What Causes a Heart Murmur in Cats? Key Causes Explained

Finding out from your vet that your cat has a heart murmur can understandably be worrying, especially since any heart-related issue can impact your cat’s wellbeing and longevity. However, it’s important to know that heart murmurs in cats vary widely in severity, and many causes offer treatment options to support your beloved pet.

A heart murmur represents an abnormal sound generated when blood flows turbulently through your cat’s heart valves. Veterinarians detect this unusual noise using a stethoscope. The bit of a “whooshing” or abnormal sound is caused by disrupted blood flow inside the heart, rather than an illness itself, but it often signals an underlying condition worth investigating.

Understanding What a Heart Murmur Is

Heart murmurs are categorised by several factors including the location within the heart where the murmur is most audible, the timing of the sound in relation to the heartbeats, and changes in its nature over time. The loudness or intensity of the murmur is graded on a scale from one to six. A grade one murmur is very faint and may even not be present consistently, whereas a grade six is loud and easily heard, often indicating more serious disruption.

Top 10 Causes of Heart Murmurs in Cats

Heart murmurs can stem from a variety of causes. Knowing these helps in understanding your cat's diagnosis and what treatments might be necessary. Below are the most common causes:

1. Hereditary Heart Defects

Some cats are born with structural defects in their heart walls or valves. Such congenital abnormalities may present as murmurs early on. Interestingly, some kittens may outgrow these murmurs as their heart grows and matures. These birth defects are relatively frequent but often do not severely impair heart function in adult cats.

2. Anaemia

Severe anaemia, which can be triggered by heavy flea or tick infestations, reduces red blood cell levels. This affects the heart's functioning and can create murmurs due to increased workload and altered blood flow. Resolving the underlying cause, alongside nutritional support, often improves the murmur.

3. Pulmonic Stenosis

This condition involves narrowing of the pulmonary artery where blood exits the right ventricle. It creates turbulent blood flow and consequently a heart murmur. Pulmonic stenosis is a congenital condition and its severity varies.

4. Aortic Stenosis

Similar to pulmonic stenosis, aortic stenosis is narrowing of the aortic outflow tract. This obstructs blood flow leaving the left ventricle and can generate a loud murmur. Both stenosis conditions may require careful monitoring and management by your vet.

5. Ventricular Septal Defect

A hole in the wall separating the heart’s chambers (ventricular septal defect) causes abnormal blood flow that can result in a murmur. Surprisingly, small holes may produce more turbulence and louder murmurs than larger holes where blood flows more freely.

6. Mitral Valve Dysplasia

This heart valve malformation causes leakage (regurgitation) of blood backward, disrupting normal flow and causing turbulence that your vet hears as a murmur. It is a congenital abnormality and can vary in impact.

7. Patent Ductus Arteriosus

Shortly after birth, a special fetal heart vessel called the ductus arteriosus should close. When it remains open (patent), blood shunts abnormally between the heart vessels, causing audible murmurs. This defect may require surgical correction in some cats.

8. Tetralogy of Fallot

This is a complex congenital defect involving multiple simultaneous heart abnormalities, including pulmonic stenosis. It results in heart murmurs and requires specialised veterinary care.

9. Hyperthyroidism

One of the most common causes of heart murmurs in older cats, hyperthyroidism leads to excessive thyroid hormone production. This hormone excess increases heart workload and can induce murmurs. Treating hyperthyroidism often reduces or resolves related murmurs.

10. Cardiomyopathy

Cardiomyopathy is a disease affecting the heart muscle itself, causing it to change shape and function less efficiently. It is a leading heart disease in cats and often results in murmurs. While incurable, medication can help strengthen heart muscle function and support your cat's quality of life.

Approach to Diagnosis and Management

Since the causes of heart murmurs are diverse, your vet will often recommend diagnostic tests like echocardiography (heart ultrasound) to pinpoint the exact problem. Blood tests may also be needed to identify systemic causes such as hyperthyroidism or anaemia.

Treatment depends on the underlying cause—ranging from medication to control symptoms, nutritional support, to surgical repair in certain congenital defects. Regular monitoring is crucial to detect any progression early and adjust care accordingly.

Remember, some murmurs are “innocent” or benign, especially in young kittens or stressed cats, resolving naturally without treatment. However, any heart murmur should prompt a thorough veterinary examination to ensure the best outcomes for your feline friend.

By understanding what causes a heart murmur and how it can be managed, cat owners can be empowered to provide attentive care and make informed decisions in partnership with their vet to maintain their cat’s heart health and happiness.