Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

194 Chapter 7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.4

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.3

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.6

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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196 Chapter 7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.8

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Rate: ___________________________________________ QRS: ___________________________________________

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7.10

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Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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198 Chapter 7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.12

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Rate: ___________________________________________ QRS: ___________________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Rate: ___________________________________________ QRS: ___________________________________________

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204 Chapter 7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.24

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Rate: ___________________________________________ QRS: ___________________________________________

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Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

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Rate: ___________________________________________ QRS: ___________________________________________

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206 Chapter 7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.28

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

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Rate: ___________________________________________ QRS: ___________________________________________

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208 Chapter 7

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Rate: ___________________________________________ QRS: ___________________________________________

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210 Chapter 7

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Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

7.36

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212 Chapter 7

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Heart Blocks 213

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7.41

214

Overview

IN THIS CHAPTER, you will learn the characteristics of rhythms that originate within the

ventricles, and you will find out what things are common to all ventricular arrhythmias. You will

then learn the names and features of five different arrhythmias that originate in the ventricles.

For each of these arrhythmias, you will learn about the etiology, conduction, and resulting EKG

features (regularity, rate, P waves, PR intervals, and QRS complexes).

Ventricular Rhythms

1. All of the arrhythmias you have learned so far are classified as supraventricular

arrhythmias, because they originate above the ventricles. All supraventricular

arrhythmias have one thing in common: they all have QRS complexes of less than

 second duration. This classification has a very scientific

basis. We know from physiological measurements that an impulse originating above

the ventricles, and that follows the normal conduction pathways, can depolarize the

ventricles in less than 0.12 second. We also learned that it is possible for a supraventricular impulse to produce a QRS that is wider than 0.12 second, but that this would

indicate some kind of delay in through the ventricles and

0.12

conduction

Ventricular Rhythms

8

Ventricular Rhythms 215

thus would be considered an abnormality to be noted. Generally speaking, a normal

supraventricular complex will have a measurement of less

than second.

2. We say that a supraventricular arrhythmia should have a QRS complex of less than

0.12 second, but we also acknowledge that an abnormality could cause the QRS to

be wider than that. However, we can say with certainty that a ventricular impulse

cannot depolarize the ventricles in less than 0.12 second. Hence, a basic rule for ventricular arrhythmias is that the QRS measurement will be 0.12 second or greater. If a

complex measures less than 0.12 second, we know that it must have been initiated by

a impulse. But if it is 0.12 second or greater, it might have originated in the , or it could have originated above the ventricles

but encountered a conduction disturbance along the way.

3. Ventricular arrhythmias are very serious for several reasons. First, the heart was

intended to depolarize from the top down. The were meant

to contract before the in order to pump blood effectively.

When an impulse originates in the ventricles, this process is reversed, and the heart’s

efficiency is greatly reduced. Further, since the ventricles are the lowest site in the

conduction system, there are no more fail-safe mechanisms to back up a ventricular

arrhythmia. Ventricular arrhythmias are the most serious arrhythmias because the

heart has lost its and because it is functioning on its last level

of backup support.

4. In this section, we will be learning five ventricular arrhythmias:

• Premature Ventricular Complex (PVC)

• Ventricular Tachycardia

• Ventricular Fibrillation

• Idioventricular Rhythm

• Asystole

Although their mechanisms differ, each of these arrhythmias originates

in the and thus will have a QRS measurement of

 second or more.

Premature Ventricular Complex (PVC)

5. The first arrhythmia is not a rhythm itself, but is instead a single ectopic beat

originating from an irritable ventricular focus. Since it arises from an irritable focus,

the complex will come than expected in the cardiac cycle

and will interrupt the regularity of the underlying rhythm. PVCs (Figure  61) are

single that come earlier than expected and interrupt

the underlying .

6. Because PVCs originate in the ventricles, the QRS will be than

normal. But another important feature of a ventricular focus is that there is no P wave

preceding the QRS complex. Since the SA node did not precipitate the ventricular depolarization, there will be no P wave. On the EKG, you will see a very wide, bizarre QRS

complex that is not preceded by a wave.

