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Electrocardiography-
Part III
Kathy Glaze, RVT
Department of Small Animal Surgery College of Veterinary Medicine Texas A&M University College Station. Texas
KEY POINTS
Four steps are essential for the proper diagnosis of cardiac arrhythmias.
Ventricular arrhythmia can be determined by the duration of the QRS complex.
Assessment of the P-R interval helps determine whether the atria and ventricles are working in synchrony.
This article is the third part of a three-pan presentation. The first part discussed the essentials of
basic electrocardiography (ECG), such as the physiology of depolarization and repolarization and
the anatomy of the heart. The second part discussed the production and interpretation of the
electrocardiogram (ECG). This part reviews the use of the ECG to diagnose cardiac arrhythmias,
such as premature beats and escape beats, supraventricular rhythms, ventricular rhythms, and
disorders of atrioventricular conduction.
There are four steps involved in interpreting an ECG. Nearly any ECG can be interpreted by
following these steps.
The first step is to evaluate the P waves. P waves indicate whether the atrial rhythm is
normal. For proper diagnosis of an ECG, technicians should ask themselves the following
questions: Are the P waves occurring at regular intervals? Do the P waves all have the same
appearance on the ECG? Are the P waves visible at all? Normal sinus rhythm and respiratory sinus
arrhythmia are two normal rhythms that apply to the first two questions; rhythms that do not have
recognizable P waves are discussed later in this article. If the answer to any of these questions is
"no," further investigation is necessary to diagnose the arrhythmia.
P waves appear on the ECG at regular intervals during normal sinus rhythm. This means that the
interval from one P wave to the next is the same throughout the rhythm. In dogs, respirator, sinus
arrhythmia is a normal rhythm in which the P waves have irregular interval. In both normal sinus
rhythm and respiratory sinus arrhythmia, the P waves have the same appearance. P waves with
different appearances can be seen in a rhythm with a wandering pacemaker. If the P waves are
unrecognizable in a rhythm that has a low amplitude in the waveforms, doubling the sensitivity to
1 mV = 2 cm occasionally makes the P waves more apparent.
The second step is to ascertain whether the ventricles were activated from an impulse originating in
the ventricles themselves or from some other location. This can be determined by evaluating the
duration of the QRS complex. A QRS complex of normal duration (0.04 to 0.06 seconds) indicates
that conduction through the ventricles has occurred along the normal pathway of specialized
conduction tissues. A QRS complex of longer duration indicates that conduction has left the
normal pathway and is occurring within the ventricular myocardium. When the specialized conduction pathways are not used and depolarization and repolarization occur on a cell-to-cell
basis within the myocardium, the process takes much longer. This is called a ventricular
complex or beat and causes the QRS complex to have a wide and bizarre appearance on the
ECG.
The third step is to determine the relationship between the P waves and the QRS complexes. This
is a means of assessing atrioventricular conduction and whether the atria and ventricles are
working in synchrony. It must be determined whether the P waves are always, sometimes, or never
associated with the QRS complex. On tracings of normal sinus rhythm, there is a P wave for every
QRS complex and vice versa; in addition, the P-R interval is within normal limits for each beat,
demonstrating that the atria and ventricles are working in synchrony. However, this is not always
the case.
Finally, the tracing should be examined for anything that looks unusual. Such unusual elements on
the ECG demonstrate a true arrhythmia or an artifact. Arrhythmias include premature beats, escape
beats, supraventricular arrhythmias, ventricular arrhythmias, and disorders in atrioventricular
conduction.
Premature Beats:
Atrial Premature Contraction
Atrial premature contraction (Figure 1) is a beat that is not synchronized with the rest of the rhythm.
The ECG appearance of P waves associated with atrial premature contraction usually differs from that of
normal sinus rhythm because of the different origin and conduction through the atria. The P-R intervals
may be short, normal, or long, depending on the site of origin of the premature beat.
Sites of origin include the sinoatrial node or ectopic locations in the atria. Atrial premature
contractions may or may not be conducted to the ventricles, depending on when the impulse reaches
the atrioventricular node. If depolarization is conducted to the ventricles, the QRS complexes usually
have a normal configuration because impulse has traveled along the normal conduction system within
the ventricles. If depolarization travels to the ventricles and reaches the atrioventricular node before it
has repolarized, premature P waves without QRS complexes appear on the ECG.
Ventricular Premature Contraction
Ventricular premature contraction (Figure 2) is characterized by wide, bizarre QRS complexes without
associated P waves. P waves are absent because depolarization does not originate from the
sinoatrial node The wide configuration of the QRS complex results from repolarization and
depolarization occurring on a cell-to-basis within the ventricular myocardium; the appearance of
this arrhythmia on the ECG differs greatly from that of other QRS complexes of sinus origin.
The origin of ventricular premature contraction can be determined by looking at lead II and
applying a knowledge of vectors (see Part I). If the impulse originated in the left ventricle, the wave
of depolarization would travel upward and to the right on the ECG; as a result, the ventricular
premature contraction would have a downward deflection on the ECG tracing. Conversely, a
ventricular premature contraction that originated in the right ventricle would travel downward
and to the left and thus have an upward deflection on the ECG.
