Ask the Expert
DIAGNOSTICS / CARDIOLOGY
Peer Reviewed
Interpreting ECGs with
Confidence: Part 1
This is the first installment of a two-part
series outlining ECG
interpretation. Part 2
will address bradyarrhythmias and
tachyarrhythmias.
REQUIREMENTS
FOR SUCCESS
● Obtaining accurate ECG
diagnosis
● Considering possible
underlying mechanism
or cause of arrhythmia
● Determining presence of
underlying heart disease
● Understanding the
goals of arrhythmia
treatment:
◗ In patients with
clinical signs: Reducing
hemodynamic compromise caused
by arrhythmia and
preventing death
◗ In asymptomatic
patients: Preventing
arrhythmia from
progressing to cause
clinical signs or death
Barret Bulmer, DVM, MS,
DACVIM (Cardiology)
Tufts University
You have asked….
How do I interpret ECGs with confidence?
The expert says…
A
rrhythmias are clinically important because of their ability to compromise cardiac
output and oxygen delivery to the body. The level of cardiac performance during an
arrhythmia depends on rate, site of origin, and duration of the arrhythmia, as well
as the presence of underlying cardiac or systemic diseases that may adversely affect the
patient. Consequences of an arrhythmia may be clinically undetectable, produce signs of
inadequate cardiac output (eg, weakness, fainting, shock), or lead to complete collapse of
the circulatory system and sudden death.
The 4 general steps for ECG assessment include evaluation of heart rate, cardiac rhythm,
mean electrical axis, and assessment for patterns of chamber enlargement. The following
review assesses and interprets common canine and feline arrhythmias.
A systematic approach to interpreting ECGs will ensure an accurate diagnosis. The initial
steps should include evaluating paper speed and sensitivity followed by labeling the individual waveforms recorded on the ECG. Afterward, the following questions should be
addressed:
Q
A
IS THERE A P WAVE FOR EVERY QRS COMPLEX?
Atrial depolarization normally propagates through the AV node and subsequently
produces depolarization of the ventricles, resulting in a fixed relationship of one
P wave preceding every QRS complex.
Meanwhile, ectopic QRS complexes often originate without a preceding atrial event.
Premature ectopic complexes occur <1 R-R interval from the preceding sinus beat,
while escape complexes generally occur 1.5 to 2 R-R intervals from the preceding
sinus beat.
Q
A
IS THERE A QRS COMPLEX FOR EVERY P WAVE?
After a brief delay (PR interval) ventricular depolarization normally follows every
atrial depolarization, resulting in a QRS complex for every P wave. However, in
some instances the electrical impulse may be blocked at the level of the AV node or
His-Purkinje system, preventing propagation and subsequent depolarization of the
ventricles.
AV = atrioventricular, SV = supraventricular, VPC = ventricular premature complex
32 ..............................................................................................................................................................................NAVC Clinician’s Brief / May 2012 / Ask the Expert
FOR MORE…
See Management Tree on page 37 for an algorithm outlining the
diagnosis and management of rhythms associated with normal
heart rates (dogs, 70-160 bpm; cats, 140-220 bpm).
Q
A
ARE P WAVES ASSOCIATED WITH
QRS COMPLEXES?
Normally there is a fixed or nearly fixed physiologic PR interval between the P waves and QRS
complexes, but in some instances there appear to
be P waves that contribute to generating ventricular depolarization while they are instead merely
fortuitous and QRS complexes are unrelated to the
preceding atrial activity.
Q
A
ARE THE QRS COMPLEXES SUPRAVENTRICULAR OR VENTRICULAR?
If the electrical impulse originates from the sinus
node, atrial myocardium, or atrioventricular (AV)
node, the QRS complex will generally be tall,
upright, and narrow (ie, supraventricular [SV])
because it utilizes the normal His-Purkinje system.
Circumstances in which the QRS complex is SV
but appears abnormal include ventricular enlargement patterns or bundle branch blocks.
