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Arrhythmias and Other Electrocardiographic

Abnormalities

Key Points

• Although at opposite ends of the heart rate spectrum, tachycardias and bradycardias may produce similar overt clinical signs
(e.g., syncope); however, most cardiac arrhythmias in cats produce no detectable signs and instead are incidental findings.
• The electrocardiogram (ECG, EKG) remains the diagnostic test of choice to confirm or rule out cardiac arrhythmias. If a resting
ECG is insufficient, portable or implantable ECG monitors may confirm the diagnosis.
• Myocardial disease is often present in cats with ventricular tachyarrhythmias (premature ventricular complexes, ventricular
tachycardia) or atrial fibrillation.

INTRODUCTION indistinguishable in their clinical manifestations from
those arrhythmias that cause an excessively slow heart
Cardiac arrhythmias (dysrhythmias) are defined as dis- rate (bradycardias, also called bradyarrhythmias). The
turbances in the rhythm of the heartbeat (Tilley 1993; rapid fluctuations in heart rate brought on by sporadic
Mirvis and Goldberger 2008; Fox and Harpster 1999; arrhythmias can alter cardiac output faster than a
Côté 2010). In cats, as in other species, a range of sever- patient’s homeostatic mechanisms can offset these
ity exists in terms of the significance of an arrhythmia changes. Therefore, forward cardiac failure, manifested
to the patient: arrhythmias may be normal physiologic classically as syncope, is the chief complaint of cardiac
variants, such as sinus tachycardia in the exam room; arrhythmias in cats, in contrast to chronic mechanical
individual variants of no concern, such as right bundle pump failure, such as with the cardiomyopathies or con-
branch block; markers for underlying structural heart genital malformations, where backward (congestive)
disease, such as many cases of third-degree AV block or heart failure occurs as a result of the lesion exceeding
atrial fibrillation; or the principal cause of the patient’s the body’s longstanding compensatory mechanisms.
chief complaint: episodic weakness, syncope, or even The rarity of syncope in the cat attests to the severity
sudden death. of arrhythmia needed to cause an observable change.
When bradycardias limit cardiac output, they do so by
ETIOLOGY, PATHOPHYSIOLOGY, AND
GROSS PATHOLOGY compromising heart rate (cardiac output [ml/min] =
heart rate [beats/min] × stroke volume [ml/beat]).
Broadly, arrhythmias can be divided into those disorders When tachycardias limit cardiac output, they do so by
where the creation or conduction of impulses is greater limiting stroke volume: the physiologic maximum heart
than normal, and those where it is less than normal. rate (the fastest rate of sinus tachycardia possible in a
Both situations can cause identical clinical manifesta- healthy cat, subjectively 260–280 beats/min for periods
tions: arrhythmias that involve an excessively high heart of several seconds to minutes) ensures adequate dia-
rate (tachycardias, also called tachyarrhythmias) are stolic filling time for the ventricles, but pathologic

Feline Cardiology, First Edition. Etienne Côté, Kristin A. MacDonald, Kathryn M. Meurs, Meg M. Sleeper.
© 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.

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