Page 125 - Feline Cardiology

124 Section D: Cardiomyopathies

the cardiac electrical rhythm. Rather, it is created by an degrees) suggestive of left anterior fascicular block or left
extra heart sound (S 3 or S 4 ) that occurs with every heart ventricular enlargement is another relatively common
beat, giving the auscultatory impression of a galloping finding, occurring in 10–33% of cats with HCM (Figure
horse irrespective of the heart’s rhythm (see Chapter 1). 18.22) (Fox et al. 1999b; Bright et al. 1992). However,
However, if the clinician is unsure whether there is an left axis deviation is not specific for HCM, because 6–8%
arrhythmia versus an extra heart sound, an electrocar- of cats with hyperthyroidism have left axis deviation,
diogram is indicated. Arrhythmias are variably present and the mean electrical axis may be altered substantially
in cats diagnosed with HCM. There is a wide variation by simple factors such as limb position and electrode
in reported incidence of arrhythmias in cats with HCM. clip placement (see Chapter 18) (Fox et al. 1999a).
The electrocardiogram is an insensitive test for detec-
Arrhythmias occur in only ∼7% of cats in one case series tion of cardiac chamber enlargement in cats and should
Cardiomyopathies diagnosed with HCM reported higher incidences of not be used as a screening test for HCM. Only 25% of
of 61 cats with HCM, whereas another study of 46 cats
cats with HCM in one study had a left ventricular
arrhythmias as follows: ventricular premature com-
plexes in 41% of cats, ventricular tachycardia in 10% of
cats, atrial premature complexes in 10% of cats, atrial
tachycardia or atrial fibrillation in 10% of cats (see
Chapter 18) (Ferasin et al. 2003; Fox et al. 1999b).
Differences in arrhythmia incidence may be affected by
the duration of ECG monitoring, patient selection
(whether all cats are evaluated compared to only cats
with an auscultable arrhythmia), or severity of disease
within the studied population (proportion of asymp-
tomatic versus symptomatic patients evaluated).
Ventricular premature complexes appear to be more
common in cats with HCM suffering from arterial
thromboembolism than in all cats with HCM combined A
(31% versus 9%, respectively) (Fox and Harpster 1999).
Atrial fibrillation occurs when there is significant left
atrial dilation, and reflects an advanced stage of disease
(Côté et al. 2004). Tachyarrhythmias worsen diastolic
dysfunction and lead to higher diastolic filling pressures
in HCM (Gwathmey et al. 1991). That is, heart rates
sustained above a certain threshold, such as 240–260
beats/min in the cat, may limit diastolic ventricular
filling to a substantial degree (see Figure 18.1). This
impairment in diastolic filling may trigger development
of heart failure in cats otherwise compensated with
HCM. Third-degree atrioventricular block is rare in cats
with HCM (1/61 cats in one study). In a pathologic
study of 13 cats with HCM and third-degree atrioven-
tricular block, all cats had marked degeneration and
fibrous replacement of the atrioventricular conduction
system, along with endomyocardial and myocardial
fibrosis of the basilar interventricular septum (see
Chapter 18) (Kaneshige et al. 2006). However, a majority B
of age-matched control cats (mean age 12.9 years old)
also had evidence of slight to moderate fibrosis of the Figure 11.7. Thoracic radiographs of a cat with moderate hy-
atrioventricular conduction system, but changes were pertrophic cardiomyopathy but no congestive heart failure. There
less severe and less extensive. It was hypothesized that is left atrial enlargement (arrow) seen on both lateral (A) and dor-
soventral (B) views in this cat with moderate HCM. The moderate
abnormal mechanical forces in HCM as well as left ven- left atrial dilation causes a classic “valentine” heart shape on the
tricular obstruction due to SAM may amplify age-related dorsoventral view. The pulmonary vasculature is not distended,
fibrotic changes to the atrioventricular conduction and the pulmonary parenchyma is unremarkable, with no evi-
system. Left axis deviation (0 degrees to minus 90 dence of heart failure at this time.