If the electrical impulse originates from the ventricular myocardium, the QRS complex will be
wide and bizarre (ie, ventricular) because it does
not utilize the normal His-Purkinje system.
Q
A
WHAT IS THE UNDERLYING RHYTHM
OF P WAVES & QRS COMPLEXES?
A regular rhythm generally has <10% variation in the
R-R intervals, while a regularly irregular rhythm has
QRS complexes that vary by >10% but there is a repeating pattern to rate variation. An irregularly irregular
rhythm is chaotic, usually fast, and without pattern to
the irregular nature.
A paroxysmal rhythm disturbance is usually characterized
by a sudden outburst of arrhythmia that may last for as
short as 3 to 4 beats or as long as minutes to hours.
Q
A
WHAT ARE THE MOST COMMON RHYTHMS
WITH NORMAL HEART RATES?
The most common cardiac rhythms that occur while the
heart rate is normal include sinus rhythm, sinus arrhythmia, SV premature complexes, ventricular premature
complexes (VPCs), or accelerated idioventricular rhythm.
Sinus rhythm, sinus arrhythmia, and SV premature complexes tend to have tall, upright, narrow QRS complexes.
Sinus rhythm with VPCs or accelerated idioventricular
rhythm may have a normal heart rate with intermittent
or sustained ventricular morphology QRS complexes;
because these impulses originate from the ventricular
myocardium, they are conducted aberrantly and have
abnormal morphology.
CONTINUES
SINUS RHYTHM & CARDIAC ARRHYTHMIA
Sinus rhythm represents the normal
sequence of cardiac electrical activity.
Pacemaker cells within the sinoatrial
node display automaticity and serve as
the origin for the normal cardiac
impulse. The P wave is recorded on the
ECG as the wave of electrical activity
depolarizes the atrial myocardium. The
electrical impulse then traverses the AV
node and His-Purkinje system, recorded
as the PR interval, followed by depolarization of the ventricular myocardium
producing the QRS complex. The T
wave is recorded during ventricular
repolarization (Figure 1). The average
heart rate (ie, ventricular heart rate) can
be determined on an ECG strip
recorded at 50 mm/sec by counting the
number of QRS complexes in 15 large
boxes (75 mm) and multiplying by 40.
1
P wave (P), QRS complex
(QRS), and T wave (T). In
this ECG, the average
heart rate would be 120
bpm. If the ECG strip is
recorded at 25 mm/sec,
the number of QRS complexes can still be counted
in 15 large boxes (75 mm)
but must be multiplied by
20 to determine the bpm.
Ask the Expert / NAVC Clinician’s Brief / May 2012 ..............................................................................................................................................................................33
NEXT ISSUE...
Ask the Expert
Part 2: Common tachyarrhythmias,
bradyarrhythmias, algorithms, and
more.
CONTINUED
SINUS RHYTHM
Sinus rhythm (Figure 2) maintains a regular underlying rhythm with P waves present for every QRS
complex, QRS complexes present for every P wave,
a relatively fixed PR interval, and SV QRS complexes. Sinus rhythm is normal for dogs and cats.
2
PR interval
SINUS ARRHYTHMIA
Variations in R-R intervals
Sinus arrhythmia (Figure 3) is a phasic
variation in heart rate in which R-R
3A
intervals often vary by >10%. There
continues to be a P wave for every
QRS complex and a QRS complex
for every P wave, although on occasion
the P wave morphology will vary or
be difficult to visualize because of a
phenomenon called wandering
3B
pacemaker (Figure 3B).
●
●
Sinus arrhythmia is usually respiratory with the heart rate increasing
in late inspiration and early expiration and decreasing in mid to late expiration. However, any other cause of
increased vagal tone may also contribute to sinus arrhythmia even in the absence of respiratory variation. The
underlying mechanism of sinus arrhythmia is depressed normal automaticity of the SA node.
