Heart murmurs in cats are a somewhat common problem, and can vary drastically in severity and scope. A heart murmur refers to an abnormal heart sound that the veterinarian picks up while listening to the cat's heartbeat with a stethoscope. Since a variety of different issues can cause a heart murmur, the prognosis for the feline will vary.
Since heart murmurs vary so widely, vets grade them on a scale of I-VI, with a Grade I heart murmur being the least intense. These heart murmurs will be very quiet, whereas a Grade VI heart murmur is very loud and may even be felt or heard without a stethoscope.
Some murmurs are benign and have no impact on a cat’s health because they are not caused by disease. These murmurs are called "innocent" or "physiologic" heart murmurs and are typically found in young growing kittens, although they may also be detected intermittently in adult cats. Most innocent murmurs are of low intensity and are usually Grade I or II. Vets will not prescribe any specific treatment for these heart murmurs because they have no effect on the cat's health, and will typically disappear on their own over time.
Heart disease in cats may also cause murmurs. Structural heart problems may be congenital—meaning a cat is born with a defect, or may be acquired—meaning it is a disease which develops later in life. The magnitude of the heart murmur will vary in these conditions but often will be grade III or higher in intensity.
The two most common congenital defects are ventricular septal defect (VSD) or atrial septal defect (ASD), both of which will cause heart murmurs in young cats. Cats may also be born with defective heart valves which can also cause murmurs. The most common acquired disease seen in older cats is called hypertrophic cardiomyopathy which is a thickening of the heart muscle which causes a disturbance in the flow of blood, which results in a heart murmur. Murmurs can also be caused by a variety of fevers and infections, though these are less common.
Owners should realize the heart murmur itself is not what's harmful. Instead, the murmur is possibly indicative of an underlying issue in their cat. Your veterinarian will likely recommend chest radiographs (x-rays) or possibly an ultrasound of your cat’s heart in order to identify the underlying cause of the heart murmur. Then, if necessary, medication or other therapy will be prescribed to address the underlying heart disease, if present.