7. One of the things that gives a PVC such a bizarre appearance, in addition to

the width of the QRS complex, is the tendency for PVCs to produce a T wave that

QRS

0.12

supraventricular

ventricles

atria

ventricles

effectiveness

ventricles

0.12

earlier

ectopics

rhythm

wider

P

216 Chapter 8

extends in the opposite direction of the QRS complex (Figure 62). That is, if the QRS is

negative, the T wave will be . This is not a hard-and-fast rule,

but is a very frequent finding that contributes to an overall

appearance of a PVC.

8. PVCs are usually easy to spot because they are wide and bizarre, with a QRS

complex measurement of second or more, and they are not

preceded by a wave (Figure 63). Another feature common to

many PVCs is that the T wave is in the opposite direction of the

complex.

9. PVCs come earlier than expected, but they don’t conduct impulses back through the

AV node and into the atria. Therefore, the atria are not depolarized. This leaves the sinus

node undisturbed; it continues to discharge regularly. The result is that the distance

between the normal complex preceding the PVC and the normal complex following

the PVC will measure exactly twice the distance of one R–R interval in the underlying

rhythm (Figure  64). This feature is called a compensatory pause. If an ectopic is

followed by a pause, it is a good indication that the ectopic is

a . The presence of a compensatory pause helps identify the

ectopic as a PVC.

upright

bizarre

0.12

P

QRS

 

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

 .

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

 

arrhythmia; describe

P

rate

regular

P; rate

impossible

Figure 45 Overlapping Rates in Supraventricular Tachycardia

RATE (bpm)

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250

Sinus Tachycardia

Atrial Tachycardia

Atrial Flutter

Junctional

Tachycardia

Junctional Rhythms 149

48. You now know about four more arrhythmias—those that originate in the AV junction. You have learned the characteristics that these arrhythmias share and how they differ from rhythms that originate in other pacemaker sites. You also learned that several

of the tachycardias cannot always be differentiated, because they might have similar

rate ranges and obscured P waves. When this situation exists, you can only describe the

rhythm as a Supraventricular Tachycardia, rather than giving it a specific name. Now

you must practice applying this knowledge to actual interpretation of rhythm strips.

Turn to the Practice Strips at the end of this chapter and practice applying your new

knowledge until you feel very comfortable in this area.

Practice Strips

150

KEY POINTS

■ Rhythms that originate in the AV junction include:

• Premature Junctional Complex

• Junctional Escape Rhythm

• Accelerated Junctional Rhythm

• Junctional Tachycardia

■ Junctional arrhythmias will create an inverted

P wave because the atria are depolarized via retrograde

conduction.

■ A junctional impulse will depolarize the ventricles in a

normal manner at about the same time that the atria are

being depolarized with retrograde conduction.

■ A junctional rhythm can have the inverted P wave occurring before, during, or after the QRS complex.

■ All junctional arrhythmias will create an inverted P wave,

but some low atrial impulses can also cause inverted

P waves.

■ All junctional arrhythmias will have a PRI of less than

0.12 second.

■ The rules for Premature Junctional Complex are:

Regularity: depends on regularity of underlying

arrhythmia

Rate: depends on rate of underlying arrhythmia

P Waves: will be inverted; can fall before, during, or

after the QRS complex

PRI: can only be measured if the P wave precedes the QRS complex; if measurable,

will be less than 0.12 second

QRS: less than 0.12 second

■ A PJC is an irritable ectopic.

■ The normal inherent rate of the AV junction is 40–60 bpm.

■ If higher pacemaker sites fail, a junctional escape pacemaker might take over control of the heart. This is called

Junctional Escape Rhythm.

■ The rules for Junctional Escape Rhythm:

Regularity: regular

Rate: 40–60 bpm

P Waves: will be inverted; can fall before, during, or

after the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if measurable,

will be less than 0.12 second

QRS: less than 0.12 second

■ If the AV junction becomes irritable, it can speed up and

override higher pacemaker sites. This arrhythmia is

called Junctional Tachycardia.

