Page 211 - Feline Cardiology

216 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

• Complete blood count change over time (Alings and Bowman 1993). There is
• Serum biochemistry profile no established gender difference in heart rate.
• Urinalysis It should be remembered that normal sinus rhythm
may constitute a bradycardia if the patient is in an
Serum thyroxine levels, feline leukemia testing, and environment or has a medical status that would nor-
feline immunodeficiency virus testing may be appropri- mally be expected to elicit sinus tachycardia. An example
ate depending on age, environment, and physical exam is the cat that appears aroused or anxious but whose
findings. Additional testing is pursued based on results exam-room heart rate is only 140 beats/minute. While
and on the particular features of a case. These diagnostic some healthy cats may have naturally high vagal tone
evaluations allow the clinician to identify underlying or even when anxious, any cat with a resting heart rate
concurrent disorders and develop a treatment plan and <160 beats/minute in an examination room should be
prognosis. The specific prioritization of these tests is evaluated like a sinus bradycardia patient, beginning
detailed below for each arrhythmia. with an ECG to rule out pathologic bradycardias (see
below).
Heart Rate An elevated heart rate alone rarely is indicative of
The adult cat’s intrinsic heart rate (where the heart is heart disease in the cat. Although it is true that cats with
Arrhythmias pathetic inputs by concurrent propranolol and atropine minute, healthy cats or cats with noncardiac illness also
heart disease may have heart rates exceeding 240 beats/
artificially deprived of both sympathetic and parasym-
routinely have sinus tachycardia at such rates. For
treatment) is 158 ± 4 beats/minute, as compared to
218 ± 3 beats/minute in the same cats in a laboratory
cardiomyopathy and 87 did not, but their mean heart
setting prior to treatment (Diepstra et al. 1980). These example, in a study of 103 overtly healthy cats, 16 had
values confirm what is apparent to most clinicians, rates were essentially identical: 189 vs 188 beats/minute
namely that the autonomic input to the heart of a cat (Paige et al. 2009). Thus, rate alone cannot be used for
undergoing examination is usually characterized by a suspecting or ruling out heart disease in cats. Exceptions
predominance of sympathetic tone over parasympa- include 1) the concurrent presence of an irregular
thetic tone, likely through the same increase in cardiac rhythm, and 2) when the heart rate is sustained at >240
sympathetic nerve activity that is noted in cats during beats/minute for 60 seconds or more despite a calm
physical exercise (Tsuchimochi et al. 2002). Most normal environment and no identifiable systemic illness. With
cats seen in the veterinary setting have a heart rate that such unrelentingly high heart rates, pathologic tachycar-
ranges from 140 to 260 beats/minute (Côté et al. 2004), dias including ventricular tachycardia, atrial tachycardia,
with a mean of 150 ± 23 beats/minute at rest in a veteri- atrial fibrillation, and macroreentry need to be ruled out
nary teaching hospital and 187 ± 25 beats/minute during with an ECG.
restraint for echocardiography (Abbott 2005). By con-
trast, normal cats at home have a mean heart rate of SPECIFIC ARRHYTHMIAS
118 ± 11 bpm when ausculted by their veterinary
student-owners (Hamlin 1989) and routinely have heart Normal Sinus Rhythm
rates <100 beats/minute (presumably during sleep) The normal heartbeat originates in the sinoatrial (SA)
when assessed by Holter monitor (Abbott 2005; Hanås node, a region of specialized impulse-forming cells
2009; Ware 1999). Therefore, while some sources located in the dorsolateral right atrium. In the domestic
describe normal sinus rhythm and its defined rate of cat, a cluster of fewer than 2000 (possibly <500) cells
110–180 beats/minute as the normal cardiac rhythm for comprises the primary pacemaker within the SA node
the cat, context is critical and normal sinus rhythm, (Opthof et al. 1986). Unifocal impulse generation, a pre-
sinus bradycardia, and sinus tachycardia are to be dominance of collagen, and a paucity of gap junctions
expected in normal cats in appropriate environments. are characteristics of the feline SA node. Its morphology
Kittens have a higher heart rate (median: 247 beats/ resembles that of the dog and human but not the rabbit
minute during the first week of life) (Gomes Lourenço or guinea pig; however, electrophysiologic properties of
and Ferreira 2003) whereas adult cats conserve a similar the SA node of the cat and rabbit are similar (Opthof
range of heart rates across all ages, from young adult- et al. 1986).
hood to geriatric age. As cats age from kittenhood (6 A combination of sodium influx through the I f
weeks) to geriatric (18 years), SA nodal cycle length current, decay in potassium influx through I Kr and I Ks
increases, dependent on diastolic depolarization rate currents, and intramyocyte sodium-potassium exchange
and action potential duration (Alings and Bowman (transmembrane I Na–Ca current) in SA nodal cells raises
1993). The size of the adult feline SA node does not the cells’ membrane potential past the threshold level.