ETIOLOGY AND PATHOPHYSIOLOGY
cardiomyopathy (HC) is the most prevalent feline cardiac disorder, its
relative importance having increased with the decline in the incidence
of dilated cardiomyopathy (DC). It is a disease of middle-aged
cats (average 6.5 years), but all ages are affected. There is a
male predisposition, with > 75% of cases being male. In humans,
there is an important hereditary predisposition for HC in 55% of cases.
In people, this disorder may be congenital or acquired, and probably represents
a group of diseases. Although the etiology of feline HC is unknown,
the Persian and Maine coon cat have appeared to be predisposed in some
case series, suggesting a genetic influence. A controlled study
in our laboratory showed a trend toward a predisposition for Maine coon
cats to HC. Interesting work by Kittleson and associates suggests
a potential etiologic role for excessive growth hormone secretion in some
cases. As is the case with systemic hypertension, hyperthyroidism,
and aortic stenosis, HC is associated with marked left ventricular hypertrophy,
but in this instance, no underlying cause can be identified.
lesions are typified by severe left ventricular concentric hypertrophy and
secondary left atrial dilatation. Asymmetric septal hypertrophy (ASH), present
in the majority of dogs and humans with HC, is present in only 30% of cats
with HC. Histological cardiac myofiber disarray is reported in 27%
of affected cats and only in those with asymmetric septal hypertrophy.
Other histological features of feline HC include myocardial and endocardial
fibrosis and narrowed coronary arteries. Dynamic aortic outflow obstruction,
secondary mitral insufficiency, myocardial ischemia, and systemic arterial
embolism (SAE) may complicate this syndrome.
The left heart is predominately affected and clinical
signs manifested as sudden death or, more commonly, acute left heart failure
due to diastolic dysfunction. Systolic function is usually adequate
or enhanced. Tilley and Lord demonstrated an elevated resting left
ventricular end diastolic pressure (LVEDP) in feline HC. With the
administration of isoproterenol, mimicking endogenous, stress-related
sympathoadrenal activity, the LVEDP pressure doubled. Left ventricular
end diastolic pressure is indicative of pressures in the left atrium and
pulmonary veins, which reflect the tendency for the development of pulmonary
edema. In addition, during stressful situations, acceleration of the
heart rate reduces cardiac filling time and myocardial perfusion.
The former further diminishes cardiac volume and the latter results in relative
myocardial ischemia in a rapidly beating heart with high oxygen needs, thereby,
aggravating diastolic dysfunction. Stressful incidents, such as a
car ride, restraint for an ECG, confrontation with a dog, or an embolic
event may precipitate in left heart failure and pulmonary edema.
the aid of ECG, thoracic radiographs, and echocardiography, a high percentage
of cases of HC are diagnosed prior to the onset of symptomatology.
Suspicion is raised in such instances when the attending clinician discovers
a murmur, gallop, or arrhythmia. At the other end of the spectrum,
cats may die unexpectedly with no prior signs. The most common clinical
sign is the sudden onset of dyspnea, with or without evidence of SAE (the
prevalence of which has ranged from 16 to 48%, in clinical and autopsy studies,
respectively). Physical examination typically reveals a well-fleshed,
dyspneic cat with audible pulmonary crackles, murmur (50% of cases) typically
loudest at the left apex, gallop (40%, usually S4), and/or arrhythmia (25
to 40% of cases). Heart sounds may be muffled. The oral mucosa
is ashen, the pulses normal, weak, or absent (SAE), the apex beat may be
hyperdynamic, and the liver may rarely be palpably enlarged. Cats
with HC are generally not hypothermic, providing information useful in differentiation
of HC is not difficult, but does require special testing to confirm clinical
suspicions. Without the aid of echocardiography, dilated and restrictive
(RC) cardiomyopathies can be difficult to distinguish from HC. This
distinction is especially important in the case of DC, because it requires
an entirely different therapeutic approach and prognosis. Other disorders
that produce left ventricular and septal hypertrophy, such as hyperthyroidism,
systemic hypertension, and aortic stenosis, must also be ruled out.
The ECG is abnormal in 35 to 70% of cases and can
provide useful diagnostic information. Many ECG findings are not specific,
but left axis deviation and left anterior fascicular block are strongly
suggestive of HC, but also may be recognized in RC, hyperkalemia, hyperthyroidism,
and, rarely, DC. Other ECG abnormalities include P-mitrale and
P-pulmonale (10% and 20%, respectively), tall R waves (40%), wide
QRS complexes (35%), conduction disturbances (50%, including left axis deviation
in 25% and left anterior fascicular block in 15%), and arrhythmias (55%,
usually ventricular in origin).
radiographic findings suggestive of HC include cardiomegaly with a prominent
left ventricle and atrium, and pulmonary congestion and/or edema.
In the ventrodorsal projection, the heart may appear "valentine-shaped,"
reflecting the concentric ventricular hypertrophy and enlarged left auricle.
Additionally, the apex is often shifted to the right. On the lateral
view, the heart is enlarged with increased sternal contact, left atrial
prominence, left ventricular convexity, and a prominent caudal cardiac waist.