■ Junctional Tachycardia is usually divided into two categories, depending on rate:

60–100 bpm Accelerated Junctional Rhythm

100–180 bpm Junctional Tachycardia

■ The rules for Accelerated Junctional Rhythm are:

Regularity: regular

Rate: 60–100 bpm

P Waves: will be inverted; can fall before, during, or

after the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if measurable,

will be less than 0.12 second

QRS: less than 0.12 second

■ The rules for Junctional Tachycardia are:

Regularity: regular

Rate: 100–180 bpm

P Waves: will be inverted; can fall before, during, or

after the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if measurable,

will be less than 0.12 second

QRS: less than 0.12 second

■ If a rapid arrhythmia is regular, has no visible P waves,

and has a rate range common to other arrhythmias,

thereby making more accurate identification impossible, the arrhythmia is termed Supraventricular

Tachycardia.

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.

Junctional Rhythms 151

Questions Referenced Frames Answers

1. What does the P wave look like in a junctional

rhythm?

5, 6, 7, 8, 10, 11, 17, 44 The P is always inverted, even

though it is often hidden in the

QRS complex.

2. How do you explain the unusual configuration of the

P wave in junctional rhythms?

2, 3, 4, 5, 6, 7 The atria are depolarized via

retrograde conduction. Since the

electrical flow will be traveling

away from the positive electrode

in Lead II, the wave form will be

negative.

3. Will the QRS complex be normal in a junctional

rhythm?

3, 5, 10 Yes, even though atrial

depolarization is retrograde,

ventricular depolarization will be

normal. The QRS complex should

measure less than 0.12 second.

4. Does an inverted P wave always indicate a junctional

rhythm?

8, 9, 12, 13, 14 No, atrial rhythms can also have

inverted P waves if the impulse

originated low enough in the atria.

You can differentiate between

the two by looking at the PRI; in

a junctional rhythm it will be less

than 0.12 second.

5. Where will the P wave be located in a junctional

rhythm?

6, 7, 10, 15, 16, 17, 41 It can fall before or after the QRS

complex, or it can be hidden

within the QRS complex. In the

latter, it will appear as if the

rhythm has no P wave at all.

6. What is the biggest clue to a junctional rhythm? 8 the inverted P wave

7. What is a PJC? 19, 20, 21, 25, 43 A Premature Junctional Complex

is a single beat that originates

from an irritable focus in the AV

junction.

8. Will a PJC have a P wave? 22, 23, 25 Yes, the P wave will be inverted,

either before or after the QRS

complex or it can be hidden

within the QRS complex.

9. What will the PRI be for a PJC? 23, 25 If the P wave is in front of the

QRS complex, the PRI will be

less than 0.12 second. Otherwise,

there will be no PRI.

10. Does a PJC come earlier or later than expected in the

cardiac cycle?

19, 20, 25 earlier

11. What will the QRS measurement be for a PJC? 24, 25 Since conduction through the

ventricles is normal, the QRS

measurement should be less than

0.12 second.

12. What is the normal, inherent rate for the AV junction? 26, 27, 28, 44 40–60 bpm

13. What is a Junctional Escape Rhythm? 26, 27, 28, 33, 34, 42 It is an escape mechanism that

occurs when a higher pacemaking

site fails and the AV junction has

to take over at its own inherent

rate of 40–60 bpm.

152 Chapter 6

Questions Referenced Frames Answers

14. Is Junctional Escape Rhythm regular or irregular? 29, 33 regular

15. What will the P waves look like for this arrhythmia? 30, 33 They will be inverted: they can fall

before or after the QRS complex

or can be hidden within the QRS

complex.

16. Will the PRI measurement be normal for a Junctional

Escape Rhythm?

31, 33 No, if the P wave precedes the

QRS, the PRI will be less than

0.12 second. If it falls within the

QRS or after the QRS, there will

be no PRI.

