There are approximately 60 million cats in the US, of which it is estimated that 11% have heart disease. Of the heart diseases
approximately 85% are hypertrophic cardiomyopathy (hCMy); the prevalence of systemic arterial hypertension (SAH) is unknown
but probably more prevalent than hCMy. The vast majority of other diseases affecting the heart of cats occur rarely. Hyperthyroidism
is the exception and may occur in as many as 1 in 300 cats, with some estimates that 1/5th of them may develop heart failure.
hCMy refers to idiopathic hypertrophy of one or more portions (septum, free-wall, both) of the left ventricle in which myofibers
may be more numerous than normal and in disarray, and with arteriole sclerosis leads to patches of myocardial fibrosis. Hypertrophy
of the septum may result in obstruction of the flow of blood from the left ventricle—termed dynamic outflow tract obstruction—often
accompanied displacement of the anterior leaflet of the mitral valve—termed systolic anterior motion (SAM). The 2 together
may limit outflow from the left ventricle and produce a systolic ejection-type murmur, and may produce mitral regurgitation
with a typical murmur of that physiology. Typically the left ventricle is excessively stiff, fills less and/or more slowly,
and terminates filling more abruptly, giving rise to abnormal gallop sounds. Hypertrophy may be attributed to excessive angiotensin-II
or other cytokines that stimulate proliferation of muscle components, and it certainly, in many cases, a genetic disease.
Systemic arterial hypertension refers to elevation of pressure within the systemic arteries. In humans arterial pressure fluctuates
from approximately 120 mmHg in systole when the ventricle contracts to 80 mmHg in diastole when the ventricle relaxes. There
are no absolute limits of systemic arterial pressure in cats which if exceeded constitute SAH. Upper limits of systolic pressure
may be as high as 180 mmHg (in part because cats become agitated when blood pressure is taken, in part depending upon how
the pressure is measured), and normal diastolic pressure has been described as anywhere from 50 mmHg to 120 mmHg. Arterial
blood pressure is a balance between how much blood the heart ejects (i.e., cardiac output, CO) and the hindrance to flow imposed
by the aorta (impedance) or systemic arterioles (systemic vascular resistance). Thus SAH may result from an elevated CO (but
rarely so), but more so from stiffening of the arterial walls due to constriction of vascular smooth muscle. In cats, the
most common mechanism is probably stiffening of the system arterioles due to primary kidney disease in which the kidney produces
renin which activates vasopressor neuroendocrines (e.g., arginine vasopressin, angiotensin-II) and the adrenergic portion
of the sympathetic nervous system. This is often called renoprival hypertension. Hyperthyroidism results most often from thyroid
adenomas of 12 or both lobes of the gland. These may be palpable in many cats, but of course the detection depends upon experience.
Cats with either hCMy or SAH and with hyperthyroidism may present with similar symptoms/signs. They may be dyspneic due to
either pulmonary edema (with louder than usual breath sounds) or more likely pleural effusion (with softer than usual breath
sounds), heart rate may be elevated, they may have "extra" heart sounds produced by S3 and/or S4 gallops, and they usually
have left ventricular hypertrophy and left atrial enlargement. Those with more severe edema and effusion may be cyanotic and
may "mouth" breath. Pressure pulses are unremarkable, and they will (not infrequently) have arrhythmias. Often cats with hCMy
will present with thromboembolism affecting (usually but not always) the pelvic limbs causing them excruciating pain and paresis.
The greatest number of cats with either hCMy or SAH are asymptomatic; cats with hCMy probably remain asymptomatic for years
or for their entire lives. Cats with SAH often develop; retinal hemorrhages, detached retinae, retinal edema, tortuous retinal
arteries. Their arterial pulses are usually of normal intensity. Because most often SAH in cats is secondary to other diseases,
it is not unusual to observe azotemia from primary renal disease, or denuding and a pendulous abdomen from hyperadrenocorticism,
or hyperglycemia from diabetes.