Overview of Feline Heart Murmurs
Heart murmurs are sounds created by high velocity, turbulent or disturbed blood flow in the heart or blood vessels. Heart sounds and murmurs are detected by listening to the heart with a stethoscope, an examination called “cardiac auscultation”. Murmurs are associated with a variety of causes including congenital malformations (birth defects), heart diseases acquired during life such as cardiomyopathy, or by diseases of other organs that affect the heart indirectly, as with hyperthyroidism. Importantly, nearly half of all heart murmurs in cats are caused by the sympathetic nervous system (or adrenaline) acting on an otherwise normal heart. Heart murmurs may affect cats of any age, sex, or breed although some cats breeds are more prone to certain heart diseases.
From a practical standpoint, feline heart murmurs can be classified into four broad categories:
1) Functional murmurs associated with a normal heart;
2) Murmurs due to congenital heart disease;
3) Murmurs associated with cardiomyopathies or heart muscle disease; and
4) Murmurs related to diseases of blood vessels.
Unlike dogs, heart murmurs in cats are rarely caused by degenerative valve problems or by infection of the valve.
The term “functional murmur” indicates that the heart is structurally and functionally normal based on a cardiac ultrasound examination. Some functional murmurs detected in kittens are probably due to an immature heart and these murmurs disappear as the pet reaches about 4 months of age. These functional murmurs are often called “innocent” by veterinarians.
However, the majority of functional murmurs are thought to develop from the cardiac effects of the sympathetic nervous system. Most people recognize the effects of adrenaline and sympathetic nerve chemicals (nor-epinephrine) on their own hearts. These increase with stress, exercise, and illness, making the heart beat faster and stronger. The increase in strength of contraction causes a faster ejection of blood from the ventricles, often creating a murmur. Since most cats removed from their home experience some degree of stress, functional murmurs are frequently detected during veterinary examinations. Functional murmurs can also develop with some illnesses, including fever, anemia, and high levels of thyroid hormone (hyperthyroidism). These murmurs are indistinguishable from those caused by congenital or acquired heart diseases.
Congenital heart disease means the heart did not form properly prior to birth. Collectively these are called “cardiac malformations”. Most common in cats are holes in the heart (ventricular and atrial septal defects) and malformations of the mitral valve. Congenital heart defects allow blood to move in an abnormal direction or cause a valve to malfunction, resulting in high-velocity and turbulent blood flow. These are usually detected during a kitten’s wellness examination. The majority of cat owners have no idea there is anything wrong with their cat’s heart.
Cardiomyopathies are common and represent the most important acquired heart diseases in this species. The word “cardiomyopathy” simply means that the heart muscle is diseased. The most common form is genetically determined and results in a thickened left ventricle (hypertrophic cardiomyopathy). There are other forms of cardiomyopathy as well, including some “secondary” cardiomyopathies associated with chronically high blood pressure (hypertension), hyperthyroidism, or diabetes mellitus (sugar diabetes). These cardiomyopathies are discussed under separate topics on this site. Cardiomyopathies can induce heart valves to leak or create obstruction to blood flow within the ventricular chambers, dysfunctions that can generate a murmur. Cardiomyopathies in some cats are very serious, leading to heart failure, dangerous blood clots (thromboembolism), or sudden death. This is the main reason veterinarians are so concerned when they detect a heart murmur in a mature cat and why they recommend additional diagnostic tests.
Diseases centered in the blood vessels sometimes lead to heart murmurs although the precise reasons are not completely understood. As in humans, systemic hypertension (high blood pressure) is common in older cats. This can lead to dilation of the aorta, thickening of the left ventricle, and a heart murmur. In older cats the aorta sometimes dilates for uncertain reasons (a condition called “idiopathic aortic dilation” or “aortoannular ectasia”): many of these cats also have a cardiac murmur. Pulmonary hypertension (high pressure in the arteries of the lung) can also cause a murmur. This condition can follow heartworm infection or chronic lung disease.
Murmurs are often classified based by their location, timing, duration, character and intensity or grade. For example:
Although the classification of a murmur might help your veterinarian prioritize diagnostic considerations, feline auscultation is rarely sufficient to make a definitive diagnosis or to reliably separate a functional murmur – one with a structurally normal heart – from a malformation or cardiomyopathy. Additional tests are needed and might include a blood test for anemia or hyperthyroidism, or a highly specialized blood test that measures a heart hormone, NTpro-BNP. Most older cats with murmurs should have a blood pressure measured. Definitive diagnosis requires an echocardiogram (cardiac ultrasound) with Doppler studies. The prognosis (outlook) for animals with heart murmurs depends on the underlying cause of the murmur such as the type and extent of any existing heart disease.