Page 210 - Feline Cardiology

Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 215

bradycardias requiring treatments that increase the experts and medical device companies support such
heart rate (bottom left of yellow box). screening, while others, including many cardiologists,
• Extreme increases in heart rate may render the ven- do not advocate this extent of preemptive use of the
tricles essentially non-functional; so little time is ECG. Certain changes are indeed apparent on ECG that
available between beats for diastolic filling that the are associated with underlying heart disease, and detect-
ventricles ultimately quiver rather than contracting ing these can lead to earlier diagnosis of heart disease.
and filling. Examples of such a situation are very rapid For example, 22% of cats with atrial fibrillation (AF) are
ventricular tachycardia (e.g., >400 beats/minute in the diagnosed with this arrhythmia incidentally during a
cat) or ventricular fibrillation. Cardiac output falls routine physical examination (Côté et al. 2004) and the
below a level that can sustain perfusion of vital organs vast majority of cats with AF have myocardial disease,
and cardiac arrest occurs (segment of the curve at the typically cardiomyopathy. Perhaps more cases of AF and
bottom right corner, outside the yellow box). Treatment cardiomyopathy could be identified through a more
requires immediate elimination of the tachycardia, widespread use of ECG in cats. Similarly, third-degree
such as with electrical defibrillation, so that normal AV block is detected as an incidental finding in other-
sinus rhythm may reemerge spontaneously. wise normal-appearing cats in 29% of cases (Kellum and
Stepien 2006); a wider application of early identification
This curve illustrates the relationship and concepts asso- could conceivably lead to pacemaker implantation and
ciated with heart rate and cardiac output in the cat, but prevention of sudden death in some patients. However,
exact numerical values for each parameter vary from these arrhythmias are often detected on auscultation Arrhythmias
one cat to the next depending on the structure of the and the expense associated with early diagnosis and
myocardium, presence of euvolemia/hypovolemia, and intervention may be reasonable when ECGs are per-
electromechanical association, among other factors. formed on a select population (e.g., those cats with an
Pathophysiologic mechanisms underlying arrhyth- audible arrhythmia); the expense and other drawbacks
mias are varied and are discussed for each individual may become dramatically—perhaps prohibitively—
arrhythmia. greater when applied to the feline hospital population as
a whole. Another example is ST segment changes, which
CLINICAL ELECTROCARDIOLOGY occur on the ECGs of 43–47% of cats with cardiomy-
opathy (Schwerin et al. 2002a, b). Although this high
Diagnostic Decision-Making and
Electrocardiography prevalence is important, its clinical application as a
screening test means that trusting the ECG to accurately
The ECG is the uncontested cornerstone of evaluation identify a cardiomyopathic cat amounts to the flip of a
of the cardiac rhythm. Two central questions govern its coin (50% likelihood). Therefore, despite the technical
use: when should one be performed, and what is to be simplicity and appeal of an ECG, these examples dem-
done if it reveals an abnormality? onstrate that the role of ECG as a screening tool is not
An ECG is warranted under any of the following supported by current data. Its use should be confined to
circumstances: patients meeting one or more of the criteria in the list,
above, until studies identifying the cost-to-benefit ratio
• When an irregular heart rhythm is ausculted or pal-
pated during physical exam of systematic ECG screening are performed in the feline
• When the heart rate is noted to be unusually rapid or population.
unusually slow considering the cat’s immediate envi- Evaluating the Arrhythmic Feline Patient
ronment and medical status
• When overt clinical signs suggest compromised cir- When the ECG has identified an abnormality, defined as
culatory function (e.g., syncope or near-syncope) a clinically relevant finding not explained by normal
• As part of monitoring a patient under general anes- physiologic changes (excluding sinus tachycardia caused
thesia, in an intensive care unit, or in any setting simply by excitement, for example), then the following
where acute changes in cardiovascular function are diagnostic investigations are appropriate in all cats (Côté
considered likely and Harpster 2009):
• As part of a comprehensive cardiovascular evaluation
• History
The value of ECG as a clinical screening test remains • Physical exam
debated. Some practitioners obtain ECGs from cats as • Thoracic radiographs
part of routine geriatric, preoperative, asymptomatic • Echocardiography
murmur, or other stable patient evaluations; some • Arterial blood pressure