SA node

junctional

Junctional Rhythms

6

138 Chapter 6

2. When electrical impulses originate in the AV junction, the heart is depolarized in a somewhat unusual fashion. With the pacemaker located in the middle of

the heart, the electrical impulses spread in two directions simultaneously. This is

unusual because the heart is normally depolarized by a single force spreading downward toward the . However, when the AV junction assumes

 responsibility, the atria and the ventricles will be depolarized

at very nearly the same time because the impulse spreads in

directions at one time. This concept is pictured in Figure 34.

3. As you recall, electrode positions for Lead II place the electrode above the right atria and the electrode below the ventricle

(see Figure 35). In the normal heart, the major thrust of electrical flow is toward the ventricles (and toward the positive electrode in Lead II), thus producing an upright P wave

and an upright QRS complex. In a junctional rhythm, the ventricles are depolarized by

an impulse traveling down the conduction system toward the positive electrode; thus,

the QRS complex usually is . But at the same time, the impulse

can spread upward through the atria toward the electrode.

When the atria are depolarized in this “backward” fashion, it’s called retrograde conduction because the electrical impulse travels in the opposite direction it usually takes.

The mechanism that enables the AV junction to depolarize the atria with a backward

flow of electricity is called conduction.

4. In junctional rhythms, the impulse that depolarizes the ventricles is traveling

toward the electrode (in Lead II), thus producing a QRS

complex that usually is upright. When retrograde conduction occurs in AV junctional rhythms, the electrical impulse that depolarizes the atria is traveling toward

the electrode. Thus, we can deduce that the atrial activity will

produce a negative deflection on the EKG. In other words, the P wave of an AV junctional

arrhythmia should be inverted because it was produced by an impulse traveling toward

the electrode.

5. In AV junctional arrhythmias, the atria are depolarized via

conduction at approximately the same time as the ventricles are depolarized normally. The two simultaneous electrical force flows, one retrograde and the other

normal, result in an inverted wave and an upright

 complex.

6. In junctional arrhythmias, a single impulse originates in the AV junction and

causes electricity to flow in two directions. One electrical force flows upward (retrograde) to depolarize the , while the other flows downward to depolarize the . Even though both electrical forces

ventricles

pacemaking

two

negative

positive

upright

negative

retrograde

positive

negative

negative

retrograde

P

QRS

atria

ventricles

Figure 34 Electrical Flow in Junctional Arrhythmias

Junctional Rhythms 139

originate from a single impulse in the junction, the force that depolarizes the atria is

not the same force that depolarizes the ventricles. For this reason, you will not always

see a consistent relationship between the inverted P wave and the QRS complex.

The wave will not have a consistent relationship to

the complex because the force that depolarizes the atria is

not the same force that depolarizes the .

Junctional P Wave

7. In junctional arrhythmias, the P wave does not always have to precede the QRS

complex because it is possible for the ventricles to be depolarized before the atria, if

the force reaches them first. The position of the P wave in relation to the QRS complex

will depend on whether the atria or the ventricles were first.

If the ventricles are depolarized before the atria, the QRS will come before the P wave.

If the atria depolarize first, the P wave will precede the QRS complex. If they both

depolarize simultaneously, the wave will be hidden within

the QRS complex. In junctional arrhythmias, the P wave isn’t always visible, but when

it is, it will be because the atria are depolarized via retrograde conduction. The P wave in junctional rhythms can come before, during, or after

the complex, depending on which depolarize first, the atria or

the ventricles (see Figure 36).

8. The biggest clue to a junctional rhythm is the inverted P wave. But this same

phenomenon occurs with some atrial arrhythmias when the impulse originates so

low in the atria that it is very near the AV junction. In such cases, the impulse will

have to depolarize parts of the atria with retrograde conduction, thus producing an

inverted P wave. Therefore, while junctional rhythms characteristically have inverted

P waves, a rhythm with an inverted P wave can be either or

 in origin.

