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:
- Location refers to the area over the heart in which the sound appears to originate or has maximal intensity. While this is classically described by proximity to a valve area, in cats the heart is so small that isolating valve areas is difficult. Accordingly, most describe the location relative to the left or right sternal border and indicate if the location is caudal (apical) or more cranial.
- Timing simply indicates when in the heart beat cycle the murmur is heard. Murmur can develop during ventricular pumping (systolic murmurs) and rarely during filling (diastolic murmurs). Continuous murmurs most often indicate a congenital heart defect, patent ductus arteriosus, which can be corrected surgically. Some systolic murmurs are shorter whereas others are detected throughout pumping (holosystolic).
- Duration refers to how long the sound lasts within the timing phase.
- Character of the murmur refers to the quality of the sound. Sometimes functional or innocent murmurs are vibratory or musical, but most have a harsh (mixed-frequency) character.
- Grade refers to the intensity or loudness of the murmur with murmurs graded from 1 (softest) to 6 (loudest). Some murmurs are accompanied by a “thrill” which indicates that the murmur is so loud it causes a vibration that can be felt over the chest wall. Except for very loud (grade 5 to 6) murmurs, the grade does not predictably relate to the cause of the murmur or to the severity of any underlying heart disease.
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.
How are Heart Murmurs Created?
Each heartbeat originates as an electrical impulse and eventually generates a muscular movement. The heart is a muscle that’s “job” is to pump blood. Blood is pumped from the body to the heart, processed through the lungs, returned back to the heart and pumped out to the body. As the blood is pumped, it goes through four different parts of the heart or heart chambers. Between these chambers are “valves”. A valve is a membrane that opens and closes to prevent backward flow of blood.
Heart murmurs can be caused from either heart muscle or valve abnormalities. If either of these structures do not function properly, the flow of blood is disturbed. This disturbance can cause “turbulence” in the blood flow that creates an abnormal sound that we refer to as a “murmur”.
What to Watch For
A murmur is an abnormal heart sound and only discovered by listening to the heart with a stethoscope. This requires some skill because of the very fast feline heart rate and the fact that some murmurs come and go. Infrequently a vibration or “thrill” can be felt over the heart. Murmurs do not cause any symptoms, but if the underlying cause is a heart or body system disease, the signs associated with a murmur might include:
- Many cats display no symptoms, including some with serious heart disease (!)
- Nonspecific signs such as reduced activity or interest in surroundings
- Collapse or sudden loss of consciousness (syncope)
- Sudden limb paralysis (thromboembolism)
- Slow heart rate (bradycardia)
- Fast heart rate (tachycardia)
- Erratic heart rate (arrhythmia)
- Rapid or difficult breathing (respiratory distress, “tachypnea”, “dyspnea”)
- Lack of appetite (anorexia)
Diagnosis of Cause of Murmurs in Cats
When a murmur is present, the goal is to determine the underlying cause and in particular to exclude the presence of a serious heart disease that might require therapy. Preliminary tests based on the cat’s age and other findings may include blood tests, urinalysis, chest x-rays (radiographs), or an electrocardiogram (EKG). Although x-rays and an EKG can detect moderate to severe heart disease, or complications of heart failure, these two tests are often normal in mild heart disease and in cats with hypertrophic cardiomyopathy. For these reasons, a proBNP test and an echocardiogram most often recommended when a heart murmur is uncovered in a completely healthy cat.
- The heart produces a hormone called B-type natriuretic peptide, which contributes to the body’s normal fluid regulation. Disease of the heart often increases the formation and release of this hormone into the blood. The hormone is split into an active form and inactive portion (NT-proBNP) that can be measured at a reference laboratory or with a new in-hospital test. Due to the higher cost of an echocardiogram, many veterinarians will perform this test first. This is NOT a definitive test for heart disease, but helps to establish the “risk” for heart disease. In general, a low value means the cat is unlikely to have serious heart disease. A high level means the risk of serious heart disease is greater, but it should be appreciated that some normal cats also have higher levels for uncertain reasons. Any cat with a high level should have an echocardiogram with Doppler, if possible.
- The “gold standard” for diagnosing the underlying cause of a heart murmur is a cardiac ultrasound (echocardiogram) optimally performed with a color Doppler study to evaluate blood flow. A full “Echo” can identify and further evaluate heart size, function, and valve function and usually identify the likely cause of a murmur. Murmurs that occur in puppies and in cats can be very difficult to diagnose without a detailed Echo. For that reason, even for the most experienced veterinarians will often refer pets to a board certified veterinary cardiologist for further evaluation.
- Thoracic (chest) radiographs (X-rays) may help determine if heart disease or heart failure is present. However these have a low sensitivity for mild heart disease or a thickened left ventricle. In a healthy cat, the value of this test is lower.
- Blood tests, including a complete blood count and biochemical profile, may be performed to look for any underlying abnormalities. Some cats may be anemic, have an elevated white blood cell count, or have other organ dysfunction.
Possible Causes of Cat Murmurs
Murmurs can be caused by a variety of cardiac and non-cardiac diseases. Some diseases can affect the heart secondarily but are not caused by an underlying heart disease.
Cardiac Causes of a Murmur
- Subaortic stenosis
- Pulmonary stenosis
- Septal defects
- Atrioventricular dysplasia
- Tetralogy of Fallot
- Mitral Stenosis
- Aortic Valvular Insufficiency
- Pulmonic Valvular insufficiency
- Patent Ductus Arteriosus
- Arteriovenous fistula
- Chronic Valvular Heart Disease in Cats – Endocardiosis
Non-cardiac Causes of a Murmur
- Anemia in Cats
- Infection causing endocarditis
- Heartworm Disease
- Pericardial Disease
Treatment of Cat Heart Murmurs
Treatment depends on the underlying disease. There are a variety of causes of heart murmurs and each is managed differently. Some are serious and require life-long medications or even surgery. Others are innocuous and do not require any treatment at all.
Prognosis for Murmurs in Cats
The prognosis of pets with a murmur is variable. It depends on the underlying cause for the murmur.
Home Care and Prevention
There is no home care for heart murmurs, except that you should administer any medications your veterinarian prescribes. If you suspect that your pet has a heart murmur, you should discuss diagnostic options with your veterinarian. Animals with heart murmurs should be monitored periodically to determine if disease is progressive.