17. What is the rate range for an Accelerated Junctional

Rhythm?

35, 36, 37, 40 60–100 bpm

18. Is Accelerated Junctional Rhythm regular or irregular? 37, 40 regular

19. What is the P wave like in Accelerated Junctional

Rhythm?

36, 37, 40 Just like the P wave in all other

junctional rhythms: it is inverted

and can fall before, during, or

after the QRS complex.

20. Is the QRS measurement normal for Accelerated

Junctional Rhythm?

36, 37, 40 Yes, since the conduction through

to the ventricles is normal, the

QRS should measure less than

0.12 second.

21. What is the rate range for Junctional Tachycardia? 35, 39, 40 100–180 bpm

22. Is a Junctional Tachycardia regular or irregular? 38, 39, 40 regular

23. What does the P wave look like in Junctional

Tachycardia?

38, 39, 40 It is probably not visible since

the rate is so fast that it might

be hidden in the T waves, or it

might be hidden within the QRS

complex. If it is visible, it will be

inverted either before or after the

QRS complex.

24. Is the QRS measurement normal for Junctional

Tachycardia?

38, 39, 40 Yes, it should be less than

0.12 second.

25. Is the PRI measurement normal for Junctional

Tachycardia?

38, 39, 40 No, if the P wave precedes the

QRS complex, the PRI will be

less than 0.12 second. Otherwise,

there will be no PRI.

26. When can you call an arrhythmia a Supraventricular

Tachycardia?

45, 46, 47, 48 only when you have a regular

rhythm, in a tachycardia range

with no visible P waves, and at a

rate that is common to more than

one arrhythmia, thereby making

more accurate identification

impossible

27. Which arrhythmias commonly need to be described

as Supraventricular Tachycardias?

45, 46, 47, 48 The most common ones are

Atrial Tachycardia and Junctional

Tachycardia, although the term

is also used to describe Sinus

Tachycardia and Atrial Flutter.

28. At what rate would you expect to have trouble

discerning P waves and might consider calling an

arrhythmia Supraventricular Tachycardia?

47 usually 150–250 bpm, although

you may lose the P waves at

slower rates

Junctional Rhythms 153

PRACTICE STRIPS (answers can be found in the Answer Key on page 557)

6.1

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.2

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

154 Chapter 6

6.3

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.4

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 155

6.5

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.6

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

156 Chapter 6

6.7

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.8

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 157

6.9

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.10

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

158 Chapter 6

6.11

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.12

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 159

6.13

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.14

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

160 Chapter 6

6.15

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.16

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 161

6.17

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.18

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

162 Chapter 6

6.19

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.20

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 163

6.21

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.22

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

164 Chapter 6

6.23

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.24

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 165

6.25

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.26

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

166 Chapter 6

6.27

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.28

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 167

6.29

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.30

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

168 Chapter 6

6.31

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.32

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Junctional Rhythms 169

6.33

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.34

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

170 Chapter 6

6.35

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.36

Junctional Rhythms 171

6.37

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.38

172 Chapter 6

6.39

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

6.40

Junctional Rhythms 173

6.41

Regularity: ______________________________________ PRI: ____________________________________________

Rate: ___________________________________________ QRS: ___________________________________________

P Waves: _______________________________________ Interp: _______________________________________

174

Overview

IN THIS CHAPTER, you will learn to recognize arrhythmias that are manifestations of conduction defects (blocks) at the AV node, and you will learn to describe characteristics that are common to this category of arrhythmia. You will then learn the names and features of four different

arrhythmias that are in this category called Heart Blocks. For each of these arrhythmias, you will

learn about the etiology, conduction, and resulting EKG features (regularity, rate, P waves, PR

intervals, and QRS complexes).

Conduction Through the AV Node

 

Accelerated Junctional Rhythm

36. Let’s take Accelerated Junctional Rhythm separately first. This is an irritable arrhythmia that originates in the AV junction and fires at a rate of bpm.

