Page 123 - Feline Cardiology

122 Section D: Cardiomyopathies

occurred after two-thirds of systole had been completed thoracic radiographs until moderate left atrial dilation
(Doi et al. 1983). Given the high heart rates present in develops. Thoracic radiographic evidence of cardiomeg-
cats, it may be very difficult to distinguish these subtle aly in cats may help justify to clients the importance of
differences noted in human medicine. It is likely that left further evaluation by echocardiography if they were ini-
ventricular geometry—in particular, cavity size and tially reluctant to proceed with an echocardiogram.
myocardial mass, and the effect of endogenous and Thoracic radiographs are essential for diagnosis of con-
exogenous catecholamines can play an important part gestive heart failure, which may be present in cats with
in the pathogenesis of SAM (Luckie and Khattar 2008). severe HCM or in less severe cases with an inciting
For example, SAM was produced in 9 of 14 (64%) trigger such as intravascular fluid administration.
healthy, echocardiographically normal dogs given dobu- Because left ventricular morphologic changes involve
Cardiomyopathies magnitude of SAM and left ventricular outflow tract ventricular chamber size, thoracic radiographs are not
increased diastolic wall thickness but not increase in
tamine (Sakurai et al. 1995). In 7 of these dogs, the
useful for identifying left ventricular changes, which
pressure gradient varied in a dose-dependent manner.
must be evaluated by echocardiography. Routine bio-
Although it occurs rarely, the authors have identified
chemical tests are not helpful to diagnose HCM, but
SAM on echocardiography in anxious (catecholamine-
driven) cats with structurally normal hearts, and these
and for identification of concurrent disease processes.
cats may not necessarily develop HCM over time but they are useful for overall systemic evaluation of the cat
require follow-up echocardiograms to monitor for pos- Electrocardiography is an insensitive test for diagnosis
sible emergence of HCM. of HCM (see Chapter 18), but is essential for evaluation
The most common differential for SAM of the mitral of an arrhythmia. Systolic blood pressure measurement
valve is mitral valve dysplasia (see Chapter 10). The dys- is necessary to rule out systemic hypertension as a cause
plastic mitral valve is malformed, sometimes elongated of concentric left ventricular hypertrophy.
and thickened, with thick, short chordae tendinae and
abnormally positioned or shaped papillary muscles. The General Diagnostic Tests
mitral valve structure is normal in SAM, but it is pulled
anteriorly (ventrally) into the left ventricular outflow Key Points
tract by hypertrophied papillary muscles. Fixed subaor-
tic stenosis is another differential for SAM of the mitral • Basic comprehensive blood work including complete
valve and is a congenital heart defect. SAS is caused by blood count, serum chemistry, total thyroxine level, and
a fibrotic ring, band, or fibromuscular tunnel in the left urinalysis is recommended in cats diagnosed with HCM,
ventricular outflow tract just below the aortic cusps. especially in middle-aged to older cats, or in cats with
With SAS and SAM, the degree of obstruction changes heart failure.
exponentially based on heart rate and contractility, but • Baseline renal function is important to assess prior to
with SAM, obstruction becomes disproportionately medical treatment of heart failure.
more severe as these parameters increase because the • Significant anemia should be identified, as this may
obstruction is dynamic in nature and most severe in worsen ventricular volume overload (i.e., preload) in
cats with HCM.
mid- or late-systole. In dynamic obstructions such as • Total thyroxine concentration is an essential diagnostic
SAM of the mitral valve and dynamic right ventricular test in middle-aged to older cats with echocardiographic
outflow obstructions causing nonpathologic murmurs, evidence of concentric hypertrophy, because
the murmur often becomes louder when the cat becomes hyperthyroidism is a secondary cause of concentric left
agitated. The Doppler tracing of fixed obstructions such ventricular hypertrophy.
as SAS shows a peak systolic velocity in mid-systole, with
a symmetrical, cone-shaped blood flow velocity pattern.
In dynamic obstructions such as SAM, there is a char-
acteristic late systolic peak velocity, creating a dagger Basic comprehensive blood work including complete
shape with a peak velocity at end-systole (see Figure blood count, serum chemistry, total thyroxine level, and
11.18, later in this chapter). urinalysis is recommended for overall assessment of
aged cats or ill cats. It is important to identify other
DIAGNOSTIC TESTING concurrent disease processes in cats with HCM, because
they may influence treatment recommendations. A total
Overview thyroxine level (and free thyroxine if total thyroxine
Echocardiography is the most useful and sensitive test levels are equivocal) must be measured in all mature cats
for diagnosis of HCM in cats. Because the hypertrophy with concentric hypertrophy of the left ventricle, because
is concentric, overall cardiac size may be normal on hyperthyroidism can cause reversible secondary concen-