9. When you see an arrhythmia with an inverted P wave following the QRS complex,

you know that the rhythm originated in the AV . But if the

inverted P wave precedes the QRS complex, you need to determine whether it originated

in the AV junction or in the . The important clue will come from

the PR interval. If the impulse originated in the atria, the impulse would take the normal

P

QRS

ventricles

depolarized

P

inverted

QRS

atrial

junctional

junction

atria

Figure 35 Normal Electrical Flow in Lead II

+

–

140 Chapter 6

length of time getting through the node and into the ventricles. Thus, the PRI would

be normal, or second. But if the impulse originated in the AV

junction, it would take less time to get to the ventricles and thus would have a PRI of

less than 0.12 second. If the rhythm has an inverted P wave and a normal PRI measurement, you would know that it originated in the ; whereas if

the PRI is less than 0.12 second, it must have originated in the .

10. You now know quite a bit about junctional rhythms in general. You know

that the QRS measurement is and that the P wave will

be . The P wave can be seen before, during, or after the QRS

complex but may not be visible at all if it is hidden within the QRS complex. Finally,

you know that the PRI must be less than 0.12 second; if it is greater than 0.12 second,

the arrhythmia would be in origin. All of these rules pertain

to every junctional rhythm, regardless of whether it is a tachycardia, bradycardia, or a

single ectopic beat. For each of the four AV junctional arrhythmias you will now learn,

you already know that all of the preceding rules apply.

11. All junctional arrhythmias will have an inverted P wave because the atria are depolarized via conduction.

12. Atrial arrhythmias can also have P waves since they can

be produced by retrograde conduction.

13. Junctional arrhythmias will have a PRI of less than second;

atrial arrhythmias will have a PRI of second.

14. An inverted P wave that precedes the QRS complex and has a PRI of less than 0.12

second indicates that the pacemaker impulse originated in the ,

and the atria depolarized the ventricles.

15. If the junctional impulse reached the ventricles first and depolarized the ventricles

before the atria, it would produce an inverted P wave the QRS

complex.

0.12–0.20

atria

AV junction

normal

inverted

atrial

retrograde

inverted

0.12

0.12–0.20

AV junction

before

following

Figure 36 P Wave Placement in Junctional Rhythms

The P wave will precede the QRS complex

if the atria are depolarized before the

ventricles. In such a case, the PRI will be

less than 0.12 seconds.

If the atria and the ventricles are depolarized

simultaneously, there will be no visible P wave,

since it is hidden within the QRS complex.

The P wave will follow the QRS complex if the

ventricles are depolarized before the atria.

 INVERTED P WAVE

 INVERTED P WAVE

 INVERTED P WAVE

(hidden)

Junctional Rhythms 141

16. You would not see a P wave if the impulse originated in the junction but reached

the atria and the ventricles simultaneously, since this would cause the P wave to

be within the QRS complex.

17. If visible, a junctional P wave will be , but it can

be hidden within the QRS complex if both the atria and the ventricles are

 simultaneously.

18. The junctional pacemaker site can produce a variety of arrhythmias, depending on

the mechanism employed. We will discuss four basic mechanisms common to the AV

junction:

• Premature Junctional Complex

• Junctional Escape Rhythm

• Accelerated Junctional Rhythm

• Junctional Tachycardia

Although these are four different mechanisms, each of these arrhythmias originates in

the AV .

Premature Junctional Complex

19. The first junctional arrhythmia we will learn about is called a Premature Junctional

Complex, or PJC (Figure 37). A PJC is not an entire rhythm; it is a single ectopic beat.

A PJC is similar in many ways to a PAC. In the case of the PJC, the irritable focus

comes from the AV junction to stimulate an early cardiac cycle, which interrupts the

underlying rhythm for a single . When such a premature ectopic originates in the atria, it is called a Premature Atrial Complex, or PAC. But when

the irritable focus is in the AV junction, it is called a PJC, or .

A PJC is a single ectopic beat that comes in the cardiac cycle

to interrupt the underlying rhythm.

