How are 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 Weakness 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)
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.