Page 384 - Feline Cardiology

402 Section N: Endocrine Diseases Affecting the Heart

abnormalities of the heart and/or lungs, or other roidism is reserved for identifying arrhythmias in
respiratory signs should be evaluated with thoracic patients known or suspected to have them based on
radiographs. history and physical examination. Arrhythmias and con-
Historically, hyperthyroidism has produced radio- duction disturbances noted in hyperthyroid cats histori-
graphic cardiomegaly, increased QRS amplitude and cally have included prolonged QRS complex duration
other ECG changes, and thickening of the left ventricle (7/45 cats, 16%) (Peterson et al. 1982), atrial premature
on echocardiography (Moïse and Dietze 1986; Moïse et complexes (5/45, 11%; Peterson et al. 1982), short QT
al. 1986). Radiographic changes are limited in their sen- interval (5/45, 11%) (Peterson et al. 1982), left anterior
sitivity and specificity; that is, radiographs are poor fascicular block (2/45, 4%) (Peterson et al. 1982), pre-
screening tools and poor confirmatory tools for hyper- mature ventricular complexes (2/45, 4%; Peterson et al.
thyroidism, respectively, because radiographic changes 1982), 2 cases of ventricular preexcitation (Peterson et
are only noted when cardiac changes are very substan- al. 1982; Riesen and Lombard 2005) and one case each
tial, and there is much overlap between such changes of ventricular tachycardia and bigeminy, right bundle
with other nonthyroid diseases such as hypertrophic branch block, atrial tachycardia (all Peterson et al. 1982),
cardiomyopathy. Radiographic cardiomegaly was noted second-degree atrioventricular block with right bundle-
in 8/20 (40%) (Moïse et al. 1986) and 5/11 (45%) (Liu branch block (Rishniw et al. 1982), and third-degree AV
et al. 1984) of hyperthyroid cats in 2 studies published block (Jacobs and Otto 1988). The AV block and bundle
in the mid-1980s, and even this prevalence of cardio- branch block cases may have been caused by ossification
megaly is likely much lower now given our earlier con- of the central fibrous body and fibrosis and degenera-
sideration of hyperthyroidism in the differential tion of conduction fibers in and around the atrioven-
diagnosis of cats with compatible clinical signs tricular node, as was found in 3 hyperthyroid cats (Liu
(Broussard et al. 1995). However, in cats showing overt et al. 1984).
clinical signs of respiratory embarrassment (mainly Echocardiography is the diagnostic test of choice for
dyspnea), thoracic radiographs remain the diagnostic assessing myocardial structure and function clinically in
test of choice for ruling in or ruling out pulmonary the cat. An echocardiogram is indicated in any cat with
edema, for identifying pleural effusion, and for identify- suspected or confirmed hyperthyroidism if a history of
ing other causes of respiratory signs that might be unre- syncope, presyncope (self-resolving ataxia and disorien-
lated to hyperthyroidism. The current standard of care tation of a few seconds’ duration), or respiratory distress
should include radiographs of the thorax in all cats with is present; if physical examination reveals a heart
suspected or confirmed hyperthyroidism, not princi- murmur, gallop sound, dyspnea, an arrhythmia, or
pally for assessing the heart but for identifying other pulselessness of a limb; if radiography identifies cardio-
intrathoracic abnormalities that could be responsible megaly and/or pulmonary infiltrates or pleural effusion;
for clinical signs otherwise attributed to hyperthyroid- or if an electrocardiogram demonstrates a cardiac
ism (e.g., metastatic neoplasia, primary respiratory arrhythmia. In the absence of all of these signs, the diag-
disease). nostic yield of echocardiography is low and unlikely to
Endocrine Diseases common arrhythmia in cats with hyperthyroidism, cats described structural changes in a majority of them:
alter treatment or prognosis.
Sinus tachycardia has been recognized as the most
The first report of echocardiography in hyperthyroid
occurring in 29–70% of cases (Broussard et al. 1995;
hypertrophy of the left ventricular free wall in 74/103
Peterson et al. 1982; Liu et al. 1984). However, a newer
cats (72%), and interventricular septal hypertrophy in
study identified sinus tachycardia in only 14/202 cats
(14%) (Broussard et al. 1995). Electrocardiographic evi-
dence of left ventricular enlargement consists mainly of 41/103 cats (40%). Such changes resolved in 30–50% of
cats after antithyroid treatment (Bond et al. 1988). A
increased QRS amplitude in the left-sided and caudal more recent survey of 91 hyperthyroid cats, published
ECG leads (II, III, aVF, V2). Historically, such evidence 20 years after the report mentioned above, identified one
was apparent in 22% (Liu et al. 1984), 49% (Peterson et or more abnormal echocardiographic measurements in
al. 1982), 65% (Moïse 1986), and 66% (Broussard et al. only 37% of cases (Weichselbaum et al. 2005). Even with
1995) of hyperthyroid cats; a more recent comparative this lower prevalence of echocardiographic changes, the
study revealed a lower prevalence (34% [Broussard et al. abnormalities mostly appeared to be trivial: for example,
1995]). Overall, this decreasing trend in radiographic the mean interventricular septal thickness in diastole in
and ECG abnormalities is consistent with the general the hyperthyroid cats was 4.4 ± 0.7 mm compared to
decrease in the severity of lesions in feline hyperthyroid- 4.2 ± 0.7 mm in healthy controls, and mean left ven-
ism over time (Broussard et al. 1995). Therefore, the tricular free wall thickness in diastole was 4.7 ± 1 mm
current role of electrocardiography in feline hyperthy- compared to 4.1 ± 0.7 mm. By comparison, the pre- and