Escape Beats
The sinoatrial node is the normal and primary pacemaker of the heart. Occasionally, the
sinoatrial node fails to depolarize on time. A brief pause is referred to as a sinus pause. A pause
that lasts longer than 2 seconds is called sinus arrest. In most cases, the sinoatrial node resumes a
normal rhythm after a pause. When the sinoatrial node does not resume a normal rhythm, there are
failsafe mechanisms within the heart that enable continued functioning.
The atrioventricular node (also called the atrioventricular junction) depolarizes automatically if
the sinoatrial node fails to depolarize. Depolarization of other areas of cardiac anatomy resulting
from failure of the sinoatrial node to depolarize is called an escape beat (Figure 3). Escape beats
occur as a safety mechanism to preclude negative consequences of arrested normal sinus rhythm and
are considered beneficial ectopic beats.
When the atrioventricular node depolarizes, the current travels upward into the atria and
downward into the ventricles along the normal conduction system (Figure 3A). Depolarization
through the ventricles produces a QRS complex of normal duration. Depolarization
through the atria causes a downward tracing of the P wave in lead II of the ECG. Depending on
the speed at which this upward wave of depolarization travels, the P wave may appear before,
during, or after the QRS complex.
If the atrioventricular node fails to depolarize, the ectopic focus within the ventricles takes over.
The resultant ECG tracing strongly resembles that of ventricular premature contraction, with a
wide, bizarre QRS complex and no associated P waves. The difference between a ventricular
premature contraction and a ventricular escape beat is very important. Ventricular premature
contraction is an irritable occurrence within the ventricle that disturbs a usually healthy rhythm. An
escape beat is a failsafe mechanism that interrupts a life-threatening sinus arrest.
Supraventricular Arrhythmias
Supraventricular arrhythmias are those arrhythmias that originate above the ventricles. Such
arrhythmias include sinus bradycardia, sinus tachycardia, atrial flutter, atrial fibrillation,
junctional rhythm, and bundle branch blocks.
Sinus Bradycardia
Sinus bradycardia (Figure 4) is normal sinus rhythm that is less than 60 beats/min in dogs and
70 to 80 beats/min in cats. Sinus bradycardia originates from the sinoatrial node. Athletic dogs
may have heart rates below 60 beats/min; it must be determined whether bradycardia is a
normal or abnormal state for the particular patient.
Sinus Tachycardia —
Sinus tachycardia (Figure 5) is normal sinus rhythm that is faster than 160 beats/min in dogs
and faster than 240 beats/mm in cats.
Atrial Flutter
Atrial flutter (Figure 6) is rapid atrial depolarizations that occur at a rate of 250 to 350 beats/min.
Atrial flutter appears on an ECG as regular, sawtooth waveforms between the QRS complexes.
During this arrhythmia, the atrioventricular node is flooded with depolarizations from the atria.
These depolarizations may arrive before the atrioventricular node has been able to repolarize,
and it thus cannot accept the impulses from the atria. After repolarization, the atrioventricular
node accepts the impulses sent from the atria and allows the impulses to pass to the ventricles.
Thus, the ventricular rate differs from the atrial rate.
This rhythm is sometimes difficult to identify on the ECG. If ventricular rates are
approximately 140 to 150 beats/min and if bizarre P waves appear between the QRS complexes,
flutter is a likely diagnosis. Flutter may be chronic or transient and may mark a transition to
atrial fibrillation.
Atrial Fibrillation
On the ECG, atrial fibrillation (Figure 7) is characterized by no recognizable P waves, a
regularly irregular rhythm, and a very fast rate for the atria and ventricles. The atrial rate ranges
from 350 to 600 beats/min. Effective atrial contraction is lost, as is the contribution of the atria to
ventricular filling; the result is decreased cardiac output. Ventricular rate is irregular because the
atrioventricular node receives countless impulses from the atria at erratic intervals. The ventricular
rate characteristically falls between 220 and 240 beats/min.
Junctional Rhythm
Impulses that arise in the area of the atrioventricular node and the bundle of His are called
junctional rhythms (Figure 8). These impulses may be escape beats, premature beats, or an
entire rhythm of junctional beats. In dogs, the heart rate associated with junctional rhythm is
approximately 40 to 60 beats/mm as compared with the normal sinus rhythm rate of 100 to 120
beats/min. Accelerated junctional rhythm is characteri z e d by a heart rate of approximately 60 to
100 beats/mm. Junctional tachycardia has a rate greater than 100 beats/mm.
