Page 188 - Small Animal Internal Medicine, 6th Edition

160 PART I Cardiovascular System Disorders

asymptomatic HCM is estimated at 5 years. Between 20% alveolar pulmonary edema infiltrates (Fig. 8.1, C and D). The
to 40% of cats diagnosed with HCM will eventually develop radiographic distribution of pulmonary edema is variable; a
VetBooks.ir CHF; between 5% to 10% suffer an arterial thromboembo- diffuse or multifocal distribution throughout the lung fields
is common, in contrast to the characteristic perihilar distri-
lism; and approximately 20% of cats with HCM experience
sudden cardiac death.
effusion also is common (Fig. 8.1, E and F).
Subclinical (occult) HCM can be detected using echocar- bution of cardiogenic pulmonary edema seen in dogs. Pleural
diography. However, echocardiography is typically only
undertaken in cats where a murmur, arrhythmia, or gallop ELECTROCARDIOGRAPHY
sound is heard on routine examination. Heart murmur prev- Most cats with HCM have an underlying normal sinus
alence in apparently healthy cats ranges from 20% to more rhythm. Sinus tachycardia is common in hospital environ-
than 40% (see p. 11). Among cats with heart murmurs, the ments; either sinus tachycardia or bradycardia can occur in
reported prevalence of HCM based on echocardiography has cats with CHF. Electrocardiogram (ECG) complex abnor-
ranged from about 33% to more than 50%. Conversely, malities that can be seen in cats with HCM include criteria
among cats diagnosed echocardiographically with HCM, the for LA or LV enlargement and a left anterior fascicular block
prevalence of heart murmurs varies from 30% to 80% in pattern (see Fig. 8.2 and Chapter 2). Occasional ventricular
different reports. Most murmurs in HCM are caused by arrhythmias are common. A small study of Holter monitor-
dynamic left ventricular outflow tract (LVOT) obstruction. ing in cats showed that all cats with asymptomatic HCM had
Murmur auscultation is thus a crude screening test for sub- at least 1 ventricular premature complex (VPC) per day,
clinical HCM, and tends to preferentially identify cats with though the overall number of VPCs was relatively low (geo-
HOCM yet missing cases of nonobstructive HCM. metric mean of 124 ventricular complexes/24 hours). More
Symptomatic cats most often are presented for respiratory clinically significant arrhythmias such as AF or high-grade
signs (indicating CHF) or acute thromboembolism (see AV block can occur in cats with severe HCM (often with
p. 224). Respiratory signs include tachypnea, open-mouth concurrent CHF). An ECG is too insensitive to be useful as
breathing associated with activity, or dyspnea; unlike in a screening test for HCM but can be useful to characterize
dogs, coughing is an uncommon clinical sign of CHF in cats. concurrent arrhythmias.
Disease onset can seem acute in sedentary cats, even though
pathologic changes have developed gradually. Occasionally, ECHOCARDIOGRAPHY
lethargy or anorexia is the only evidence of CHF. Some cats Echocardiography is the best means of diagnosis and dif-
have syncope or sudden death in the absence of other signs. ferentiation of HCM from other disorders. The extent of
Stresses such as anesthesia, surgery, fluid administration, hypertrophy and its distribution within the ventricular wall,
systemic illnesses (e.g., fever, anemia), recent injection of septum, and papillary muscles is shown by two-dimensional
long-acting corticosteroids, or boarding can precipitate CHF (2-D) and M-mode echo studies (Fig. 8.3). Doppler tech-
in an otherwise compensated cat. niques can demonstrate LV diastolic or systolic abnormali-
ties. Widespread myocardial thickening is common, and the
RADIOGRAPHY hypertrophy is often asymmetrically distributed among
various LV wall, septal, and papillary muscle locations. Focal
Diagnosis areas of hypertrophy also occur. Use of 2-D–guided M-mode
Although the cardiac silhouette appears normal in most cats echocardiography helps ensure proper beam position. Stan-
with mild HCM, radiographic features of advanced HCM dard M-mode views and measurements are obtained, but
include prominent LA and variable LV enlargement (Fig. 8.1, thickened areas outside these standard positions also should
A and B). Radiographically apparent LA enlargement gener- be measured (using 2-D or perpendicularly aligned M-mode
ally occurs only when echocardiographic measurements images). The 2-D right parasternal long-axis view is useful
suggest severe LA enlargement (ratio of LA to aortic size > for measuring basilar IVS thickness. The diagnosis of early
2.0). The classic valentine-shaped appearance of the heart on disease may be questionable in cats with mild or only focal
dorsoventral or ventrodorsal views is not always present, thickening. Falsely increased thickness measurements (pseu-
although usually the point of the LV apex is maintained. dohypertrophy) can occur with dehydration and sometimes
Vertebral heart score (VHS) can be useful when differentiat- tachycardia. Spurious diastolic thickness measurements also
ing cardiac versus noncardiac causes of respiratory distress arise when the beam does not transect the wall/septum per-
in cats; VHS of > 9.3v suggests significant heart disease pendicularly, when the septal tricuspid leaflet is included in
(normal VHS in cats is about 7.5v). Enlarged and tortuous the IVS measurement, or when the measurement is not taken
pulmonary veins might be noted in cats with chronically at the end of diastole, as can happen without simultaneous
high LA and pulmonary venous pressure. However, the ECG recording or when using 2-D imaging of insufficient
pattern of pulmonary vascular change is inconsistent in frame rate. A (properly obtained) end-diastolic LV wall or
feline CHF; pulmonary artery enlargement reportedly occurs septal thickness greater than 6.0 mm is considered diagnos-
in approximately two thirds of cats with CHF and may be tic for LV hypertrophy in cats, though a thickness of 5.5 to
even more common than lobar vein enlargement. Left-sided 5.9 mm is likely abnormal except in cats of very large body
CHF produces variable degrees of patchy interstitial or size. Cats with severe HCM can have diastolic LV wall or