It will have the inverted P wave typical of junctional arrhythmias, or it may have no

P wave if the atria and ventricles depolarize . If the P wave

precedes the QRS complex, the PRI should be less than

second. Conduction through the ventricles is normal, so the QRS complex should have

a measurement of less than 0.12 second.

37. Here are the rules for Accelerated Junctional Rhythm (Figure 43):

Regularity: regular

Rate: 60–100 bpm

P Wave: will be inverted; can fall before or after the QRS complex or can be

hidden within the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if

measurable, will be less than 0.12 second

QRS: less than 0.12 second

Tachycardia

60–100

simultaneously

0.12

normal

Figure 41 Mechanism of Accelerated Junctional Rhythm

Retrograde

Pacemaker: AV junction Conduction

speeds up to override

higher sites

Rate: 60–100 bpm

Regularity: regular

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

An irritable focus in the AV junction speeds up to override the SA node for control of the heart. The

atria are depolarized via retrograde conduction. Conduction through the ventricles is normal.

Figure 42 Mechanism of Junctional Tachycardia

Retrograde

Pacemaker: AV junction Conduction

speeds up to override

higher sites

Rate: 100–180 bpm

Regularity: regular

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

A very rapid irritable focus in the AV junction overrides the SA node for control of the heart. The atria

are depolarized via retrograde conduction. Conduction through the ventricles is normal.

146 Chapter 6

38. When the AV junction fires in the tachycardia range (100–180 bpm) the rhythm

will remain regular. The P waves will be inverted and can fall before or after the

QRS complex, or they might be absent if they are within

the QRS complex. When the P wave precedes the QRS complex, the PRI will

be than 0.12 second. Since conduction through the ventricles

is normal, the QRS complex will be less than second.

39. The rules for Junctional Tachycardia (Figure 44) are:

Regularity: regular

Rate: 100–180 bpm

P Wave: will be inverted; can fall before or after the QRS complex or can be

hidden within the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if

measurable, will be less than 0.12 second

QRS: less than 0.12 second

40. The only difference you will see on the EKG among Junctional Escape Rhythm,

Accelerated Junctional Rhythm, and Junctional Tachycardia is the rate. The rates are:

Junctional Escape Rhythm 40–60 bpm

Accelerated Junctional Rhythm 60–100 bpm

Junctional Tachycardia 100–180 bpm

Each of these rhythms originates in the , and will thus produce an inverted P wave because of retrograde conduction. Depending on whether

the atria or ventricles depolarize first, the P wave can come before, during, or after

hidden

less

0.12

AV junction

Figure 43 Rules for Accelerated Junctional Rhyth

ate: 40–60 bpm

P Wave: will be inverted: can fall before or after the QRS complex or can be

hidden within the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if

measurable, will be less than 0.12 second

QRS: less than 0.12 second

Junctional Tachycardia

34. Junctional Escape Rhythm is a fail-safe mechanism rather than an irritable arrhythmia. However, the AV junction is capable of irritability and is known to produce an irritable arrhythmia called Junctional Tachycardia. This rhythm occurs when the junction

initiates impulses at a rate than its inherent rate of 40–60 bpm,

thus overriding the SA node or other higher pacemaker sites for control of the heart rate.

Junctional Escape Rhythm is an escape mechanism, whereas Junctional Tachycardia is

an rhythm.

35. Junctional Tachycardia is usually divided into two categories, depending on how

fast the irritable site is firing. If the junction is firing between 60 and 100 bpm, the arrhythmia is termed an Accelerated Junctional Rhythm (Figure 41) because a rate below

100 can’t really be considered a tachycardia. When the junctional rate exceeds 100 bpm,

the rhythm is considered a Junctional Tachycardia (Figure 42). Junctional Tachycardia

can be as fast as 180 bpm, but at this rapid rate, it is extremely difficult to identify positively since P waves are superimposed on preceding T waves. When an AV junctional

focus fires at a rate of 60–100 bpm, it is termed an Junctional

faster

irritable

Accelerated

Figure 40 Rules for Junctional Escape Rhythm

Junctional Escape Rhythm

Regularity: The R–R intervals are constant. The rhythm is regular.

