Page 220 - Feline Cardiology

Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 225

Historically perceived as a series of spontaneous elec- may be considered a marker for underlying structural
trical discharges in either or both atria or a single large heart disease in the cat.
reentrant circuit, AF is now understood to be a series of The hallmark physical examination finding in AF is
microreentrant electrical circuits that propagate in the an irregularly irregular rhythm. That is, auscultation of
atria in a spiral fashion, with important additional para- the heart reveals that the time between heartbeats varies
mechanisms including cranial and caudal vena caval, chaotically, and an inconsistent, patternless rhythm is
pulmonary venous, and left atrial contributions that heard. Given the rapid heart rate (>200 beats/minute)
continue to be investigated (Nattel 2002). of many cats in the hospital environment, underdiagno-
The loss of atrial contribution to ventricular filling, sis of AF is likely because at faster heart rates, the irregu-
and especially the high ventricular rate that often results larity of AF may be difficult to appreciate during
from AF, can cause decompensation (congestive heart auscultation. Pulse deficits, so commonly noted in AF in
failure and/or syncope) in previously compensated patients of other species, are possible but less common
patients with preexisting heart disease: AF can push a in cats with AF, perhaps because of the smaller diameter
patient “over the edge” from asymptomatic to symptom- femoral artery and variable pulse quality in this species,
atic (Nattel 2002). This phenomenon is well-recognized and because the difference in diastolic filling time is not
in humans and dogs, but appears less common in cats, substantially different if the ventricular response rate in
perhaps because of the comparative rarity of AF in this AF is similar to the rate of sinus tachycardia normally
species, or because ventricular rates are not dramatically experienced by cats in the exam room. A heart murmur, Arrhythmias
higher in cats with AF versus cats in normal sinus gallop sound, or both, may be ausculted as a result of
rhythm/sinus tachycardia. underlying structural heart disease (with which AF is
Equally or more important than the loss of effective commonly associated), but AF itself produces no aus-
atrial contraction to ventricular filling is the rapid ven- cultable abnormalities in the cat other than the irregular
tricular response rate (heart rate) that commonly occurs rhythm.
in AF. A patient’s AV node represents the filter that deter- On physical exam, the differential diagnosis for AF
mines whether the heart rate in a cat with atrial fibrilla- includes premature atrial complexes and premature
tion will be less than 260 beats/minute (resulting in ventricular complexes; any of the three arrhythmias
fairly normal cardiac output) or greater than 260 beats/ may produce an irregularly irregular rhythm and an
minute (where the reduced time for diastolic filling ECG is required to make the definitive diagnosis.
between beats outweighs the benefit of more beats per Electrocardiographically, the most important differen-
minute and cardiac output falls; see Figure 18.1). Thus, tial diagnosis for AF is motion artifact. Purring (Figure
treatment for atrial fibrillation in cats is generally 18.10) and shivering (Figure 18.11) are two common
reserved for those cats with a ventricular response rate such artifacts, and the rapid, tiny baseline undulations
(heart rate) >260 beats/minute, and such treatment con- they produce can easily be mistaken for AF if care is not
sists of administering medications that delay AV nodal given to noting whether the rhythm is irregular.
conduction. The confirmatory test for AF is the ECG. Any cat
The signalment of cats with AF reflects the underlying suspected or confirmed of having AF also should
cause: most cats with AF are male (82%) and have undergo tests to identify underlying heart structure and
underlying myocardial disease (restrictive or unclassi- extracardiac organ function in anticipation of medical
fied cardiomyopathy: 38%; hypertrophic cardiomyopa- treatment (see the section “Evaluating the Arrhythmic
thy or left ventricular concentric hypertrophy of other Feline Patient,” above). The ECG features of AF are
origin: 36%; dilated cardiomyopathy: 12%, including absence of P waves in all ECG leads, fine baseline undu-
cases from the pre-taurine era) (Côté et al. 2004; Boyden lations representing atrial fibrillatory activity (often too
et al. 1984). Thirty-three percent of cats with arrhyth- small to be clearly appreciated in the cat), and an irregu-
mogenic right ventricular cardiomyopathy had AF in larly irregular rhythm, such that R-R intervals vary from
one case series (Fox et al. 2000). A wide age range one beat to the next (see Figure 18.9). In cats with AF,
through adulthood exists in feline AF patients the mean ventricular response rate is 223 ± 36 beats/
(mean ± SD: 10.2 ± 3.7 years), and there is no known minute (Côté et al. 2004), which is increased but still
breed predisposition for AF specifically. overlaps with the expected heart rate range of healthy
The owners’ chief presenting complaints for cats with cats in a similar setting (Abbott 2005; Hamlin 1989). Of
AF are consistent with cardiac disease (dyspnea, signs of note, the R-R intervals of AF in any species may be
aortic thromboembolism, lethargy) but 22% of cats trivially variable when the heart rate is high—the QRS
with AF are found to have the arrhythmia during routine complexes are sufficiently close together when the heart
examination, indicating that AF, as an incidental finding, rate is fast that variation in R-R interval is difficult to