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DIAGNOSTICS / CARDIOLOGy
Peer Reviewed
Interpreting ECGs with
Confidence: Part 2
This is the second
installment of a twopart series outlining
ECG interpretation.
Part 1, which appeared
in the May 2012 issue
of Clinician’s Brief,
discussed the general
steps for ECG assessment and common
Barret Bulmer, DVM, MS,
DACVIM (Cardiology)
Tufts University
Q
A
WHAT ARE THE MOST COMMON TACHYARRHYTHMIAS?
The most common sustained tachyarrhythmias include atrial fibrillation (AF), ventricular
tachycardia (VT), sinus tachycardia, and supraventricular tachycardia (SVT).
ATRIAL FIBRILLATION
AF (Figure 1) is the classic irregularly irregular, fast cardiac rhythm produced by disorganized and chaotic depolarization of atrial myocardial tissue, resulting in the inability to
produce effective atrial contractions. AF is one of the most important arrhythmias in dogs
but is uncommon in cats. Most cases are secondary to severe atrial enlargement and underlying heart disease.
rhythms associated
with normal heart
rates.
AF = atrial fibrillation, AV = atrioventricular, SA = sinoatrial, SVT = supraventricular tachycardia, VPCs = ventricular
premature complexes, VT = ventricular
tachycardia
1
●
Propagation of AF requires a critical amount of atrial mass and, in general, the larger the
animal the easier it is to induce and maintain AF. Because of their size, giant-breed dogs
can develop AF without other objective evidence of cardiac disease. Hallmark findings
include an irregularly irregular rhythm with variation in heart sound intensity and arterial
pulse quality.
●
The 3 ECG hallmarks of AF include absence of identifiable P waves in all leads,
supraventricular QRS complexes, and an irregularly irregular rhythm. The baseline may
frequently display oscillations but the organization is ill defined.
102............................................................................................................................................................................NAVC Clinician’s Brief / June 2012 / Ask the Expert
VENTRICULAR TACHYCARDIA
VT is defined as 3 or more ventricular premature
2A
complexes (VPCs) occurring in succession. VT produces wide and bizarre QRS complexes and displays a
range of rate, morphology, and regularity; it is generally
considered a serious and potentially life-threatening
arrhythmia, especially in patients with underlying heart
disease or clinical signs (weakness, collapse, shock). A
variety of mechanisms can produce VT, including reentry, abnormal automaticity, and triggered activity. P
waves may still be visible (Figure 2A) but there is no 2B
consistent temporal relationship with QRS complexes.
SVT with abnormal ventricular conduction (eg, bundle
branch block) can be confused with VT.
●
Unifocal VT is the most common type and (when
sustained) produces a fast, regular cardiac rhythm.
●
Paroxysmal VT (Figure 2B) may produce irregular bursts of tachycardia that interrupt an otherwise normal rhythm.
SINUS TACHYCARDIA
Sinus tachycardia (Figure 3) maintains all criteria for sinus rhythm except the heart rate is above normal. The most common
cause of sinus tachycardia is high sympathetic tone, usually associated with excitement or stress during examination. Other causes
include fever, pain, hyperthyroidism, anemia, shock, heart failure, and treatment with sympathomimetic agents such as bronchodilators or catecholamines.
●
The underlying mechanism of sinus
tachycardia is enhanced normal
automaticity of the sinoatrial (SA) 3
node. In general, there are P waves
for every QRS complex and the PR
interval is consistent. However, at
faster rates the P waves may become superimposed on the preceding T wave and the rate may be fast enough to
cause intermittent atrioventricular (AV) block. The heart rate may gradually slow during a vagal maneuver, ultimately
allowing identification of P waves.
SUPRAVENTRICULAR TACHYCARDIA
SVT (atrial or junctional) may lack easily identifiable P waves or have P waves with slightly different morphology than normal
sinus beats. Paroxysmal SVT often displays abrupt onset and termination. AF is a specific form of SVT, as previously discussed.
●
SVT (Figure 4) is an infrequent arrhythmia
defined as 3 or more SV beats in succession.
SVT is usually associated with significant heart 4
disease and atrial enlargement. A unique form
of SVT is associated with an accessory pathway
connecting the atria and ventricles. When occurring as a paroxysm, the rhythm is fast and regular, often with abrupt onset and termination. The first heart sound is often accentuated while audible murmurs diminish in
intensity during the paroxysm as a result of decreased stroke volume. SVT usually results from a reentry circuit, although it
may also occur subsequent to enhanced normal or abnormal automaticity.