Rate: Atrial and ventricular rates are equal. The inherent rate of the AV junction is 40–60 bpm.

P Waves: The P wave can come before or after the QRS complex, or it can be lost entirely within the QRS complex.

If visible, the P wave will be inverted.

PRI: If the P wave precedes the QRS complex, the PRI will be less than 0.12 second. If the P wave falls within

the QRS complex or follows it, there will be no PRI.

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

Junctional Rhythms 145

Rhythm. If the rate exceeds 100 bpm, up to a rate of 180 bpm, the rhythm is called a

Junctional .

Accelerated Junctional Rhythm

36. Let’s take Accelerated Junctional Rhythm separately first. This is an irritable arrhythmia that originates in the AV junction and fires at a rate of bpm.

It will have the inverted P wave typical of junctional arrhythmias, or it may have no

P wave if the atria and ventricles depolarize . If the P wave

precedes the QRS complex, the PRI should be less than

second. Conduction through the ventricles is normal, so the QRS complex should have

a measurement of less than 0.12 second.

37. Here are the rules for Accelerated Junctional Rhythm (Figure 43):

Regularity: regular

Rate: 60–100 bpm

P Wave: will be inverted; can fall before or after the QRS complex or can be

hidden within the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if

measurable, will be less than 0.12 second

QRS: less than 0.12 second

Tachycardia

60–100

simultaneously

0.12

normal

Figure 41 Mechanism of Accelerated Junctional Rhythm

Retrograde

Pacemaker: AV junction Conduction

speeds up to override

higher sites

Rate: 60–100 bpm

Regularity: regular

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

An irritable focus in the AV junction speeds up to override the SA node for control of the heart. The

atria are depolarized via retrograde conduction. Conduction through the ventricles is normal.

Figure 42 Mechanism of Junctional Tachycardia

Retrograde

Pacemaker: AV junction Conduction

speeds up to override

higher sites

Rate: 100–180 bpm

Regularity: regular

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

A very rapid irritable focus in the AV junction overrides the SA node for control of the heart. The atria

are depolarized via retrograde conduction. Conduction through the ventricles is normal.

146 Chapter 6

38. When the AV junction fires in the tachycardia range (100–180 bpm) the rhythm

will remain regular. The P waves will be inverted and can fall before or after the

QRS complex, or they might be absent if they are within

the QRS complex. When the P wave precedes the QRS complex, the PRI will

be than 0.12 second. Since conduction through the ventricles

is normal, the QRS complex will be less than second.

39. The rules for Junctional Tachycardia (Figure 44) are:

Regularity: regular

Rate: 100–180 bpm

P Wave: will be inverted; can fall before or after the QRS complex or can be

hidden within the QRS complex

PRI: can be measured only if the P wave precedes the QRS complex; if

measurable, will be less than 0.12 second

QRS: less than 0.12 second

40. The only difference you will see on the EKG among Junctional Escape Rhythm,

Accelerated Junctional Rhythm, and Junctional Tachycardia is the rate. The rates are:

Junctional Escape Rhythm 40–60 bpm

Accelerated Junctional Rhythm 60–100 bpm

Junctional Tachycardia 100–180 bpm

Each of these rhythms originates in the , and will thus produce an inverted P wave because of retrograde conduction. Depending on whether

the atria or ventricles depolarize first, the P wave can come before, during, or after

hidden

less

0.12

AV junction

Figure 43 Rules for Accelerated Junctional Rhythm

Accelerated Junctional Rhythm

Regularity: The R–R intervals are constant. The rhythm is regular.

Rate: Atrial and ventricular rates are equal. The rate will be faster than the AVjunction’s inherent rate but not yet

into a true tachycardia range. It will be in the 60–100 bpm range.

P Waves: The P wave can come before or after the QRS complex, or it can be lost entirely within the QRS complex.

If visible, the P wave will be inverted.

