus, the QRS complex will measure less than

0.12 second. If the focus is ventricular, the QRS will measure 0.12 second or greater.

Heart Blocks 189

48. Type II Second-Degree Heart Block intermittently conducts some impulses

through the AV node, whereas others are blocked. This means that some P waves

will not produce a QRS complex. However, those that do will have a PR interval that

is across the strip.

49. First-Degree Heart Block is actually a feature within a rhythm rather than an

arrhythmia itself. Therefore, a rhythm with First-Degree Heart Block can be regular or

irregular, depending on the regularity of the rhythm.

50. Wenckebach always has an R–R interval due to the

progressively lengthening PRIs and the dropped QRS complexes. This type of

Second-Degree Heart Block often has a visible pattern of “grouping” of the QRS complexes, emphasized by the missing QRS complex. This is frequently the feature that separates Wenckebach from CHB because CHB has R–R intervals.

51. You now have a very good foundation for approaching the heart blocks. As you

go over the arrhythmias in the Practice Strips at the end of this chapter, remember

to use your systematic approach for gathering all of the data available for each strip.

To differentiate between block types, pay particular attention to the PR intervals,

because this will give you the most information about AV nodal activity.

constant

underlying

irregular

regular

Practice Strips

P–P R–R PRI CONDUCTION

FIRSTDEGREE

Regular Usually regular

(depending on

underlying

rhythm)

Greater than

0.20 second,

constant

One P wave for every

QRS complex

SECONDDEGREE

Type I Wenckebach Regular Irregular Increasingly

longer until

one P wave is

blocked

More P waves than

QRS complexes

Type II Regular Usually regular

(can be irregular

if conduction

ratio varies)

Constant on

conducted beats

(can be greater

than 0.20 second)

More P waves than

QRS complexes

THIRDDEGREE

(CHB)

Regular Regular PRI not constant;

no relation of

P waves to QRS

complexes

(P waves march

through)

More P waves than

QRS complexes

Figure 60 The Heart Blocks

190

KEY POINTS

■ The arrhythmias categorized as heart blocks are caused

by conduction disturbances at the AV node.

■ The four types of heart block we learned in this chapter

are:

First-Degree: not actually a block; merely a delay in

conduction

Second-Degree Type I (Wenckebach): an intermittent

block; each beat is progressively delayed until one is

blocked

Second-Degree Type II: an intermittent block; the

node selectively lets some beats through and blocks

others

Third-Degree (CHB): a complete block; none of the

supraventricular pacemaker impulses is conducted

through the node to the ventricles; the ventricles are

depolarized by a dissociated pacemaker from below

the site of the block

■ A First-Degree Heart Block is not a rhythm itself but is

a condition that is superimposed on another rhythm.

Therefore, when identifying a First-Degree Heart Block,

you must also identify the underlying rhythm.

■ Here are the rules for First-Degree Heart Block:

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

■ Wenckebach is a characteristic cyclic pattern in which

the PRIs get longer and longer until one P wave does not

produce a QRS complex. This cycle repeats itself, producing grouping of the R waves.

■ The rules for Wenckebach are:

Regularity: irregular in a pattern of grouped beating

Rate: usually slightly slower than normal

P Waves: upright and uniform; some P waves are

not followed by QRS complexes

PRI: progressively lengthens until one P wave

is blocked

QRS: less than 0.12 second

■ In Type II Second-Degree Heart Block, the AV node

blocks many of the impulses, creating two, three, four,

or even more P waves for every QRS complex.

■ The rules for Type II Second-Degree Heart Block are:

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

P–P interval is regular

Rate: usually in the bradycardia range; can be

one-half to one-third 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

■ In Third-Degree Heart Block (CHB), there is a total

obstruction at the AV node, resulting in A–V dissociation. The atria and ventricles are totally dissociated from

each other.

■ In CHB, the ventricles can be controlled by either a junctional or a ventricular escape rhythm. The lower pacemaker site can be identified by looking at the ventricular

rate and the width of the QRS.

■ The rules for Complete Heart Block are:

Regularity: regular

Rate: Atrial: usually 60–100 bpm; Ventricular:

40–60 if focus is junctional; 20–40 if focus

is ventricular

P Waves: upright and uniform; more P waves than

QRS complexes

PRI: no relationship between P waves and

QRS complexes; P waves can occasionally be found superimposed on QRS

complexes

QRS: less than 0.12 second if focus is junctional; 0.12 second or greater if focus is

ventricular

SELF-TEST

Directions: Complete this self-evaluation of the information

you have learned from this chapter. If your answers are all

correct and you feel comfortable with your understanding

of the material, proceed to the next chapter. However, if you

miss any of the questions, you should review the referenced

frames before proceeding. If you feel unsure of any of the

underlying principles, invest the time now to go back over

the entire chapter. Do not proceed with the next chapter

until you are very comfortable with the material in this

chapter.

Heart Blocks 191

Questions Referenced Frames Answers

1. What kind of disturbance causes the arrhythmias you

learned in this chapter?

1 conduction disturbances in the

AV node

2. Which of the arrhythmias you learned in this chapter

is not a true block?

2, 3, 4, 46 First-Degree Heart Block is

not a true block; it is a delay in

conduction.

3. Which of the wave patterns on the EKG will yield

information about the AV node?