●
Sustained SVT can provoke loss of normal myocardial contractility and eventual heart failure, while paroxysmal SVT
can cause episodic weakness and syncope. Therapeutic interventions can be administered to acutely terminate SVT
and prevent recurrence.
CONTINUES
Ask the Expert / NAVC Clinician’s Brief / June 2012............................................................................................................................................................................103
Ask the Expert
CONTINUED
FOR MORE...
See Management Tree on page 106 for algorithms
outlining diagnostic and management approaches to
bradyarrhythmia and tachyarrhythmia.
Q
A
WHAT ARE THE MOST COMMON BRADYARRHYTHMIAS?
The most common bradyarrhythmias include atrial standstill, sinus arrest, sinus bradycardia, and various forms of AV
block (second- or third-degree).
ATRIAL STANDSTILL
Atrial standstill (Figure 5) is an uncommon arrhythmia caused by idiopathic destruction of atrial myocardium or severe hyperkalemia. The atrial myocardium becomes unresponsive to sinus impulses and does not depolarize. A junctional or ventricular
escape rhythm takes over control of the ventricles. The prognosis in dogs with idiopathic disease is guarded, as many experience
progressive myocardial
disease and eventual
heart failure. How5
ever, some dogs will
live several years with
pacemaker implantation
and prevention of congestive heart failure.
SINUS ARREST
If a P wave is not present for every QRS complex, sinus arrest (Figure 6) may be present. Some patients with sinus arrest may
have sick sinus syndrome, which may be complicated with periods of AV block and/or SVT. Sinus arrest occurs when the SA
node fails to depolarize because of deceased normal automaticity, resulting in a pause in the heart rhythm that can last from
<1 second to many seconds. Longer periods of sinus arrest may be accompanied by signs of weakness or syncope.
●
Historically, the
criteria for sinus
arrest involved a
6
pause that lasted
>2 R-R intervals;
however, dogs with
pronounced sinus
arrhythmia can also
display pauses of this
duration, resulting in
some overlap between
the diagnoses. Possible causes of sinus arrest include excessive vagal tone, sick sinus syndrome, and hyperkalemia. In general,
treatment is not required in asymptomatic patients.
AV = atrioventricular, SA = sinoatrial, SVT = supraventricular tachycardia
104............................................................................................................................................................................NAVC Clinician’s Brief / June 2012 / Ask the Expert
SINUS BRADYCARDIA
Sinus bradycardia (Figure 7) occurs when criteria for sinus rhythm are met but the heart rate is slower than normal. The exact
rate at which sinus bradycardia is considered pathologic is subjective: 40 bpm in a sleeping dog is considered normal, but the
same would be considered abnormal during examination. A common cause of sinus bradycardia is elevated vagal tone caused by
athletic conditioning, increased intracranial pressure, severe GI or respiratory disease, or administration of parasympathomimetic
or sympatholytic drugs.
Other causes include
sedative or anesthetic
7
agents, hypothermia,
hyperkalemia, sick sinus
syndrome, and severe
hypothyroidism.
SECOND- & THIRD-DEGREE AV BLOCK
Second-degree (Figure 8) and third-degree (Figure 9) AV block have some (second-degree) or all (third-degree) P waves blocked
at the level of the AV node or His-Purkinje system. This alteration will produce P waves without an associated QRS complex.
●
Second-degree AV
block occurs when
a proportion of
8
atrial impulses are
prevented from
reaching the ventricles. High-grade second-degree AV block
is often secondary to
idiopathic conduction system disease and may cause transient weakness, syncope, or sudden death. Other causes include high
vagal tone, treatment with digoxin, calcium-channel blockers, β-blockers, or underlying heart disease or electrolyte abnormalities (uncommon).
●
Third-degree or
complete AV block
is present when
9
none of the atrial
impulses reach the
ventricles, indicating
complete AV dissociation. Instead, ventricular depolarization
relies on subsidiary
pacemaker cells that depolarize more slowly than the sinus node or AV node. Canine patients with third-degree AV block
usually present with weakness, exercise intolerance, syncope, congestive heart failure, and/or dyspnea; some have experienced
sudden death. Dogs reportedly may also be asymptomatic and the arrhythmia will be detected during routine or preoperative
examination. AV block is often idiopathic, although other causes include degenerative, infiltrative, inflammatory, infectious,
and/or immune-mediated heart disease; hyperkalemia or severe digoxin toxicity can also be a cause.
●
Cats with third-degree AV block rarely display clinical signs as a result of an adequate ectopic pacemaker rate to maintain cardiac output. Some cats with transient AV block will present with syncope. Atropine or oral sympathomimetics can be administered but are often ineffective.
See Aids & Resources, back page, for references & suggested reading.
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