20. Since a PJC is a single beat, it will interrupt the pattern of the underlying rhythm.

The R–R interval can be regular or irregular, depending on the regularity of the underlying rhythm, but the PJC will come earlier than expected and thus will cause the overall

rhythm to be irregular. Because a PJC is a single early beat, it will cause the overall

rhythm to be .

hidden

inverted

depolarized

junction

beat

Premature Junctional Complex

early

irregular

Figure 37 Mechanism of Premature Junctional Complex

Retrograde

Conduction

Pacemaker: a single

irritable focus within

the AV junction

Rate: depends on

underlying rhythm

Regularity: ectopic

interrupts underlying

rhythm

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

The pacemaker is an irritable focus within the AV junction that fires prematurely and produces a

single ectopic beat. The atria are depolarized via retrograde conduction. Conduction through the

ventricles is normal. This is a single beat, not an entire rhythm; the underlying rhythm also must be

identified.

142 Chapter 6

21. As with regularity, the rate will depend on the rate of the underlying arrhythmia.

Being a single beat, a PJC does not have a rate of its own. To determine heart rate, you

would have to look at the overall rate of the rhythm.

22. The P wave of a PJC will be consistent with the P waves of all other junctional arrhythmias. Because atrial depolarization is retrograde, the P wave will be

 and can fall before, during, or after the

complex.

23. If the P wave of the PJC precedes the QRS complex, the PRI will be less

than second.

24. Conduction through the ventricles should be normal with a PJC. Therefore, the QRS

complex should have a normal duration of .

25. The rules of PJCs (Figure 38) are:

Regularity: depends on regularity of underlying arrhythmia

Rate: depends on rate of underlying arrhythmia

P Wave: 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

26. The normal, inherent rate for the AV junction is 40–60 bpm. A PJC occurs when the

junction becomes irritable and overrides higher sites. But the junction can also take over

underlying

inverted; QRS

0.12

less than 0.12 second

Figure 38 Rules for Premature Junctional Complex

Premature Junctional Complex

Regularity: Since this is a single premature ectopic beat, it will interrupt the regularity of the underlying rhythm.

The R–R interval will be irregular.

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

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 following it, there will be no PRI.

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

Junctional Rhythms 143

pacemaking responsibility if higher sites fail. The junction would then “escape” and

assume pacemaking functions at its own inherent rate of bpm.

27. As you recall, a premature beat is a sign of irritability, whereas an

 beat comes later than you would expect it and is a fail-safe

mechanism to protect the heart. When the AV junction is allowed to assume that pacemaking responsibility, it functions at its inherent rate of 40–60 bpm, this is an example

of mechanism rather than irritability.

Junctional Escape Rhythm

28. When you see Junctional Escape Rhythm (Figure 39), you would expect it to

have a rate of bpm, since this is the inherent rate of the AV

junction. Junctional Escape Rhythm is sometimes referred to as “Passive” Junctional

Rhythm.

29. The AV junction is normally a very regular pacemaker. In a Junctional Escape

Rhythm, you would find a regular R–R interval. AV Junctional Escape Rhythm is

a rhythm with a rate of 40–60 bpm.

30. As with other junctional arrhythmias, Junctional Escape Rhythm has inverted

P waves, which can fall before or after the QRS complex. It is also possible that

there would be no P wave, since the P wave can be hidden within the QRS complex.

Junctional Escape Rhythm always has inverted P waves, either before or after the

QRS complex, or the P wave might be hidden within the

complex.

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

 second. If the PRI is greater than 0.12 second, you would suspect that the rhythm originated in the .

32. As with other junctional arrhythmias, you would expect ventricular conduction to be , and thus the QRS measurement should be less

than second in a Junctional Escape Rhythm.

40–60

escape

escape

40–60

regular

QRS

0.12

atria

normal

0.12

Figure 39 Mechanism of Junctional Escape Rhythm

Retrograde

Pacemaker: AV junction Conduction

steps in when higher

sites fail

Rate: 40–60 bpm

Regularity: regular

Conduction: Atria are

depolarized by retrograde

conduction, while conduction

through the ventricles

proceeds normally

When higher pacemaker sites fail, the AV junction is left with pacemaking responsibility. The atria are

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

144 Chapter 6

33. The rules of Junctional Escape Rhythm (Figure 40) are:

Regularity: regular

Rate: 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 .