9 the PR interval (specifically, the

PR segment), since it will tell you

the relationship between the atria

and the ventricles

4. What will the PRI be like in a First-Degree Heart

Block?

4, 9, 10, 11, 14, 46 It will be longer than normal,

greater than 0.20 second.

5. What is the rate of a First-Degree Heart Block? 12, 14 First-Degree Heart Block is not

a rhythm in itself; thus, it cannot

have a rate. The rate of the rhythm

will depend on the underlying

rhythm.

6. Is a First-Degree Heart Block regular or irregular? 11, 14, 49 Again, this will depend on the

regularity of the underlying

rhythm.

7. In addition to identifying a First-Degree Heart Block,

what other information must you provide in order for

your interpretation to be complete?

12, 13 the identity of the underlying

rhythm

8. Does the PRI in First-Degree Heart Block vary from

one beat to the next?

10, 11, 14 No, it remains constant across the

strip.

9. In First-Degree Heart Block, how many P waves will

you see for every complex?

14 One; all beats are eventually

conducted QRS through to the

ventricles, even though each

one encounters a delay at the

AV node.

10. Is the QRS measurement also prolonged in FirstDegree Heart Block?

11, 35 No; once the impulse passes

through the AV node, conduction

through the ventricles is normal.

11. In Wenckebach, do any of the sinus impulses get

through the AV node to depolarize the ventricles?

2, 5, 15, 18, 19, 20, 29,

32, 45, 47

Yes, most of them do; but the

AV node holds each one a little

longer than the preceding one,

until one is blocked completely.

Then the cycle starts over.

12. What is the ventricular rate of a Wenckebach? 34, 35 It’s usually just a little bit slower

than normal, since most of the

impulses are conducted.

13. Is the R–R interval regular in a Wenckebach? 33, 35, 50 No; it is irregular in a pattern of

grouped beating.

14. Does a Wenckebach have a regular P–P interval? 29, 35 Yes; even though the PRIs and

the R–Rs change, the

P–P remains regular.

15. Is the R–R interval grossly irregular in a Wenckebach? 33, 35, 50 No; it has a distinctive cyclic

pattern of grouped beating.

16. Does a Wenckebach produce one P wave for every

QRS complex?

5, 15, 20, 29, 30, 31,

35, 45

No; most P waves are followed

by QRS complexes, but some

P waves are not conducted

through to the ventricles.

192 Chapter 7

Questions Referenced Frames Answers

17. What is the key feature of a Wenckebach? 21, 30 progressively lengthening PRIs

with eventual blocked impulses

18. Does Type II Second-Degree Heart Block have an

equal number of P waves and QRS complexes?

17, 20, 21, 22, 23, 24,

28, 45, 48

No; a Type II Second-Degree

Heart Block will always have more

P waves than QRS complexes.

19. Is the PRI of a Type II Second-Degree Heart Block

constant, or does it vary between beats?

24, 25, 28, 48 It’s constant. This is a key

diagnostic feature that helps

distinguish it from Wenckebach

and CHB.

20. Is the PRI measurement normal in Type II SecondDegree Heart Block?

25, 28 It can be normal or it can

be prolonged. Whatever the

measurement, however, it will

always be constant.

21. What is the usual rate range for a Type II SecondDegree Heart Block?

26, 28 Because most of the P waves

are being blocked, it will be in

the bradycardia range; usually

one-half to one-third the normal

rate.

22. What is meant by a variable conduction ratio? 27 It means that the AV node is

varying the pattern in which sinus

impulses are being conducted

to the ventricles. It changes from

one beat to the next (e.g., 4:3, 5:4,

4:3, 5:4, etc.).

23. Is the R–R interval regular or irregular in a Type II

Second-Degree Heart Block?

27, 28 It will be regular unless the

conduction ratio is variable, in

which case the rhythm will be

irregular.

24. Is the QRS measurement normal or abnormal in a

Type II Second-Degree Heart Block?

18, 19, 28 It should be normal because

those impulses that are allowed

to pass through the AV node are

expected to continue on through

the ventricles in a normal way.

25. In Third-Degree Heart Block (CHB), do any of the

impulses from the SA node penetrate the AV node to

depolarize the ventricles?

36, 37, 40, 44 No. In CHB, the block at the AV

node is complete. None of the

sinus impulses passes through to

the ventricles.

26. In CHB, will there be more P waves or more QRS

complexes on the EKG?

37, 43 There will be more P waves.

27. If none of the sinus impulses is able to depolarize the ventricles, what focus is producing the QRS

complexes?

38, 40 A lower site will take over at an

escape rate. This rhythm can be

either junctional or ventricular in

origin.

28. How would you differentiate between a junctional

focus and a ventricular focus in a CHB?

39, 40, 43 Junctional focus: rate 40–60 bpm,

QRS less than 0.12 second;

Ventricular focus: rate 20–40 bpm,

QRS 0.12 second or more.

29. Is CHB regular or irregular? 42, 43 Regular; this will help you

distinguish it from Wenckebach.

30. What will the PRI be in a CHB? 40, 43, 44 There is no PRI because the

P waves have no relationship to

the QRS complexes; the atria and

ventricles are dissociated.

Heart Blocks 193

PRACTICE STRIPS (answers can be found in the Answer Key on page 559)

7.1

7.2