Page 217 - Feline Cardiology

222 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

QRS

P
T

Figure 18.8. Supraventricular tachycardia/atrial tachycardia in a 4-year-old Persian cat with unclassified cardiomyopathy. QRS com-
plexes are narrow and upright in this lead II tracing, consistent with a supraventricular origin of the arrhythmia. P waves are sometimes
difficult to see because of fine artifact altering the baseline. The heart rate is 320 beats/minute. 25 mm/sec, 1 cm = 1 mV.

may explain the occurrence of atrial arrhythmias nodal reentrant tachycardia (AVNRT), orthodromic
through microreentry: heterogeneous depolarization atrioventricular reciprocating tachycardia (OAVRT),
Arrhythmias poses to reentry, especially if an obstacle exists (inexcit- these, sinus tachycardia has been discussed (above).
and repolarization of diseased atrial myocytes predis-
and automatic junctional tachycardia (Wright 2009). Of
Atrial flutter, atrial fibrillation, AVNRT, and OAVRT (the
able atrial myocytes) around which the reentry may
occur (Boyden et al. 1984).
separately (see below). The remaining arrhythmias are
Premature atrial complexes are thus associated with latter two being forms of preexcitation) are presented
diseases that can cause atrial enlargement, cardiomyopa- generally grouped in the broad category of “atrial tachy-
thy, or both: a case series of 59 cats with cardiomyopathy cardia.” This simplification would be excessive for
from the early 1970s revealed a prevalence of PACs of patients whose tachycardia might be cured with radio-
12% (Harpster 1977). Premature atrial complexes also frequency catheter ablation, but is adequate for practical
occur in association with noncardiomyopathic disor- purposes in feline cardiology given the current practical
ders, including 11% (Peterson et al. 1982) to 15% (Moïse limitations to performing therapeutic catheter ablations
et al. 1986) of cats with hyperthyroidism; feline endo- in cats due to their small body size. An atrial tachycardia
carditis (Kovacevic et al. 2002); and in association with may be defined as a series of 3 or more consecutive PACs
ventricular preexcitation (Rishniw 2000). Healthy cats (Figure 18.8), some or all of which may successfully
have 0–21 PACs daily, with the higher number occurring cross the AV node. When atrial activity is excessively
in older individuals (Ware 1999). The clinical signifi- rapid, the filtering function of the normal AV node may
cance of PACs in cats primarily is their role as a marker limit the number of P’ waves admitted into the ventri-
for atrial disease. The underlying cause must be managed, cles, a protective mechanism called physiologic AV block.
but antiarrhythmic treatment is not warranted for PACs For example, a cat with an ectopic atrial focus that fires
alone, unless clinical signs such as syncope exist and it at a rate of 420 beats/minute would have a ventricular
is suspected that PACs are only the tip of the supraven- rate (heart rate) of 420 beats/minute if each atrial
tricular tachyarrhythmia iceberg. In such cases, extended impulse crossed the AV node normally into the ventri-
ECG monitoring such as Holter assessment or in- cles (“1 : 1 conduction”). Such a rapid heart rate could
hospital telemetry is recommended to confirm that be expected to cause a substantial decrease in cardiac
clinical signs coincide with a sustained arrhythmia, and output due to severely limited diastolic filling time (see
to identify the causative arrhythmia at the time of clini- Figure 18.1). Physiologic 2 : 1 or 3 : 1 second-degree AV
cal episodes. block in this patient would commonly occur, producing
heart rates much closer to the normal range: 210 or 140
Atrial Tachycardia (AT) beats/minute, respectively, and such physiologic block
Supraventricular tachycardias are broadly defined as any has a beneficial effect in AT that explains why some
tachycardia originating from the SA node, atrial myo- patients may tolerate AT better than others.
cardium, AV node/junction, or great vessels connecting The mechanism for AT is the same as for PACs. In
to the atria (venae cavae, pulmonary veins, coronary cats, atrial tachycardia has been associated with hyper-
vein) (Blömstrom-Lundqvist et al. 2003). Specifically trophic cardiomyopathy (Boyden et al. 1984; Liu and
included are sinus tachycardia, sinus node reentrant Tilley 1980), dilated cardiomyopathy (Liu and Tilley
tachycardia, automatic atrial tachycardia, intra-atrial 1980), and hyperthyroidism (Peterson et al. 1982;
reentrant tachycardia, atrial flutter, atrial fibrillation, AV Kovacevic et al. 2004), and is always considered abnor-