Download Scoring Center: Scoring Cardiac Dysrhythmias - Part 2

Scoring Center: Scoring Cardiac Dysrhythmias - Part 2
By Jon Atkinson, BS, RPSGT
Editor’s Note: This is the second of a two-part series on
scoring cardiac dysrhythmias. The first part was published
in A2Zzz volume 16, number 4. For more information,
including detailed features and figures of brief graphic
samples, refer to chapter 36, “Cardiac Arrhythmias,” in
the Fundamentals of Sleep Technology textbook (Lippincot
Williams & Wilkins, 2007).
The key to identifying cardiac dysrhythmias is first to
understand what is normal. This begins with knowledge
of the conducting system of the heart and the relationship
of the electrophysiologic phenomena to the electrographic
representation of the cardiac cycle.
Figure 1 is an exterior view of the heart, and Figure
2 is a cross-section showing a representation of the
conducting system of the heart. The normal cardiac
cycle begins with a discharge of the sinoatrial (SA) node.
This causes a wave of excitation across the atrium (atrial
depolarization) that is represented by the P wave of the
normal EKG complex. See Figure 3.
Superior
Vena Cava
Aorta
Pulmonary Artery
Right Atrium
Left Atrium
Pulmonary
Veins
Right Ventricle
Left Ventricle
Figure 1. Exterior view of the heart
Interatrial
Septum
Right Atrium
Left Atrium
AV Bundle
(His)
SA Node
Intermodal
Pathways
AV Node
Left Ventricle
Left Bundle Branch
Right Ventricle
Right Bundle
Branch
Interventricular
Septum
Purkinje Fibers
Figure 2. The conducting system of the heart
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Atrial depolarization
Ventricular repolarization
R
T
P
Q
S
PR interval
QRS interval
Figure 3. The cardiac cycle
There is a brief delay in conduction at the
atrioventricular (AV) node, located near the junction of the
atria and the ventricles in the interatrial septum separating
the right and left atria. The P wave and this pause
comprise the P-R interval. The normal P-R interval is 0.12
to 0.20 seconds duration. Once the AV node is stimulated,
it sends a wave of depolarization down the atrioventricular
bundle (bundle of His) and then to the right and left bundle
branches of the conducting system. The left bundle branch
bifurcates into an anterior and a posterior limb. These
and the right bundle branch terminate into the Purkinje
fibers, which terminate on all the cells of the myocardium,
essentially allowing the entire muscle mass of the
ventricles to depolarize and contract simultaneously. This
is electrographically represented as the QRS complex.
A key concept in cellular physiology is that in cyclical
phenomena, if cells depolarize, then they must then
repolarize to prepare for the next depolarization cycle.
The T wave of the EKG complex represents repolarization
of the ventricle. The waveform associated with
repolarization of the atria is not seen because of its low
amplitude and its simultaneous occurrence during the
much higher amplitude QRS complex. In the adult human
heart, this cyclical depolarization and repolarization occurs
at a rate of 60 to 100 times per minute.
This is related to another concept: “the rules of
morphology.” When recorded from the same two points,
an electrical phenomenon that starts at the same location
and takes the same pathway will always look the same
electrographically. This is the “rule of same morphology.”
Conversely, if the phenomenon starts in a different
location or travels a different pathway, the waveform will
look different; this is the “rule of differing morphology.”
The Normal Sinus Rhythm (NSR)
In NSR for the adult, the P wave will always look the
same, and the QRS complexes will always be identical. If
depolarization always starts in the same place and takes the
same pathway, the repolarization waveform (T wave) also will
look the same. The ratio of P waves to QRS complexes will
always be 1 to 1; i.e., there is a P wave for every QRS complex
and a QRS complex for every P wave. The P-R interval will
range from 0.12 to 0.20 seconds, and the QRS interval will
be between 0.04 and 0.12 seconds in duration. The rhythm
will be regular; i.e., the P to P interval and the R to R interval
will be constant. The rate will be 60 to 100 per minute.
Table 1 describes the parameters or waveforms and their
characteristics or values for NSR.
