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 .