Bundle Branch Blocks
The right and left bundle branches of the ventricular conduction system convey impulses between the
atrioventricular node and the Purkinje's fibers at the apex of the ventricles. When one of the branches
gets blocked by a lesion, the impulse from the atrioventricular node travels exclusively down the other
bundle branch; it activates the ventricle on that side, crosses the septum, and travels through the
musculature to activate the ventricle on the opposite side. The ventricles are activated sequentially
rather than simultaneously, nearly doubling the duration of the QRS complex. Bundle branch blocks
are often confused with ectopic ventricular contraction because of the increased duration of the QRS
complex. The presence of normal P waves and P-R intervals on the ECG indicates a supraventricular
rhythm with a conduction abnormality. Left- and right-bundle branch blocks are shown in Figures 9
and 10, respectively.
Ventricular Arrhythmias
Ventricular arrhythmias are those that originate within the ventricles. These arrhythmias
include idioventricular arrhythmias, ventricular tachycardia, ventricular fibrillation, and disorders of atrioventricular conduction. Other abnormal rhythms include atrioventricular block
and atrioventricular dissociation.
Idioventricular Arrhythmias
Failure of both the sinoatrial node and the atrioventricular node results in an ectopic focus
within the ventricle that acts as the second failsafe mechanism to ensure a continuing heartbeat.
The rate of idioventricular arrhythmia (Figure 11) is 1 to 40 beats/min. No P waves are present
because the sinoatrial node is not functioning. Idioventricular arrhythmia can be lifethreatening
because an ectopic focus within the ventricles is very unstable.
Ventricular Tachycardia
Ventricular tachycardia (Figure 12) is three or more consecutive ectopic ventricular complexes at
a rate of 140 beats/mm or faster. Ventricular tachycardia may appear and disappear during the
patient's normal rhythm (called paroxysmal ventricular tachycardia) or may be sustained. This type of
arrhythmia indicates an irritable ventricular myocardium and may precede ventricular fibrillation.
Ventricular Fibrillation
Ventricular fibrillation (Figure 13) results from chaotic depolarization of the ventricles with a loss
of organized contractions. It is associated with small or large undulations of the baseline but no
true QRS complexes. Unless it is controlled immediately, ventricular fibrillation will result in
cardiac arrest.
Disorders of Atrioventricular Conduction
Atrioventricular conduction is assessed by examining the relationship between the P waves and
QRS complexes. It must be determined whether the P wave is always, sometimes, or never
associated with the QRS complex. This can be done by examining the P-R interval. If the P-R
interval is always within normal limits, then the atria and ventricles are working in unison. If the
duration of the P-R interval varies, a disorder of atrioventricular conduction is a likely diagnosis.
First-Degree Atrioventricular Block
First-degree atrioventricular block (Figure 14) is diagnosed if a P wave precedes every QRS
complex but the P-R interval is longer than normal. This type of atrioventricular block is usually a
minor conduction defect because the atrial stimulus, although delayed, is always conducted to the
ventricles.
Second-Degree Atrioventricular Block
In second-degree atrioventricular block, some of the atrial impulses are not conducted to the
ventricles. There are two types of second-degree atrioventricular block. Type I second-degree
atrioventricular block (also referred to as Mobitz type I or Wenckebach's disease) (Figure 15) is
characterized by progressive lengthening of the P-R interval on successive beats and then P waves
occurring without QRS complexes. This occurrence is called a dropped beat. Conduction has failed
at the atrioventricular node, and the atrial impulse is not conducted to the ventricles. Type II seconddegree atrioventricular block (Figure 16) is characterized by a constant P-R interval that is usually of
normal duration with random dropped beats.
Third-Degree Atrioventricular Block
Also known as complete heart block, third-degree atrioventricular block (Figure 17) is
characterized by lack of association between the P waves and QRS complexes. A P wave may
occasionally seem to be associated with a QRS complex, but careful measurement of the distance
between individual P waves and between individual QRS complexes shows two separate,
independent rhythms: an atrial rhythm and a ventricular rhythm.
The atrial rhythm is seen as isolated P waves during third-degree atrioventricular block. These P
waves usually occur on the ECG as a very regular rhythm because the sinoatrial node is
functioning; however, the atrioventricular node is blocked. As a result, the impulses are unable to
pass through. If the site of the conduction block is at or above the atrioventricular node, the
ventricular rhythm is represented by junctional (or atrioventricular) escape beats. As mentioned,
an entire rhythm of these beats is called a junctional rhythm. The appearance of QRS complexes
associated with this type of rhythm is normal. If the block is located below the atrioventricular
node, the ventricles become the pacemaker. This appears on the ECG as wide QRS complexes.
Ventricular rate is approximately 20 to 40 beats/min.
Atrioventricular Dissociation
Atrioventricular dissociation (Figure 18) is a form of complete heart block; however, the
ventricular rate is faster than the atrial rate. In cases of atrioventricular dissociation, P waves and QRS
complexes occur independently of one another. This dissociation generally results from an irritable
ventricular musculature.
Conclusion
Diagnosis of cardiac arrhythmias does not have to be mysterious. By applying a knowledge of
vectors and evaluating the timing and appearance of the P waves and QRS complexes, most ECG
tracings can be diagnosed with only a small amount of effort. The four steps to diagnosis of cardiac
arrhythmias can provide the technician with an easy-to-follow algorithmic approach.