Parameter/
waveform
Value/characteristic
P wave:
Present, each appears the same
QRS complex:
Present, each appears the same
PR interval:
0.12-0.20 seconds
QRS interval:
0.04-0.12 sounds
P:QRS ratio:
1:1
Rate:
60-100 bpm
Rhythm:
Regular
Table 1. Characteristics of NSR
Arrhythmia Analysis Steps
This section presents routine steps you should follow for
a comprehensive and accurate analysis of cardiac rhythm
disturbances. Whenever possible you should add a second,
different EKG dipole to the montage. This may reveal
differences in P wave or QRS complex configuration. It can
be difficult to identify these differences with only a singlechannel recording. Use either a 10-second window or a sixsecond window when examining the EKG for detail. This
will enable you to analyze the intervals and look for subtle
changes in morphology.
Following these routine steps will help you identify
arrhythmias with greater accuracy:
Step 1: Examine the P wave.
Examine the tracing to detect the absence or presence of P
waves. If there is no distinct P wave, then you know that the
arrhythmia is not of atrial origin, except in these three cases:
Atrial fibrillation: The P waves are replaced by
fibrillatory waves.
Atrial flutter: The P waves are replaced by Sawtoothappearing flutter waves
Sinus pauses: Both P wave and corresponding QRS
complexes are absent.
Next, examine the P wave configuration. Do the P waves
all look the same? In other words, do they have the same
morphology? Remember the rules of differing morphology
and same morphology that are described above.
Step 2: Examine the QRS complex.
Check for the absence or presence of the QRS complex.
An absent QRS is a sign of some type of second-degree or
third-degree AV block. It also may indicate that there is a
severe ventricular disturbance such as ventricular fibrillation
or asystole. Scrutinize the QRS complexes to see if they
have an identical appearance. QRS complexes with different
appearances indicate a shift from the normal pathway for
ventricular contraction to a different pathway progressing
through the ventricle (e.g., a bundle branch block). This also
may be a sign of a different origin of the ventricular beat
(e.g., PVCs or other beats of ventricular origin).
Step 3: Examine the relationship between P
waves and QRS complexes.
Determine if the P waves and QRS complexes are
“married.” Is there a P for every QRS? Is there a QRS for
every P? Is there a 1:1 P:QRS ratio? If there are more P
waves than QRS complexes (i.e., a P:QRS ratio of more
than 1), then some sort of AV block is present. If there
are more QRS complexes than P waves (i.e., a P:QRS ratio
of less than 1), then a junctional or ventricular arrhythmia
is present.
Step 4: Measure the PR and QRS intervals.
Inspect the PR interval. Is it too long or too short,
or does it have a normal duration? There may be a
junctional beat (retrograde P wave) if the PR interval is
shortened. The presence of an accessory pathway [such
as in Wolff-Parkinson-White (WPW) syndrome] also may
explain the shortened PR interval. If the PR interval
is lengthened, then there is some type of AV block.
Determine if the QRS interval is normal or too long; only
in rare cases will it be too short. If the QRS complex has
an increased duration, then either bundle branch block
or a beat of ventricular origin is likely present. In some
cases, a QRS complex with an increased duration may be a
sign of an aberrantly conducted beat of supraventricular
origin; this occurs when the beat originates before the
ventricular conductive pathway is repolarized (during the
relative refractory period).
Step 5: Detect a regular or irregular rhythm.
Study the intervals from P wave to P wave and from R
wave to R wave. If the intervals are constant, then the
rhythm is regular. If the intervals vary, then the rhythm is
irregular. To determine the regularity of these intervals,
simply print out a screenshot and measure with calipers
or a ruler. You can even use a 3x5 index card or a piece of
paper; hold it up on a stationary screen view and mark a P
wave and then the next one. Then move the card from P to
P to P or R to R to R and see if the marks fall in line.
Step 6: Determine the rate.
Do not rely on the cardiotachogram to determine
the heart rate, which is usually derived from the pulse
oximeter. It is prone to errors such as a decreased or
increased peripheral pulse sensing, or an undercount or
overcount of the heart rate. Count the actual rate in a 10second period and multiply by six; or in a 15-second period
and multiply by four.
Arrhythmias
Arrhythmias are present if the rate is too fast or too
slow, the site of origin is other than the SA node, the
pathway of electrical conduction is altered, or there is a
combination of any of these features.
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Continued from page 31...
To learn more about specific arrhythmias in each of
the following categories, see the detailed features for
arrhythmia recognition and figures of brief graphic
samples in chapter 36, “Cardiac Arrhythmias,” in the
Fundamentals of Sleep Technology textbook (Lippincot
Williams & Wilkins, 2007).