PRI: If the P wave precedes the QRS complex, the PRI will be less than 0.12 second. If the P wave falls within

the QRS complex or follows it, there will be no PRI.

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

Junctional Rhythms 147

the QRS complex. If the P wave precedes the QRS complex, the PRI will be less than

0.12 second. The QRS measurement will be normal. If the rate is 40–60 bpm, the rhythm

is called Rhythm. If the rate is between 60 and 100 bpm, the

rhythm is termed Junctional Rhythm, and the rhythm is called

Junctional Tachycardia if the rate is bpm.

41. A junctional impulse that reaches the atria before the ventricles will produce an

inverted P wave that falls the QRS complex. Such a beat

would have a PRI of less than second. If the PRI were greater

than 0.12 second, you would suspect that the impulse originated in the

.

42. A regular rhythm with a QRS complex of less than 0.12 second and a rate

of 50 bpm, which did not have any visible P waves, would fit the rules for

a Rhythm.

43. A single premature ectopic beat originating from an irritable focus in the AV

junction would be called a PJC, or . Such a beat would have

an inverted P wave that falls , ,

or the QRS complex.

44. All junctional arrhythmias have the same general characteristics; that is,

they all have P waves that can occur before, during, or

after a QRS complex; the PRI will be ; and the QRS will

be . However, not all junctional arrhythmias have the same

mechanism. PJCs, Junctional Tachycardia, and Accelerated Junctional Rhythm are all

caused by irritability, whereas a junctional rhythm within its inherent rate of 40–60 bpm

would be an indication of an mechanism.

Junctional Escape

Accelerated

100–180

before

0.12

atria

Junctional Escape

Premature Junctional Complex

before; during

after

inverted

shortened

normal

escape

Figure 44 Rules for Junctional Tachycardia

Junctional Tachycardia

Regularity: The R–R intervals are constant. The rhythm is regular.

Rate: Atrial and ventricular rates are equal. The rate will be in the tachycardia range but does not usually exceed

180 bpm. Usual range is 100–180 bpm.

P Waves: The P wave can come before or after the QRS complex, or it can be lost entirely within the QRS complex.

If visible, the P wave will be inverted.

PRI: If the P wave precedes the QRS complex, the PRI will be less than 0.12 second. If the P wave falls within

the QRS complex or follows it, there will be no PRI.

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

148 Chapter 6

Supraventricular Tachycardia

45. You have now learned several arrhythmias that are regular and beat at such a

rapid rate that the P wave might not be discernible from the T wave. If you include

Junctional Tachycardia, which might not have a visible P wave, you have a group of

tachycardias that are regular and don’t have visible P waves. The ventricular rates for

these arrhythmias are:

Sinus Tachycardia 100–160 bpm

Atrial Tachycardia 150–250 bpm

Atrial Flutter 150–250 bpm

Junctional Tachycardia 100–180 bpm

From these rate ranges you can see that as the rate exceeds 150 or 160 bpm, a rate at which

the P wave could very well be encroaching on the preceding

wave, you would have no way to distinguish between these arrhythmias. Since you

can’t accurately identify the rhythm, you would instead give it a descriptive identification. The term that’s used to describe this category of indistinguishable arrhythmias is

Supraventricular Tachycardias (Figure 45).

46. A Supraventricular Tachycardia (SVT) is not the name of a specific

. It is a term that’s used to a category

of several regular tachyarrhythmias that can’t be identified more accurately because

they have indistinguishable waves and fall within a common

 range.

47. The rates at which you most commonly need to use the term Supraventricular

Tachycardia is the 150–250 range, although sometimes a slower rate will still have

obscured P waves. An SVT is usually a toss-up between Atrial Tachycardia and

Junctional Tachycardia, although Sinus Tachycardia and, less commonly, Atrial

Flutter can also be in the running. These arrhythmias can only be called SVT if

they cannot be identified more accurately. It is not a catch-all phrase. To be

called SVT, an arrhythmia must be , have no visible

 waves, and have a range

common to other arrhythmias, thereby making further and more accurate

identification .

T