Sinus arrhythmia is most common in normal resting dogs but uncommon in cats. Treatment is not required.
ACCELERATED IDIOVENTRICULAR RHYTHM
Also known as slow ventricular tachycardia (Figure 4), this is a unique
form of ventricular tachycardia
4
typified by slow heart rates (dogs,
70–150 bpm). The rate of the accelerated idioventricular rhythm is
usually within 10 to 15 beats of the
normal sinus rate, and control of the
heart rhythm alternates between the two sites. Because the rate of the idioventricular focus is slow, there is little hemodynamic
consequence and patients generally remain asymptomatic. These rhythms are commonly seen with noncardiac disease and also
in dogs with traumatic myocarditis and neurologic disease. Specific treatment of an accelerated idioventricular rhythm is rarely
indicated and should be directed toward underlying causes.
SV = supraventricular, SVT = SV tachycardia, VPC = ventricular premature complex
34 ..............................................................................................................................................................................NAVC Clinician’s Brief / May 2012 / Ask the Expert
VENTRICULAR PREMATURE COMPLEXES
VPCs (Figure 5) can be associated with
significant inflammatory, structural, toxic,
infiltrative, traumatic, or ischemic heart
disease. VPCs are also common in
5A
patients with noncardiac disease (eg,
gastric dilatation-volvulus [GDV], splenic
disease/surgery, hyperthyroidism, sepsis,
anemia, hypoxia, myocarditis/infectious
disease, coagulopathies). Patients with
elevated sympathetic tone from extreme
excitement or pain may also display VPCs,
as well as drug therapy with digoxin,
anesthetics (especially short-acting
thiobarbiturates), and catecholamines.
VPC
Note: No P wave
5B
●
VPCs generally occur <1 R-R interval
from the preceding sinus beat (vs ventricular escape beats [Figure 5B] that
generally occur >2 R-R intervals from
the preceding sinus beat) and are not associated with preceding P waves. Care must be taken to exclude these
arrhythmias from sinus rhythm with chamber enlargement patterns or sinus rhythm with conduction disturbances.
●
The decision to treat VPCs can be difficult. Frequent and multiform VPCs in the presence of underlying heart
disease, especially with clinical signs, will necessitate treatment. Infrequent, uniform VPCs without clinical signs
or heart disease uncommonly require treatment. Treatment for VPCs secondary to underlying metabolic disease is
usually not indicated and therapy should instead be aimed at correcting the underlying condition.
SUPRAVENTRICULAR PREMATURE COMPLEXES
Underlying sinus rhythm with SV premature complexes will have QRS complexes that occur early (<1 R-R interval
from the preceding sinus beat), maintain an SV morphology, and either lack P waves (junctional) or have P waves with
aberrant morphology (atrial). SV premature complexes can be difficult to distinguish from sinus arrhythmia with a
wandering pacemaker, although the former usually occurs at an instantaneous rate suggestive of tachycardia. The
instantaneous rate is determined by counting the number of millimeters between 2 successive QRS complexes, and
if recorded at 50 mm/sec, dividing that number into 3000.
●
●
<1 R-R interval
SV premature complexes
(Figure 6) are often produced
subsequent to atrial enlargement associated with under6
lying myocardial, valvular, or
congenital heart disease. SV
premature complexes are
occasionally seen in patients
without clinical evidence of
cardiac disease. The presence of these beats may precede development of further atrial arrhythmias, including SV
tachycardia (SVT), atrial flutter, or atrial fibrillation.
In most cases, SV premature complexes are infrequent, do not cause clinical signs, and do not require treatment.
In patients with paroxysms of SVT, treatment with digitalis, calcium-channel blockers, or β-blockers may
be instituted. Treatment for underlying heart disease may also be necessary.
See Aids & Resources, back page, for references & suggested reading.
Ask the Expert / NAVC Clinician’s Brief / May 2012 ..............................................................................................................................................................................35