Atrial Arrhythmias
Atrial arrhythmias are characterized by ectopic P waves
(i.e., those originating from locations in the atria some
place other than the normal SA node) and therefore have
a different morphology or shape. The exceptions are
atrial fibrillation (i.e., no distinct P waves), atrial flutter
(i.e., appearance of saw tooth waves instead of P waves)
and sinus pauses (i.e., absence of P waves due to arrest of
the SA node or blockage of the discharge of the SA node
preventing depolarization of the atrium).
Junctional Arrhythmias
In junctional arrhythmias, the QRS complex will have
the same morphology as that in the patient’s sinus rhythm
because the pathway from the AV node to the ventricular
myocardium is not altered. The P waves will either be
absent (i.e., buried in the QRS complex), inverted, or
perhaps, will follow the QRS complex. If the inverted P wave
is seen, the P-R interval will also be shorter than normal.
References
American Academy of Sleep Medicine. The AASM manual
for the scoring of sleep and associated events: rules,
terminology and technical specifications. Westchester, Ill:
American Academy of Sleep Medicine; 2007.
Atkinson JW. Cardiac arrhythmias. In: Butkov N, LeeChiong T, editors. Fundamentals of sleep technology.
Philadelphia: Lippincot Williams & Wilkins; 2007. p. 314-332.
Atkinson, J. Appendix II Cardiac Arrhythmias. In:
Barkoukis T, Avidon A, editors. Review of sleep medicine.
2nd ed. Burlington, Mass.: Butterworth-Heinemann; 2007.
p. 545-560.
Atkinson JW. Cardiac arrhythmias. Respir Care Clin N
Am 2005;11(4):635-662.
Thaller M. The only EKG book you’ll ever need. 4th ed.
Philadelphia: Lippincot Williams & Wilkins; 2003.
Dubin D. Rapid interpretation of EKG. 6th ed. Tampa,
Fla.: Cover Publishing; 2000.
Jon Atkinson, BS, RPSGT, is the AAST President. He has
been in the sleep field for 27 years, and he currently works
as a self-employed consultant in sleep medicine technology.
AV Blocks
With AV blocks the electrical conduction between the
atria and the ventricles is either delayed (i.e., prolonged P-R
interval) or totally absent.
In 1st-degree AV block, the P-R interval is greater than
0.20 seconds. There are no lost QRS complexes.
Second-degree blocks are always accompanied by
the loss of at least one QRS complex. The Wenckebach
phenomenon (2nd-degree AV block, Mobitz type I)
demonstrates a progressively lengthening P-R interval as
the AV node becomes increasingly refractory to conduction
until a QRS complex is no longer generated.
In 2nd-degree AV block type II, the P-R interval remains
constant (i.e., it may be either normal or prolonged) and
there is a sudden loss of one or more QRS complexes (i.e.,
visible P waves without corresponding QRS complexes).
In AV dissociation (i.e., complete AV block, 3rd degree AV
block), there is no relationship between the P wave and the
QRS complexes. The P-P interval is usually quite constant as
is the R-R interval; however, the duration of the P-P interval
and the R-R interval is different giving the appearance of P
waves “marching” through the rest of the EKG waveforms.
Ventricular Arrhythmias
Ventricular arrhythmias demonstrate the absence of P
waves and widened, bizarre-appearing QRS complexes as
they are generated from ventricular foci that circumvent
the normal “rapid transit” pathway from the AV node to
the Purkinje fibers. This causes a slower depolarization
of the ventricle and thus a prolonged QRS interval. It
should be noted that the T wave will not have its normal
morphology either since the repolarization waveform must
necessarily be aberrant as well.
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CORRECTION
In Figure 1 of “Scoring Center: Scoring Cardiac
Dysrhythmias – Part 1 of 2” (A2Zzz 2007;16(4):31), the
arrow showing the direction of electrical impulse for
modified lead 1 points in the wrong direction. The arrow
should point from negative to positive, which would be
from the patient’s right subclavicular to left subclavicular.
The corrected figure is shown here.
I
(-,-)
(+,-)
III
II
(+,+)
FIGURE 1. Locations for electrode placements
The red line is a modified lead 2 (recommended), the blue
line is a modified lead 1, and the green line is a modified
lead 3. The polarity of the electrode in each dipole is
indicated in parentheses.