Pleural effusion may be noted in 25 to 33% of cases in heart failure, but
is usually of much less volume than that noted in DC. Nonselective
angiography is of less risk in HC than in DC. This procedure typically
reveals normal or enhanced circulation, pulmonary venous tortuosity, left
atrial enlargement, small left ventricular lumen, thickening of the left
ventricular wall, and papillary muscle enlargement. The diagnosis
of SAE (usually located at the aortic trifuracton: saddle thrombus) can
be confirmed by the finding of an abrupt termination of the dye column in
the aorta at its trifurcation.
is extremely useful for distinguishing HC from DC, but, because of overlap
of echocardiographic reference values, differentiation of normal from asymptomatic
HC and HC from RC may be difficult. Concentric left ventricular hypertrophy
and left atrial enlargement are features useful in confirming the diagnosis
of HC. Cardiac function is normal to exaggerated, due to diminished
afterload and possibly hypercontractility. Systolic anterior mitral
valve motion may be evident, suggesting dynamic aortic outflow obstruction.
If present, ASH, left atrial thrombi, pleural effusion, and/or pericardial
effusion may be evident.
case of sudden death, the diagnosis is made at necropsy by disclosure of
typical gross and histologic cardiac pathology. Other laboratory findings,
with the exception of hypotaurinemia, are similar to that described for
feline DC. Differential diagnoses are also similar with the addition
of restrictive pericarditis and emphasis of systemic hypertensive and hypertrophic
thyrotoxic heart disease.
of HC is different than that of DC and entails the goals of reducing LVEDP,
abolishing sinus tachycardia and other arrhythmias, improving myocardial
oxygenation, and alleviating and preventing pulmonary edema. Positive inotropic
agents are not needed and generally contraindicated because they may increase
LVEDP and aggravate outflow obstruction. The latter precaution should
be exercised in the use of arterial vasodilators and, to a lesser degree,
preload reducing agents (diuretics and mixed or venodilators).
therapy is indicated to eliminate pulmonary edema. Furosemide is the
diuretic of choice in emergencies because it reduces LVEDP and, hence, left
atrial, and pulmonary venous pressures through diuresis and venodilation.
In the emergency situation, treatment with parenteral furosemide (2-4
mg/kg IV or IM) is accompanied by the use of topical nitroglycerin (1/8-1/4
inch tid-qid for first 24 hours, then “8 hours on, 8 off” only if
necessary) and oxygen supplementation (40%). Although furosemide diuresis
is usually successful, the addition of enalapril (.25-.5 mg/kg sid) is indicated
in refractory cases or when biventricular failure (pleural effusion) ensues.
that enhance ventricular relaxation and slow the heart include the beta
adrenergic (atenolol), and calcium channel (diltiazem) blockers. Such
therapy is indicated in treatment of the diastolic failure of HC.
Beta blockers improve diastolic indirectly, enhancing ventricular filling
by reducing heart rate and improving myocardial perfusion. Traditionally,
beta-blockers have been administered orally after stabilization (24 to 36
hours after institution of diuretic therapy) to reduce and prevent elevations
in LVEDP, to lower systolic pressure gradients and myocardial oxygen requirements,
to prevent stress-induced tachycardia and reduce resting heart rate,
and for its antiarrhythmic effects. When arrhythmias are present,
this drug may be initiated earlier in the disease course. This is
the author’s treatment of choice for asymptomatic HC, for cats with documented
outflow obstruction (HOCM), and when tachycardia persists..
channel blocking agents have been effective in human HC by reducing heart
rate, myocardial oxygen consumption, and diastolic dysfunction. In
addition to directly enhancing myocardial relaxation, these drugs dilate
peripheral and coronary arteries. Bright has demonstrated the utility
of diltiazem (3-7.5 mg po tid) in the treatment feline HC, including those
cases refractory to the beta-blocker, propranolol. Unfortunately,
current packaging for human use, makes accurate feline dosing of diltiazem
difficult. Long-acting diltiazem may be substituted and includes Cardizem
CD (45 PO sid; requires disassembling capsules) or Dilacor (30 mg PO bid;
requires disassembling capsules). Combining a calcium channel blocker
and a beta blocker has theoretical advantages and is often done, using a
long-acting form of each drug, one in the morning and one in the evening.
report by Rush, et al. demonstrated a reduction in wall thickness with the
administration of enalapril to cats with HCM. This suggests a potential
role for ACE-inhibitors in the treatment of HCM. These drugs are generally
safe and do play a role in cases which are refractory or in which pleural
effusion is present. In asymptomatic patients, it is logical
that the renin-angiotensin-aldosterone system is not pathologically activated,
and hence ACE-inhibitors would not be useful. The study referred to
argues that they may play a role, however. Further studies are being
planned. Enalapril is used at .5 mg/kg daily.
other than those described above should be used sparingly and with
caution (see section on DC). Digoxin, while generally contraindicated
in HC, may be used when supraventricular arrhythmias are refractory to calcium
channel and beta adrenergic blocking agents.
therapies, including oxygen, aspirin, home confinement, and moderate salt
restriction should be instituted as needed. Taurine supplementation
is not necessary in the treatment of HC. In asymptomatic cats with
HC, the author advises home confinement, moderate salt restriction, Beta-
and/or calcium channel blockade, and aspirin indefinitely.
with asymptomatic HC should be evaluated at 12 month intervals, while those
with symptoms should ideally be seen more frequently until stabilized for
a period of time. The prognosis for asymptomatic HC is guarded to
good, with a median survival of over 5 years. Cats presented in heart
failure survive a median of approximately 3 months (though 20% live longer
than 3 years), while cats with emboli survive a median of approximately